CONSTRIBUTIONS TO THE CONFERENCE

Quantum Optics III, Pucón – Chile, November 27th - 30 th, 2006

 

 

Abanto Peralta Miguel

Ficek Zbigniew

Orozco Luis

Bachor Hans - Albert

Fonseca-Romero Karen

Orszag Miguel

Blackburn Paul

Gilchrist Alexei

Padúa Sebastiao

Barreiro Julio T

Guzmán Robert

Paz Irismar

Barreto Lemos Gabriela

Hennrich Markus

Paz Juan Pablo

Becker Wilhelm

Hor-Meyll Malena

Polzik Eugene

Bergou János

Jauregui Rocio

Pozo Rogelio

Bernardes Nadja Kolb

Jiménez Omar

Rempe Gerhard

Bjork Gunnar

Khoury Antonio

Rodríguez-Lara Blas Manuel

Blatt Rainer

Kimble H. Jeff

Romero Guillermo

Bouda Jan

Klimov Andrei B.

Salles Alejo

Buchleitner Andreas

Ladrón de Guevara María Loreto

Sánchez-Soto Luis

Buhmann Stefan Yoshi

Lastra Freddy

Santos Marcelo França

Cacheffo Alexandre

Lezama Arturo

Savels Tom

Céleri Lucas

Leuchs Gerd

Schmid Christian

Corbalán Ramón

Lin Hai Qing

Spehner Dominique

Cosme da Silva Olavo

López Carlos

Steinberg Aephraim M.

Cotta Eduardo

Lucio M. José Luis

Tabosa José

Davidovich Luiz

Maccone Lorenzo

Tasca Daniel

Delgado Aldo

Martinelli Marcelo

Torres Fabián

de Martini Francesco

Meiser Dominic

Toschek Peter E.

de Matos Filho L.

Ming Yang

Valente Paulo

de Melo Fernando

Molina Mario

VanDevender Aaron

de Sousa Villar Alessandro

Monken  Carlos

Villas Boas Celso

Dodonov Alexander

Mundarain  Douglas

Vogel Werner

Drumond Raphael

Nadyne Cassemiro Katiuscia

Walborn Stephen

Eberly Joseph H.

Nemes María Carolina

Wallentowitz Sasha

Eckart Michael

Nic Chormaic Sile

White Andrew

Eschner Jürgen

Olaya-Castro Alexandra

Zhu Shi-Yao

Ether Diney

Olivares-Rentería Georgina

Zoller Peter

Fassioli Olsen Francesca

Oliveira José Geraldo

 

 

 

 

Abanto Peralta Miguel

Centro de Ciências Biológicas e da Natureza, Campus de Cruzeiro do Sul, Universidade Federal do Acre, Cruzeiro do Sul, Acre, Brazil.

and Instituto de Física, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil.

 

Physical limits for the implementation of quantum computers in Si far infrared cavities

 

We study the possible implementation of quantum optical architectures, like the ones currently implemented at the few-qubit level in trapped ion systems, to the silicon semiconductor environment. We estimate lower bounds for two critical physical quantities required for the implementation o four proposals: (i) The Si cavity quality factor and (ii) The spin-orbit coupling of an electronic excited state of the donor involved in a spin flip Raman transition. We show that if these hurdles are surpassed the errors in the implementation of logic gates would be well bellow the threshold established by Preskill for scalable quantum computation. Our study is also of relevance for the development of other solid-state quantum computer architectures, systematically establishing criteria for the applicability of quantum control operations already demonstrated in atomic and optical systems to a solid-state environment.

    

 

Bachor Hans- Albert           

ARC COE for Quantum-Atom Optics, Australian National University, Canberra, Australia

 

Experiments with spatial squeezing: opportunities for quantum information and precision experiments

 

New techniques have been developed for the generation of squeezed light directly in the higher order spatial modes [1]. At the same time we have built a complete theoretical framework for the optimization of spatial measurements. We have techniques for analyzing the performance for various spatial detectors and design systems for the close to perfect detection of the spatial properties of light. We find that the displacement (position) and tilt (momentum) of  a laser beam are very practical parameters for optical quantum information transmission.

By combining these advances in optical physics now have all the tools required to encode spatial quantum information on CW laser beams, to transmit and to detect the information with very high efficiency. In this talk we are presenting these building blocks and show demonstration of measurements that go beyond the limit set by classical optics.  We show spatial measurements below the quantum noise limit [2] and our attempts to show EPR correlation of the position and momentum of laser beams.

[1] M. Lassen, V. Delaubert, C. C. Harb, P. K. Lam, N. Treps, and H-A. Bachor Generation of squeezing in higher order Hermite-Gaussian modes with an optical parametric amplifier  J. European Optical Society,    rapid publications 1, 06003 (2006).

[2] V. Delaubert, N. Treps, C. C. Harb, P. K. Lam, and H-A. Bachor Quantum measurements of spatial conjugate variables: displacement and tilt of a Gaussian beam: Optics Letters 31, 1537 (2006).

    

 

Blackburn Paul

Facultad de Física, Pontificia Universidad Católica de Chile, Santiago, Chile

 

A single-ion stochastic quantum processor

 

We propose a scheme for implementing a single-ion Stochastic Quantum Processor using a single cold trapped ion. The processor implements an arbitrary rotation around the z axis of the Bloch sphere of a data qubit, given two program qubits; that is, the operation realized on the data is determined by using different program qubits and not by varying the gate itself. Unfortunately this cannot be done deterministically, and must be necessarily stochastic. In this proposal the operation is applied successfully with probability p = 3/4..

    

 

Barreiro Julio T.

Department of Physics, University of Illinois, USA

 

Hyperentanglement for Quantum Communication

 

Most quantum communication protocols rely on the creation and analysis of entangled photons.  Advanced and efficient protocols are possible by encoding multiple qubits per photon.  For example, unambiguous, 100%-efficient Bell-state analysis is only possible with linear elements by incorporating hyperentanglement-simultaneous quantum correlations in more than one degree of freedom.  Using spontaneous down conversion in type-I phase-matched crystals, we recently generated pairs of photons whose state is a tensor product of polarization, orbital angular momentum (OAM) and energy-time entangled states.  We utilize this source for remote entangled-state preparation (RESP), a novel quantum communication protocol in which, conditional on the positive outcome of a measurement on her qubit, Alice can remotely prepare Bob's photon in an arbitrary 2-qubit state.  RESP is potentially simpler than teleportation because Alice prepares Bob's photon without Bell-state analysis and Bob retrieves his state with only one bit of classical communication.  Using our hyperentangled source, we remotely prepared a variety of states, including a full set of single-photon polarization-OAM Bell states, with fidelities greater than 90%, and tangle greater than 80%. A similar experimental setup enables the realization of hyperentanglement-assisted Bell-state analysis.

    

 

Barreto Lemos Gabriela

Departamento de Física, ICEx, UFMG, Belo Horizonte, Brazil

 

Quantum dynamics of bosons in a double-well potential: Josephson oscillations, self-trapping and ultralong tunneling times

 

We investigate the tunneling process of a Bose-Einstein Condensate (BEC) in a double well potential from the point of view of a many body Hamiltonian. Although the model for the process is not realistic for large enough number of particles, it points out a completely different explanation for the non-observation of tunneling. We also observe a “phase transition” at the same value of the population imbalance as in Michael Albiez et al, Phys. Rev. Lett. 95, 010402 (2005). However, in the present work, this phase transition is related to spectral properties of the Hamiltonian and the initial conditions which, for large enough initial population imbalance, presents occupation probabilities concentrated in doublets. The period is approximately given by t ~ / ∆E. Since ∆E becomes smaller as N increases, we show that tunneling will be in principal present, although far from realistic experimental observation times.

    

 

Becker Wilhelm

Max-Born-Institut, Berlin, Germany

 

At to second dynamics of intense-laser induced atomic processes

 

Laser-induced electronic processes in atoms proceed on a time scale of the order of a fraction of the laser period. For the commonly used titanium-sapphire laser with its period of 2.7 fs, this is well within the at to second regime. The S-matrix formalism used to describe these processes is usually believed to yield no insight into the internal time-dependent dynamics. However, if an S-matrix element is expanded in terms of "quantum orbits", time-dependent information can be extracted. The dynamics of processes that are well understood then provide an internal clock, which can be used to time other processes. Near-infrared few-cycle and/or UV at to second pulses are helpful for this purpose, but neither is required. The formalism will be sketched and examples will be given.

    

 

Bergou János

Department of Physics, CUNY Hunter College, Now York, USA

 

Discrimination of quantum states with selected applications

 

It has long been assumed that, in order to discriminate between quantum states, the states to be discriminated must be known.  In this talk I’ll focus on recent developments in the area of discrimination between unknown quantum states and its relation to the discrimination of mixed quantum states.  Possible linear optical implementations of the generalized measurements (POVMs) that are optimal for discrimination will also be given. The usefulness of the discrimination between unknown states will be illustrated on the examples of quantum key distribution protocols and operator discrimination.

    

 

Bernardes Nadja Kolb

Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil

 

Polarization of light in multimode quantum states

 

In this work we characterize the polarization states using the Stokes parameters. More specifically, we are interested in the two photon states of polarization generated by the parametric down-conversion. Furthermore, we clarify the concept of a reduced matrix which in the multimode regime is not very well defined and we calculate the effective reduced matrix for the twin photon states. Finally, we show that a scalar treatment for the polarization of these states is very adequate.

    

 

Bjork Gunnar

School of Information and Communication Technology, Royal Institute of Technology (KTH), Kista, Seeden

 

Detecting entanglement through correlations between local observables

 

Detection, and in particular, quantification of entanglement is a difficult experimental task. Recently, it has been proposed to use the joint uncertainties of locally measured observables as a means of detecting \cite{Hofmann} and quantifying \cite{Khan} bipartite entanglement. These proposals are based on the variance of sums of local measurements, that is, these schemes rely on {\em covariances} between the local observables. Using this insight, we have proposed a measure of bipartite entanglement that is invariant under local unitary transformations, and which is based on such covariances \cite{Kothe}. Analyzing the mathematical foundation of the measurement, it measures the Hilbert-Schmidt distance between the state and the product state obtained by multiplying the local density matrices (found by tracing out the respective subsystem). The measure has the benefit that the experimentalist need not have any {\it a priori} knowledge of the state to make the measurements. For pure states, the measure provides the state's concurrence directly, without resorting to state tomography. For statistically mixed states, the measure provides bounds for the concurrence.

    

 

Blatt Rainer

Institut für Experimentalphysik, Universität Innsbruck, Innsbruck, Austria,

and Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der  Wissenschaften, Innsbruck, Austria.

 

Quantum Information Processing with Trapped Ca+ Ions

 

Trapped strings of cold ions provide an ideal system for quantum information processing. The quantum information can be stored in individual ions and these qubits can be individually prepared, the corresponding quantum states can be manipulated and measured with nearly 100% detection efficiency. With a small ion-trap quantum computer based on two and three trapped Ca+ ions as qubits we have generated in a pre-programmed way genuine quantum states. These states are of particular interest for the implementation of an ion quantum register: we have demonstrated selective read-out of single qubits and manipulation of single qubits of the register conditioned on the read-out results. Moreover, entangled states of up to eight particles were generated using an algorithmic procedure and the resulting states were analyzed using state tomography proving genuine multi-partite entanglement. With Bell states as a resource, entangled states are applied for teleportation and for improved precision measurements.

    

 

Buchleitner Andreas

Max Planck Institute for the Physics of Complex Systems, Dresden, Germany.

 

Atomic current across an optical lattice

 

We study the spectral structure of the Bose-Hubbard problem, which governs the microscopic dynamics of ultracold atoms loaded into optical lattices. On the basis of these results, we devise a microscopic model for the emergence of a fermionic atomic current across a tilted optical lattice, where the necessary relaxation processes are provided by collisions of the fermions with a bosonic "bath". Tuning the - experimentally controllable - parameters of the microscopic dynamics allows to switch from Ohmic to negative differential conductance.

    

 

Bouda Jan

Faculty of Informatics, Masaryk University, Czech Republic

 

Anonymous transmission of quantum information

 

We propose first protocol for anonymous distribution of quantum information based on standard cryptographic primitives, in our case bipartite shared secret key and anonymous distribution of classical information. It can be used to implement either quantum channel with anonymous sender or quantum channel with anonymous receiver. We prove that our protocol achieves anonymity and message secrecy with unconditional security. It tolerates disruption (i.e. denial of service attack by group of participants) of the protocol, but the number of disrupters must be limited by the quantum Gilbert-Varshamov bound. This bound can be exceeded provided a specific entanglement distillation procedure will be used. A different version of the protocol tolerates any number of disrupters, but is secure only when receiver does not actively cooperate with other corrupted participants. Simplified version of this protocol was experimentally realized for 5 participants, but with different motivation, see Zhao et al.: Experimental demonstration of five-photon entanglement and open-destination teleportation, Nature 430, 54 - 58 (2004).

    

 

Buhmann Stefan Yoghi

Theoretisch-Physikalisches Institut, Friedrich-Schiller Universität Jena, Germany

 

Dynamical theory of Casimir-Polder forces

 

The Casimir-Polder force experienced by a single atom in the presence of magnetodielectric bodies is a well-known consequence of QED. Following the ideas of Casimir and Polder, it is usually regarded as a potential force derivable from the position-dependent part of the atom-field coupling energy, which is calculated within the framework of normal-mode QED by means of time-independent perturbation theory.

Such an approach has two fundamental limitations: (i) Methods based on normal-mode QED are not applicable in the presence of absorbing bodies, and (ii) time-dependent perturbation theory is not able to give an account of the dynamics present for atoms initially prepared in an excited state. To overcome these problems, we (i) work within the framework of the recently developed macroscopic quantization scheme for the electromagnetic field in the presence of absorbing and dispersing magnetodielectric media and (ii) start our calculation from the operator-valued Lorentz force, as given in the Heisenberg picture. We thus derive a general formula for the dynamical Casimir-Polder force, which can be further evaluated by solving the coupled atom-field dynamics.

For weak coupling the Markov approximation applies, and the resulting Casimir-Polder force on an atom initially prepared in an excited state can be written as a linear combination of components which depend on the shifted and broadenend the atomic transition frequencies, and whose dynamics follows that of the associated atomic density matrix elements. The case of strong atom-field coupling, which may arise if an atomic transition resonantly interacts with the body-assisted electromagnetic field, can be treated by deriving an effective Hamiltonian for a two-level atom and solving the dynamics nonperturbatively. It is found that the behaviour of the force is influenced by Rabi oscillations.

     

 

Cacheffo Alexandre

Departamento de Física, Universidade Federal de São Carlos, Brazil

 

Path integral approach to two interacting dissipative harmonic oscillators

 

In the present work we treat the double Caldeira-Leggett model: The path integral approach to two interacting dissipative harmonic oscillators. We consider a general form for the interaction between the oscillators in both cases: i) when the oscillators are coupled to a common reservoir, and when ii) each oscillator is coupled to its own reservoir. We analyze the emergence of a decoherence free subspace (DFS) in both cases, showing that for distinct reservoirs such a DFS occurs only when the coupling between the oscillators are made sufficiently strong. Pursuing the program of engineering reservoirs, we also analyze the effect of one of the oscillator on the decoherence process of a superposition state prepared in the other oscillator. These analysis result on interesting insights about coherence and decoherence dynamics on networks of dissipative quantum systems, a central subject for the theory of quantum information.

    

 

Céleri Lucas

Departamento de Física, Universidade Federal de São Carlos, Brazil

 

Switching off the reservoir through no stationary quantum systems

 

In the present work we demonstrate that the inevitable action of the environment can be substantially weakened when considering appropriate no stationary quantum systems. Beyond protecting quantum states against decoherence, an oscillating frequency can be engineered to make the system-reservoir coupling almost negligible. We show, in the domain of cavity quantum electrodynamics, how to engineer such no stationary cavity mode through its dispersive interaction with a driven two-level atom.

     

 

Corbalán Ramón

Departamento de Física, Universidad Autónoma de Barcelona, Spain

 

Cavity-QED-based entangled photon pair gun

 

We describe a cavity-QED scheme to deterministically generate polarization entangled photon pairs by using a single atom successively coupled to two single longitudinal mode cavities presenting polarization degeneracy. The cavities are initially prepared either in the vacuum state or in a single photon Fock state for each orthogonal polarization. Sharing the same basic elements, the source can operate on different physical processes. For a V-type three-level atom initially prepared in the ground state two implementations of the source are possible using either: i) two truncated Rabi oscillations, or ii) a counterintuitive Stimulated Raman Adiabatic Passage (STIRAP) process. Although slower than the former implementation, this second one is very efficient and robust under fluctuations of the experimental parameters and, particularly interesting, almost insensitive to atomic decay. For a four-level atom in a diamond configuration initially prepared in the upper state, the source can produce entangled photon pairs even in the bad cavity limit via a STIRAP process. We have performed Monte Carlo wave function simulations to characterize these sources by means of: (i) the success probability P of producing the desired entangled state, (ii) the fidelity F, and (iii) the S parameter of the CHSH inequality to quantify the entanglement capability.

    

 

Cotta Eduardo

Universidade Federal de Minas Gerais Belo Horizonte, Brazil

 

Optical Bistability and Self-Oscillations in a Semiconductor Microcavity

 

Optical bistability (OB) has already been observed in quantum well microcavities, but in the weak-coupling regime. Using a high finesse semiconductor microcavity with a single quantum well (100A GaAs) embedded in two DBR's, we can measure an OB effect under strong-coupling regime, that are followed of the self-oscillations (~80us). This slowly oscillations are associated at the behaviour analogous to the Kerr effect, but here the refraction index depends on the polariton number instead of the photon number. Using a theoretical model, we can estimate the threshold and their photons number of this bistable regime. This effect is sensitive by the energy of excitation and the cavity detuning concerning exciton-polariton energy that change the bistability curve. The oscillations are faster in the threshold laser of the microcavity, where the cavity mode changes the bistable branches. The thermal effects are negligible under current experimental conditions.

    

 

Cosme da Silva Olavo

Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil.

 

Multi-photon Interference: Hong-Ou-Mandel interferometer with two and four photons state.

 

The study of the multi-photons interference was made with the use of the Hong-Ou-Mandel interferometer. In our calculations we consider the spontaneous parametric down-conversion of two photons from the pumping laser generating one and two pairs of photons simultaneously [1]. In this work, we study too the relation between the visibility of the fourth order interference pattern with the power of the pumping laser. Our results are in good agreement with experimental results.

[1] Z. Y. Ou, J.-K. Rhee, and L. J. Wang, Phys. Rev. A 60, 593 (1999).

    

 

Davidovich Luiz

Instituto de Física, Universidade Federal do Rio de Janeiro, Brazil

 

Entanglement, hyperentanglement, and decoherence

 

I show how polarization and momentum degrees of freedom of twin photons can be used to realize a projective measurement of entanglement, and to simulate the decay dynamics of entangled qubits.

       

 

Delgado Aldo

Center for Quantum Optics and Quantum Information, Departamento de Física, Universidad de Concepción, Concepción, Chile

 

Quantum state distribution

 

We propose a scheme for the deterministic sharing of arbitrary states of a qudit among three distant parties and characterize the set of ideal quantum channels. We also show that the use of non-ideal quantum channels for quantum state sharing can be related to the problem of quantum state discrimination. This allows us to formulate a protocol which leads to perfect quantum state sharing with a finite success probability.

       

 

de Martini Francesco

Universita’ di Roma “La Sapienza”, Italy

 

Einstein-Podolsky–Rosen  correlations in mesoscopic photon quantum systems

 

The quantum injected optical parametric amplifier (QIOPA) [1] reproduces closely all relevant properties of the paradigmatic “Schrödinger Cat”, according to its original 1935 formulation. By this apparatus important fundamental theorems of quantum mechanics and quantum information have been recently investigated in our laboratories [2]. By this system, operating in a very high-gain, collinear configuration the possibility of transfer of meaningful information exclusively by a non-local channel is presently being investigated experimentally. This result, if realized, would imply a striking violation of the fundamental “non signalling” principle in quantum information.

[1] F. De Martini, Phys. Rev. Lett. 81,2842 (1998)

[2] F. De Martini, D. Pelliccia and F. Sciarrino, Phys. Rev. Lett. 92, 067901 (2004).  

      

 

de Matos Filho L.

Institute of Physics – Universidade Federal do Rio de Janeiro, Brazil

 

Scheme for quantum computing with donor-based qubits in Si far infrared cavities

 

We study the possible implementation of quantum optical architectures, like the ones currently implemented at the few-qubit level in trapped ion systems, to the silicon semiconductor environment. Our scheme combines the Si substitutional-donor quantum computing architecture, where qubits are encoded in the donor electron spin states, with the optical initialization and manipulation processes already demonstrated in ion traps and other atomic schemes.

The qubits are taken as the donor ground state Zeeman-split levels, which result from the interaction of the electron spin with a uniform magnetic field, strong enough to overcome the hyperfine coupling. Two-qubit operations are mediated by the vacuum field of the Si material cavity, which couples to the donor electronic states.

We estimate lower bounds for two critical physical quantities required for the implementation of our proposal: (i) The Si cavity quality factor, and (ii) The spin-orbit coupling of an electronic excited state of the donor involved in a spin-flip Raman transition. We show that if these hurdles are surpassed the errors in the implementation of logic gates would be well bellow the threshold established by Preskill for scalable quantum computation.

     

 

de Melo Fernando

Instituto de Física, Universidade Federal do Rio de Janeiro, Brazil

 

Quantum Non-Demolition Test of Bipartite Complementarity: A Universal Device

 

We present a universal quantum circuit that implements a non-demolition measurement of the complementarity aspects of a pure bipartite qubit system: its entanglement and its single particle features, visibility and predictability. This device can be easily implemented in many systems of interest to quantum information.

    

 

de Sousa Villar Alessandro

Instituto de Física, Universidade São Paulo, Brazil.

 

Tripartite Pump-Signal-Idler Entanglement in the Above-Threshold OPO

 

We theoretically demonstrate that the above-threshold Optical Parametric Oscillator directly generates tripartite continuous-variable entanglement between pump, signal, and idler bright optical fields.  The fields can be all at very different frequencies, opening a new avenue of multi-color entanglement to be explored in quantum information.

Reference: A. S. Villar, M. Martinelli, C. Fabre, and P. Nussenzveig, "Direct Production of Tripartite Pump-Signal-Idler Entanglement in the Above-Threshold OPO", to appear in Phys. Rev. Lett.

    

 

Dodonov Alexander

Departamento de Física, Universidade Federal de São Carlos, Brazil

 

Continuous photodetection model: quantum jumps engineering and ways for experimental verification

 

Here we examine some aspects of the Continuous Photodetection Model (CPM), which describes the photocounting processes in cavities. We derive the model's main ingredient - the Quantum Jump Superoperator (QJS) from microscopic considerations and expose simple measures for verifying the theory in realistic experiments.

First, we work out a microscopic model that describes the field-detector interaction and deduce a general expression for the QJS, that describes the detector's post-action on the field upon a detection. Then we show that in particular cases our model recovers the QJS's proposed previously \emph{ad hoc}. Moreover, we point out that by adjusting the detector parameters one could in principle engineer the forms of the QJS, thus modifying the dynamics of the photodetection, as well as the post-measurement field state.

Second, we treat the issue of verifying experimentally the CPM. By taking into account the non-idealities present in photodetection experiments (such as quantum efficiency, dark counts, etc), we show that by measuring the lower moments of the photocounts and waiting time statistics one could check the theory and determine which one of the proposed QJS's better describes the photocounting phenomenon.

    

 

Drumond Raphael

Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil

 

Induced Rabi Oscillations in the presence of Dissipation

 

Although the revival of the Rabi oscillations of a Rydberg atom due to an interaction with a coherent field has been predicted many years ago in the context of JCM, his experimental observation is, up to date, a difficult task, mainly because of the dissipation that this kind of system is usually subjected. This implies that is hard to determine, experimentally, in what extent the dynamics of the atom is dominated by the JMC or by a dissipative source. In a recent experiment, Meunier et al., Phys. Rev. Lett. 94, 010401, (2005)} used a "trick" to distinguish these two situations. After a certain time interval of the Rabi oscillations collapse they apply a π pulse in the state of the atom, which they called "kick pulse". In the JCM this pulse would induce a "time reversal" in the dynamics of the system so that the initial Rabi oscillations would come back after the kick. They actually observed this return which permits us to conclude that the collapse is not only due to dissipation but also, and significantly, by the JCM dynamics. The main objective of this work was to obtain approximate expressions of the dynamics of this system, with the applied kick pulse in a arbitrary (but small) time and in the presence of dissipation. The dissipation is modelled by the usual way, i.e., by considering dissipation only of the field by the cavity in zero temperature. The approximation is strongly based on the paper by Banacloche, Phys. Rev. A 47, 2221–2234 (1993) and require that, in an certain sense, that the initial coherent field is large and that the dissipation and observation time is both small.

    

 

Eberly Joseph H.

University of Rochester, Rochester NY, USA

 

Entanglement Sudden Death (ESD) in Quantum Optics: What is it, and what to do about it?

 

Experts have said: "...it seems fair to say that the study of entanglement is in its infancy, and it is not entirely clear what ... can be expected as a result of the study of quantitative measures of entanglement."  Now we are learning that time dependent measures can predict a sudden ending for even two-qubit atomic entanglement, despite smooth exponential decay of all density matrix elements. Abrupt 'sudden death' can occur sooner than basic T1 and T2 lifetimes. The same issues are open for photonic qubits. arXiv:quant-ph/0602196, quant-ph/0602206 and quant-ph/0603256.

    

 

Eckart Michael

Abteilung Quantenphysik, University Ulm, Germany

 

How correlation functions illuminate the frontiers of an extended mean-field theory in a quasi-1D Bose gas

 

In this contribution we use an extended mean-field theory in order to calculate the correlation functions up to third order of a homogeneous as well as a trapped quasi-1D gas of bosons at zero and finite temperature. The significance of our results arises from the fact that experiments in quasi-1D geometries have been conducted in the recent past and up to now there is no satisfying theoretical description for the transition from the weakly correlated Gross-Pitaevskii regime to the strongly correlated Tonks-Girardeau regime.

With our results we are also able to study the limits of an extended mean-field theory and give a clear indication where it has to be replaced by a different approach. In addition to this we also present an approximation of our extended mean-field theory which has an explicit solution and manages to give a good description of the physical reality in its intended regime of validity.

     

 

Eschner Jürgen

ICFO - Institut de Ciències Fotòniques, Mediterranean Technology Park, Barcelona, Spain

 

Electronic and optical feedback control of a single trapped ion

 

Based on a real-time measurement of the motion of a single ion in a Paul trap, we demonstrate its electromechanical cooling below the Doppler limit by homodyne feedback control (cold damping). The feedback cooling results are well described by a model based on a quantum mechanical master equation.

     

 

Ether Diney

Instituto de Física, Universidade Federal do Rio de Janeiro, Brazil

 

Effects of spatial transverse correlations in second-harmonic generation

 

We put up with a theoretical study of second-harmonic generation concerning spatial transverse correlations. We show that the intensity of second-harmonic crucially depends on the spatial fourth-order correlations of the fundamental. Furthermore, if these correlations describe spatial photon antibunching, the second-harmonic intensity can be zero, whichever the intensity of the fundamental is.

Reference: D. S. Ether, P. H. Souto Ribeiro, C. H. Monken, and R. L. de Matos Filho, Phys. Rev. A 73, 053819 (2006).

    

 

Fassioli Olsen Francesca

Lincoln Collage, Oxford, United Kingdom

 

Dynamics of two-qubit entanglement in a self-interacting spin-bath

 

It has recently been shown that interactions among the spins of a bath can result in an effective suppression of decoherence of a single qubit embedded in the spin environment. However, the effects of such intra-bath interaction on the dynamics of entanglement are still not well understood. Here we study the dynamics of a system composed of two entangled qubits, initialized in pure and mixed states, under the influence of a self-interacting spin bath. We consider the cases when the two qubits interact with independent and common baths. We find that, as in the case of a single qubit, strong interactions among the spins in the bath may induce an effective decoupling between the bath and the system -- this in turn suppresses the disentangling effects that a non-, or weakly, interacting bath would typically exhibit. We compare the cases when (1) the two qubits share a common bath and (2) each qubit is coupled to an independent bath. We find that in the latter case (2), the system is more sensitive to the interactions within the environment. These results can be understood as an entanglement-sharing condition where the entanglement within the bath -- which increases with increasing intra-bath interaction strength -- limits the entanglement between the bath and the system, resulting in a conservation of the system's entanglement.

    

 

Ficek Zbigniew

Department of Physics, The University of Queensland, Brisbane, Australia

 

Diffraction effects in entanglement creation in an optical cavity

 

We consider the mechanism involved in creation of entanglement between distant atoms coupled to a single-mode cavity field. Our treatment closely follows the approach that was used Refs. [1-4], but we have considered one essential difference. We include a possible variation of the coupling constant g(r) with the location of the atoms in a standing-wave cavity mode. We are particularly interested in the consequences of this variation on entanglement creation between the atoms, since this will be very pertinent to any practical experimental arrangements as the distances involved are very small. We assume the atoms are far enough apart that the direct dipole-dipole coupling or other direct interactions between the atoms can be neglected. To quantify the degree of entanglement, we use the concurrence that is the widely accepted measure of two-atom entanglement. Simple analytical expressions are obtained for the concurrence that is valid for arbitrary initial conditions and arbitrary positions of the atoms inside the cavity mode. We obtain the interesting result that spatial variations of the coupling constants actually help to create transient entanglement which may appear on the time scale much longer than that predicted for the case of equal coupling constants. We explain this effect in terms of the degree of localization of the energy induced in the field by the interacting atoms. We also find that the entanglement exhibits an interesting phenomenon of diffraction when the atoms are located between the nodes and antinodes of the cavity mode. The diffraction formula shows explicitly the trend of the modification of the entanglement with the localization of the atoms when the observation time increases. For a short time the entanglement is seen to occur over a wide range of positions centred about the antinodes of the cavity mode. As time progresses, oscillations appear and consequently the spatial region where the optimum entanglement occurs, becomes narrower. This effect is explained in terms of the quantum property of complementarity, which is manifested as a trade off between the knowledge of energy of the exchanged photon versus the evolution time of the system.

[1] C. C. Gerry, Phys. Rev. A 53, 2857 (1996).

[2] S.-B. Zheng and G.-C. Guo, Phys. Rev. Lett. 85, 2392 (2000).

[3] J.-B. Xu and S.-B. Li, New Journal of Physics 7, 72 (2005).

[4] Z. Ficek and R. Tanas, Phys. Rep. 329, 369 (2002).

    

 

Fonseca-Romero Karen M.

Universidad Nacional. Departamento de Física, Bogota, Colombia.

 

Entanglement production in microwave cavities

 

We study the dynamics of one and two atoms depressively coupled to a loss microwave cavity kept at zero temperature, and of one atom coupled to two cavities. We find the dispersive Hamiltonian in the case of two identical atoms and show the appearance of an effective atom-atom interaction. The evolution (super) operator, which is analytically computed in all three cases, is used to find the evolution of states which are the product of (separable) atomic states and a field coherent state. We verify the production of atom-field, atom-atom and cavity-cavity entanglement and employ the concurrence to characterize it, showing that appropriate values of the parameters allow for (almost) maximally entangled mixed states. The conditions and time intervals for large values of entanglement are investigated.

    

 

Gilchrist Alexei

Department of Physics, University of Queensland, Australia

 

Quantum Control of a Single Qubit

 

Measurements in quantum mechanics cannot perfectly distinguish all states and necessarily disturb the measured system. We present and analyse a proposal to demonstrate fundamental limits on quantum control of a single qubit arising from these properties of quantum measurements.  We consider a qubit prepared in one of two non-orthogonal states and subsequently subjected to dephasing noise. The task is to use measurement and feedback control to attempt to correct the state of the qubit.  We demonstrate that projective measurements are not optimal for this task, and that there exists a non-projective measurement with an optimum measurement strength which achieves the best trade-off between gaining information about the system and disturbing it through measurement back-action. We study the performance of a quantum control scheme that makes use of this weak measurement followed by feedback control, and demonstrate that it realises the optimal recovery from noise for this system. We contrast this approach with various classically inspired control schemes.

    

Guzmán Robert1 and Retamal Juan Carlos2

1Departamento de Ciencias Física, Universidad de La Frontera, Casilla 54-D, Temuco, Chile

2Departamento de Física, Universidad de Santiago, Casilla 307 Correo 2 Santiago de Chile.

 

Two photon generation in optical cavities

 

We investigate the feasibility of two photon state wave packet generation in a bimodal cavity QED interacting with a trapped ion in double lambda electronic configuration.

    

 

Hennrich Markus

ICFO - Institut de Ciencies Fotoniques, Castelldefels, Barcelona, Spain

 

Towards probabilistic entanglement of distant atoms

 

Non-local entanglement is considered a key resource in future quantum information processing. One of its applications is the implementation of quantum communication schemes over long distances [1]. Such non-local entanglement has been realized e.g. between two photons [2] and between an atom and a photon [3]. The proximate step towards distributed quantum information processing is the generation of entanglement of distant massive particles like single atoms. First steps towards atom-atom entanglement at micrometer distance without direct interaction have been achieved in [4]. One possible scheme proposed by Cabrillo et al. [5] relies on the projective measurement of photons scattered from two distant atoms after excitation by a common laser pulse. Here we describe details of our experimental setup towards the realization of this scheme.

[1] H.-J. Briegel et al., Phys. Rev. Lett. 81, 5932 (1998).

[2] P. G. Kwiat et al., Phys. Rev. Lett. 75, 4337 (1995).

[3] B. B. Blinov et al., Nature 428, 153 (2004).

[4] J. Beugnon et al., Nature 440, 779 (2006) ; P. Maunz et al., quant-ph/0608047.

[5] C. Cabrillo et al., Phys. Rev. A 59, 1025 (1999).

    

 

Hor-Meyll Malena

Universidade Federal do Rio de Janeiro, Brazil

 

Investigation of global x local dynamics of a bipartite two-level system using linear optics

 

It has been shown that disentanglement due to the interaction of a system with the environment may occur in a finite time in contrast with the asymptotic decoherence time of individual parties. We present an experimental setup to observe this effect for both amplitude and phase decays using linear optics combined with parametric down conversion. The momenta of the photons generated by the parametric down conversion act as the environment while the photon polarization represents the two-level system.

    

 

Jauregui Rocio

Instituto de Física, U. N. A. M., Mexico D.F., Mexico

 

Coherent states for anharmonic systems: the Morse potential

 

A brief review of current generalizations of the concept of coherent states for anharmonic systems is given. Group theoretical methods, the introduction of generalized displacement operators, algebra deformations and axiomatic formulations are considered. Applying these generalizations to systems with discrete and continuum spectra is not a trivial task. In this work we propose how this can be done for the Morse potential that in fact has a finite discrete spectrum. The properties of the resulting states within the different generalizations are compared and their feasibility in current experiments is analyzed.

    

 

Jiménez Omar

Center for Quantum Optics and Quantum Information, Departamento de Física, Universidad de Concepción, Concepción, Chile

 

Distribution of D-dimensional quantum states

 

We study a scheme for the deterministic sharing of arbitrary states of a qudit among three distant parties. The set of ideal quantum channels is characterized. We also show that the use of non-ideal quantum channels for quantum state sharing can be related to the problem of quantum state discrimination. This allows us to formulate a protocol which leads to perfect quantum state sharing with a finite success probability.

    

 

Khoury Antonio

Instituto de Física da Universidade Federal Fluminense, RJ, Brazil.

 

Geometric phases and injection of orbital angular momentum in an optical parametric oscillator

 

Optical beams bearing orbital angular momentum (OAM), also called {\it optical vortices}, have attracted a considerable attention recently, both for its fundamental aspects as well as for its potential applications to quantum information processing. The OAM transfer in nonlinear optical processes has been widely discussed in the literature with a special attention to parametric down conversion where entangled photon pairs bearing OAM were experimentally observed. Recently, E. J. Galvez and co-workers [Phys. Rev. Lett. 90, 203901 (2003)] demonstrated for the first time the geometric phase acquired by an optical vortice when it passes through a cyclic sequence of astigmatic mode conversion. This geometric phase is analogous to the so called Pancharatnam phase observed in cyclic transformations of the polarization state of a monochromatic beam. In this work we discuss the operation of an optical parametric oscillator injected with an incoming signal prepared in arbitrary states of OAM. A special attention is given to the role of the geometric phase acquired when adiabatic mode transformations are implemented in the injected beam.

 

Topological phase for entangled two-qubit states of a single photon

 

It has already been shown by P. Milman and R. Mosseri [Phys. Rev. Lett. 90, 230403 (2003)] that two-qubit maximally entangled states (MES) can be used to display the double connectedness of the SO(3) rotation group. A geometric representation of the MES can be built and a topological phase associated with each homotopy class appears when different paths are followed by the state evolution. One interesting point is that this topological phase appears even when only one of the qubits is operated.

In this work, the SO(3) rotation group topology is considered in the context of entangled two-qubit states of a single photon. Besides the usual polarization qubit state, we take into account the orbital angular momentum (OAM) state of the photon. Therefore a single photon is considered as a two-qubit carrier. This kind of "spin-orbit" entanglement has already been proposed as a possible photonic implementation of a C-NOT gate for quantum computation. Here we propose an intereferometric measurement that displays the topological phase difference between the two homotopy classes in the geometric representation of the MES evolution.

    

 

Kimble H. Jeff

Norman Bridge Laboratory of Physics, California Institute of Technology, Pasadena, California, USA

 

Quantum Optics with Atomic Ensembles and Single Atoms in Cavities

 

After a brief overview, I will focus attention on three recent advances in the Caltech Quantum Optics Group. (1) A single Cesium atom trapped within an optical cavity in the regime of strong coupling has been cooled to the ground state of axial motion by way of coherent Raman transitions. This result culminates a long quest to achieve control over the external center-of-mass degree of freedom in cavity QED, which together with the existing strong coupling for the internal degrees of freedom, enables a new set of phenomena to be explored at the light-matter interface. (2) Strong coupling has been observed for single Cesium atoms falling through the evanescent field of a high-Q toroidal microresonator, with g0/2π ≈ (50±12) MHz for interactions near the surface of the resonator, where 2g0 is the single-photon Rabi frequency. This is the first realization of the strong interaction of single atoms and photons within a lithographically fabricated microresonator, and opens new avenues for cavity QED, including the implementation of quantum networks, scalable quantum logic with photons, and quantum information processing on atom chips. (3) Quantum entanglement has been achieved between a pair of atomic ensembles separated by 2.8 meters, with the entangled state involving one spin excitation within a collective system of roughly 10 atoms at two different sites. This work significantly extends laboratory capabilities for entanglement generation and provides a new capability for the distribution of entanglement in quantum information science.

    

 

Klimov Andrei B.

Departamento de Física, Universidad de Guadalajara, Mexico

 

Mutually Unbiased Basis and discrete phase-space structure

 

We show that different nontrivial sets of Mutually Unbiased Basis (MUB) for systems of power prime dimension are related to Abelian curves (which in the simplest case are degenerated to straight lines) in the corresponding discrete phase space. A simplest classification of such curves is given in the particular case of 2^n dimensional systems. Examples of 4 and 8 dimensional systems are analyzed in details.

    

 

Ladrón de Guevara María Loreto

Departamento de Física, Universidad Católica del Norte, Antofagasta, Chile

 

Measurement-driven quantum evolution from a known state

 

In this article we deal with the control of a quantum system when it is not possible to resort unitary transformations. In particular, we study the problem of driving a known, possibly mixed, state onto a known pure state by means of a sequence of von Neumann measurements with the highest possible probability of success. In the proposed scheme, the evolution from the initial to the target state is carried out by introducing N different observables, which are consecutively measured.

We first analyze the problem of just two observables, and then we show that a new observable can be added to the process in order to achieve a further increase in the success probability.  We prove by means of a numerical simulation that the probability of projecting onto the target state can be meaningfully increased by adding suitable observables to the process, converging to 1 when N increases. 

We discuss the physical implementation of this scheme of quantum evolution in the problem of keeping an initial flux of a beam of systems in the same state, each of the exposed to a postselection measurement.

 

Phase discontinuities without zeros in transmission

 

B. Solis, Ladrón de Guevara M. L., and P. A. Orellana

Departamento de Física, Universidad Católica del Norte, Antofagasta, Chile

 

In this article we study the transmission phase of the electron transport in a double quantum dot coupled to leads. We model our system by a non-interacting two impurity Anderson Hamiltonian. We show that the transmission phase displays abrupt changes in p, which are associated to zeros in the real part of the transmission amplitude, keeping the module of the amplitude finite. This contrasts with all previous works, where discontinuities in the transmission phase are associated the existence of zeros in transmission and Fano resonances.

    

 

Lastra Freddy

Departamento de Física, Universidad de Santiago, Chile

 

Entanglement Evolution of Bipartite m×n-dimensional System

 

We study the entanglement evolution of several bipartite m×n-dimensional systems. The entanglement is studied through a recently proposed analytical lower bound for the Entanglement of Formation (EOF) given by Chen, et al. In particular, we study analytically the case of two qutrits under the effects of a thermal bath at zero temperature. We analyze the robustness of high dimensional maximally entangled stated under this dissipative effect. We also study the case of N two-level atoms interacting with quantum electromagnetic field. Analytical expressions for the entanglement are obtained for the non dissipative with N=3, 4 atoms.

    

 

Lezama Arturo

Universidad de La Republica, Uruguay

 

Light storage with sudden light switching

 

Light storage (LS) may be understood as the controlled release of a light pulse by an atomic sample dependent on the past presence of a signal pulse. This may be seen as a “weak" version of the storage concept in which the information carried by the state of the signal pulse is not necessarily retrieved. Such a definition of LS falls short with respect to the important goal of full quantum information preservation but can nevertheless accommodate many LS experimental observations to date.

As shown by Lukin and coworkers, LS (with complete information preservation) is in principle possible in the limit of adiabatic field switching and negligible ground state coherence relaxation. We show here that under opposite assumptions, i.e. sudden change in the light-atom coupling and non-negligible relaxation, LS can also occur. We present a simple picture for the underlying physical mechanism and discuss the process for an ideal ¤ system and realistic atomic transitions driven by sharp-edge pulses. We address the question of the information loss occurring for sudden-switch light storage and show that useful applications are nevertheless possible. Connection between the mechanism presented here and already reported LS observations is made and extension of the proposed mechanism to the single photon limit is foreseen.

    

 

Leuchs Gerd

Institute of Optics, Information and Photonics, Max Planck Research Group, University Erlangen-Nürnberg, Erlangen, Germany

 

Continuous variable amplification and quantum cloning

 

Low noise optical amplifiers with high gain (G ~ 1000) are used in optical communication while low gain quantum noise limited optical amplifiers (G ~ 2) are relevant for implementing quantum communication protocols [1]. The fundamental quantum limit of an optical amplifier is reviewed and discussed with respect to various experimental realizations. So far the only implementation of a low gain quantum limited amplifier is the quantum electro-optic feed forward amplifier [2]. It has been used for quantum cloning [3] and minimal disturbance measurements [4].

References

[1] G. Leuchs, U.L. Andersen, and C. Fabre, Advances in Atomic, Molecular and Optical Physics 53, 139-149 (2006).

[2] V. Josse, M. Sabuncu, N.J. Cerf, G. Leuchs, and U.L. Andersen, Phys. Rev. Lett. 96, 163603 (2006).

[3] U.L. Andersen, V. Josse and G. Leuchs, Phys. Rev. Lett. 94, 240503 (2005).

[4] U.L. Andersen, M. Sabuncu, R. Filip and G. Leuchs, Phys. Revs Lett. 96, 020409 (2006).

    

 

Lin Hai Qing

Department of Physics and Institute of Theoretical Physics, The Chinese University of Hong Kong, Hong Kong, China

 

Entanglement and quantum phase transitions

 

In this talk, I report our group recent efforts in exploring the relation between quantum phase transition (QPT) and entanglement. First we briefly discuss measures on the entanglement. Then we concentrate on the concurrence, a measure of pairwise entanglement for spin S=1/2 systems, and study its behaviour in several well known quantum spin models. We classify the behaviours of the concurrence into three types and emphasize the important role played by the low-lying excitation spectra reconstruction near the QPT. We also study other measures of the entanglement and apply them to high spin (S ³ 1) systems and fermionic systems such as the extended Hubbard model. It is hoped that a unified and intuitive picture on the relation between the QPT and the entanglement could be given.

    

 

López Carlos

Departamento de Física, Universidad de Santiago, Chile

 

Concurrence in the Inhomogeneous Tavis-Cummings Model

 

In this work we study the properties of the bipartite atomic concurrence in the ground state of the inhomogeneous Tavis-Cummings Model. The in homogeneity is present in the coupling among the atoms with quantum electromagnetic field. Due to the inhomogeneous character of this coupling the bipartite concurrence is dependent on the pair of atoms we consider. In this sense a remarkable difference with the standard Tavis-Cummings Model appear, where the bipartite concurrence is independent on the pair of particles involved. The essential difference between both cases is the symmetry in the system, on the one hand the homogeneous case is correctly described by symmetric Dicke states associated with the SU(2) group of symmetry. On the other hand in the inhomogeneous case this symmetry is broken, leading to an enlarged total Hilbert space. We have found that the concurrence is strongly dependent on the inhomogeneity. Analytical expressions for the bipartite atomic Concurrence are obtained in some special situations.

 

Simulation of flip-flop spin dynamics in trapped ions: generation and detection of Dicke states

 

We propose an implementation of generalized selective interactions in trapped ions to simulate flip-flop spin dynamics. We show that selectivity of collective ionic-motional subspaces, based on the suitable tuning of motion-dependent AC Stark shifts, allows for a motional mode to be reduced to a spin 1/2 system. We discuss the case of inhomogeneous coupling for simulating contact hyperfine interactions in quantum dots, and propose the generation of ionic Dicke states in the homogeneous and inhomogeneous cases. In addition, we present a method for detecting Dicke states via the measurement of the concurrence between one ion (first qubit) and the rest of the ions (second effective qubit).

    

 

Lucio M. José Luis

Instituto de Física, Universidad de Guanajuato, Guanajuato, Mexico

 

On the analogy between continuous variable and spin 1/2 systems

 

We point out limitations to the analogy between the continuous variable and spin 1=2 systems and show that the maximal violation of Bell inequality is related to the infinite degeneracy. We quantify non-maximal violation of the Bell-CHSH inequality.

    

 

Maccone Lorenzo

QUIT and Dip. Fisica “A. Volta”, Pavia, Italy

 

Quantum Metrology

 

We point out a general framework that encompasses most cases in which quantum effects enable an increase in precision when estimating a parameter (quantum metrology). The typical quantum precision-enhancement is of the order of the square root of the number of times the system is sampled. We prove that this is optimal and we point out the different strategies (classical and quantum) that permit to attain this bound.

    

 

Martinelli Marcelo

Instituto de Física, Universidade São Paulo, Brazil

 

Colouring entanglement with an Optical Parametric Oscillator

 

Entanglement is at the heart of quantum mechanics, and has been widely observed in discrete and continuous variable systems. In the last case, it can be produced from unitary operations on squeezed states, but this is limited to frequency degenerate states. Non-linear processes, such as Parametric Oscillation, can generate entangled fields without this frequency constraint. In the above threshold OPO, intense entangled fields are produced, with a wavelength difference of the order of nanometers. We present the latest results in this system, with the generation of bright entangled fields of different frequencies. Fluctuations of the difference of the amplitudes and the sum of the phases are reduced (respectively 0.50 and 0.73 relative to vacuum fluctuations), and satisfy the conditions of inseparability of two fields. We discuss the limits of the noise observed in this system and the applications of this entanglement in quantum information.

    

 

Meiser Dominic and Meystre Pierre

Department of Physics, University of Arizona, Arizona, USA

 

Ultra-cold atoms in radiation pressure driven interferometers

 

The improved capabilities to micromachine sub-micrometer sized mechanical structures with well defined mechanical and optical properties enables the creation of moveable mirrors with small damping whose motion is strongly affected by the radiation pressure due to the light field inside an optical cavity. The coupled nonlinear dynamics found in those systems is very rich. In this talk we analyze the coupled motion of a cavity end-mirror, the light field inside the cavity and ultra-cold atoms trapped in the light field. Field and mirror are coupled to each other through the radiation pressure and the atoms collectively act back on the light field through their polarizability. The dipole potential changes qualitatively as a result of the strong coupling between the light field and the atoms due to their collective back-action, notables feature being that the atomic position becomes bistable and that the resonances of the cavity are broadened and develop a double peak structure in the limit of high atomic reflectivity. The sidebands of the light transmitted through the cavity can serve as an indicator of the coupled motion of the mirror and the atoms.

    

 

Ming Yang

Center for Quantum Optics and Quantum Information, Departamento de Física, Universidad de Concepción, Concepción, Chile.

And School of Physics & Material Science, Anhui University, Hefei, 230039, Peoples Republic of China

 

Generation and Purification for Ionic Entangled States via Cavity QED and Linear Optical Elements

 

In cavity QED, the atoms would be sent through the sequential arrays of cavities for the generation of multi-cavity entanglement, or several atoms would be sent into the same cavity mode one bye one for the generation of multi-atom entanglement. The complexity of these processes will impose limitations on the experimental feasibility of it. So, here, we will propose an alternative scheme for the preparation of multi-cavity W state via cavity QED, which uses the geometrical method to do what other authors have proposed previously using sequential arrays of cavities. Due to the impossibility that one quantum system can be isolated from the environment absolutely, the entanglement of the entangled objects will decrease exponentially with the propagating distance of the objects, and the practically available quantum entangled states are all non-maximally entangled states or the more general case---mixed states. Here, we will propose an entanglement generation and purification scheme for atomic or ionic system, which is mainly based on Cavity QED and linear optical elements. This purification process avoids the controlled-NOT (C-NOT) operations needed in the original purification protocol, which simplifies the whole purification process.

    

 

 

Molina Mario

Departamento de Física, Facultad de Ciencias, Universidad de Chile

 

Discrete surface solitons in semi-infinite waveguide arrays

 

We discuss the formation and stability properties of localized optical modes at the interface between a homogeneous and periodic dielectric media and at the interface between two different periodic dielectric media. Among other cases, we discuss guided modes near the edge of semi-infinite monoatomic and binary arrays of nonlinear optical waveguides, surface multi-gap vector solitons, hybrid surface solitons residing at the interface between two different periodic photonic lattices, and the first experimental observation of nonlinear optical Tamm states in a truncated defocusing LiNbO3 waveguide array.

    

 

Monken  Carlos

Departamento de Física, Universidad Federal de Minas Gerais, Belo Horizonte, Brazil

 

Encoding four orthogonal polarization states in a monochromatic two-photon beam

 

We will show how to encode four orthogonal polarization states in a monochromatic two-photon beam. The four polarization states are the Bell states. Discriminating the four Bell states is not a problem in our case, since the anti-symmetric polarization state occupies an anti-symmetric spatial mode, which can be separated from the other three states by a simple interferometer. We will also discuss some possible applications in communication through noisy channels and cryptography.

    

 

Mundarain  Douglas

Departamento de Física, Universidad Simón Bolívar, Venezuela

 

Total Quantum Zeno Effect for two-level system in squeezed bath

 

In this work we show that by frequent measurements of adequately chosen observables, a complete suppression of the decay in an exponentially decaying two level system interacting with a squeezed bath is obtained. The observables for which the effect is observed depend on the squeezing parameters of the bath. The initial states which display Total Zeno Effect are intelligent states of two conjugate observables associated to the electromagnetic fluctuations of the bath.

    

 

Nadyne Cassemiro Katiuscia

Instituto de Física, Universidade São Paulo, Brazil.

 

Experimental investigation of three-field quantum correlations in the OPO

 

We experimentally observed quantum correlations between pump, signal, and idler fields in the above-threshold Optical Parametric Oscillator.  The fields' frequencies are all different.  To our knowledge, this is the first observation of three-color quantum correlations.

    

 

Nemes María Carolina

Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil.

 

Dispersion and Uncertainty in multislit matter wave diffraction

 

We show that single and multislit experiments involving matter waves may be constructed to assess correlations between the position and momentum of a single free particle. These correlations give rise to position dependent phases which develop from the collimator to the grating and therefore play an important role in the interference patterns. For large enough transverse correlation length such interference patterns are noticeably different from those of a classical dispersion free wave. We also show that the same phenomenon can be observed with light provided the conditions for the parametric approximation are met.

    

 

Nic Chormaic Sile

Department of Applied Physics and Instrumentation, Cork Institute of Technology, Ireland

 

Coherent superposition of atomic states using a STIRAP type process in a lambda-configuration

 

We present an atom optics analogue to the generation of a coherent superposition of atomic states using a STIRAP type process in a lambda-configuration where the final state is twofold. In our setup a system of four microtraps is used and the tunneling between traps is treated as being equivalent to the interaction between the internal atomic states and the control and pump laser fields. By ensuring a counter-intuitive positioning of the microtraps relative to each other, a coherent superposition of a single atomic wavefunction can be obtained with no population of the intermediate trap. In particular, we show that it is possible to create two orthogonal maximally coherent superposition states with equal amplitude but inverse relative phase. This system is experimentally feasible and can easily be expanded to more than two final traps. This is of interest for the generation of quantum bits and the spatial manipulation of the same for quantum processing.

    

 

Olaya-Castro Alexandra

Clarendon Laboratory, University of Oxford, United Kingdom

 

Quantum optical properties of the radiation field in the Dicke model

 

We study the optical signatures of the quantum critical behaviour associated with the zero-temperature quantum phase transition exhibited by the Dicke model in the thermodynamic limit. We obtain an effective Hamiltonian for the radiation field, which resembles a degenerate parametric amplifier and which retains the main critical features of the original phase transition. We identify the state of the radiation field in the sub-radiant and super-radiant phases and show that the optical squeezing, photon statistics and correlation functions present striking behaviour in the vicinity of the phase transition. Our results are relevant to recent experimental proposals that hope to realize the quantum phase transition in the Dicke model.

    

 

Olivares-Rentería Georgina

Center for Quantum Optics and Quantum Information, Departamento de Física, Universidad de Concepción, Concepción, Chile

 

Decoherence assisting a measurement-driven quantum evolution process

 

We study the problem of driving an unknown initial mixed quantum state onto a known pure state without using unitary transformations. This can be achieved, in an efcient manner, with the help of sequential measurements on at least two unbiased bases. However here we found that, when the system is affected by a decoherence mechanism, only one observable is required in order to achieve the same goal. In this way the decoherence can assist the process. We show that, depending on the sort of decoherence, the process can converge faster or slower than the method implemented by means of two complementary observables.

    

 

Oliveira José Geraldo

Departamento de Física, Universidad Federal de Minas Gerais, Belo Horizonte, Brazil

 

Entanglement versus energy in the entanglement transfer problem

 

We study the relation between energy and entanglement in an entanglement transfer problem. We first analyze the general setup of two entangled qubits (a and b) exchanging this entanglement with two other independent qubits (A and B). Qubit a (b) interacts with qubit A (B) via a spin exchange-like unitary evolution. A physical realization of this scenario could be the problem of two--level atoms transferring entanglement to resonant cavities via independent Jaynes-Cummings interactions. We study the dynamics of entanglement and energy for the second pair of qubits (tracing out the originally entangled ones) and show that these quantities are closely related. For example, the allowed quantum states occupy a restricted area in a phase diagram entanglement vs. energy. Moreover the curve which bounds this area is exactly the one followed if both interactions are equal and the entire four qubit system is isolated. We also consider the case when the target pair of qubits is subjected to losses and can spontaneously decay.

    

 

Orozco Luis

Department of Physics, University of Maryland, USA

 

Spontaneous Emission and Conditional Dynamics in cavity QED

 

Spontaneous emission of atoms in free space is different from that in optical cavities. We are studying the probe spectrum of light generated by spontaneous emission into the mode of a cavity QED system. Our system is strongly coupled, which means that fluctuations can be larger than the steady state average, and consists of an optical cavity with two orthogonally polarized modes and a collection of Rb atoms prepared in the m=0 magnetic sublevel. When the fluctuation is a single photon, as is the case in cavity QED, the return to the steady state after the detection of a single photon follows conditional dynamics measurable with quantum optical correlations. Those conditional dynamics are different depending on the information gained from the detection. Cross-correlations of the light that originates from fluorescence and that from transmission in cavity QED open an avenue for study of entanglement in this system.

    

 

Orszag Miguel

Facultad de Física,  Pontificia Universidad Católica de Chile, Santiago, Chile

 

Decoherence Free Subspaces by interaction with common bath

 

We find the Decoherence Free Subspace (DFS) of two atoms in a common squeezed bath. Also strong entanglement can be generated. We also study the effect of coupling between the atoms in the DFS.

    

 

Padúa Sebastiao

Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil.

 

Generating, distributing and characterizing two-photon entangled spatial qudits

 

The interest in studying higher dimensional entangled states comes both from the foundations of quantum mechanics and from the development of new protocols in quantum communication. For instance, it was demonstrated that maximally entangled states of two quantum systems in D-dimensional Hilbert space, qudits, violate local realism stronger than qubits. In quantum cryptography, the use of entangled qutrits, D = 3, or qudits instead of qubits is more secure against eavesdropping attacks. Moreover, one knows that the protocols like quantum teleportation or quantum cryptography, work best for maximally entangled states. All these facts motivate the development of techniques to generate entangled states among quantum systems in higher dimensional Hilbert space with a good quality of entanglement. We demonstrate the experimental generation of entangled states of qudits by using the transverse

Spatial correlations of the photon pairs biphotons produced by SPDC. Biphotons are sent through apertures with D-slits. The D possible paths (slits) followed by each photon of the pair are defined as our qudit space. Due to a transference of information from the pump laser beam to the two-photon state, we can control the transverse correlations of the photon pairs passing by the slits, by manipulating the pump beam. A proper manipulation of it allow us to make the biphotons pass only by symmetrically opposite slits, generating entangled states between these different transverse spatial modes. Results for qudits with D = 4 and 8 are shown and the scheme described here can be extended to higher dimensions.

We also perform the propagation of entangled states of high-dimensional quantum systems, where a high-fidelity to the original states was attained. We report the experimental free-space propagation of two entangled 4-dimensional qudits or ququarts. The qudits states were generated using the transverse correlation of the twin photons produced by spontaneous parametric down-conversion. Their free-space distribution was performed at the laboratory scale and with the theory developed, the work shows that it can be implemented through long distances. The use of entangled qudits allows an increase in the robustness of the shared entanglement and in the quantity of information that can be transmitted. Therefore, this is an important step for the effort of building global quantum communication network.

A necessary step for the use of the two entangled qudits is its characterization: the measurement of its quantum state. We have used a tomography protocol for measuring the density operator of two spatial mixed qubit states. Although we have done the experiment for the dimension D= 2, this can be generalized for higher dimension D > 2. We chose to characterize the mixed state because of the practical importance of these states. A pure propagated state due to some decoherence mechanism can become a mixed state, being therefore important to be able to reconstruct these states, measuring its density operator. In principle, by knowing the density operator, we would be able to measure its entanglement.

    

 

Paz Irismar

Departamento de Física, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil.

 

Gouy phase in matter waves: an experimental proposal

 

The propagation of gaussian packets of free particles presents a phase term that we identify analogous to the well-known Gouy phase observed in the paraxial propagation of light. This phase undergoes shift in the focus proximities equal to an integer multiple of π/2. Such phase shift is an intrinsically wavelike phenomenon.

The current state-of-art in atom optics allows for the control, focusing and the realization of interference experiments involving atoms/particles beams. In this work, we propose an experiment to measure the Gouy effect in the propagation of matter waves. We analyze the feasibility of the experiment in terms of the limitations of the devices (lenses, interferometers) and of the different species of particles that form the beam.

    

 

Paz Juan Pablo

Departamento de Física, FCEyN, Universidad de Buenos Aires, Buenos Aires, Argentina.

 

Discrete Wigner Functions

 

We analyze properties of the class of discrete Wigner functions that was recently introduced by Wooters and others to represent quantum states of systems with power-of-prime dimensional Hilbert spaces [Phys. Rev. A 70, 062101 (2004)]. We show that these discrete Wigner functions have remarkably different properties from the usual (continuous) one: we consider “cat” states obtained as coherent superpositions of states with positive Wigner function; for such states we show that the oscillations of the discrete Wigner function typically spread over the entire discrete phase-space (including the regions where the two interfering states are localized). We show that this is a generic property, which is in sharp contrast with the usual properties of Wigner functions that make them useful candidates to display the existence of quantum coherence through oscillations. However, we also show that it is always possible to find a subset of cat states with a natural phase-space representation, in which the oscillatory regions remain localized. We show that this can be done for interesting families of stabilizer states which include the ones used in quantum error-correcting codes. We illustrate this by analyzing the phase-space representation of the five-qubit error-correcting code.

    

 

Polzik Eugene

Niels Bohr Institute, Copenhagen University

 

Quantum teleportation between light and matter

 

Quantum teleportation is a transfer of a quantum state between two objects. It is performed using a quantum (entangling) channel and a classical communication channel. Teleportation is an important ingredient in distributed quantum networks, and can also serve as an elementary operation in quantum computers. Here we demonstrate for the first time teleportation between objects of a different nature—light and matter, which respectively represent ‘flying’ and ‘stationary’ media. A quantum state of a few-photon pulse is teleported onto a macroscopic object (an atomic ensemble containing 1012 caesium atoms). The fidelity higher than any classical communication can possibly achieve has been achieved. Besides being of fundamental interest, teleportation using a macroscopic atomic ensemble is relevant for a practical implementation of a quantum repeater. An important factor for the implementation of quantum networks is the teleportation distance between transmitter and receiver; this is 0.5 metres in the present experiment. As our experiment uses propagating light to achieve the entanglement of light and atoms required for teleportation, the present approach should be scalable to longer distances.

 

Jacob F. Sherson, Hanna Krauter, Rasmus K. Olsson, Brian Julsgaard, Klemens Hammerer, Ignacio Cirac,  and Eugene S. Polzik, Nature, 443, 557- 560, October 5, 2006.

    

 

Pozo Rogelio and Utreras Paola

Departamento de Física, Instituto Tecnológico de Monterrey, Monterrey, Mexico.

Center for Quantum Optics and Quantum Information, Departamento de Física, Universidad de Concepción, Concepción, Chile.

 

Bipartite entanglement by means of dispersive interactions

 

We study how the entanglement of formation between two qubits can be created and modified even when they do not interact directly but when each of them interacts dispersively with a single mode field. Specifically we consider two two-level systems interacting weak and far from the resonance with single boson mode. Considering that regime we apply the method makes use of a nonlinear small deformation of the usual SU(2) algebra in order to obtain the effective Hamiltonian of describing correctly the dynamics for any initial states. In particular we study two cases: when each qubit is initially in a pure state and when they start in a Werner state. We find that both qubit can reach, periodically, a maximal entanglement state if each of them starts in any eigenstate of sx. That is achieved independent of the initial boson number state. For a high intensity energy mode state we find a periodic exponential decay of the non-diagonal terms of the two qubits density operator. This behaviour is also found analytically in the bipartite concurrence.

    

 

Rempe Gerhard

Max-Planck Institute for Quantum Optics, Hans-Kopfermann-Str. 1, D-85748 Garching, Germany

 

Particles and Waves in Cavity QED

 

The novel radiation environment provided by cavity QED is ideal to investigate fundamental phenomena concerning the wave and particle aspects of light. It makes possible to observe in one-and-the-same experimental setting a cornerstone of theoretical quantum optics, namely photon antibunching (highlighting the particle aspect) and photon bunching (highlighting the wave aspect). A transition between the two regimes occurs when the number of radiating atoms increases from one to many. As has recently been observed, such a transition features unexpected phenomena [1]. Another aspect of cavity QED is the motional dynamics of cold atoms [2,3,4]. To understand the physics of light forces in a cavity, an intuitive corpuscular picture based on cavity-enhanced photon scattering can be employed. However, non-intuitive phenomena occurs that find no explanation in terms of photon scattering and that can be attributed to the wave aspect of light [5]. Both the particle and the wave picture provide a complementary description of the main features of atomic motion in a cavity.

 

[1] M. Hennrich et al., Phys. Rev. Lett. 94, 053604 (2005).

[2] P. Maunz et al., Nature 428, 50 (2004).

[3] P. Maunz et al., Phys. Rev. Lett. 94, 033002 (2005).

[4] S. Nußmann et al., Nature Phys. 1, 122 (2005).

[5] K. Murr et al., Phys. Rev. A 74, 043412 (2006).

    

 

Rodríguez-Lara Blas Manuel

Computación y Sistemas Electrónicos, Universidad Autónoma de Tamaulipas, Mexico

 

Detection of non-Gaussian entangled states via uncertainty relations in multi-photon representations of su(2) and su(1,1) algebras

 

We present a generalization of Nha and Kim class of inequalities for detecting non-Gaussian entangled states [Phys. Rev. A 7, 012317 (2006)]. The new class of inequalities is derived via the uncertainty relations in su(2), and su(1,1) using a realization of these groups based in the Brandt-Greenberg multi-photon operators. This class of inequalities includes that derived by Nha and Kim. This class of inequalities is tested using the su(2) minimum-uncertainty states.

    

 

Romero Guillermo

Departamento de Física, Universidad de Santiago, Santiago, Chile

 

Direct Measurement of Concurrence in Matter Qubits

 

The question of how to detect entanglement is a hard and interesting problem in the burgeoning field of quantum information. Entanglement measures such as the Wootters's concurrence are nonlinear functions of the parameters associated with the physical state. In this work, we propose an experimental procedure to measure the concurrence af an arbitrary pure state of two qubits. Two physical realizations are discussed for our proposal. First, we consider physical qubits defined as the electronic states of two trapped ions cooled down to their collective motional ground state. The second physical system is composed of superconducting qubits, in particular, rf-SQUID (radio frequency superconducting quantum interference devices), which are coupled to a superconducting coplanar wave guide resonator (cavity). Provided that two copies of a two-qubit state are available, we show how to extract the information about the entanglement content of the state of interest, and to encode it onto the state af an ancillary system initially prepared in the ground state. By measuring the probability to find the ancilla in the upper state, we obtain the value of the concurrence with no prior knowledge of the quantum state.

    

 

Salles Alejo

Instituto de Fisica, UFRJ, Brazil

 

Single observable concurrence measurement without copies

 

We present a protocol that allows us to obtain the concurrence of any two qubit pure state by performing the tomography of one of the subsystems through measuring a single observable of an ancillary four dimensional qudit. This protocol is well suited for trapped ions systems, and its feasibility is here analyzed.

    

 

Sánchez-Soto Luis

Departamento de Optica. Universidad Complutense. Madrid, Spain

 

Quantum information processing with vortex beams

 

We present an accurate description of the conjugate pair angle--angular momentum in terms of the exponential of the angle instead of the angle itself, which leads to dispersion as a natural measure of angle resolution. Intelligent states minimizing the uncertainty product under the constraint of a given uncertainty in angle or in angular momentum turn out to be given by Mathieu wave functions. The theory is successfully applied to the spatial degrees of freedom of a photon in an experiment that employs computer-controlled spatial light modulators both at the state preparation and analyzing stages. This opens a way to a full quantum information processing with vortex states.

    

 

Santos Marcelo França

Departamento de Física, Universidade Federal de Minas Gerais,Belo Horizonte, Brazil

 

Useful entanglement from the Pauli principle

 

We report a scheme to extract entanglement from semiconductor quantum wells. Two independent photons excite non-interacting electrons in the semiconductor. As the electrons relax to the bottom of the conduction band, the Pauli exclusion principle forces quantum correlations between their spins. We show that after the electron-hole recombination this correlation is transferred to the emitted photons as entanglement in polarization, which can subsequently be used for quantum information tasks. This process solves an important conundrum in quantum information theory: identical particle entanglement is indeed a useful resource for quantum information processing.

    

 

Savels Tom

FOM Institute for Atomic and Molecular Physics, Amsterdam, Netherlands

 

Gain narrowing in few-atom systems

 

Using a density matrix approach, we study the simplest systems that display both gain and feedback: clusters of 2 to 5 atoms, one of which is pumped. The other atoms supply feedback through multiple scattering of light. We show that, if the atoms are in each other's near-field, the system exhibits large gain narrowing and spectral mode redistribution. The observed phenomena are more pronounced if the feedback is enhanced. Our system is to our knowledge the simplest exactly solvable microscopic system which shows the approach to laser oscillation.

    

 

Schmid Christian

Max-Planck-Institute of Quantum Optics, Garching, Germany

 

Experimental observation and characterization of entangled photon states with respect to their applications in quantum information processing

 

Spontaneous parametric down conversion together with linear optics and conditioned detection provides a versatile tool for the experimental generation of (multi)-photon entanglement. We report on recent experiments on the observation of entangled photon states and characterize their particular properties with respect to applications in quantum information processing. In particular we focus on the so called symmetric four photon Dicke state with two excitations, D_42 (quant-ph/0606234). This state is the equally weighted superposition of all six permutations of the separable state of two horizontally and two vertically polarized photons. Besides its symmetry, it has a variety of appealing properties, important for quantum information applications. Most remarkably it connects the two non-equivalent classes of genuine tri-partite entanglement (GHZ and W) via projection measurements on one of the four photons. Furthermore, an examination of the state's entanglement persistency shows that it can be applied as resource for quantum telecloning. Experimentally, the state was observed in a comparatively simple setup at high count rate and fidelity. We characterized it by quantum state tomography and novel, especially tailored, entanglement witnesses which exploit the symmetry of the state. The used analysis tools might be of general interest for future multi-photon experiments.

    

 

Spehner Dominique

Université Joseph Fourier,  France

and Fachbereich Physik, Universit¨at Duisburg–Essen, Essen, Germany

 

Quantum Measurement without Schrödinger Cat States

 

The following three-partite system serves as a model for a quantum measurement: the object of the measurement is some microscopic system

S; a single-degree-of-freedom macroscopic pointer (P) allows read-out; coupling with a bath (B) serves to decoherence distinct pointer readings. Relevant dynamical variables are the observable S of S to be measured, and the pointer position X and momentum P. P and S interact via the Hamiltonian HSP = ε SP (ε coupling constant). S, P and B are initially uncorrelated, P having a single-peaked density matrix in position representation and B being at thermal equilibrium (the case of a joint equilibrium of P and B is also considered). As time evolves, due to HSP, each eigenvector of S get entangled with a specific pointer position. Coherences between well-separated pointer positions are damped because of some P-B coupling. Both processes run simultaneously. For strong decoherence, superpositions of distinct pointer positions produced by HSP are killed before they become macroscopically distinguishable. Our main result concerns with decoherence time τdec after which a classical probabilistic interpretation of the measurement result is possible. Different decays are found in the short time τdec << τcal B and the Markov τdec >> τcal B regimes, where τcal B is the bath correlation time.

    

 

Steinberg Aephraim M.

Centre for Quantum Information & Quantum Control and Institute for Optical Sciences Department of Physics, University of Toronto, Toronto, Canada

 

Measuring and manipulating the information in multi-photon entangled states

 

Throughout the 20th century, the question of quantum measurement has confused and intrigued physicists. At the dawn of the 21st, these issues have taken on new practical importance due to the birth of the interdisciplinary science of quantum information. Motivated in part by these considerations, my lab has been carrying out a variety of experiments on controlling simple quantum systems and comparing different techniques for "measuring" their wave functions, density matrices, or phase-space distributions. I will describe some of the current issues in measurement and characterisation of quantum systems, and show the results of some of our recent experiments. In particular, I will discuss various schemes for creating path-entangled "noon" states of more than 2 photons, and some interesting problems which arise when one tries to perform characterisations of these states, either in terms of density matrices or in terms of Wigner functions.

.

    

 

Tabosa José

Departamento de Física, Universidade Federal de Pernambuco, Cidade Universitária Recife, PE, Brasil

 

Coherent spectroscopy of thermal and laser cooled atoms

 

We will present an overview on the coherent interaction of light modes carrying orbital angular momentum (OAM) with a sample of atoms either at room temperature or cooled from a magneto-optical trap. As it is well known, light beams carrying OAM present, in addition to the usual axial Doppler shift, a rotational frequency shift which is associated with its twisted wavefront and depends on the azimuthal atomic velocity. In this talk we will in particular review some results on the transfer of orbital angular momentum between different light beams via a four-wave mixing process performed in cold cesium atoms and present for the first time the spectroscopic observation of the rotational Doppler effect. This was achieved through the measurement of the broadening of a Hanle resonance induced in the Zeeman ground states of thermal rubidium atoms. We will also describe the application of an electromagnetically induced resonance to perform cold atom velocimetry.

    

 

Tasca Daniel

Instituto de Física, UFRJ, Rio de Janeiro, Brazil

 

Violation of Bell's Inequality with Transverse Spatial Variables Using Fractional Fourier Transforms

 

We report a novel Bell's inequality experiment using optical fractional Fourier transforms of transverse spatial degrees of freedom of photons pairs. Simple optical lens systems were used to implement variable-order fractional Fourier transforms of an input plane, while the detection plane was divided into two regions, resulting in a variable dichotomic detection system. We obtained a violation of the Clauser-Horne-Shimony-Holt inequality of more than 14 standad deviations.

    

 

Torres Fabián

Center for Quantum Optics and Quantum Information, Departamento de Física, Universidad de Concepción, Concepción, Chile

 

Two-photon entangled spatial qudits states reconstruction

 

We present a scheme for reconstructing states of a composite system of two spatial qudits, which are generated by using transverse correlations of parametric down-converted photons. The proposed scheme aplies both for pure states as well as mixed states. By means of local operations on down-converted photons, we are able to perform a reconstruction process by measuring intereference patterns behind the multi slit’s plane.

    

 

Toschek Peter E.

Institut für Laser-Physik, Universität Hamburg, Germany

 

Does an onlooker stop an evolving quantum system?

 

The evolution of quantum mechanics has followed the critical analysis of “gedanken” experiments. Many of these concrete speculations can become implemented today in the laboratory - thanks to now available techniques. A key experiment is concerned with the time evolution of a quantum system under repeated or continuing observation. Here, three problems overlap: 1. The microphysical measurement by a macroscopic device, 2. the system’s temporal evolution, and 3. the emergence of macroscopic reality out of the microcosmos.

A well-known calculation shows the evolution of a quantum system being slowed down, or even obstructed, when the system is merely observed. An experiment designed to demonstrate this “quantum Zeno effect” and performed in the late eighties on an ensemble of identical atomic ions confirmed its quantum description, but turned out inconclusive with respect to the very origin of the impediment of evolution.

During the past years, experiments on individual electrodynamically stored and laser-cooled ions have been performed that unequivocally demonstrate the observed system’s quantum evolution being impeded. Strategy and results exclude any physical reaction on the measured object, but reveal the effect of the gain of information as put forward by the particular correlation of the ion state with the detected signal. They shed light on the process of measurement as well as on the quantum evolution and allow an epistemological interpretation.

    

 

Valente Paulo

Universidad de Sao Paulo, Sao Paulo, Brazil

 

Correlations and anti-correlations in EIT: dependence on the input fields' statistics

 

We present recent measurements of field correlations generated by an atomic vapour presenting EIT.  Different level- configurations were investigated, involving one or two ground state hyperfine levels, either employing diode lasers or by using a CW Ti: Sapphire laser.

    

 

VanDevender Aaron

Department of Physics, University of Illinois, Urbana, IL, USA

 

Deterministic single-photon source

 

High quality single-photon Fock states are highly desirable for many quantum information applications, frequently offering improved performance over attenuated classical coherent states, e.g., in realizing secure and robust quantum cryptography. Another more challenging requirement is that the single photons be produced at a particular time, i.e., on-demand. Such a single-photon source (SPS) seems to be an essential requirement for linear optics quantum computing. We describe an improved approach for creating deterministic single-photon states. Single photons are conditionally prepared by measuring one member of a photon pair spontaneously emitted via downconversion from a pulsed 355-nm frequency-tripled Nd:YAG laser. The 355-nm pulse is cycled though a non-linear crystal so that with each pass, there is a chance of downconversion into a 718-nm trigger photon and a 702-nm conditionally prepared photon. By allowing multiple creation times, the probability of creating a single par can be enhanced, while reducing the probability of double-pair events. If we detect the presence of one (and only one) trigger photon during a particular cycle, we use a Pockels Cell to switch the corresponding prepared photon into a low-loss variable-delay storage cavity, until some predetermined later time when we release it.

    

 

Villas Boas Celso

Universidade Federal de São Carlos, São Carlos, Brazil

 

Engineering of Hamiltonians and quantum state protection in cavity QED

 

In the present work we study the Hamiltonian engineering in the cavity quantum electrodynamics domain. For this purpose we employ the interaction between two-level atoms, driven by classical fields, and the modes of a high-Q cavity. We analyze how it is possible to build different interaction Hamiltonians useful, for example, for the generation of squeezed states and entangled states of the cavity modes. We have also investigated how to simulate Kerr medium and Hamiltonians as “Bose-Einstein condensates” in a two-mode cavity. Finally we verify how it is possible to adjust a kind of interaction between an atom and a cavity mode leading to a partial decoupling of the cavity mode and the environment, inhibiting partially the dissipation and decoherence processes.

    

 

Vogel Werner

University of Rostock, Institute of Physics, Rostock, Germany

 

Nonclassicality and Entanglement: Observable Conditions

 

Necessary and sufficient conditions are formulated for characterizing the nonclassicality of quantum states of harmonic oscillator systems. They can be expressed either by characteristic functions [1] or by moments [2]. For characterizing entanglement, the partial transposition map is used to formulate a hierarchy of entanglement conditions in terms of the moments of bipartite [3] and multipartite systems [4]. Measurement principles are considered, which allow one to observe the basic quantities that are needed in our conditions [5].

[1] Th. Richter and W. Vogel, Phys. Rev. Lett. 89, 283601 (2002).

[2] E. Shchukin, Th. Richter, and W. Vogel, Phys. Rev. A 71, 011802(R) (2005); E. Shchukin and W. Vogel, Phys. Rev. A 72, 043808 (2005).

[3] E. Shchukin and W. Vogel, Phys. Rev. Lett. 95, 230502 (2005).     

[4] E. Shchukin and W. Vogel, Phys. Rev. A, Rap. Comm., in press (see also quant-ph/0605154).

[5] E. Shchukin and W. Vogel, Phys. Rev. Lett. 96, 200403 (2006).

     

 

Walborn Stephen

Instituto de Física, Universidade Federal do Rio de Janeiro, Brazil

 

Direct Measurement of the Entanglement of a Bipartite Pure State

 

We report on the first direct measurement of entanglement for bipartite pure states. Our method is based on two identical copies of the state, and a simple relation between the concurrence, introduced by Wootters, and the probability of finding two subsystems, one from each copy, in an antisymmetric state. The two identical copies are prepared on the same pair of photons, obtained from parametric down-conversion, using polarization and momentum degrees of freedom. Therefore, we do not need to generate two identical photon pairs: measurement of concurrence is achieved by direct measurement on one of the photons of a single pair. Our results provide a simple method of quantifying the entanglement of an unknown bipartite quantum state.

    

 

Wallentowitz Sasha

Departamento de Física, Pontificia Universidad Católica de Chile, Santiago, Chile

 

Model of photo detection with a Bose-Einstein condensate

 

A model for photo detection is developed that employs as absorbing device a Bose-Einstein condensed, quasi one-dimensional atomic gas, as implemented on atom chips. Photon absorption is accompanied by a transition to a non-trapped internal state, by which excited atoms may be detected as leaving from the trap. It is shown how a photon-counting formula can be derived and how this formula deviates from the well-known Mandel formula.

    

 

White Andrew

Department of Physics and Centre for Quantum Computer Technology, University of Queensland

 

Optical entangling gates

 

Entangling gates lie at the heart of quantum information. We discuss our recent efforts building, applying and characterising optical entangling gates in quantum computing and metrology protocols. These include: multi-qubit logic gates; nonlocality without entanglement; weak-valued and non-demolition quantum measurements; and phase super resolution.

    

 

Zhu Shi-Yao

Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong

 

Non-Markovian spontaneous decay of an atom in one-dimensional photonic crystals

 

The dynamics of spontaneous decay of a two-level atom in one dimension photonic crystals (1DPC) is investigated by using the mode functions for the 1DPC. Our attention is focused on the non-Markovian evolution of the spontaneous decay in the first several optical cycles. The integral-differential dynamic equation is directly solved without using the Markovian approximation, (1) by numerical calculation, (2) by a quasi-Markovian approximation. The interference of the multi-reflected fields plays an important role in the non-Markovian processes. Due to the interference of the fields successive arrived at the atom due to the reflection at the interfaces, the decay rate shows a series of pulse-like peaks. If the interference is constructive, the spontaneous decay is enhanced, while if the interference is destructive, the spontaneous decay is suppressed. The results from exact numerical calculation, the quasi-Markovian approximation and the Markovian approximation are compared, and the quasi-Markovian approximation.

    

 

Zoller Peter

Institute for Theoretical Physics of the University of Innsbruck, and Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences,

Austria

 

Quantum optics and quantum information processing with polar molecules

 

Polar molecules in their electronic and vibrational ground states have large permanent electric dipole moments associated with rotational (low energy) excitations in the microwave domain. We discuss two scenarios of quantum information processing with polar molecules based on the coupling of rotational excitations to microwave radiation.

(i) Hybrid Quantum Processors with single polar molecules, or molecular ensembles as quantum memory for solid state circuits [1,2].

(ii) 2D spin models with polar molecules in an optical lattice, as a first step towards topological quantum memory and topological quantum computing with quantum optical systems [3].

[1] P. Rabl, D. DeMille, J. M. Doyle, M. D. Lukin, R. J. Schoelkopf, P. Zoller, quant-ph/0604140 and Phys. Rev. Lett in press.

[2] A. Andre, D. DeMille, J. M. Doyle, M. D. Lukin, S. E. Maxwell, P. Rabl, R. Schoelkopf, P. Zoller, quant-ph/0605201, and Nature Physics in press.

[3] A. Micheli, G. K. Brennen, P. Zoller, Nature Physics 2, 341 (2006).