CONSTRIBUTIONS TO THE CONFERENCE
Quantum Optics III, Pucón –
Zoller Peter^{} |
||
^{ } |
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,
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).
Facultad de Física, Pontificia Universidad Católica de
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..
Department of Physics,
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,
Departamento de Física, ICEx, UFMG,
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.
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.
Department of Physics,
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.
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.
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.
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
Max Planck Institute for
the Physics of Complex Systems,
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.
Faculty of Informatics,
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).
Theoretisch-Physikalisches Institut,
Friedrich-Schiller
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.
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
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
Universidade
Federal de Minas Gerais Belo Horizonte, Brazil
Optical
Bistability and Self-Oscillations in a Semiconductor Microcavity
Optical bistability (
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).
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.
Center
for Quantum Optics and Quantum Information, Departamento de Física, Universidad
de Concepción,
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 “
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).
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.
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.
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.
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.
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.
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.
Abteilung Quantenphysik, University
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.
ICFO - Institut de Ciències Fotòniques,
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.
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).
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.
Department of Physics, The
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).
Universidad
Nacional. Departamento de Física,
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.
Department of Physics,
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 Robert^{1} and Retamal Juan Carlos^{2}
^{1}Departamento de Ciencias Física, Universidad de
^{2}Departamento
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.
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).
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.
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.
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.
Instituto de Física da Universidade Federal
Fluminense, RJ,
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.
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 g_{0}/2π ≈
(50±12) MHz for interactions near the surface of the resonator, where 2g_{0}
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
Departamento de Física,
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,
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.
Departamento de Física,
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
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.
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).
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.
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).
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
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.
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,
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.
Center for Quantum Optics and Quantum Information,
Departamento de Física, Universidad de Concepción, Concepción, Chile.
And School of Physics & Material Science,
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.
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 LiNbO_{3} waveguide array.
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
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.
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.
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.
Department of Applied Physics and Instrumentation,
Cork Institute of
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.
Clarendon Laboratory,
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.
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 efﬁcient 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.
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.
Department of Physics,
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.
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.
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.
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.
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.
Niels Bohr Institute,
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
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 s_{x}. 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.
Max-Planck Institute for Quantum Optics,
Hans-Kopfermann-Str. 1, D-85748
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.
Departamento de Física, Universidad de Santiago,
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.
Instituto
de Fisica, UFRJ,
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.
Departamento de Optica. Universidad Complutense.
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.
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.
FOM Institute for Atomic and Molecular Physics,
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.
Max-Planck-Institute
of Quantum Optics,
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_4^{2} (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.
Université Joseph Fourier, France
and Fachbereich Physik, Universit¨at Duisburg–Essen,
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
H_{SP} = 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 H_{SP}, 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 H_{SP} are killed before they
become macroscopically distinguishable. Our main result concerns with
decoherence time τ_{dec} after which a classical probabilistic
interpretation of the measurement result is possible. Different decays are
found in the short time τ_{dec} << τ_{cal B}
and the Markov τ_{dec} >> τ_{cal B} regimes,
where τ_{cal B} is the bath correlation time.
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.
.
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.
Instituto
de Física, UFRJ, Rio de Janeiro, Brazil
Violation of
We report
a novel
Center for Quantum Optics and Quantum Information,
Departamento de Física, Universidad de Concepción,
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,
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.
Universidad de Sao Paulo,
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,
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.
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.
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
[5] E.
Shchukin and W. Vogel, Phys. Rev. Lett. 96, 200403 (2006).
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.
Departamento de Física, Pontificia Universidad
Católica de 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.
Department of Physics and Centre for Quantum Computer
Technology,
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.
Department of Physics,
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.
Institute for Theoretical Physics of the
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).