49 research outputs found
Quantum state engineering and reconstruction in cavity QED: An analytical approach
The models of a strongly-driven micromaser and a one-atom laser are developed. Their analytical solutions are obtained by means of phase space techniques. It is shown how to exploit the model of a one-atom laser for simultaneous generation and monitoring of the decoherence of the atom-field "Schrödinger cat" states. The similar machinery applied to the problem of the generation of the maximally-entangled states of two atoms placed inside an optical cavity permits its analytical solution. The steady-state solution of the problem exhibits a structure in which the two-atom maximally-entangled state correlates with the vacuum state of the cavity. As a consequence,
it is demonstrated that the atomic maximally-entangled state, depending on a coupling regime, can be produced via a single or a sequence of no-photon measurements. The question of the implementation of a quantum memory device using a dispersive interaction between the collective internal ground state of an atomic ensemble and two orthogonal modes of a cavity is addressed. The problem of quantum state reconstruction in the context of cavity quantum electrodynamics is considered. The optimal operational definition of the Wigner function of a cavity field is worked out. It is based on the Fresnel
transform of the atomic invertion of a probe atom. The general integral transformation for the Wigner function reconstruction of a particle in an arbitrary symmetric potential is derived
Quantum Computing, Metrology, and Imaging
Information science is entering into a new era in which certain subtleties of
quantum mechanics enables large enhancements in computational efficiency and
communication security. Naturally, precise control of quantum systems required
for the implementation of quantum information processing protocols implies
potential breakthoughs in other sciences and technologies. We discuss recent
developments in quantum control in optical systems and their applications in
metrology and imaging.Comment: 11 pages, 6 figures; Proceedings of SPIE: Fluctuations and Noise in
Photonics and Quantum Optics III (2005
Strengthened Bell Inequalities for Entanglement Verification
Bell inequalities were meant to test quantum mechanics vs local hidden
variable models, but can also be used to verify entanglement. For entanglement
verification purposes one assumes the validity of quantum mechanics as well as
quantum descriptions of one's measurements. With the help of these assumptions
it is possible to derive a strengthened Bell inequality whose violation implies
entanglement. We generalize known examples of such inequalities by relating the
expectation value of the Bell operator to a particular quantitative measure of
entanglement, namely the negativity. Moreover, we obtain statistics
illustrating the fact that violating a given (strengthened or not) Bell
inequality is a much more rare feat for a quantum state of two qubits than it
is to be entangled.Comment: This submission, together with the previous one, supersedes
arXiv:0806.416
Generating the local oscillator "locally" in continuous-variable quantum key distribution based on coherent detection
Continuous-variable quantum key distribution (CV-QKD) protocols based on
coherent detection have been studied extensively in both theory and experiment.
In all the existing implementations of CV-QKD, both the quantum signal and the
local oscillator (LO) are generated from the same laser and propagate through
the insecure quantum channel. This arrangement may open security loopholes and
also limit the potential applications of CV-QKD. In this paper, we propose and
demonstrate a pilot-aided feedforward data recovery scheme which enables
reliable coherent detection using a "locally" generated LO. Using two
independent commercial laser sources and a spool of 25 km optical fiber, we
construct a coherent communication system. The variance of the phase noise
introduced by the proposed scheme is measured to be 0.04 (rad^2), which is
small enough to enable secure key distribution. This technology also opens the
door for other quantum communication protocols, such as the recently proposed
measurement-device-independent (MDI) CV-QKD where independent light sources are
employed by different users.Comment: 11 pages, 10 figure
Hamiltonian Assignment for Open Quantum Systems
We investigate the problem of determining the Hamiltonian of a locally
interacting open-quantum system. To do so, we construct model estimators based
on inverting a set of stationary, or dynamical, Heisenberg-Langevin equations
of motion which rely on a polynomial number of measurements and parameters. We
validate our Hamiltonian assignment methods by numerically simulating
one-dimensional XX-interacting spin chains coupled to thermal reservoirs. We
study Hamiltonian learning in the presence of systematic noise and find that,
in certain time dependent cases, the Hamiltonian estimator accuracy increases
when relaxing the environment's physicality constraints.Comment: 5 pages, 3 figures, updated Fig 1 and reference
Quantum description and properties of electrons emitted from pulsed nanotip electron sources
We present a quantum calculation of the electron degeneracy for electron
sources. We explore quantum interference of electrons in the temporal and
spatial domain and demonstrate how it can be utilized to characterize a pulsed
electron source. We estimate effects of Coulomb repulsion on two-electron
interference and show that currently available nano tip pulsed electron sources
operate in the regime where the quantum nature of electrons can be made
dominant
Continuous Measurement of the Energy Eigenstates of a Nanomechanical Resonator without a Nondemolition Probe
We show that it is possible to perform a continuous measurement that continually projects a nanoresonator into its energy eigenstates by employing a linear coupling with a two-state system. This technique makes it possible to perform a measurement that exposes the quantum nature of the resonator by coupling it to a Cooper-pair box and a superconducting transmission line resonator