1,354 research outputs found
Physical realizations of quantum operations
Quantum operations (QO) describe any state change allowed in quantum
mechanics, such as the evolution of an open system or the state change due to a
measurement. We address the problem of which unitary transformations and which
observables can be used to achieve a QO with generally different input and
output Hilbert spaces. We classify all unitary extensions of a QO, and give
explicit realizations in terms of free-evolution direct-sum dilations and
interacting tensor-product dilations. In terms of Hilbert space dimensionality
the free-evolution dilations minimize the physical resources needed to realize
the QO, and for this case we provide bounds for the dimension of the ancilla
space versus the rank of the QO. The interacting dilations, on the other hand,
correspond to the customary ancilla-system interaction realization, and for
these we derive a majorization relation which selects the allowed unitary
interactions between system and ancilla.Comment: 8 pages, no figures. Accepted for publication on Phys. Rev.
Dynamics of quantum correlations in colored environments
We address the dynamics of entanglement and quantum discord for two non
interacting qubits initially prepared in a maximally entangled state and then
subjected to a classical colored noise, i.e. coupled with an external
environment characterized by a noise spectrum of the form . More
specifically, we address systems where the Gaussian approximation fails, i.e.
the sole knowledge of the spectrum is not enough to determine the dynamics of
quantum correlations. We thus investigate the dynamics for two different
configurations of the environment: in the first case the noise spectrum is due
to the interaction of each qubit with a single bistable fluctuator with an
undetermined switching rate, whereas in the second case we consider a
collection of classical fluctuators with fixed switching rates. In both cases
we found analytical expressions for the time dependence of entanglement and
quantum discord, which may be also extended to a collection of flcutuators with
random switching rates. The environmental noise is introduced by means of
stochastic time-dependent terms in the Hamiltonian and this allows us to
describe the effects of both separate and common environments. We show that the
non-Gaussian character of the noise may lead to significant effects, e.g.
environments with the same power spectrum, but different configurations, give
raise to opposite behavior for the quantum correlations. In particular,
depending on the characteristics of the environmental noise considered, both
entanglement and discord display either a monotonic decay or the phenomena of
sudden death and revivals. Our results show that the microscopic structure of
environment, besides its noise spectrum, is relevant for the dynamics of
quantum correlations, and may be a valid starting point for the engineering of
non-Gaussian colored environments.Comment: 8 pages, 3 figure
Quantum erasure of decoherence
We consider the classical algebra of observables that are diagonal in a given
orthonormal basis, and define a complete decoherence process as a completely
positive map that asymptotically converts any quantum observable into a
diagonal one, while preserving the elements of the classical algebra. For
quantum systems in dimension two and three any decoherence process can be
undone by collecting classical information from the environment and using such
an information to restore the initial system state. As a relevant example, we
illustrate the quantum eraser of Scully et al. [Nature 351, 111 (1991)] as an
example of environment-assisted correction. Moreover, we present the
generalization of the eraser setup for d-dimensional systems, showing that any
von Neumann measurement on a system can be undone by a complementary
measurement on the environment.Comment: 10 pages, 1 figur
Distilling entanglement from arbitrary resources
We obtain the general formula for the optimal rate at which singlets can be
distilled from any given noisy and arbitrarily correlated entanglement
resource, by means of local operations and classical communication (LOCC). Our
formula, obtained by employing the quantum information spectrum method, reduces
to that derived by Devetak and Winter, in the special case of an i.i.d.
resource. The proofs rely on a one-shot version of the so-called "hashing
bound," which in turn provides bounds on the one-shot distillable entanglement
under general LOCC.Comment: 24 pages, article class, no figure. v2: references added, published
versio
A minimum-disturbing quantum state discriminator
We propose two experimental schemes for quantum state discrimination that
achieve the optimal tradeoff between the probability of correct identification
and the disturbance on the quantum state.Comment: 9 pages, 1 figure, OSID style. Submitted to the special issue of
"Open Systems and Information Dynamics", Proceedings of the "38th Symposium
on Mathematical Physics", Torun, Poland, June 200
Entanglement dynamics of electron-electron scattering in low-dimensional semiconductor systems
We perform the quantitative evaluation of the entanglement dynamics in
scattering events between two insistinguishable electrons interacting via
Coulomb potential in 1D and 2D semiconductor nanostructures. We apply a
criterion based on the von Neumann entropy and the Schmidt decomposition of the
global state vector suitable for systems of identical particles. From the
timedependent numerical solution of the two-particle wavefunction of the
scattering carriers we compute their entanglement evolution for different spin
configurations: two electrons with the same spin, with different spin, singlet,
and triplet spin state. The procedure allows to evaluate the mechanisms that
govern entanglement creation and their connection with the characteristic
physical parameters and initial conditions of the system. The cases in which
the evolution of entanglement is similar to the one obtained for
distinguishable particles are discussed.Comment: 22 pages, 7 figures, submitted to Physical Review
Improving information/disturbance and estimation/distortion trade-offs with non universal protocols
We analyze in details a conditional measurement scheme based on linear
optical components, feed-forward loop and homodyne detection. The scheme may be
used to achieve two different tasks. On the one hand it allows the extraction
of information with minimum disturbance about a set of coherent states. On the
other hand, it represents a nondemolitive measurement scheme for the
annihilation operator, i.e. an indirect measurement of the Q-function. We
investigate the information/disturbance trade-off for state inference and
introduce the estimation/distortion trade-off to assess estimation of the
Q-function. For coherent states chosen from a Gaussian set we evaluate both
information/disturbance and estimation/distortion trade-offs and found that non
universal protocols may be optimized in order to achieve better performances
than universal ones. For Fock number states we prove that universal protocols
do not exist and evaluate the estimation/distortion trade-off for a thermal
distribution.Comment: 10 pages, 6 figures; published versio
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