1,490 research outputs found
Equivalence between two-qubit entanglement and secure key distribution
We study the problem of secret key distillation from bipartite states in the
scenario where Alice and Bob can only perform measurements at the single-copy
level and classically process the obtained outcomes. Even with these
limitations, secret bits can be asymptotically distilled by the honest parties
from any two-qubit entangled state, under any individual attack. Our results
point out a complete equivalence between two-qubit entanglement and secure key
distribution: a key can be established through a one-qubit channel if and only
if it allows to distribute entanglement. These results can be generalized to
higher dimension for all those states that are one-copy distillable.Comment: 5 pages, REVTEX. Accepted version + added appendix. Proof of the main
result and discussion improved, conclusions unchange
Quantum correlations and secret bits
It is shown that (i) all entangled states can be mapped by single-copy
measurements into probability distributions containing secret correlations, and
(ii) if a probability distribution obtained from a quantum state contains
secret correlations, then this state has to be entangled. These results prove
the existence of a two-way connection between secret and quantum correlations
in the process of preparation. They also imply that either it is possible to
map any bound entangled state into a distillable probability distribution or
bipartite bound information exists.Comment: 4 pages, published versio
Continuous stochastic Schrodinger equations and localization
The set of continuous norm-preserving stochastic Schrodinger equations
associated with the Lindblad master equation is introduced. This set is used to
describe the localization properties of the state vector toward eigenstates of
the environment operator. Particular focus is placed on determining the
stochastic equation which exhibits the highest rate of localization for wide
open systems. An equation having such a property is proposed in the case of a
single non-hermitian environment operator. This result is relevant to numerical
simulations of quantum trajectories where localization properties are used to
reduce the number of basis states needed to represent the system state, and
thereby increase the speed of calculation.Comment: 18 pages in LaTeX + 6 figures (postscript), uses ioplppt.sty. To
appear in J. Phys.
From quantum trajectories to classical orbits
Recently it has been shown that the evolution of open quantum systems may be
``unraveled'' into individual ``trajectories,'' providing powerful numerical
and conceptual tools. In this letter we use quantum trajectories to study
mesoscopic systems and their classical limit. We show that in this limit,
Quantum Jump (QJ) trajectories approach a diffusive limit very similar to the
Quantum State Diffusion (QSD) unraveling. The latter follows classical
trajectories in the classical limit. Hence, both unravelings show the rise of
classical orbits. This is true for both regular and chaotic systems (which
exhibit strange attractors).Comment: 7 pages RevTeX 3.0 + 2 figures (postscript). Submitted to Physical
Review Letter
Experimental investigation of a coherent quantum measurement of the degree of polarization of a single mode light beam
A novel method for the direct measurement of the degree of polarization is
described. It is one of the first practical implementations of a coherent
quantum measurement, the projection on the singlet state. Our first results
demonstrate the successful operation of the method. However, due to the
nonlinear crystals used presently, its application is limited to spectral
widths larger than ~8nm.Comment: 23 pages, 9 figures, submitted to Journal of Modern Optic
The no-signaling condition and quantum dynamics
We show that the basic dynamical rules of quantum physics can be derived from
its static properties and the condition that superluminal communication is
forbidden. More precisely, the fact that the dynamics has to be described by
linear completely positive maps on density matrices is derived from the
following assumptions: (1) physical states are described by rays in a Hilbert
space, (2) probabilities for measurement outcomes at any given time are
calculated according to the usual trace rule, (3) superluminal communication is
excluded. This result also constrains possible non-linear modifications of
quantum physics.Comment: 4 page
Atomic frequency comb memory with spin wave storage in 153Eu3+:Y2SiO5
153Eu3+:Y2SiO5 is a very attractive candidate for a long lived, multimode
quantum memory due to the long spin coherence time (~15 ms), the relatively
large hyperfine splitting (100 MHz) and the narrow optical homogeneous
linewidth (~100 Hz). Here we show an atomic frequency comb memory with spin
wave storage in a promising material 153Eu3+:Y2SiO5, reaching storage times
slightly beyond 10 {\mu}s. We analyze the efficiency of the storage process and
discuss ways of improving it. We also measure the inhomogeneous spin linewidth
of 153Eu3+:Y2SiO5, which we find to be 69 \pm 3 kHz. These results represent a
further step towards realising a long lived multi mode solid state quantum
memory.Comment: 7 pages and 7 figure
Single-photon-level optical storage in a solid-state spin-wave memory
A long-lived quantum memory is a firm requirement for implementing a quantum
repeater scheme. Recent progress in solid-state rare-earth-ion-doped systems
justifies their status as very strong candidates for such systems. Nonetheless
an optical memory based on spin-wave storage at the single-photon-level has not
been shown in such a system to date, which is crucial for achieving the long
storage times required for quantum repeaters. In this letter we show that it is
possible to execute a complete atomic frequency comb (AFC) scheme, including
spin-wave storage, with weak coherent pulses of photons
per pulse. We discuss in detail the experimental steps required to obtain this
result and demonstrate the coherence of a stored time-bin pulse. We show a
noise level of photons per mode during storage, this
relatively low-noise level paves the way for future quantum optics experiments
using spin-waves in rare-earth-doped crystals
Analysis of a quantum memory for photons based on controlled reversible inhomogeneous broadening
We present a detailed analysis of a quantum memory for photons based on
controlled and reversible inhomogeneous broadening (CRIB). The explicit
solution of the equations of motion is obtained in the weak excitation regime,
making it possible to gain insight into the dependence of the memory efficiency
on the optical depth, and on the width and shape of the atomic spectral
distributions. We also study a simplified memory protocol which does not
require any optical control fields.Comment: 9 pages, 4 figures (Accepted for publication in Phys. Rev. A
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