1,236 research outputs found
Nonlinear quantum state transformation of spin-1/2
A non-linear quantum state transformation is presented. The transformation,
which operates on pairs of spin-1/2, can be used to distinguish optimally
between two non-orthogonal states. Similar transformations applied locally on
each component of an entangled pair of spin-1/2 can be used to transform a
mixed nonlocal state into a quasi-pure maximally entangled singlet state. In
both cases the transformation makes use of the basic building block of the
quantum computer, namely the quantum-XOR gate.Comment: 12 pages, LaTeX, amssym, epsfig (2 figures included
Nonlinearity without Superluminality
Quantum theory is compatible with special relativity. In particular, though
measurements on entangled systems are correlated in a way that cannot be
reproduced by local hidden variables, they cannot be used for superluminal
signalling. As Czachor, Gisin and Polchinski pointed out, this is not true for
general nonlinear modifications of the Schroedinger equation. Excluding
superluminal signalling has thus been taken to rule out most nonlinear versions
of quantum theory. The no superluminal signalling constraint has also been used
for alternative derivations of the optimal fidelities attainable for imperfect
quantum cloning and other operations.
These results apply to theories satisfying the rule that their predictions
for widely separated and slowly moving entangled systems can be approximated by
non-relativistic equations of motion with respect to a preferred time
coordinate. This paper describes a natural way in which this rule might fail to
hold. In particular, it is shown that quantum readout devices which display the
values of localised pure states need not allow superluminal signalling,
provided that the devices display the values of the states of entangled
subsystems as defined in a non-standard, but natural, way. It follows that any
locally defined nonlinear evolution of pure states can be made consistent with
Minkowski causality.Comment: References update
From Bell's Theorem to Secure Quantum Key Distribution
Any Quantum Key Distribution (QKD) protocol consists first of sequences of
measurements that produce some correlation between classical data. We show that
these correlation data must violate some Bell inequality in order to contain
distillable secrecy, if not they could be produced by quantum measurements
performed on a separable state of larger dimension. We introduce a new QKD
protocol and prove its security against any individual attack by an adversary
only limited by the no-signaling condition.Comment: 5 pages, 2 figures, REVTEX
Heisenberg picture operators in the quantum state diffusion model
A stochastic simulation algorithm for the computation of multitime
correlation functions which is based on the quantum state diffusion model of
open systems is developed. The crucial point of the proposed scheme is a
suitable extension of the quantum master equation to a doubled Hilbert space
which is then unraveled by a stochastic differential equation.Comment: LaTeX2E, 6 pages, 3 figures, uses iopar
Tunable Up-Conversion Photon Detector
We introduce a simple approach for a tunable up-conversion detector. This
scheme is relevant for both single photon detection or anywhere where low light
levels at telecom wavelengths need to be detected with a high degree of
temporal resolution or where high count rates are desired. A system combining a
periodically poled Lithium niobate waveguide for the nonlinear wavelength
conversion and a low jitter Silicon avalanche photodiode are used in
conjunction with a tunable pump source. We report more than a ten-fold increase
in the detectable bandwidth using this tuning scheme.Comment: 3 pages, 3 figures, Accepted for publication in AP
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
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