8,028 research outputs found
Teleportation Criteria: Form and Significance
Our criteria for continuous variable quantum teleportation [T.C.Ralph and
P.K.Lam, Phys.Rev.Lett. {\bf 81}, 5668 (1998)] take the form of sums, rather
than products, of conjugate quadrature measurements of the signal transfer
coefficients and the covariances between the input and output states. We
discuss why they have this form. We also discuss the physical significance of
the covariance inequality.Comment: 6 pages, to appear in Lecture Notes in Physics; Dan Walls Memorial
Volume (Springer 2000
Unitary Solution to a Quantum Gravity Information Paradox
We consider a toy model of the interaction of a qubit with an exotic
space-time containing a time-like curve. Consistency seems to require that the
global evolution of the qubit be non-unitary. Given that quantum mechanics is
globally unitary, this then is an example of a quantum gravity information
paradox. However, we show that a careful analysis of the problem in the
Heisenberg picture reveals an underlying unitarity, thus resolving the paradox.Comment: 5 page
Quantum communication in the presence of a horizon
Based on homodyne detection, we discuss how the presence of an event horizon
affects quantum communication between an inertial partner, Alice, and a
uniformly accelerated partner, Rob. We show that there exists a low frequency
cutoff for Rob's homodyne detector that maximizes the signal to noise ratio and
it approximately corresponds to the Unruh frequency. In addition, the low
frequency cutoff which minimizes the conditional variance between Alice's input
state and Rob's output state is also approximately equal to the Unruh
frequency. Thus the Unruh frequency provides a natural low frequency cutoff in
order to optimize quantum communication of both classical and quantum
information between Alice and Rob.Comment: 7 pages, 6 figure
Improving entanglement concentration of Gaussian states by local displacements
We investigate entanglement concentration of continuous-variable Gaussian
states by local single-photon subtractions combined with local Gaussian
operations. We first analyze the local squeezing-enhanced entanglement
concentration protocol proposed very recently by Zhang and van Loock [e-print:
arXiv:1103.4500 (2011)] and discuss the mechanism by which local squeezing
before photon subtraction helps to increase the entanglement of the output
state of the protocol. We next show that a similar entanglement improvement can
be achieved by using local coherent displacements instead of single-mode
squeezing.Comment: 6 pages, 5 figures, REVTeX4, accepted for publication in Phys. Rev.
Entanglement decoherence in a gravitational well according to the event formalism
The event formalism is a non-linear extension of quantum field theory
designed to be compatible with the closed time-like curves that appear in
general relativity. Whilst reducing to standard quantum field theory in flat
space-time the formalism leads to testably different predictions for
entanglement distribution in curved space. In this paper we introduce a more
general version of the formalism and use it to analyse the practicality of an
experimental test of its predictions in the earth's gravitational well
Quantum Key Distribution without sending a Quantum Signal
Quantum Key Distribution is a quantum communication technique in which random
numbers are encoded on quantum systems, usually photons, and sent from one
party, Alice, to another, Bob. Using the data sent via the quantum signals,
supplemented by classical communication, it is possible for Alice and Bob to
share an unconditionally secure secret key. This is not possible if only
classical signals are sent. Whilst this last statement is a long standing
result from quantum information theory it turns out only to be true in a
non-relativistic setting. If relativistic quantum field theory is considered we
show it is possible to distribute an unconditionally secure secret key without
sending a quantum signal, instead harnessing the intrinsic entanglement between
different regions of space time. The protocol is practical in free space given
horizon technology and might be testable in principle in the near term using
microwave technology
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