174 research outputs found
Continuous variable entanglement sharing in non-inertial frames
We study the distribution of entanglement between modes of a free scalar
field from the perspective of observers in uniform acceleration. We consider a
two-mode squeezed state of the field from an inertial perspective, and
analytically study the degradation of entanglement due to the Unruh effect, in
the cases of either one or both observers undergoing uniform acceleration. We
find that for two observers undergoing finite acceleration, the entanglement
vanishes between the lowest frequency modes. The loss of entanglement is
precisely explained as a redistribution of the inertial entanglement into
multipartite quantum correlations among accessible and unaccessible modes from
a non-inertial perspective. We show that classical correlations are also lost
from the perspective of two accelerated observers but conserved if one of the
observers remains inertial.Comment: 19 pages, 13 EPS figures (most low-res due to oversize); terminology
revise
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
Stimulating uncertainty: Amplifying the quantum vacuum with superconducting circuits
The ability to generate particles from the quantum vacuum is one of the most
profound consequences of Heisenberg's uncertainty principle. Although the
significance of vacuum fluctuations can be seen throughout physics, the
experimental realization of vacuum amplification effects has until now been
limited to a few cases. Superconducting circuit devices, driven by the goal to
achieve a viable quantum computer, have been used in the experimental
demonstration of the dynamical Casimir effect, and may soon be able to realize
the elusive verification of analogue Hawking radiation. This article describes
several mechanisms for generating photons from the quantum vacuum and
emphasizes their connection to the well-known parametric amplifier from quantum
optics. Discussed in detail is the possible realization of each mechanism, or
its analogue, in superconducting circuit systems. The ability to selectively
engineer these circuit devices highlights the relationship between the various
amplification mechanisms.Comment: 27 pages, 10 figures, version published in Rev. Mod. Phys. as a
Colloquiu
Observer dependent entanglement
Understanding the observer-dependent nature of quantum entanglement has been
a central question in relativistic quantum information. In this paper we will
review key results on relativistic entanglement in flat and curved spacetime
and discuss recent work which shows that motion and gravity have observable
effects on entanglement between localized systems.Comment: Ivette Fuentes previously published as Ivette Fuentes-Guridi and
Ivette Fuentes-Schulle
Boundary conditions in the Unruh problem
We have analyzed the Unruh problem in the frame of quantum field theory and
have shown that the Unruh quantization scheme is valid in the double Rindler
wedge rather than in Minkowski spacetime. The double Rindler wedge is composed
of two disjoint regions (- and -wedges of Minkowski spacetime) which are
causally separated from each other. Moreover the Unruh construction implies
existence of boundary condition at the common edge of - and -wedges in
Minkowski spacetime. Such boundary condition may be interpreted as a
topological obstacle which gives rise to a superselection rule prohibiting any
correlations between - and - Unruh particles. Thus the part of the field
from the -wedge in no way can influence a Rindler observer living in the
-wedge and therefore elimination of the invisible "left" degrees of freedom
will take no effect for him. Hence averaging over states of the field in one
wedge can not lead to thermalization of the state in the other. This result is
proved both in the standard and algebraic formulations of quantum field theory
and we conclude that principles of quantum field theory does not give any
grounds for existence of the "Unruh effect".Comment: 31 pages,1 figur
The Gauge Fields and Ghosts in Rindler Space
We consider 2d Maxwell system defined on the Rindler space with metric
ds^2=\exp(2a\xi)\cdot(d\eta^2-d\xi^2) with the goal to study the dynamics of
the ghosts. We find an extra contribution to the vacuum energy in comparison
with Minkowski space time with metric ds^2= dt^2-dx^2. This extra contribution
can be traced to the unphysical degrees of freedom (in Minkowski space). The
technical reason for this effect to occur is the property of Bogolubov's
coefficients which mix the positive and negative frequencies modes. The
corresponding mixture can not be avoided because the projections to positive
-frequency modes with respect to Minkowski time t and positive -frequency modes
with respect to the Rindler observer's proper time \eta are not equivalent. The
exact cancellation of unphysical degrees of freedom which is maintained in
Minkowski space can not hold in the Rindler space. In BRST approach this effect
manifests itself as the presence of BRST charge density in L and R parts. An
inertial observer in Minkowski vacuum |0> observes a universe with no net BRST
charge only as a result of cancellation between the two. However, the Rindler
observers who do not ever have access to the entire space time would see a net
BRST charge. In this respect the effect resembles the Unruh effect. The effect
is infrared (IR) in nature, and sensitive to the horizon and/or boundaries. We
interpret the extra energy as the formation of the "ghost condensate" when the
ghost degrees of freedom can not propagate, but nevertheless do contribute to
the vacuum energy. Exact computations in this simple 2d model support the claim
made in [1] that the ghost contribution might be responsible for the observed
dark energy in 4d FLRW universe.Comment: Final version to appear in Phys. Rev. D. Comments on relation with
energy momentum computations and few new refs are adde
Quantum Communication with an Accelerated Partner
An unsolved problem in relativistic quantum information research is how to
model efficient, directional quantum communication between localised parties in
a fully quantum field theoretical framework. We propose a tractable approach to
this problem based on solving the Heisenberg evolution of localized field
observables. We illustrate our approach by analysing, and obtaining approximate
analytical solutions to, the problem of communicating coherent states between
an inertial sender, Alice and an accelerated receiver, Rob. We use these
results to determine the efficiency with which continuous variable quantum key
distribution could be carried out over such a communication channel.Comment: Additional explanatory text and typo in Eq.17 correcte
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