1,198 research outputs found
Irreversible degradation of quantum coherence under relativistic motion
We study the dynamics of quantum coherence under Unruh thermal noise and seek
under which condition the coherence can be frozen in a relativistic setting. We
find that the frozen condition is either (i) the initial state is prepared as a
incoherence state, or (ii) the detectors have no interaction with the external
field. That is to say, the decoherence of detectors' quantum state is
irreversible under the influence of thermal noise induced by Unruh radiation.
It is shown that quantum coherence approaches zero only in the limit of an
infinite acceleration, while quantum entanglement could reduce to zero for a
finite acceleration. It is also demonstrated that the robustness of quantum
coherence is better than entanglement under the influence of the atom-field
interaction for an extremely large acceleration. Therefore, quantum coherence
is more robust than entanglement in an accelerating system and the coherence
type quantum resources are more accessible for relativistic quantum information
processing tasks.Comment: 6 pages, 2 figure
Radiative process of two entanglement atoms in de Sitter spacetime
We investigate the radiative processes of a quantum system composed by two
identical two-level atoms in the de Sitter spacetime, interacting with a
conformally coupled massless scalar field prepared in the de Sitter-invariant
vacuum. We discuss the structure of the rate of variations of the atomic energy
for two static atoms. Following a procedure developed by Dalibard, Dupont-Roc
and Cohen-Tannoudji, our intention is to identify in a quantitative way the
contributions of vacuum fluctuations and the radiation reaction to the
generation of quantum entanglement and to the degradation of entangled states.
We find that when the distance between two atoms larger than the characteristic
length scale, the rate of variation of atomic energy in the de Sitter-invariant
vacuum behaves differently compared with that in the thermal Minkowski
spacetime. In particular, the generation and degradation of quantum
entanglement can be enhanced or inhibited, which are dependent not only on the
specific entangled state but also on the distance between the atoms.Comment: 9 pages, 4 figures. Accepted by PRD for publication. arXiv admin
note: text overlap with arXiv:1512.02886 by other author
Influence of relativistic effects on satellite-based clock synchronization
Clock synchronization between the ground and satellites is a fundamental
issue in future quantum telecommunication, navigation, and global positioning
systems. Here, we propose a scheme of near-Earth orbit satellite-based quantum
clock synchronization with atmospheric dispersion cancellation by taking into
account the spacetime background of the Earth. Two frequency entangled pulses
are employed to synchronize two clocks, one at a ground station and the other
at a satellite. The time discrepancy of the two clocks is introduced into the
pulses by moving mirrors and is extracted by measuring the coincidence rate of
the pulses in the interferometer. We find that the pulses are distorted due to
effects of gravity when they propagate between the Earth and the satellite,
resulting in remarkably affected coincidence rates. We also find that the
precision of the clock synchronization is sensitive to the source parameters
and the altitude of the satellite. The scheme provides a solution for
satellite-based quantum clock synchronization with high precision, which can be
realized, in principle, with current technology.Comment: 7 pages, 3 figures, to appear in Phys. Rev.
Protecting quantum coherence of two-level atoms from vacuum fluctuations of electromagnetic field
In the framework of open quantum systems, we study the dynamics of a static
polarizable two-level atom interacting with a bath of fluctuating vacuum
electromagnetic field and explore under which conditions the coherence of the
open quantum system is unaffected by the environment totally. For both a
single-qubit and two-qubit systems, we find that the quantum coherence can not
be protected from noise when the atom interacts with a non-boundary
electromagnetic field. However, with the presence of a boundary, the dynamical
conditions for the insusceptible of quantum coherence are fulfilled only when
the atom is close to the boundary and is transversely polarizable. Otherwise,
the quantum coherence can only be protected in some degree in other polarizable
direction.Comment: 7 pages, 2 figure
Optimal estimation of parameters for scalar fields in expanding universe exhibiting Lorentz invariance violation
We address the optimal estimation of quantum parameters, in the framework of
local quantum estimation theory, for a massive scalar quantum field in the
expanding Robertson-Walker universe exhibiting Lorentz invariance violation
(LIV). The information about the history of the expanding spacetime in the
presence of LIV can be extracted by taking measurements on the entangled state
of particle modes. We find that, in the estimation of cosmological parameters,
the ultimate bounds to the precision of the Lorentz-invariant massive scalar
field can be improved due to the effects of LIV under some appropriate
conditions. We also show that, in the Lorentz-invariant massive scalar field
and massless scalar field due to LIV backgrounds, the optimal precision can be
achieved by choosing the particles with some suitable LIV, cosmological and
field parameters. Moreover, in the estimation of LIV parameter during the
spacetime expansion, we prove that the appropriate momentum mode of field
particles and larger cosmological parameters can provide us a better precision.
Particularly, the optimal precision of the parameters estimation can be
obtained by performing projective measurements implemented by the projectors
onto the eigenvectors of specific probe states.Comment: 14 pages, 12 figure
Quantum metrology and estimation of Unruh effect
We study the quantum metrology for a pair of entangled Unruh-Dewitt detectors
when one of them is accelerated and coupled to a massless scalar field.
Comparing with previous schemes, our model requires only local interaction and
avoids the use of cavities in the probe state preparation process. We show that
the probe state preparation and the interaction between the accelerated
detector and the external field have significant effects on the value of
quantum Fisher information, correspondingly pose variable ultimate limit of
precision in the estimation of Unruh effect. We find that the precision of the
estimation can be improved by a larger effective coupling strength and a longer
interaction time. Alternatively, the energy gap of the detector has a range
that can provide us a better precision. Thus we may adjust those parameters and
attain a higher precision in the estimation. We also find that an extremely
high acceleration is not required in the quantum metrology process.Comment: 24 pages,3 figures, typos corrected, new references adde
Downlink Goodput Analysis for D2D Underlaying Massive MIMO Networks
The performance of downlink massive multiple-input-multiple-output networks
with co-channel device-to-device communications is investigated in this paper.
Specifically, we consider a cellular network with sufficient number of antennas
at the base station and typical hexagonal cell coverage, where the cell users
and device-to-device transmitters are randomly and uniformly distributed. To
obtain the analytical expressions of system-level performance, the asymptotic
signal-to-interference ratios for both downlink and device-to-device links are
first obtained, which depend on the pathloss and small-scale fading of the
interference channels. Since these information may not be available at the
service base station or device-to-device transmitters, there exists a chance of
packet outage. Therefore, we continue to derive the closed-form approximation
of the average goodput, which measures the average number of information bits
successfully delivered to the receiver. Hence, the system design trade-off
between downlink and co-channel device-to-device communications can be
investigated analytically. Moreover, the performance region in which the
co-channel device-to-device communications could lead to better overall
spectral efficiency can be obtained. Finally, it is shown by simulations that
the analytical results matches the actual performance very well.Comment: 6 pages, 4 figures, conference paper accepted by Globecom 201
Relativistic Quantum Metrology in Open System Dynamics
Quantum metrology studies the ultimate limit of precision in estimating a
physical quantity if quantum strategies are exploited. Here we investigate the
evolution of a two-level atom as a detector which interacts with a massless
scalar field using the master equation approach for open quantum system. We
employ local quantum estimation theory to estimate the Unruh temperature when
probed by a uniformly accelerated detector in the Minkowski vacuum. In
particular, we evaluate the Fisher information (FI) for population measurement,
maximize its value over all possible detector preparations and evolution times,
and compare its behavior with that of the quantum Fisher information (QFI). We
find that the optimal precision of estimation is achieved when the detector
evolves for a long enough time. Furthermore, we find that in this case the FI
for population measurement is independent of initial preparations of the
detector and is exactly equal to the QFI, which means that population
measurement is optimal. This result demonstrates that the achievement of the
ultimate bound of precision imposed by quantum mechanics is possible. Finally,
we note that the same configuration is also available to the maximum of the QFI
itself.Comment: 20 ppages, 4 figure
Entanglement Enhanced Thermometry in the Detection of the Unruh Effect
We show how the use of entanglement can enhance the precision of the
detection of the Unruh effect with an accelerated probe. We use the
Unruh-DeWitt model of a two-level atom interacting relativistically with a
quantum field and treat the atom as an open quantum system to derive the master
equation governing its evolution. By means of quantum state discrimination, we
detect the accelerated motion of the atom by examining its time evolving state.
It turns out that the optimal strategy for the detection of the Unruh effect,
to which the accelerated atom is sensitive, involves letting the
atom-thermometer equilibrate with the thermal bath. However, introducing
initial entanglement between the detector and an external degree of freedom
leads to an enhancement of the sensitivity of the detector. Also, the maximum
precision is attained within finite time, before equilibration takes place.Comment: 9 pages, 3 figures. To match with the published versio
Detecting the Curvature of de Sitter Universe with Two Entangled Atoms
Casimir-Polder interaction arises from the vacuum fluctuations of quantum
field that depend on spacetime curvature and thus is spacetime-dependent. Here
we show how to use the resonance Casimir-Polder interaction (RCPI) between two
entangled atoms to detect spacetime curvature. We find that the RCPI of two
static entangled atoms in the de Sitter-invariant vacuum depends on the de
Sitter spacetime curvature relevant to the temperature felt by the static
observer. It is characterized by a power law decay when beyond a
characteristic length scale associated to the breakdown of a local inertial
description of the two-atom system. However, the RCPI of the same setup
embedded in a thermal bath in the Minkowski universe is temperature-independent
and is always characterized by a power law decay. Therefore, although a
single static atom in the de Sitter-invariant vacuum responds as if it were
bathed in thermal radiation in a Minkowski universe, using the distinct
difference between RCPI of two entangled atoms one can in principle distinguish
these two universes.Comment: 17pages, no figures. Accepted in Scientific Report
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