1,881 research outputs found
Quantum Decoherence with Holography
Quantum decoherence is the loss of a system's purity due to its interaction
with the surrounding environment. Via the AdS/CFT correspondence, we study how
a system decoheres when its environment is a strongly-coupled theory. In the
Feynman-Vernon formalism, we compute the influence functional holographically
by relating it to the generating function of Schwinger-Keldysh propagators and
thereby obtain the dynamics of the system's density matrix.
We present two exactly solvable examples: (1) a straight string in a BTZ
black hole and (2) a scalar probe in AdS. We prepare an initial state that
mimics Schr\"odinger's cat and identify different stages of its decoherence
process using the time-scaling behaviors of R\'enyi entropy. We also relate
decoherence to local quantum quenches, and by comparing the time evolution
behaviors of the Wigner function and R\'enyi entropy we demonstrate that the
relaxation of local quantum excitations leads to the collapse of its
wave-function.Comment: 55 pages, 13 figures; v2 47 pages & 13 figs, minor revision to match
published versio
Orthogonal Constant-Amplitude Sequence Families for System Parameter Identification in Spectrally Compact OFDM
In rectangularly-pulsed orthogonal frequency division multiplexing (OFDM)
systems, constant-amplitude (CA) sequences are desirable to construct
preamble/pilot waveforms to facilitate system parameter identification (SPI).
Orthogonal CA sequences are generally preferred in various SPI applications
like random-access channel identification. However, the number of conventional
orthogonal CA sequences (e.g., Zadoff-Chu sequences) that can be adopted in
cellular communication without causing sequence identification ambiguity is
insufficient. Such insufficiency causes heavy performance degradation for SPI
requiring a large number of identification sequences. Moreover,
rectangularly-pulsed OFDM preamble/pilot waveforms carrying conventional CA
sequences suffer from large power spectral sidelobes and thus exhibit low
spectral compactness. This paper is thus motivated to develop several order-I
CA sequence families which contain more orthogonal CA sequences while endowing
the corresponding OFDM preamble/pilot waveforms with fast-decaying spectral
sidelobes. Since more orthogonal sequences are provided, the developed order-I
CA sequence families can enhance the performance characteristics in SPI
requiring a large number of identification sequences over multipath channels
exhibiting short-delay channel profiles, while composing spectrally compact
OFDM preamble/pilot waveforms.Comment: 15 pages, 4 figure
Convergence of quantum random walks with decoherence
In this paper, we study the discrete-time quantum random walks on a line
subject to decoherence. The convergence of the rescaled position probability
distribution depends mainly on the spectrum of the superoperator
. We show that if 1 is an eigenvalue of the superoperator
with multiplicity one and there is no other eigenvalue whose modulus equals to
1, then converges to a convex combination of
normal distributions. In terms of position space, the rescaled probability mass
function , , converges in
distribution to a continuous convex combination of normal distributions. We
give an necessary and sufficient condition for a U(2) decoherent quantum walk
that satisfies the eigenvalue conditions.
We also give a complete description of the behavior of quantum walks whose
eigenvalues do not satisfy these assumptions. Specific examples such as the
Hadamard walk, walks under real and complex rotations are illustrated. For the
O(2) quantum random walks, an explicit formula is provided for the scaling
limit of and their moments. We also obtain exact critical exponents
for their moments at the critical point and show universality classes with
respect to these critical exponents
Eigenvalue Problems for One-Dimensional Discrete Schrödinger Operators with Symmetric Boundary Conditions
WiRiS: Transformer for RIS-Assisted Device-Free Sensing for Joint People Counting and Localization using Wi-Fi CSI
Channel State Information (CSI) is widely adopted as a feature for indoor
localization. Taking advantage of the abundant information from the CSI, people
can be accurately sensed even without equipped devices. However, the
positioning error increases severely in non-line-of-sight (NLoS) regions.
Reconfigurable intelligent surface (RIS) has been introduced to improve signal
coverage in NLoS areas, which can re-direct and enhance reflective signals with
massive meta-material elements. In this paper, we have proposed a
Transformer-based RIS-assisted device-free sensing for joint people counting
and localization (WiRiS) system to precisely predict the number of people and
their corresponding locations through configuring RIS. A series of predefined
RIS beams is employed to create inputs of fingerprinting CSI features as
sequence-to-sequence learning database for Transformer. We have evaluated the
performance of proposed WiRiS system in both ray-tracing simulators and
experiments. Both simulation and real-world experiments demonstrate that people
counting accuracy exceeds 90%, and the localization error can achieve the
centimeter-level, which outperforms the existing benchmarks without employment
of RIS
Effect of end-stage renal disease on long-term survival after a first-ever mechanical ventilation: a population-based study
The 30-day, 6-month, and 1-, 2-, 5-, and 10-year survival rate differences in the ESRD Pos and ESRD Neg groups from the beginning. (DOCX 17 kb
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