92 research outputs found
Equilibration of quantum chaotic systems
Quantum ergordic theorem for a large class of quantum systems was proved by
von Neumann [Z. Phys. {\bf 57}, 30 (1929)] and again by Reimann [Phys. Rev.
Lett. {\bf 101}, 190403 (2008)] in a more practical and well-defined form.
However, it is not clear whether the theorem applies to quantum chaotic
systems. With the rigorous proof still elusive, we illustrate and verify this
theorem for quantum chaotic systems with examples. Our numerical results show
that a quantum chaotic system with an initial low-entropy state will
dynamically relax to a high-entropy state and reach equilibrium. The quantum
equilibrium state reached after dynamical relaxation bears a remarkable
resemblance to the classical micro-canonical ensemble. However, the
fluctuations around equilibrium are distinct: the quantum fluctuations are
exponential while the classical fluctuations are Gaussian.Comment: 11 pages, 8 figure
Increase of degeneracy improves the performance of the quantum adiabatic algorithm
We propose a strategy to improve the performance of the quantum adiabatic algorithm (QAA) on an NP-hard (nondeterministic-polynomial-time-hard) problem exact cover, by increasing the ground-state degeneracy of the problem Hamiltonian. Our strategy is based on the empirical finding that for the QAA the difficulty of random instances decreases with the degeneracy of the ground state. We increase the degeneracy by adding extra qubits to form additional clauses. Our numerical results show that on average our strategy can provide an increase in the minimum gap size along the linear interpolation path of Hamiltonian for both easy and difficult instances. The success probability at fixed total evolution time is thus increased.Massachusetts Institute of Technology. Department of Physic
Distributed Quantum Sensing Using Continuous-Variable Multipartite Entanglement
Distributed quantum sensing uses quantum correlations between multiple
sensors to enhance the measurement of unknown parameters beyond the limits of
unentangled systems. We describe a sensing scheme that uses continuous-variable
multipartite entanglement to enhance distributed sensing of field-quadrature
displacement. By dividing a squeezed-vacuum state between multiple
homodyne-sensor nodes using a lossless beam-splitter array, we obtain a
root-mean-square (rms) estimation error that scales inversely with the number
of nodes (Heisenberg scaling), whereas the rms error of a distributed sensor
that does not exploit entanglement is inversely proportional to the square root
of number of nodes (standard quantum limit scaling). Our sensor's scaling
advantage is destroyed by loss, but it nevertheless retains an rms-error
advantage in settings in which there is moderate loss. Our distributed sensing
scheme can be used to calibrate continuous-variable quantum key distribution
networks, to perform multiple-sensor cold-atom temperature measurements, and to
do distributed interferometric phase sensing.Comment: 7 pages, 3 figure
Quantifying precision loss in local quantum thermometry via diagonal discord
When quantum information is spread over a system through nonclassical
correlation, it makes retrieving information by local measurements
difficult---making global measurement necessary for optimal parameter
estimation. In this paper, we consider temperature estimation of a system in a
Gibbs state and quantify the separation between the estimation performance of
the global optimal measurement scheme and a greedy local measurement scheme by
diagonal quantum discord. In a greedy local scheme, instead of global
measurements, one performs sequential local measurement on subsystems, which is
potentially enhanced by feed-forward communication. We show that, for
finite-dimensional systems, diagonal discord quantifies the difference in the
quantum Fisher information quantifying the precision limits for temperature
estimation of these two schemes, and we analytically obtain the relation in the
high-temperature limit. We further verify this result by employing the examples
of spins with Heisenberg's interaction.Comment: 5+4 pages, 4 figures, We thank the referees and editors for helpful
opinions. Accepted by Phys. Rev. A (accepted version
Quantum ranging with Gaussian entanglement
It is well known that entanglement can benefit quantum information processing
tasks. Quantum illumination, when first proposed, is surprising as
entanglement's benefit survives entanglement-breaking noise. Since then, many
efforts have been devoted to study quantum sensing in noisy scenarios. The
applicability of such schemes, however, is limited to a binary quantum
hypothesis testing scenario. In terms of target detection, such schemes
interrogate a single polarization-azimuth-elevation-range-Doppler resolution
bin at a time, limiting the impact to radar detection. We resolve this
binary-hypothesis limitation by proposing a quantum ranging protocol enhanced
by entanglement. By formulating a ranging task as a multiary hypothesis testing
problem, we show that entanglement enables a 6-dB advantage in the error
exponent against the optimal classical scheme. Moreover, the proposed ranging
protocol can also be utilized to implement a pulse-position modulated
entanglement-assisted communication protocol. Our ranging protocol reveals
entanglement's potential in general quantum hypothesis testing tasks and paves
the way towards a quantum-ranging radar with a provable quantum advantage.Comment: 5+5 pages, 4 figures, comments are welcomed, typos correcte
Distributed quantum sensing enhanced by continuous-variable error correction
A distributed sensing protocol uses a network of local sensing nodes to estimate a global feature of the network, such as a weighted average of locally detectable parameters. In the noiseless case, continuous-variable (CV) multipartite entanglement shared by the nodes can improve the precision of parameter estimation relative to the precision attainable by a network without shared entanglement; for an entangled protocol, the root mean square estimation error scales like 1/M with the number M of sensing nodes, the so-called Heisenberg scaling, while for protocols without entanglement, the error scales like 1βM. However, in the presence of loss and other noise sources, although multipartite entanglement still has some advantages for sensing displacements and phases, the scaling of the precision with M is less favorable. In this paper, we show that using CV error correction codes can enhance the robustness of sensing protocols against imperfections and reinstate Heisenberg scaling up to moderate values of M. Furthermore, while previous distributed sensing protocols could measure only a single quadrature, we construct a protocol in which both quadratures can be sensed simultaneously. Our work demonstrates the value of CV error correction codes in realistic sensing scenarios
Large-Alphabet Encoding Schemes for Floodlight Quantum Key Distribution
Floodlight quantum key distribution (FL-QKD) uses binary phase-shift keying
(BPSK) of multiple optical modes to achieve Gbps secret-key rates (SKRs) at
metropolitan-area distances. We show that FL-QKD's SKR can be doubled by using
32-ary PSK.Comment: 2 pages, 2 figure
Entanglement-Enhanced Lidars for Simultaneous Range and Velocity Measurements
Lidar is a well known optical technology for measuring a target's range and
radial velocity. We describe two lidar systems that use entanglement between
transmitted signals and retained idlers to obtain significant quantum
enhancements in simultaneous measurement of these parameters. The first
entanglement-enhanced lidar circumvents the Arthurs-Kelly uncertainty relation
for simultaneous measurement of range and radial velocity from detection of a
single photon returned from the target. This performance presumes there is no
extraneous (background) light, but is robust to the roundtrip loss incurred by
the signal photons. The second entanglement-enhanced lidar---which requires a
lossless, noiseless environment---realizes Heisenberg-limited accuracies for
both its range and radial-velocity measurements, i.e., their root-mean-square
estimation errors are both proportional to when signal photons are
transmitted. These two lidars derive their entanglement-based enhancements from
use of a unitary transformation that takes a signal-idler photon pair with
frequencies and and converts it to a signal-idler photon
pair whose frequencies are and .
Insight into how this transformation provides its benefits is provided through
an analogy to superdense coding.Comment: 7 pages, 3 figure
- β¦