2,324 research outputs found
A simple example of "Quantum Darwinism": Redundant information storage in many-spin environments
As quantum information science approaches the goal of constructing quantum
computers, understanding loss of information through decoherence becomes
increasingly important. The information about a system that can be obtained
from its environment can facilitate quantum control and error correction.
Moreover, observers gain most of their information indirectly, by monitoring
(primarily photon) environments of the "objects of interest." Exactly how this
information is inscribed in the environment is essential for the emergence of
"the classical" from the quantum substrate. In this paper, we examine how
many-qubit (or many-spin) environments can store information about a single
system. The information lost to the environment can be stored redundantly, or
it can be encoded in entangled modes of the environment. We go on to show that
randomly chosen states of the environment almost always encode the information
so that an observer must capture a majority of the environment to deduce the
system's state. Conversely, in the states produced by a typical decoherence
process, information about a particular observable of the system is stored
redundantly. This selective proliferation of "the fittest information" (known
as Quantum Darwinism) plays a key role in choosing the preferred, effectively
classical observables of macroscopic systems. The developing appreciation that
the environment functions not just as a garbage dump, but as a communication
channel, is extending our understanding of the environment's role in the
quantum-classical transition beyond the traditional paradigm of decoherence.Comment: 21 pages, 6 figures, RevTex 4. Submitted to Foundations of Physics
(Asher Peres Festschrift
Adiabatic-Impulse approximation for avoided level crossings: from phase transition dynamics to Landau-Zener evolutions and back again
We show that a simple approximation based on concepts underlying the
Kibble-Zurek theory of second order phase transition dynamics can be used to
treat avoided level crossing problems. The approach discussed in this paper
provides an intuitive insight into quantum dynamics of two level systems, and
may serve as a link between the theory of dynamics of classical and quantum
phase transitions. To illustrate these ideas we analyze dynamics of a
paramagnet-ferromagnet quantum phase transition in the Ising model. We also
present exact unpublished solutions of the Landau-Zener like problems.Comment: 12 pages & 6 figures, minor corrections, version accepted in Phys.
Rev.
Dynamics of a quantum phase transition in the random Ising model
A quantum phase transition from paramagnetic to ferromagnetic phase is driven
by a time-dependent external magnetic field. For any rate of the transition the
evolution is non-adiabatic and finite density of defects is excited in the
ferromagnetic state. The density of excitations has only logarithmic dependence
on the transition rate. This is much weaker than any usual power law scaling
predicted for pure systems by the Kibble-Zurek mechanism.Comment: 4 pages and 2 figures; improved presentatio
Assisted finite-rate adiabatic passage across a quantum critical point: Exact solution for the quantum Ising model
The dynamics of a quantum phase transition is inextricably woven with the
formation of excitations, as a result of the critical slowing down in the
neighborhood of the critical point. We design a transitionless quantum driving
through a quantum critical point that allows one to access the ground state of
the broken-symmetry phase by a finite-rate quench of the control parameter. The
method is illustrated in the one-dimensional quantum Ising model in a
transverse field. Driving through the critical point is assisted by an
auxiliary Hamiltonian, for which the interplay between the range of the
interaction and the modes where excitations are suppressed is elucidated.Comment: 2 figures, 5 page
Testing quantum adiabaticity with quench echo
Adiabaticity of quantum evolution is important in many settings. One example
is the adiabatic quantum computation. Nevertheless, up to now, there is no
effective method to test the adiabaticity of the evolution when the
eigenenergies of the driven Hamiltonian are not known. We propose a simple
method to check adiabaticity of a quantum process for an arbitrary quantum
system. We further propose a operational method for finding a uniformly
adiabatic quench scheme based on Kibble-Zurek mechanism for the case when the
initial and the final Hamiltonians are given. This method should help in
implementing adiabatic quantum computation.Comment: This is a new version. Some typos in the New Journal of Physics
version have been correcte
Sub-Planck spots of Schroedinger cats and quantum decoherence
Heisenberg's principle states that the product of uncertainties of
position and momentum should be no less than Planck's constant . This is
usually taken to imply that phase space structures associated with sub-Planck
() scales do not exist, or, at the very least, that they do not
matter. I show that this deeply ingrained prejudice is false: Non-local
"Schr\"odinger cat" states of quantum systems confined to phase space volume
characterized by `the classical action' develop spotty structure
on scales corresponding to sub-Planck . Such
structures arise especially quickly in quantum versions of classically chaotic
systems (such as gases, modelled by chaotic scattering of molecules), that are
driven into nonlocal Schr\"odinger cat -- like superpositions by the quantum
manifestations of the exponential sensitivity to perturbations. Most
importantly, these sub-Planck scales are physically significant: determines
sensitivity of a quantum system (or of a quantum environment) to perturbations.
Therefore sub-Planck controls the effectiveness of decoherence and
einselection caused by the environment. It may also be relevant in
setting limits on sensitivity of Schr\"odinger cats used as detectors.Comment: Published in Nature 412, 712-717 (2001
Highly sensitive multichannel spectrometer for subpicosecond spectroscopy in the midinfrared
A spectrometer system is presented for time-resolved probing in the midinfrared between 5 and 11 /tLmw ith a
temporal resolution of better than 400 fs. Multichannel detection with HgCdTe detector arrays consisting of
ten elements in combination with a high repetition rate permits one to record weak absorbance changes with an
accuracy of 0.1 mOD
Decoherence from a Chaotic Environment: An Upside Down "Oscillator" as a Model
Chaotic evolutions exhibit exponential sensitivity to initial conditions.
This suggests that even very small perturbations resulting from weak coupling
of a quantum chaotic environment to the position of a system whose state is a
non-local superposition will lead to rapid decoherence. However, it is also
known that quantum counterparts of classically chaotic systems lose exponential
sensitivity to initial conditions, so this expectation of enhanced decoherence
is by no means obvious. We analyze decoherence due to a "toy" quantum
environment that is analytically solvable, yet displays the crucial phenomenon
of exponential sensitivity to perturbations. We show that such an environment,
with a single degree of freedom, can be far more effective at destroying
quantum coherence than a heat bath with infinitely many degrees of freedom.
This also means that the standard "quantum Brownian motion" model for a
decohering environment may not be as universally applicable as it once was
conjectured to be.Comment: RevTeX, 29 pages, 5 EPS figures. Substantially rewritten analysis,
improved figures, additional references, and errors fixed. Final version (to
appear in PRA
John Wheeler, relativity, and quantum information
In spring 1952, as John Wheeler neared the end of design work for the first thermonuclear explosion, he plotted a radical change of research direction: from particles and atomic nuclei to general relativity
Critical quench dynamics in confined systems
We analyze the coherent quantum evolution of a many-particle system after
slowly sweeping a power-law confining potential. The amplitude of the confining
potential is varied in time along a power-law ramp such that the many-particle
system finally reaches or crosses a critical point. Under this protocol we
derive general scaling laws for the density of excitations created during the
non-adiabatic sweep of the confining potential. It is found that the mean
excitation density follows an algebraic law as a function of the sweeping rate
with an exponent that depends on the space-time properties of the potential. We
confirm our scaling laws by first order adiabatic calculation and exact results
on the Ising quantum chain with a varying transverse field.Comment: To appear in Phys. Rev. Let
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