3,122 research outputs found

### Lyapunov Generation of Entanglement and the Correspondence Principle

We show how a classically vanishing interaction generates entanglement
between two initially nonentangled particles, without affecting their classical
dynamics. For chaotic dynamics, the rate of entanglement is shown to saturate
at the Lyapunov exponent of the classical dynamics as the interaction strength
increases. In the saturation regime, the one-particle Wigner function follows
classical dynamics better and better as one goes deeper and deeper in the
semiclassical limit. This demonstrates that quantum-classical correspondence at
the microscopic level requires neither high temperatures, nor coupling to a
large number of external degrees of freedom

### Einselection without pointer states

We consider small subsystems of large, closed quantum systems that evolve
according to the von Neumann equation. Without approximations and without
making any special assumptions on the form of the interaction we prove that,
for almost all initial states and almost all times, the off-diagonal elements
of the density matrix of the subsystem in the eigenbasis of its local
Hamiltonian must be small, whenever the energy difference of the corresponding
eigenstates is larger than the interaction energy. This proves that decoherence
with respect to the local energy eigenbasis is a natural property of weakly
interacting quantum systems.Comment: published in Phys. Rev. E, 4 pages, no figures, revised introduction
and conclusions, references revised and new references added, editorial
change

### Decoherence and entropy of primordial fluctuations. I: Formalism and interpretation

We propose an operational definition of the entropy of cosmological
perturbations based on a truncation of the hierarchy of Green functions. The
value of the entropy is unambiguous despite gauge invariance and the
renormalization procedure. At the first level of truncation, the reduced
density matrices are Gaussian and the entropy is the only intrinsic quantity.
In this case, the quantum-to-classical transition concerns the entanglement of
modes of opposite wave-vectors, and the threshold of classicality is that of
separability. The relations to other criteria of classicality are established.
We explain why, during inflation, most of these criteria are not intrinsic. We
complete our analysis by showing that all reduced density matrices can be
written as statistical mixtures of minimal states, the squeezed properties of
which are less constrained as the entropy increases. Pointer states therefore
appear not to be relevant to the discussion. The entropy is calculated for
various models in paper II.Comment: 23 page

### Long-Time Coherence in Echo Spectroscopy with $\pi/2$--$\pi$--$\pi/2$ Pulse Sequence

Motivated by atom optics experiments, we investigate a new class of fidelity
functions describing the reconstruction of quantum states by time-reversal
operations as $M_{\mathrm{Da}}(t) = | <\psi | e^{i H_2 t / 2} e^{i H_1 t / 2}
e^{-i H_2 t / 2} e^{-i H_1 t / 2} | \psi >|^2$. We show that the decay of
$M_{\mathrm{Da}}$ is quartic in time at short times, and that it freezes well
above the ergodic value at long times, when $H_2-H_1$ is not too large. The
long-time saturation value of $M_{\mathrm{Da}}$ contains easily extractable
information on the strength of decoherence in these systems.Comment: 5 pages, 3 figure

### Loschmidt echo with a non-equilibrium initial state: early time scaling and enhanced decoherence

We study the Loschmidt echo (LE) in a central spin model in which a central
spin is globally coupled to an environment (E) which is subjected to a small
and sudden quench at $t=0$ so that its state at $t=0^+$, remains the same as
the ground state of the initial environmental Hamiltonian before the quench;
this leads to a non-equilibrium situation. This state now evolves with two
Hamiltonians, the final Hamiltonian following the quench and its modified
version which incorporates an additional term arising due to the coupling of
the central spin to the environment. Using a generic short-time scaling of the
decay rate, we establish that in the early time limit, the rate of decay of the
LE (or the overlap between two states generated from the initial state evolving
through two channels) close to the quantum critical point (QCP) of E is
independent of the quenching. We do also study the temporal evolution of the LE
and establish the presence of a crossover to a situation where the quenching
becomes irrelevant. In the limit of large quench amplitude the non-equilibrium
initial condition is found to result in a drastic increase in decoherence at
large times, even far away from a QCP. These generic results are verified
analytically as well as numerically, choosing E to be a transverse Ising chain
where the transverse field is suddenly quenched.Comment: 5 pages, 6 figures; New results, figures and references added, title
change

### Shot Noise of Spin-Decohering Transport in Spin-Orbit Coupled Nanostructures

We generalize the scattering theory of quantum shot noise to include the full
spin-density matrix of electrons injected from a spin-filtering or
ferromagnetic electrode into a quantum-coherent nanostructure governed by
various spin-dependent interactions. This formalism yields the spin-resolved
shot noise power for different experimental measurement setups--with
ferromagnetic source and ferromagnetic or normal drain electrodes--whose
evaluation for the diffusive multichannel quantum wires with the Rashba (SO)
spin-orbit coupling shows how spin decoherence and dephasing lead to
substantial enhancement of charge current fluctuations (characterized by Fano
factors $> 1/3$). However, these processes and the corresponding shot noise
increase are suppressed in narrow wires, so that charge transport experiments
measuring the Fano factor $F_{\uparrow \to \uparrow \downarrow}$ in a
ferromagnet/SO-coupled-wire/paramagnet setup also quantify the degree of
phase-coherence of transported spin--we predict a one-to-one correspondence
between the magnitude of the spin polarization vector and $F_{\uparrow \to
\uparrow \downarrow}$.Comment: 8 pages, 3 figure; enhanced with 2 new figure

### Collisional decoherence reexamined

We re-derive the quantum master equation for the decoherence of a massive
Brownian particle due to collisions with the lighter particles from a thermal
environment. Our careful treatment avoids the occurrence of squares of Dirac
delta functions. It leads to a decoherence rate which is smaller by a factor of
2 pi compared to previous findings. This result, which is in agreement with
recent experiments, is confirmed by both a physical analysis of the problem and
by a perturbative calculation in the weak coupling limit.Comment: 33 pages, 4 figure

### Decoherence: Concepts and Examples

We give a pedagogical introduction to the process of decoherence - the
irreversible emergence of classical properties through interaction with the
environment. After discussing the general concepts, we present the following
examples: Localisation of objects, quantum Zeno effect, classicality of fields
and charges in QED, and decoherence in gravity theory. We finally emphasise the
important interpretational features of decoherence.Comment: 24 pages, LATEX, 9 figures, needs macro lamuphys.sty, to appear in
the Proceedings of the 10th Born Symposiu

### Reduced coherence in double-slit diffraction of neutrons

In diffraction experiments with particle beams, several effects lead to a
fringe visibility reduction of the interference pattern. We theoretically
describe the intensity one can measure in a double-slit setup and compare the
results with the experimental data obtained with cold neutrons. Our conclusion
is that for cold neutrons the fringe visibility reduction is due not to
decoherence, but to initial incoherence.Comment: 4 pages LaTeX, 2 figure

### Why the Universe Started from a Low Entropy State

We show that the inclusion of backreaction of massive long wavelengths
imposes dynamical constraints on the allowed phase space of initial conditions
for inflation, which results in a superselection rule for the initial
conditions. Only high energy inflation is stable against collapse due to the
gravitational instability of massive perturbations. We present arguments to the
effect that the initial conditions problem {\it cannot} be meaningfully
addressed by thermostatistics as far as the gravitational degrees of freedom
are concerned. Rather, the choice of the initial conditions for the universe in
the phase space and the emergence of an arrow of time have to be treated as a
dynamic selection.Comment: 12 pages, 2 figs. Final version; agrees with accepted version in
Phys. Rev.

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