289 research outputs found
Classical limit of the quantum Zeno effect
The evolution of a quantum system subjected to infinitely many measurements
in a finite time interval is confined in a proper subspace of the Hilbert
space. This phenomenon is called "quantum Zeno effect": a particle under
intensive observation does not evolve. This effect is at variance with the
classical evolution, which obviously is not affected by any observations. By a
semiclassical analysis we will show that the quantum Zeno effect vanishes at
all orders, when the Planck constant tends to zero, and thus it is a purely
quantum phenomenon without classical analog, at the same level of tunneling.Comment: 10 pages, 2 figure
A perturbative approach for the dynamics of the quantum Zeno subspaces
In this paper we investigate the dynamics of the quantum Zeno subspaces which
are the eigenspaces of the interaction Hamiltonian, belonging to different
eigenvalues. Using the perturbation theory and the adiabatic approximation, we
get a general expression of the jump probability between different Zeno
subspaces. We applied this result in some examples. In these examples, as the
coupling constant of the interactions increases, the measurement keeps the
system remaining in its initial subspace and the quantum Zeno effect takes
place.Comment: 14 pages, 3 figure
Squeezing in driven bimodal Bose-Einstein Condensates: Erratic driving versus noise
We study the interplay of squeezing and phase randomization near the
hyperbolic instability of a two-site Bose-Hubbard model in the Josephson
interaction regime. We obtain results for the quantum Zeno suppression of
squeezing, far beyond the previously found short time behavior. More
importantly, we contrast the expected outcome with the case where randomization
is induced by erratic driving with the same fluctuations as the quantum noise
source, finding significant differences. These are related to the distribution
of the squeezing factor, which has log-normal characteristics: hence its
average is significantly different from its median due to the occurrence of
rare events.Comment: 5 pages, 4 figure
Unification of Dynamical Decoupling and the Quantum Zeno Effect
We unify the quantum Zeno effect (QZE) and the "bang-bang" (BB) decoupling
method for suppressing decoherence in open quantum systems: in both cases
strong coupling to an external system or apparatus induces a dynamical
superselection rule that partitions the open system's Hilbert space into
quantum Zeno subspaces. Our unification makes use of von Neumann's ergodic
theorem and avoids making any of the symmetry assumptions usually made in
discussions of BB. Thus we are able to generalize BB to arbitrary fast and
strong pulse sequences, requiring no symmetry, and to show the existence of two
alternatives to pulsed BB: continuous decoupling, and pulsed measurements. Our
unified treatment enables us to derive limits on the efficacy of the BB method:
we explicitly show that the inverse QZE implies that BB can in some cases
accelerate, rather than inhibit, decoherence.Comment: 6 pages. To appear in Phys. Rev.
Domain wall suppression in trapped mixtures of Bose-Einstein condensates
The ground state energy of a binary mixture of Bose-Einstein condensates can
be estimated for large atomic samples by making use of suitably regularized
Thomas-Fermi density profiles. By exploiting a variational method on the trial
densities the energy can be computed by explicitly taking into account the
normalization condition. This yields analytical results and provides the basis
for further improvement of the approximation. As a case study, we consider a
binary mixture of Rb atoms in two different hyperfine states in a double
well potential and discuss the energy crossing between density profiles with
different numbers of domain walls, as the number of particles and the
inter-species interaction vary.Comment: 9 page
Slow relaxation, confinement, and solitons
Millisecond crystal relaxation has been used to explain anomalous decay in
doped alkali halides. We attribute this slowness to Fermi-Pasta-Ulam solitons.
Our model exhibits confinement of mechanical energy released by excitation.
Extending the model to long times is justified by its relation to solitons,
excitations previously proposed to occur in alkali halides. Soliton damping and
observation are also discussed
Disclosing hidden information in the quantum Zeno effect: Pulsed measurement of the quantum time of arrival
Repeated measurements of a quantum particle to check its presence in a region
of space was proposed long ago [G. R. Allcock, Ann. Phys. {\bf 53}, 286 (1969)]
as a natural way to determine the distribution of times of arrival at the
orthogonal subspace, but the method was discarded because of the quantum Zeno
effect: in the limit of very frequent measurements the wave function is
reflected and remains in the original subspace. We show that by normalizing the
small bits of arriving (removed) norm, an ideal time distribution emerges in
correspondence with a classical local-kinetic-energy distribution.Comment: 5 pages, 4 figures, minor change
Timelapse
We discuss the existence in an arbitrary frame of a finite time for the
transformation of an initial quantum state into another e.g. in a decay.
This leads to the introduction of a timelapse in analogy with
the lifetime of a particle. An argument based upon the Heisenberg uncertainty
principle suggests the value of . Consequences for the
exponential decay formula and the modifications that introduces
into the Breit-Wigner mass formula are described.Comment: 5 pages [2 figs], ReV-Te
Multipartite Entanglement and Frustration
Some features of the global entanglement of a composed quantum system can be
quantified in terms of the purity of a balanced bipartition, made up of half of
its subsystems. For the given bipartition, purity can always be minimized by
taking a suitable (pure) state. When many bipartitions are considered, the
requirement that purity be minimal for all bipartitions can engender conflicts
and frustration arises. This unearths an interesting link between frustration
and multipartite entanglement, defined as the average purity over all
(balanced) bipartitions.Comment: 15 pages, 7 figure
- …