640 research outputs found
Continuous quantum error correction by cooling
We describe an implementation of quantum error correction that operates
continuously in time and requires no active interventions such as measurements
or gates. The mechanism for carrying away the entropy introduced by errors is a
cooling procedure. We evaluate the effectiveness of the scheme by simulation,
and remark on its connections to some recently proposed error prevention
procedures.Comment: 8 pages, 5 figures. Published version. Minor change in conten
Mesoscopic interference
We analyze a double-slit experiment when the interfering particle is
"mesoscopic" and one endeavors to obtain Welcher Weg information by shining
light on it. We derive a compact expression for the visibility of the
interference pattern: coherence depends on both the spatial and temporal
features of the wave function during its travel to the screen. We set a bound
on the temperature of the mesoscopic particle in order that its quantum
mechanical coherence be maintained.Comment: 16 pages, 14 figure
Decoherence vs entropy in neutron interferometry
We analyze the coherence properties of polarized neutrons, after they have
interacted with a magnetic field or a phase shifter undergoing different kinds
of statistical fluctuations. We endeavor to probe the degree of disorder of the
distribution of the phase shifts by means of the loss of quantum mechanical
coherence of the neutron. We find that the notion of entropy of the shifts and
that of decoherence of the neutron do not necessarily agree. In some cases the
neutron wave function is more coherent, even though it has interacted with a
more disordered medium.Comment: 13 pages, 5 figure
Zeno Dynamics in Quantum Statistical Mechanics
We study the quantum Zeno effect in quantum statistical mechanics within the
operator algebraic framework. We formulate a condition for the appearance of
the effect in W*-dynamical systems, in terms of the short-time behaviour of the
dynamics. Examples of quantum spin systems show that this condition can be
effectively applied to quantum statistical mechanical models. Further, we
derive an explicit form of the Zeno generator, and use it to construct Gibbs
equilibrium states for the Zeno dynamics. As a concrete example, we consider
the X-Y model, for which we show that a frequent measurement at a microscopic
level, e.g. a single lattice site, can produce a macroscopic effect in changing
the global equilibrium.Comment: 15 pages, AMSLaTeX; typos corrected, references updated and added,
acknowledgements added, style polished; revised version contains corrections
from published corrigend
Wigner function and coherence properties of cold and thermal neutrons
We analyze the coherence properties of a cold or a thermal neutron by
utilizing the Wigner quasidistribution function. We look in particular at a
recent experiment performed by Badurek {\em et al.}, in which a polarized
neutron crosses a magnetic field that is orthogonal to its spin, producing
highly non-classical states. The quantal coherence is extremely sensitive to
the field fluctuation at high neutron momenta. A "decoherence parameter" is
introduced in order to get quantitative estimates of the losses of coherence.Comment: 6 pages, 3 figures. Contribution to the Sixth Central-European
Workshop on Quantum Optics, Chudobin near Olomouc, Czech Republic, April-May
199
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
Quantum Zeno Effect in Trapped Ions
A "continuous measurement" Quantum Zeno Effect (QZE) in the context of
trapped ions is predicted. We describe the physical system and study its exact
time evolution showing the appearance of Zeno Phenomena. New indicators for the
occurrence of QZE in oscillatory systems are proposed and carefully discussed.Comment: 12 pages, 4 figure
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