165 research outputs found
Complementarity in atomic (finite-level quantum) systems: an information-theoretic approach
We develop an information theoretic interpretation of the number-phase
complementarity in atomic systems, where phase is treated as a continuous
positive operator valued measure (POVM). The relevant uncertainty principle is
obtained as an upper bound on a sum of knowledge of these two observables for
the case of two-level systems. A tighter bound characterizing the uncertainty
relation is obtained numerically in terms of a weighted knowledge sum involving
these variables. We point out that complementarity in these systems departs
from mutual unbiasededness in two signalificant ways: first, the maximum
knowledge of a POVM variable is less than log(dimension) bits; second,
surprisingly, for higher dimensional systems, the unbiasedness may not be
mutual but unidirectional in that phase remains unbiased with respect to number
states, but not vice versa. Finally, we study the effect of non-dissipative and
dissipative noise on these complementary variables for a single-qubit system.Comment: 16 page
Dynamics of decoherence without dissipation in a squeezed thermal bath
We study a generic open quantum system where the coupling between the system
and its environment is of an energy-preserving quantum nondemolition (QND)
type. We obtain the general master equation for the evolution of such a system
under the influence of a squeezed thermal bath of harmonic oscillators. From
the master equation it can be seen explicitly that the process involves
decoherence or dephasing without any dissipation of energy. We work out the
decoherence-causing term in the high and zero temperature limits and check that
they match with known results for the case of a thermal bath. The decay of the
coherence is quantified as well by the dynamics of the linear entropy of the
system under various environmental conditions. We make a comparison of the
quantum statistical properties between QND and dissipative types of evolution
using a system of two-level atom and a harmonic oscillator.Comment: Accepted for publication in J. Phys. A: Math. Theor.; 23 pages, 8
figure
An environment-mediated quantum deleter
Environment-induced decoherence presents a great challenge to realizing a
quantum computer. We point out the somewhat surprising fact that decoherence
can be useful, indeed necessary, for practical quantum computation, in
particular, for the effective erasure of quantum memory in order to initialize
the state of the quantum computer. The essential point behind the deleter is
that the environment, by means of a dissipative interaction, furnishes a
contractive map towards a pure state. We present a specific example of an
amplitude damping channel provided by a two-level system's interaction with its
environment in the weak Born-Markov approximation. This is contrasted with a
purely dephasing, non-dissipative channel provided by a two-level system's
interaction with its environment by means of a quantum nondemolition
interaction. We point out that currently used state preparation techniques, for
example using optical pumping, essentially perform as quantum deleters.Comment: 5 pages, 3 figures, to appear in Physics Letters
Entropic Leggett-Garg inequality in neutrinos and B (K) meson systems
Entropic Leggett-Garg inequality is studied in systems like neutrinos in the
context of two and three flavor neutrino oscillations and in neutral ,
and mesons. The neutrino dynamics is described with the matter effect
taken into consideration. For the decohering meson systems, the effect of
decoherence and CP violation have also been taken into account, using the
techniques of open quantum systems. Enhancement in the violation with increase
in the number of measurements has been found, in consistency with findings in
spin- systems. The effect of decoherence is found to bring the deficit
parameter closer to its classical value zero, as expected.
The violation of entropic Leggett-Garg inequality lasts for a much longer time
in meson system than in and systems
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