81 research outputs found
Cooperative fluorescence effects for dipole-dipole interacting systems with experimentally relevant level configurations
The mutual dipole-dipole interaction of atoms in a trap can affect their
fluorescence. Extremely large effects were reported for double jumps between
different intensity periods in experiments with two and three Ba^+ ions for
distances in the range of about ten wave lengths of the strong transition while
no effects were observed for Hg^+ at 15 wave lengths. In this theoretical paper
we study this question for configurations with three and four levels which
model those of Hg^+ and Ba^+, respectively. For two systems in the Hg^+
configuration we find cooperative effects of up to 30% for distances around one
or two wave lengths, about 5% around ten wave lengths, and, for larger
distances in agreement with experiments, practically none. This is similar for
two V systems. However, for two four-level configurations, which model two Ba^+
ions, cooperative effects are practically absent, and this latter result is at
odds with the experimental findings for Ba^+.Comment: 9 pages, 5 figures, RevTeX4, to be published in Phys. Rev.
State permutations from manipulation of near level-crossings
We discuss some systematic methods for implementing state manipulations in
systems formally similar to chains of a few spins with nearest-neighbor
interactions, arranged such that there are strong and weak scales of coupling
links. States are permuted by means of bias potentials applied to a few
selected sites. This generic structure is then related to an atoms-in-a-cavity
model that has been proposed in the literature as a way of achieving a
decoherence free subspace. A new method using adiabatically varying laser
detuning to implement a CNOT gate in this model is proposed.Comment: 6 pages, 5 figures. Substantial revision and extension of the
introduction and the atoms-in-a-cavity section
Quantum Zeno and anti-Zeno effects by indirect measurement with finite errors
We study the quantum Zeno effect and the anti-Zeno effect in the case of
`indirect' measurements, where a measuring apparatus does not act directly on
an unstable system, for a realistic model with finite errors in the
measurement. A general and simple formula for the decay rate of the unstable
system under measurement is derived. In the case of a Lorentzian form factor,
we calculate the full time evolutions of the decay rate, the response of the
measuring apparatus, and the probability of errors in the measurement. It is
shown that not only the response time but also the detection efficiency plays a
crucial role. We present the prescription for observing the quantum Zeno and
anti-Zeno effects, as well as the prescriptions for avoiding or calibrating
these effects in general experiments.Comment: 4 pages, 3 figure
Computation on a Noiseless Quantum Code and Symmetrization
Let be the state-space of a quantum computer coupled with the
environment by a set of error operators spanning a Lie algebra
Suppose admits a noiseless quantum code i.e., a subspace annihilated by We show that a universal set of
gates over is obtained by any generic pair of -invariant
gates. Such gates - if not available from the outset - can be obtained by
resorting to a symmetrization with respect to the group generated by Any computation can then be performed completely within the coding
decoherence-free subspace.Comment: One result added, to appear in Phys. Rev. A (RC) 4 pages LaTeX, no
figure
Error Prevention Scheme with Four Particles
It is shown that a simplified version of the error correction code recently
suggested by Shor exhibits manifestation of the quantum Zeno effect. Thus,
under certain conditions, protection of an unknown quantum state is achieved.
Error prevention procedures based on four-particle and two-particle encoding
are proposed and it is argued that they have feasible practical
implementations.Comment: 4 pages, RevTeX, references updated and improved protocol adde
Conditional quantum logic using two atomic qubits
In this paper we propose and analyze a feasible scheme where the detection of
a single scattered photon from two trapped atoms or ions performs a conditional
unitary operation on two qubits. As examples we consider the preparation of all
four Bell states, the reverse operation that is a Bell measurement, and a CNOT
gate. We study the effect of atomic motion and multiple scattering, by
evaluating Bell inequalities violations, and by calculating the CNOT gate
fidelity.Comment: 23 pages, 8 figures in 11 file
Generation of spatial antibunching with free propagating twin beams
We propose and implement a novel method to produce a spatial anti-bunched
field with free propagating twin beams from spontaneous parametric
down-conversion. The method consists in changing the spatial propagation by
manipulating the transverse degrees of freedom through reflections of one of
the twin beams. Our method use reflective elements eliminating losses from
absorption by the objects inserted in the beams.Comment: Submitted for publication in Phys. Rev.
Comment on "Evolution of a Quasi-Stationary State"
Approximately forty years ago it was realized that the time development of
decaying systems might not be precisely exponential. Rolf Winter (Phys. Rev.
{\bf 123}, 1503 (1961)) analyzed the simplest nontrivial system - a particle
tunneling out of a well formed by a wall and a delta-function. He calculated
the probability current just outside the well and found irregular oscillations
on a short time scale followed by an exponential decrease followed by more
oscillations and finally by a decrease as a power of the time. We have
reanalyzed this system, concentrating on the survival probability of the
particle in the well rather than the probability current, and find a different
short time behavior.Comment: 8 pages, 6 figures, RevTex
Real clocks and the Zeno effect
Real clocks are not perfect. This must have an effect in our predictions for
the behaviour of a quantum system, an effect for which we present a unified
description encompassing several previous proposals. We study the relevance of
clock errors in the Zeno effect, and find that generically no Zeno effect can
be present (in such a way that there is no contradiction with currently
available experimental data). We further observe that, within the class of
stochasticities in time addressed here, there is no modification in emission
lineshapes.Comment: 12 a4 pages, no figure
Screening of qubit from zero-temperature reservoir
We suggest an application of dynamical Zeno effect to isolate a qubit in the
quantum memory unit against decoherence caused by coupling with the reservoir
having zero temperature. The method is based on using an auxiliary casing
system that mediate the qubit-reservoir interaction and is simultaneously
frequently erased to ground state. This screening procedure can be implemented
in the cavity QED experiments to store the atomic and photonic qubit states.Comment: 4 pages, 5 figure
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