16,777 research outputs found
Retrodiction with two-level atoms: atomic previvals
In the Jaynes-Cummings model a two-level atom interacts with a single-mode
electromagnetic field. Quantum mechanics predicts collapses and revivals in the
probability that a measurement will show the atom to be excited at various
times after the initial preparation of the atom and field. In retrodictive
quantum mechanics we seek the probability that the atom was prepared in a
particular state given the initial state of the field and the outcome of a
later measurement on the atom. Although this is not simply the time reverse of
the usual predictive problem, we demonstrate in this paper that retrodictive
collapses and revivals also exist. We highlight the differences between
predictive and retrodictive evolutions and describe an interesting situation
where the prepared state is essentially unretrodictable.Comment: 15 pages, 3 (5) figure
Effects of self-phase modulation on weak nonlinear optical quantum gates
A possible two-qubit gate for optical quantum computing is the parity gate
based on the weak Kerr effect. Two photonic qubits modulate the phase of a
coherent state, and a quadrature measurement of the coherent state reveals the
parity of the two qubits without destroying the photons. This can be used to
create so-called cluster states, a universal resource for quantum computing.
Here, the effect of self-phase modulation on the parity gate is studied,
introducing generating functions for the Wigner function of a modulated
coherent state. For materials with non-EIT-based Kerr nonlinearities, there is
typically a self-phase modulation that is half the magnitude of the cross-phase
modulation. Therefore, this effect cannot be ignored. It is shown that for a
large class of physical implementations of the phase modulation, the quadrature
measurement cannot distinguish between odd and even parity. Consequently, weak
nonlinear parity gates must be implemented with physical systems where the
self-phase modulation is negligable.Comment: 7 pages, 4 figure
Retrodiction as a tool for micromaser field measurements
We use retrodictive quantum theory to describe cavity field measurements by
successive atomic detections in the micromaser. We calculate the state of the
micromaser cavity field prior to detection of sequences of atoms in either the
excited or ground state, for atoms that are initially prepared in the excited
state. This provides the POM elements, which describe such sequences of
measurements.Comment: 20 pages, 4(8) figure
Dependence of the evolution of the cavity radiation of a coherently pumped correlated emission laser on dephasing and phase fluctuation
Analysis of the dynamics of the cavity radiation of a coherently pumped
correlated emission laser is presented. The phase fluctuation and dephasing are
found to affect the time evolution of the two-mode squeezing and intensity of
the cavity radiation significantly. The intensity and degree of the two-mode
squeezing increase at early stages of the process with time, but this trend
changes rapidly afterwards. It is also shown that they increase with phase
fluctuation and dephasing in the strong driving limit, however the situation
appears to be opposite in the weak driving limit. This essentially suggests
that the phase fluctuation and dephasing weaken the coherence induced by a
strong driving mechanism so that the spontaneous emission gets a chance. The
other important aspect of the phase fluctuation, in this regard, is the
relaxation of the time at which the maximum squeezing is manifested as well as
the time in which the radiation remains in a squeezed state.Comment: 10 pages, 12 figure
Difficulty of distinguishing product states locally
Non-locality without entanglement is a rather counter-intuitive phenomenon in
which information may be encoded entirely in product (unentangled) states of
composite quantum systems in such a way that local measurement of the
subsystems is not enough for optimal decoding. For simple examples of pure
product states, the gap in performance is known to be rather small when
arbitrary local strategies are allowed. Here we restrict to local strategies
readily achievable with current technology; those requiring neither a quantum
memory nor joint operations. We show that, even for measurements on pure
product states there can be a large gap between such strategies and
theoretically optimal performance. Thus even in the absence of entanglement
physically realizable local strategies can be far from optimal for extracting
quantum information.Comment: 5 pages, 1 figur
Discrimination of two mixed quantum states with maximum confidence and minimum probability of inconclusive results
We study an optimized measurement that discriminates two mixed quantum states
with maximum confidence for each conclusive result, thereby keeping the overall
probability of inconclusive results as small as possible. When the rank of the
detection operators associated with the two different conclusive outcomes does
not exceed unity we obtain a general solution. As an application, we consider
the discrimination of two mixed qubit states. Moreover, for the case of
higher-rank detection operators we give a solution for particular states. The
relation of the optimized measurement to other discrimination schemes is also
discussed.Comment: 7 pages, 1 figure, accepted for publication in Phys. Rev.
Combining real and virtual Higgs boson mass constraints
Within the framework of the standard model we observe that there is a
significant discrepancy between the most precise boson decay asymmetry
measurement and the limit from direct searches for Higgs boson production.
Using methods inspired by the Particle Data Group we explore the possible
effect on fits of the Higgs boson mass. In each case the central value and the
95% confidence level upper limit increase significantly relative to the
conventional fit. The results suggest caution in drawing conclusions about the
Higgs boson mass from the existing data.Comment: 11 pages, Latex. Citations are added and paper is otherwise
reconciled with version to be published in Physical Review Letter
On the Quantum Phase Operator for Coherent States
In papers by Lynch [Phys. Rev. A41, 2841 (1990)] and Gerry and Urbanski
[Phys. Rev. A42, 662 (1990)] it has been argued that the phase-fluctuation
laser experiments of Gerhardt, B\"uchler and Lifkin [Phys. Lett. 49A, 119
(1974)] are in good agreement with the variance of the Pegg-Barnett phase
operator for a coherent state, even for a small number of photons. We argue
that this is not conclusive. In fact, we show that the variance of the phase in
fact depends on the relative phase between the phase of the coherent state and
the off-set phase of the Pegg-Barnett phase operator. This off-set
phase is replaced with the phase of a reference beam in an actual experiment
and we show that several choices of such a relative phase can be fitted to the
experimental data. We also discuss the Noh, Foug\`{e}res and Mandel [Phys.Rev.
A46, 2840 (1992)] relative phase experiment in terms of the Pegg-Barnett phase
taking post-selection conditions into account.Comment: 8 pages, 8 figures. Typographical errors and misprints have been
corrected. The outline of the paper has also been changed. Physica Scripta
(in press
Decoherence of number states in phase-sensitive reservoirs
The non-unitary evolution of initial number states in general Gaussian
environments is solved analytically. Decoherence in the channels is quantified
by determining explicitly the purity of the state at any time. The influence of
the squeezing of the bath on decoherence is discussed. The behavior of coherent
superpositions of number states is addressed as well.Comment: 5 pages, 2 figures, minor changes, references adde
Joint measurements and Bell inequalities
Joint quantum measurements of non-commuting observables are possible, if one
accepts an increase in the measured variances. A necessary condition for a
joint measurement to be possible is that a joint probability distribution
exists for the measurement. This fact suggests that there may be a link with
Bell inequalities, as these will be satisfied if and only if a joint
probability distribution for all involved observables exists. We investigate
the connections between Bell inequalities and conditions for joint quantum
measurements to be possible. Mermin's inequality for the three-particle
Greenberger-Horne-Zeilinger state turns out to be equivalent to the condition
for a joint measurement on two out of the three quantum systems to exist.
Gisin's Bell inequality for three co-planar measurement directions, meanwhile,
is shown to be less strict than the condition for the corresponding joint
measurement
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