576 research outputs found
Selective linear or quadratic optomechanical coupling via measurement
The ability to engineer both linear and non-linear coupling with a mechanical
resonator is an important goal for the preparation and investigation of
macroscopic mechanical quantum behavior. In this work, a measurement based
scheme is presented where linear or square mechanical displacement coupling can
be achieved using the optomechanical interaction linearly proportional to the
mechanical position. The resulting square displacement measurement strength is
compared to that attainable in the dispersive case using the direct interaction
to the mechanical displacement squared. An experimental protocol and parameter
set are discussed for the generation and observation of non-Gaussian states of
motion of the mechanical element.Comment: 7 pages, 2 figures, (accepted in Physical Review X
Self-induced decoherence approach: Strong limitations on its validity in a simple spin bath model and on its general physical relevance
The "self-induced decoherence" (SID) approach suggests that (1) the
expectation value of any observable becomes diagonal in the eigenstates of the
total Hamiltonian for systems endowed with a continuous energy spectrum, and
(2), that this process can be interpreted as decoherence. We evaluate the first
claim in the context of a simple spin bath model. We find that even for large
environments, corresponding to an approximately continuous energy spectrum,
diagonalization of the expectation value of random observables does in general
not occur. We explain this result and conjecture that SID is likely to fail
also in other systems composed of discrete subsystems. Regarding the second
claim, we emphasize that SID does not describe a physically meaningful
decoherence process for individual measurements, but only involves destructive
interference that occurs collectively within an ensemble of presupposed
"values" of measurements. This leads us to question the relevance of SID for
treating observed decoherence effects.Comment: 11 pages, 4 figures. Final published versio
Delayed-choice quantum eraser for the undergraduate laboratory
In a delayed-choice quantum eraser, interference fringes are obtained by
erasing which-way information after the interfering particle has already been
irreversibly detected. Following an introductory review of delayed-choice
experiments and quantum erasure, we describe the experimental realization of an
optical delayed-choice quantum eraser, suitable for advanced undergraduates,
based on polarization-entangled pairs of single photons. In our experiment, the
delay of the erasure is implemented using two different setups. The first setup
employs an arrangement of mirrors to increase the optical path length of the
photons carrying which-way information. In the second setup, we use fiber-optic
cables to elongate the path of these photons after their passage through the
polarization analyzer but prior to their arrival at the detector. We compare
our results to data obtained in the absence of a delay and find excellent
agreement. This shows that the timing of the erasure is irrelevant, as also
predicted by quantum mechanics. The experiment can serve as a valuable
pedagogical tool for conveying the fundamentals of quantum mechanics.Comment: 13 pages, 5 figures, identical to published versio
Observation of the quantum paradox of separation of a single photon from one of its properties
We report an experimental realization of the quantum paradox of the
separation of a single photon from one of its properties (the so-called
"quantum Cheshire cat"). We use a modified Sagnac interferometer with displaced
paths to produce appropriately pre- and postselected states of heralded single
photons. Weak measurements of photon presence and circular polarization are
performed in each arm of the interferometer by introducing weak absorbers and
small polarization rotations and analyzing changes in the postselected signal.
The absorber is found to have an appreciable effect only in one arm of the
interferometer, while the polarization rotation significantly affects the
signal only when performed in the other arm. We carry out both sequential and
simultaneous weak measurements and find good agreement between measured and
predicted weak values. In the language of Aharonov et al. and in the sense of
the ensemble averages described by weak values, the experiment establishes the
separation of a particle from one its properties during the passage through the
interferometer.Comment: 9 pages, 4 figures, identical to published versio
Energy transfer in finite-size exciton-phonon systems : confinement-enhanced quantum decoherence
Based on the operatorial formulation of the perturbation theory, the
exciton-phonon problem is revisited for investigating exciton-mediated energy
flow in a finite-size lattice. Within this method, the exciton-phonon
entanglement is taken into account through a dual dressing mechanism so that
exciton and phonons are treated on an equal footing. In a marked contrast with
what happens in an infinite lattice, it is shown that the dynamics of the
exciton density is governed by several time scales. The density evolves
coherently in the short-time limit whereas a relaxation mechanism occurs over
intermediated time scales. Consequently, in the long-time limit, the density
converges toward a nearly uniform distributed equilibrium distribution. Such a
behavior results from quantum decoherence that originates in the fact that the
phonons evolve differently depending on the path followed by the exciton to
tunnel along the lattice. Although the relaxation rate increases with the
temperature and with the coupling, it decreases with the lattice size,
suggesting that the decoherence is inherent to the confinement
On the origin of probability in quantum mechanics
I give a brief introduction to many worlds or "no wavefunction collapse"
quantum mechanics, suitable for non-specialists. I then discuss the origin of
probability in such formulations, distinguishing between objective and
subjective notions of probability.Comment: 7 pages, 2 figures. This version to appear as a Brief Review in
Modern Physics Letter
Quantum Equilibration under Constraints and Transport Balance
For open quantum systems coupled to a thermal bath at inverse temperature
, it is well known that under the Born-, Markov-, and secular
approximations the system density matrix will approach the thermal Gibbs state
with the bath inverse temperature . We generalize this to systems where
there exists a conserved quantity (e.g., the total particle number), where for
a bath characterized by inverse temperature and chemical potential
we find equilibration of both temperature and chemical potential. For
couplings to multiple baths held at different temperatures and different
chemical potentials, we identify a class of systems that equilibrates according
to a single hypothetical average but in general non-thermal bath, which may be
exploited to generate desired non-thermal states. Under special circumstances
the stationary state may be again be described by a unique Boltzmann factor.
These results are illustrated by several examples.Comment: 8 pages, 1 figure, leaner presentation, to appear in PR
Towards violation of Born's rule: description of a simple experiment
Recently a new model with hidden variables of the wave type was elaborated,
so called prequantum classical statistical field theory (PCSFT). Roughly
speaking PCSFT is a classical signal theory applied to a special class of
signals -- "quantum systems". PCSFT reproduces successfully all probabilistic
predictions of QM, including correlations for entangled systems. This model
peacefully coexists with all known no-go theorems, including Bell's theorem. In
our approach QM is an approximate model. All probabilistic predictions of QM
are only (quite good) approximations of "real physical averages". The latter
are averages with respect to fluctuations of prequantum fields. In particular,
Born's rule is only an approximate rule. More precise experiments should
demonstrate its violation. We present a simple experiment which has to produce
statistical data violating Born's rule. Since the PCSFT-presentation of this
experiment may be difficult for experimenters, we reformulate consequences of
PCSFT in terms of the conventional wave function. In general, deviation from
Born's rule is rather small. We found an experiment amplifying this deviation.
We start with a toy example in section 2. Then we present a more realistic
example based on Gaussian states with very small dispersion, see section 3.Comment: The paper was completed with the description of an experiment with
Gaussian states with very small dispersion. This experiment should induce
violation of Born's rule, the fundamental law of Q
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