1,042 research outputs found
Measuring the quantum state of a single system with minimum state disturbance
Conventionally, unknown quantum states are characterized using quantum-state
tomography based on strong or weak measurements carried out on an ensemble of
identically prepared systems. By contrast, the use of protective measurements
offers the possibility of determining quantum states from a series of weak,
long measurements performed on a single system. Because the fidelity of a
protectively measured quantum state is determined by the amount of state
disturbance incurred during each protective measurement, it is crucial that the
initial quantum state of the system is disturbed as little as possible. Here we
show how to systematically minimize the state disturbance in the course of a
protective measurement, thus enabling the maximization of the fidelity of the
quantum-state measurement. Our approach is based on a careful tuning of the
time dependence of the measurement interaction and is shown to be dramatically
more effective in reducing the state disturbance than the previously considered
strategy of weakening the measurement strength and increasing the measurement
time. We describe a method for designing the measurement interaction such that
the state disturbance exhibits polynomial decay to arbitrary order in the
inverse measurement time . We also show how one can achieve even faster,
subexponential decay, and we find that it represents the smallest possible
state disturbance in a protective measurement. In this way, our results show
how to optimally measure the state of a single quantum system using protective
measurements.Comment: 7 pages, 4 figures, identical to published versio
What classicality? Decoherence and Bohr's classical concepts
Niels Bohr famously insisted on the indispensability of what he termed
"classical concepts." In the context of the decoherence program, on the other
hand, it has become fashionable to talk about the "dynamical emergence of
classicality" from the quantum formalism alone. Does this mean that decoherence
challenges Bohr's dictum -- for example, that classical concepts do not need to
be assumed but can be derived? In this paper, we'll try to shed some light down
the murky waters where formalism and philosophy mingle. To begin, we'll clarify
the notion of classicality in the decoherence description. We'll then discuss
Bohr's and Heisenberg's takes on the quantum-classical problem and reflect on
the different meanings of the terms "classicality" and "classical concepts" in
the writings of Bohr and his followers. This analysis will allow us to put
forward some tentative suggestions for how we may better understand the
relation between decoherence-induced classicality and Bohr's classical
concepts.Comment: 6 page
No-go theorem for the composition of quantum systems
Building on the Pusey-Barrett-Rudolph theorem, we derive a no-go theorem for
a vast class of deterministic hidden-variables theories, including those
consistent on their targeted domain. The strength of this result throws doubt
on seemingly natural assumptions (like the "preparation independence" of the
Pusey-Barrett-Rudolph theorem) about how "real states" of subsystems compose
for joint systems in nonentangled states. This points to constraints in
modeling tensor-product states, similar to constraints demonstrated for more
complex states by the Bell and Bell-Kochen-Specker theorems.Comment: 4 pages. v2: new title, significant revisions. v3: condensed, matches
final published versio
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
A Snapshot of Foundational Attitudes Toward Quantum Mechanics
Foundational investigations in quantum mechanics, both experimental and
theoretical, gave birth to the field of quantum information science.
Nevertheless, the foundations of quantum mechanics themselves remain hotly
debated in the scientific community, and no consensus on essential questions
has been reached. Here, we present the results of a poll carried out among 33
participants of a conference on the foundations of quantum mechanics. The
participants completed a questionnaire containing 16 multiple-choice questions
probing opinions on quantum-foundational issues. Participants included
physicists, philosophers, and mathematicians. We describe our findings,
identify commonly held views, and determine strong, medium, and weak
correlations between the answers. Our study provides a unique snapshot of
current views in the field of quantum foundations, as well as an analysis of
the relationships between these views.Comment: 17 pages, 3 figure
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