181 research outputs found
Violation of Bell's inequalities in a quantum realistic framework
We discuss the recently observed "loophole free" violation of Bell's
inequalities in the framework of a physically realist view of quantum
mechanics, which requires that physical properties are attributed jointly to a
system, and to the context in which it is embedded. This approach is clearly
different from classical realism, but it does define a meaningful "quantum
realism" from a general philosophical point of view. Consistently with Bell
test experiments, this quantum realism embeds some form of non-locality, but
does not contain any action at a distance, in agreement with quantum mechanics.Comment: This article is closely related to arxiv:1409.2120, with some parts
condensed and others expanded, in order to spell out how the present approach
explains quantum non-locality. In v2 some clarifications and improvements
following referees remark
What is quantum in quantum randomness?
It is often said that quantum and classical randomness are of different
nature, the former being ontological and the latter epistemological. However,
so far the question of "What is quantum in quantum randomness", i.e. what is
the impact of quantization and discreteness on the nature of randomness,
remains to answer. In a first part, we explicit the differences between quantum
and classical randomness within a recently proposed ontology for quantum
mechanics based on contextual objectivity. In this view, quantum randomness is
the result of contextuality and quantization. We show that this approach
strongly impacts the purposes of quantum theory as well as its areas of
application. In particular, it challenges current programs inspired by
classical reductionism, aiming at the emergence of the classical world from a
large number of quantum systems. In a second part, we analyze quantum physics
and thermodynamics as theories of randomness, unveiling their mutual
influences. We finally consider new technological applications of quantum
randomness opened in the emerging field of quantum thermodynamics.Comment: This article will appear in Philosophical Transaction A, following
the Royal Society Symposium "Foundations of quantum mechanics and their
impact on Contemporary Society
Contexts, Systems and Modalities: a new ontology for quantum mechanics
In this article we present a possible way to make usual quantum mechanics
fully compatible with physical realism, defined as the statement that the goal
of physics is to study entities of the natural world, existing independently
from any particular observer's perception, and obeying universal and
intelligible rules. Rather than elaborating on the quantum formalism itself, we
propose to modify the quantum ontology, by requiring that physical properties
are attributed jointly to the system, and to the context in which it is
embedded. In combination with a quantization principle, this non-classical
definition of physical reality sheds new light on counter-intuitive features of
quantum mechanics such as the origin of probabilities, non-locality, and the
quantum-classical boundary.Comment: 9 pages, 3 figures. In v2 extended section VI on EPR, and new section
VII on measurement
Optical drive of macroscopic mechanical motion by a single two-level system
A quantum emitter coupled to a nano-mechanical oscillator is a hybrid system
where a macroscopic degree of freedom is coupled to a purely quantum system.
Recent progress in nanotechnology has led to the realization of such devices by
embedding single artificial atoms like quantum dots or superconducting qubits
into vibrating wires or membranes, opening up new perspectives for quantum
information technologies and for the exploration of the quantum-classical
boundary. In this letter, we show that the quantum emitter can be turned into a
strikingly efficient light-controlled source of mechanical power, by exploiting
constructive interferences of classical phonon fields in the mechanical
oscillator. We show that this mechanism can be used as a novel strategy to
carry out low-background non-destructive single-shot measurement of an
optically active quantum bit state.Comment: 8 pages, 5 figure
Influence of phonons on solid-state cavity-QED investigated using nonequilibrium Green's functions
The influence of electron--phonon interactions on the dynamics of a quantum
dot coupled to a photonic cavity mode is investigated using a nonequilibrium
Green's function approach. Within a polaron frame, the self-consistent-Born
approximation is used to treat the phonon-assisted scattering processes between
the quantum dot polaron and the cavity. Two-time correlators of the quantum
dot-cavity system are calculated by solving the Kadanoff-Baym equations, giving
access to photon spectra and photon indistinguishability. The non-Markovian
nature of the interaction with the phonon bath is shown to be very accurately
described by our method in various regime of cavity-quantum electrodynamics
(cavity-QED). The indistinguishability of the emitted photons emitted at zero
temperature are found to be in very good agreement with a previously reported
exact diagonalization approach [Phys.~Rev.~B~87,~081308~(2013)]. Besides, our
method enables the calculations of photon indistinguishability at finite
temperatures and for strong electron-phonon interactions. More generally, our
method opens new avenues in the study of open quantum system dynamics coupled
to non-Markovian environments.Comment: 10 pages, 6 figure
Ultrafast QND measurements based on diamond-shape artificial atom
We propose a Quantum Non Demolition (QND) read-out scheme for a
superconducting artificial atom coupled to a resonator in a circuit QED
architecture, for which we estimate a very high measurement fidelity without
Purcell effect limitations. The device consists of two transmons coupled by a
large inductance, giving rise to a diamond-shape artificial atom with a logical
qubit and an ancilla qubit interacting through a cross-Kerr like term. The
ancilla is strongly coupled to a transmission line resonator. Depending on the
qubit state, the ancilla is resonantly or dispersively coupled to the
resonator, leading to a large contrast in the transmitted microwave signal
amplitude. This original method can be implemented with state of the art
Josephson parametric amplifier, leading to QND measurements in a few tens of
nanoseconds with fidelity as large as 99.9 %.Comment: 5 pages, 4 figure
Probing the state of a mechanical oscillator with an ultra-strongly coupled quantum emitter
We study the dynamics of a mechanical resonator parametrically coupled to a
driven dissipative quantum emitter in the ultra-strong coupling regime. We show
that this regime is fully compatible with a semi-classical treatment, and we
derive master equations for the emitter and the resonator. We show that the
fluctuations of the driven emitter's population induce the non-symmetrical
scattering of the mechanical quadratures. At long timescales, such scattering
back-acts on the emitter, which eventually decouples from the driving light.
This optical noise at the quantum limit is observable with state of the art
hybrid devices.Comment: 6 pages, 3 figures. Comments are welcom
A short story of quantum and information thermodynamics
This Colloquium is a fast journey through the build-up of key thermodynamical
concepts, i.e. work, heat and irreversibility -- and how they relate to
information. Born at the time of industrial revolution to optimize the
exploitation of thermal resources, these concepts have been adapted to small
systems where thermal fluctuations are predominant. Extending the framework to
quantum fluctuations is a great challenge of quantum thermodynamics, that opens
exciting research lines e.g. measurement fueled engines or thermodynamics of
driven-dissipative systems. On a more applied side, it provides the tools to
optimize the energetic consumption of future quantum computers.Comment: Submitted to SciPost Lecture Notes. To appear in 'Quantum Information
Machines; Lecture Notes of the Les Houches Summer School 2019', eds. M.
Devoret, B. Huard, and I. Po
Measurement back-action and spin noise spectroscopy in a charged cavity-QED device in the strong coupling regime
We study theoretically the spin-induced and photon-induced fluctuations of
optical signals from a singly-charged quantum dot-microcavity structure. We
identify the respective contributions of the photon-polariton interactions, in
the strong light-matter coupling regime, and of the quantum back-action induced
by photon detection on the spin system. Strong spin projection by a single
photon is shown to be achievable, allowing the initialization and measurement
of a fully-polarized Larmor precession. The spectrum of second-order
correlations is deduced, displaying information on both spin and quantum
dot-cavity dynamics. The presented theory thus bridges the gap between the
fields of spin noise spectroscopy and quantum optics.Comment: 12 pages, 8 figure
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