554 research outputs found
Comment on "Spontaneous collapse: A solution to the measurement problem and a source of the decay in mesonic systems"
In a recent article [Phys. Rev. A 94, 052128 (2016)], the authors compute the
predictions of two collapse models on the transition probabilities of neutral
mesons. Notably, they claim to find an influence on the decay rates and attempt
to prove that a new parameter is required to fully characterize the
noise of standard collapse models. These two claims are incorrect and motivated
by flawed computations. This comment derives the correct transition
probabilities exactly from the master equation, explains how they could be
computed perturbatively in a safe way and finally shows where the main mistake
of the authors of the original article was made.Comment: 4 page
Exact signal correlators in continuous quantum measurements
This article provides an exact formula for the signal n-point correlation
functions of detectors continuously measuring an arbitrary quantum system, in
the presence of detection imperfections. The derivation uses only continuous
stochastic calculus techniques, but the final result is easily understood from
a discrete picture of repeated interactions with qubits or from a parallel with
continuous matrix product states. This result provides a crude yet efficient a
way to estimate system parameters directly from experimental data, without
requiring non-linear state reconstruction
Does gravity have to be quantized? Lessons from non-relativistic toy models
It is often argued that gravity has to be a quantum theory simply because a
fundamentally semiclassical approach would necessarily be inconsistent. Here I
review recent Newtonian toy models of (stochastic) semiclassical gravity. They
provide one option to implement a force semiclassically without getting into
the known problems associated with mean-field. These models are not complete
theories and should not be considered too seriously, but their consistency
shows that semiclassical gravity is hard to dismiss on purely theoretical
grounds.Comment: 16 pages -- written for the proceedings of the DICE 2018 workshop in
Castiglioncello -- provides a more detailed account of the technical
arguments in arXiv:1802.0329
Ghirardi-Rimini-Weber model with massive flashes
I introduce a modification of the Ghirardi-Rimini-Weber (GRW) model in which
the flashes (or collapse space-time events) source a classical gravitational
field. The resulting semi-classical theory of Newtonian gravity preserves the
statistical interpretation of quantum states of matter in contrast with mean
field approaches. It can be seen as a discrete version of recent proposals of
consistent hybrid quantum classical theories. The model is in agreement with
known experimental data and introduces new falsifiable predictions: (1) single
particles do not self-interact, (2) the gravitational potential of
Newtonian gravity is cut-off at short (m) distances, and (3)
gravity makes spatial superpositions decohere at a rate inversely proportional
to that coming from the vanilla GRW model. Together, the last two predictions
make the model experimentally falsifiable for \emph{all} values of its
parameters.Comment: 5 pages, close to published versio
Binding quantum matter and space-time, without romanticism
Understanding the emergence of a tangible 4-dimensional space-time from a
quantum theory of gravity promises to be a tremendously difficult task. This
article makes the case that this task may not have to be carried. Space-time as
we know it may be fundamental to begin with. I recall the common arguments
against this possibility and review a class of recently discovered models
bypassing the most serious objection. The generic solution of the measurement
problem that is tied to semiclassical gravity as well as the difficulty of the
alternative make it a reasonable default option in the absence of decisive
experimental evidence.Comment: 13 pages, 4 figures, branched out from an essay for the Beyond
spacetime contest, comments welcom
Principle of least decoherence for Newtonian semi-classical gravity
Recent works have proved that semi-classical theories of gravity needed not
be fundamentally inconsistent, at least in the Newtonian regime. Using the
machinery of continuous measurement theory and feedback, it was shown that one
could construct well behaved models of hybrid quantum-classical dynamics at the
price of an imposed (non unique) decoherence structure. We introduce a
principle of least decoherence (PLD) which allows to naturally single out a
unique model from all the available options; up to some unspecified short
distance regularization scale. Interestingly, the resulting model is found to
coincide with the old --erstwhile only heuristically motivated-- proposal of
Penrose and one of us for gravity-related spontaneous decoherence and collapse.
Finally, this letter suggests that it is in the submillimeter behavior of
gravity that new phenomena might be found.Comment: 5
On GKLS dynamics for local operations and classical communication
We define a time continuous version of the concept of "local operations and
classical communication" (LOCC), ubiquitous in quantum information theory. It
allows us to construct GKLS master equations for particle systems that have (1)
an arbitrary pair potential, and (2) local decoherence terms, but that do not
entangle the constituents. The local decoherence terms take a particularly
simple form if a principle of least decoherence is applied.Comment: 5pp. Submitted to special OSID volume "40 years of the GKLS equation
Sourcing semiclassical gravity from spontaneously localized quantum matter
The possibility that a classical space-time and quantum matter cohabit at the
deepest level, i.e. the possibility of having a fundamental and not
phenomenological semiclassical gravity, is often disregarded for lack of a good
candidate theory. The standard semiclassical theory suffers from fundamental
inconsistencies (e.g.: Schr\"odinger cat sources, faster-than-light
communication and violation of the Born rule) which can only be ignored in
simple typical situations. We harness the power of spontaneous localization
models, historically constructed to solve the measurement problem in quantum
mechanics, to build a consistent theory of (stochastic) semiclassical gravity
in the Newtonian limit. Our model makes quantitative and potentially testable
predictions: we recover the Newtonian pair potential up to a short distance
cut-off (hence we predict no 1 particle self-interaction) and uncover an
additional gravitational decoherence term which depends on the specifics of the
underlying spontaneous localization model considered. We hint at a possible
program to go past the Newtonian limit, towards a consistent general
relativistic semiclassical gravity.Comment: 9 pages + refs, 1 figure, typos corrected and minor modification
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