13 research outputs found
Phase estimation without a priori knowledge in the presence of loss
We find the optimal scheme for quantum phase estimation in the presence of
loss when no a priori knowledge on the estimated phase is available. We prove
analytically an explicit lower bound on estimation uncertainty, which shows
that, as a function of number of probes, quantum precision enhancement amounts
at most to a constant factor improvement over classical strategiesComment: 8 pages, 2 figures, discussion on adaptive strategies adde
Device-independent quantum key distribution with single-photon sources
Device-independent quantum key distribution protocols allow two honest users
to establish a secret key with minimal levels of trust on the provider, as
security is proven without any assumption on the inner working of the devices
used for the distribution. Unfortunately, the implementation of these protocols
is challenging, as it requires the observation of a large Bell-inequality
violation between the two distant users. Here, we introduce novel photonic
protocols for device-independent quantum key distribution exploiting
single-photon sources and heralding-type architectures. The heralding process
is designed so that transmission losses become irrelevant for security. We then
show how the use of single-photon sources for entanglement distribution in
these architectures, instead of standard entangled-pair generation schemes,
provides significant improvements on the attainable key rates and distances
over previous proposals. Given the current progress in single-photon sources,
our work opens up a promising avenue for device-independent quantum key
distribution implementations.Comment: 20 pages (9 + appendices and bibliography), 5 figures, 1 tabl
Precision Limits in Quantum Metrology with Open Quantum Systems
The laws of quantum mechanics allow to perform measurements whose precision
supersedes results predicted by classical parameter estimation theory. That is,
the precision bound imposed by the central limit theorem in the estimation of a
broad class of parameters, like atomic frequencies in spectroscopy or external
magnetic field in magnetometry, can be overcome when using quantum probes.
Environmental noise, however, generally alters the ultimate precision that can
be achieved in the estimation of an unknown parameter. This tutorial reviews
recent theoretical work aimed at obtaining general precision bounds in the
presence of an environment. We adopt a complementary approach, where we first
analyze the problem within the general framework of describing the quantum
systems in terms of quantum dynamical maps and then relate this abstract
formalism to a microscopic description of the system's dissipative time
evolution. We will show that although some forms of noise do render quantum
systems standard quantum limited, precision beyond classical bounds is still
possible in the presence of different forms of local environmental
fluctuations.Comment: slightly modified versio
Upper bounds on key rates in device-independent quantum key distribution based on convex-combination attacks
Exploiting non-linear effects in optomechanical sensors with continuous photon-counting
Optomechanical systems are rapidly becoming one of the most promising
platforms for observing quantum behaviour, especially at the macroscopic level.
Moreover, thanks to their state-of-the-art methods of fabrication, they may now
enter regimes of non-linear interactions between their constituent mechanical
and optical degrees of freedom. In this work, we show how this novel
opportunity may serve to construct a new generation of optomechanical sensors.
We consider the canonical optomechanical setup with the detection scheme being
based on time-resolved counting of photons leaking from the cavity. By
performing simulations and resorting to Bayesian inference, we demonstrate that
the non-classical correlations of the detected photons may crucially enhance
the sensor performance in real time. We believe that our work may stimulate a
new direction in the design of such devices, while our methods apply also to
other platforms exploiting non-linear light-matter interactions and photon
detection.Comment: 25 pages, 10 figures, revised version with expanded analysi
Quantum metrology with imperfect measurements
The impact of measurement imperfections on quantum metrology protocols has
not been approached in a systematic manner so far. In this work, we tackle this
issue by generalising firstly the notion of quantum Fisher information to
account for noisy detection, and propose tractable methods allowing for its
approximate evaluation. We then show that in canonical scenarios involving
probes with local measurements undergoing readout noise, the optimal
sensitivity depends crucially on the control operations allowed to
counterbalance the measurement imperfections -- with global control operations,
the ideal sensitivity (e.g.~the Heisenberg scaling) can always be recovered in
the asymptotic limit, while with local control operations the
quantum-enhancement of sensitivity is constrained to a constant factor. We
illustrate our findings with an example of NV-centre magnetometry, as well as
schemes involving spin- probes with bit-flip errors affecting their
two-outcome measurements, for which we find the input states and control
unitary operations sufficient to attain the ultimate asymptotic precision.Comment: v2. 33 pages, 14 figures. Comments are welcom
Bayesian quantum thermometry based on thermodynamic length
In this work, we propose a theory of temperature estimation of quantum
systems, which is applicable in the regime of non-negligible prior temperature
uncertainty and limited measurement data. In this regime the problem of
establishing a well-defined measure of estimation precision becomes
non-trivial, and furthermore the construction of a suitable criterion for
optimal measurement design must be re-examined to account for the prior
uncertainty. We propose a fully Bayesian approach to temperature estimation
based on the concept of thermodynamic length, which solves both these problems.
As an illustration of this framework, we consider thermal spin- particles
and investigate the fundamental difference between two cases; on the one hand,
when the spins are probing the temperature of a heat reservoir and, on the
other, when the spins themselves constitute the sample.Comment: 12 pages, 5 figures, published versio
Fundamental Limits in Bayesian Thermometry and Attainability via Adaptive Strategies
We investigate the limits of thermometry using quantum probes at thermal
equilibrium within the Bayesian approach. We consider the possibility of
engineering interactions between the probes in order to enhance their
sensitivity, as well as feedback during the measurement process, i.e., adaptive
protocols. On the one hand, we obtain an ultimate bound on thermometry
precision in the Bayesian setting, valid for arbitrary interactions and
measurement schemes, which lower bounds the error with a quadratic
(Heisenberg-like) scaling with the number of probes. We develop a simple
adaptive strategy that can saturate this limit. On the other hand, we derive a
no-go theorem for non-adaptive protocols that does not allow for better than
linear (shot-noise-like) scaling even if one has unlimited control over the
probes, namely access to arbitrary many-body interactions.Comment: 14 pages with appendice