3,552 research outputs found
Detecting Gaussian entanglement via extractable work
We show how the presence of entanglement in a bipartite Gaussian state can be
detected by the amount of work extracted by a continuos variable Szilard-like
device, where the bipartite state serves as the working medium of the engine.
We provide an expression for the work extracted in such a process and
specialize it to the case of Gaussian states. The extractable work provides a
sufficient condition to witness entanglement in generic two-mode states,
becoming also necessary for squeezed thermal states. We extend the protocol to
tripartite Gaussian states, and show that the full structure of inseparability
classes cannot be discriminated based on the extractable work. This suggests
that bipartite entanglement is the fundamental resource underpinning work
extraction.Comment: 12 pages, 8 figure
Metrology with Unknown Detectors
The best possible precision is one of the key figures in metrology, but this
is established by the exact response of the detection apparatus, which is often
unknown. There exist techniques for detector characterisation, that have been
introduced in the context of quantum technologies, but apply as well for
ordinary classical coherence; these techniques, though, rely on intense data
processing. Here we show that one can make use of the simpler approach of data
fitting patterns in order to obtain an estimate of the Cram\'er-Rao bound
allowed by an unknown detector, and present applications in polarimetry.
Further, we show how this formalism provide a useful calculation tool in an
estimation problem involving a continuous-variable quantum state, i.e. a
quantum harmonic oscillator
Non-equilibrium readiness and accuracy of Gaussian Quantum Thermometers
The dimensionality of a thermometer is key in the design of quantum
thermometry schemes. In general, the phenomenology that is typical of
finite-dimensional quantum thermometry does not apply to infinite dimensional
ones. We analyse the dynamical and metrological features of non-equilibrium
Gaussian Quantum Thermometers: on one hand, we highlight how quantum
entanglement can enhance the readiness of composite Gaussian thermometers; on
the other hand, we show that non-equilibrium conditions do not guarantee the
best sensitivities in temperature estimation, thus suggesting the reassessment
of the working principles of quantum thermometry
An educational path for the magnetic vector potential and its physical implications
We present an educational path on the magnetic vector potential A addressed
to undergraduate students and to pre-service physics teachers. Starting from
the generalized Ampere-Laplace law, in the framework of a slowly varying
time-dependent field approximation, the magnetic vector potential is written in
terms of its empirical referent, i. e. the conduction current. Therefore, once
the currents are known, our approach allows a clear and univocal physical
determination of A overcoming the mathematical indeterminacy due to the gauge
transformations. We have no need to fix a gauge, since for slowly varying
time-dependent electric and magnetic fields, the natural gauge for A is the
Coulomb one. We stress the difference between our approach and those usually
presented in the literature. Finally, a physical interpretation of the magnetic
vector potential is discussed and some examples of calculation of A are
analysed
A dynamical model of genetic networks describes cell differentiation
Cell differentiation is a complex phenomenon whereby a stem cell becomes progressively more specialized and eventually gives rise to a specific cell type. Differentiation can be either stochastic or, when appropriate signals are present, it can be driven to take a specific route. Induced pluripotency has also been recently obtained by overexpressing some genes in a differentiated cell. Here we show that a stochastic dynamical model of genetic networks can satisfactorily describe all these important features of differentiation, and others. The model is based on the emergent properties of generic genetic networks, it does not refer to specific control circuits and it can therefore hold for a wide class of lineages. The model points to a peculiar role of cellular noise in differentiation, which has never been hypothesized so far, and leads to non trivial predictions which could be subject to experimental testing
Quantum noise in the spin transfer torque effect
Describing the microscopic details of the interaction of magnets and
spin-polarized currents is key to achieve control of such systems at the
microscopic level. Here we discuss a description based on the Keldysh
technique, casting the problem in the language of open quantum systems. We
reveal the origin of noise in the presence of both field-like and damping like
terms in the equation of motion arising from spin conductance
Nonlinear Inverse Spin Galvanic Effect in Anisotropic Disorder-free Systems
Spin transport phenomena in solid materials suffer limitations from spin
relaxation associated to disorder or lack of translational invariance.
Ultracold atoms, free of that disorder, can provide a platform to observe
phenomena beyond the usual two-dimensional electron gas. By generalizing the
approach used for isotropic two-dimensional electron gases, we theoretically
investigate the inverse spin galvanic effect in the two-level atomic system in
the presence of anisotropic Rashba-Dresselhaus spin-orbit couplings (SOC) and
external magnetic field. We show that the combination of the SOC results in an
asymmetric case: the total spin polarization considered for a small momentum
has a longer spin state than in a two-dimensional electron gas when the SOC
field prevails over the external electric field. Our results can be relevant
for advancing experimental and theoretical investigations in spin dynamics as a
basic approach for studying spin state control
Quantum Simulation of single-qubit thermometry using linear optics
Standard thermometry employs the thermalisation of a probe with the system of
interest. This approach can be extended by incorporating the possibility of
using the non-equilibrium states of the probe, and the presence of coherence.
Here, we illustrate how these concepts apply to the single-qubit thermometer
introduced by Jevtic et al. by performing a simulation of the qubit-environment
interaction in a linear-optical device. We discuss the role of the coherence,
and how this affects the usefulness of non-equilibrium conditions. The origin
of the observed behaviour is traced back to the propensity to thermalisation,
as captured by the Helmholtz free energy.Comment: 6 pages, 6 figure
Measuring coherence of quantum measurements
The superposition of quantum states lies at the heart of physics and has been
recently found to serve as a versatile resource for quantum information
protocols, defining the notion of quantum coherence. In this contribution, we
report on the implementation of its complementary concept, coherence from
quantum measurements. By devising an accessible criterion which holds true in
any classical statistical theory, we demonstrate that noncommutative quantum
measurements violate this constraint, rendering it possible to perform an
operational assessment of the measurement-based quantum coherence. In
particular, we verify that polarization measurements of a single photonic
qubit, an essential carrier of one unit of quantum information, are already
incompatible with classical, i.e., incoherent, models of a measurement
apparatus. Thus, we realize a method that enables us to quantitatively certify
which quantum measurements follow fundamentally different statistical laws than
expected from classical theories and, at the same time, quantify their
usefulness within the modern framework of resources for quantum information
technology.Comment: close to published versio
Monitoring dispersive samples with single photons: the role of frequency correlations
The physics that governs quantum monitoring may involve other degrees of
freedom than the ones initialised and controlled for probing. In this context
we address the simultaneous estimation of phase and dephasing characterizing a
dispersive medium, and we explore the role of frequency correlations within a
photon pair generated via parametric down-conversion, when used as a probe for
the medium. We derive the ultimate quantum limits on the estimation of the two
parameters, by calculating the corresponding quantum Cram\'er-Rao bound; we
then consider a feasible estimation scheme, based on the measurement of Stokes
operators, and address its absolute performances in terms of the correlation
parameters, and, more fundamentally, of the role played by correlations in the
simultaneous achievability of the quantum Cram\'er-Rao bounds for each of the
two parameters.Comment: to appear in Quantum Measurements and Quantum Metrolog
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