39 research outputs found
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
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
Speed of qubit states during thermalisation
Classifying quantum states usually demands to observe properties such as the
amount of correlation at one point in time. Further insight may be gained by
inspecting the dynamics in a given evolution scheme. Here we attempt such a
classification looking at single-qubit and two-qubit states at the start of
thermalisation with a heat bath. The speed with which the evolution starts is
influenced by quantum aspects of the state, however, such signatures do not
allow for a systematic classification
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
Measuring the time-frequency properties of photon pairs: a short review
Encoding information in the time-frequency domain is demonstrating its
potential for quantum information processing. It offers a novel scheme for
communications with large alphabets, computing with large quantum systems, and
new approaches to metrology. It is then crucial to secure full control on the
generation of time-frequency quantum states and their properties. Here, we
present an overview of the theoretical background and the technical aspects
related to the characterization of time-frequency properties of two-photon
states. We provide a detailed account of the methodologies which have been
implemented for measuring frequency correlations and for the retrieval of the
full spectral wavefunction. This effort has benefited enormously from the
adaptation of classical metrology schemes to the needs of operating at the
single-photon level
Assessing frequency correlation through a distinguishability measurement
The simplicity of a question such as wondering if correlations characterize
or not a certain system collides with the experimental difficulty of accessing
such information. Here we present a low demanding experimental approach which
refers to the use of a metrology scheme to obtain a conservative estimate of
the strength of frequency correlations. Our testbed is the widespread case of a
photon pair produced per downconversion. The theoretical architecture used to
put the correlation degree on a quantitative ground is also described
Multiparameter quantum estimation of noisy phase shifts
Phase estimation is the most investigated protocol in quantum metrology, but
its performance is affected by the presence of noise, also in the form of
imperfect state preparation. Here we discuss how to address this scenario by
using a multiparameter approach, in which noise is associated to a parameter to
be measured at the same time as the phase. We present an experiment using
two-photon states, and apply our setup to investigating optical activity of
fructose solutions. Finally, we illustrate the scaling laws of the attainable
precisions with the number of photons in the probe state
Geometrical bounds on irreversibility in open quantum systems
Clausius inequality has deep implications for reversibility and the arrow of
time. Quantum theory is able to extend this result for closed systems by
inspecting the trajectory of the density matrix on its manifold. Here we show
that this approach can provide an upper and lower bound to the irreversible
entropy production for open quantum systems as well. These provide insights on
the thermodynamics of the information erasure. Limits of the applicability of
our bounds are discussed, and demonstrated in a quantum photonic simulator
Quantum sensors for dynamical tracking of chemical processes
Quantum photonics has demonstrated its potential for enhanced sensing.
Current sources of quantum light states tailored to measuring, allow to monitor
phenomena evolving on time scales of the order of the second. These are
characteristic of product accumulation in chemical reactions of technologically
interest, in particular those involving chiral compounds. Here we adopt a
quantum multiparameter approach to investigate the dynamic process of sucrose
acid hydrolysis as a test bed for such applications. The estimation is made
robust by monitoring different parameters at once
Bridging thermodynamics and metrology in non-equilibrium Quantum Thermometry
Single-qubit thermometry presents the simplest tool to measure the
temperature of thermal baths with reduced invasivity. At thermal equilibrium,
the temperature uncertainty is linked to the heat capacity of the qubit,
however the best precision is achieved outside equilibrium condition. Here, we
discuss a way to generalize this relation in a non-equilibrium regime, taking
into account purely quantum effects such as coherence. We support our findings
with an experimental photonic simulation.Comment: 7 pages, 4 figure