50 research outputs found
Unbiased estimation of an optical loss at the ultimate quantum limit with twin-beams
Loss measurements are at the base of spectroscopy and imaging, thus perme-
ating all the branches of science, from chemistry and biology to physics and
material science. However, quantum mechanics laws set the ultimate limit to the
sensitivity, constrained by the probe mean energy. This can be the main source
of uncertainty, for example when dealing with delicate system such as
biological samples or photosensitive chemicals. It turns out that ordinary
(clas- sical) probe beams, namely with Poissonian photon number distribution,
are fundamentally inadequate to measure small losses with the highest
sensitivity. Conversely, we demonstrate that a quantum-correlated pair of
beams, known as twin-beam state, allows reaching the ultimate sensitivity for
all energy regimes (even less than one photon per mode) with the simplest
measurement strategy. One beam of the pair addresses the sample, while the
second one is used as a reference to compensate both for classical drifts and
for uctuation at the most fundamental quantum level. This scheme is also
absolute and accurate, since it self-compensates for unavoidable instability of
the sources and detectors, which could otherwise lead to strongly biased
results. Moreover, we report the best sensitivity per photon ever achieved in
loss estimation experiments
Quantum differential ghost microscopy
Quantum correlations become formidable tools for beating classical capacities
of measurement. Preserving these advantages in practical systems, where
experimental imperfections are unavoidable, is a challenge of the utmost
importance. Here we propose and realize a quantum ghost imaging protocol able
to compensate for the detrimental effect of detection noise and losses. This
represents an important improvement as quantum correlations allow low
brightness imaging, desirable for reducing the absorption dose. In particular,
we develop a comprehensive model starting from a ghost imaging scheme
elaborated for bright thermal light, known as differential ghost imaging and
particularly suitable in the relevant case of faint or sparse objects. We
perform the experiment using SPDC light in microscopic configuration. The image
is reconstructed exploiting non-classical intensity correlation rather than
photon pairs detection coincidences. On one side we validate the theoretical
model and on the other we show the applicability of this technique by
reconstructing a biological object with 5 micrometers resolution
Photon number correlation for quantum enhanced imaging and sensing
In this review we present the potentialities and the achievements of the use
of non-classical photon number correlations in twin beams (TWB) states for many
applications, ranging from imaging to metrology. Photon number correlations in
the quantum regime are easy to be produced and are rather robust against
unavoidable experimental losses, and noise in some cases, if compared to the
entanglement, where loosing one photon can completely compromise the state and
its exploitable advantage. Here, we will focus on quantum enhanced protocols in
which only phase-insensitive intensity measurements (photon number counting)
are performed, which allow probing transmission/absorption properties of a
system, leading for example to innovative target detection schemes in a strong
background. In this framework, one of the advantages is that the sources
experimentally available emit a wide number of pairwise correlated modes, which
can be intercepted and exploited separately, for example by many pixels of a
camera, providing a parallelism, essential in several applications, like wide
field sub-shot-noise imaging and quantum enhanced ghost imaging. Finally,
non-classical correlation enables new possibilities in quantum radiometry, e.g.
the possibility of absolute calibration of a spatial resolving detector from
the on-off- single photon regime to the linear regime, in the same setup
Optimal estimation of entanglement and discord in two-qubit states
Recently, the fast development of quantum technologies led to the need for
tools allowing the characterization of quantum resources. In particular, the
ability to estimate non-classical aspects, e.g. entanglement and quantum
discord, in two-qubit systems, is relevant to optimise the performance of
quantum information processes. Here we present an experiment in which the
amount of entanglement and discord are measured exploiting different
estimators. Among them, some will prove to be optimal, i.e., able to reach the
ultimate precision bound allowed by quantum mechanics. These estimation
techniques have been tested with a specific family of states ranging from
nearly pure Bell states to completely mixed states. This work represents a
significant step in the development of reliable metrological tools for quantum
technologies
Experimental quantum cryptography scheme based on orthogonal states
Since, in general, non-orthogonal states cannot be cloned, any eavesdropping
attempt in a Quantum Communication scheme using non-orthogonal states as
carriers of information introduces some errors in the transmission, leading to
the possibility of detecting the spy. Usually, orthogonal states are not used
in Quantum Cryptography schemes since they can be faithfully cloned without
altering the transmitted data. Nevertheless, L. Goldberg and L. Vaidman [\prl
75 (1995) 1239] proposed a protocol in which, even if the data exchange is
realized using two orthogonal states, any attempt to eavesdrop is detectable by
the legal users. In this scheme the orthogonal states are superpositions of two
localized wave packets travelling along separate channels. Here we present an
experiment realizing this scheme
Anomalous Weak Values and the Violation of a Multiple-measurement Leggett-Garg Inequality
Quantum mechanics presents peculiar properties that, on the one hand, have
been the subject of several theoretical and experimental studies about its very
foundations and, on the other hand, provide tools for developing new
technologies, the so-called quantum technologies. The nonclassicality pointed
out by Leggett-Garg inequalities has represented, with Bell inequalities, one
of the most investigated subject. In this letter we study the connection of
Leggett-Garg inequalities with a new emerging field of quantum measurement, the
weak values. In particular, we perform an experimental study of the four-time
correlators Legget-Garg test, by exploiting single and sequential weak
measurements performed on heralded single photons. We show violation of a
four-parameters Leggett-Garg inequality in different experimental conditions,
demonstrating an interesting connection between Leggett-Garg inequality
violation and anomalous weak values
Nonclassical noise features in a correlation plenoptic imaging setup
Sub-shot-noise imaging and correlation plenoptic imaging are two quantum imaging techniques that enable to overcome different problems of classical imaging systems. Combining the two techniques is not trivial, since the former is based on the detection of identical corresponding modes to subtract noise, while the latter requires the detection of different modes to perform directional reconstruction. In this paper, we experimentally show the possibility to obtain a noise-reduction factor smaller than one, a necessary condition to perform sub-shot-noise imaging, in a setup that can be adapted to correlation plenoptic imaging
Temporal teleportation with pseudo-density operators: how dynamics emerges from temporal entanglement
We show that, by utilising temporal quantum correlations as expressed by
pseudo-density operators (PDOs), it is possible to recover formally the
standard quantum dynamical evolution as a sequence of teleportations in time.
We demonstrate that any completely positive evolution can be formally
reconstructed by teleportation with different temporally correlated states.
This provides a different interpretation of maximally correlated PDOs, as
resources to induce quantum time-evolution. Furthermore, we note that the
possibility of this protocol stems from the strict formal correspondence
between spatial and temporal entanglement in quantum theory. We proceed to
demonstrate experimentally this correspondence, by showing a multipartite
violation of generalised temporal and spatial Bell inequalities and verifying
agreement with theoretical predictions to a high degree of accuracy, in
high-quality photon qubits.Comment: preprin