75 research outputs found
Measurement of Coupling PDC photon sources with single-mode and multimode optical fibers
We investigate the coupling efficiency of parametric downconversion light
(PDC) into single and multi-mode optical fibers as a function of the pump beam
diameter, crystal length and walk-off. We outline two different theoretical
models for the preparation and collection of either single-mode or multi-mode
PDC light (defined by, for instance, multi-mode fibers or apertures,
corresponding to bucket detection). Moreover, we define the mode-matching
collection efficiency, important for realizing a single-photon source based on
PDC output into a well-defined single spatial mode. We also define a multimode
collection efficiency that is useful for single-photon detector calibration
applications.Comment: 13 pages, 12 figure
A Molecule‐Based Single‐Photon Source Applied in Quantum Radiometry
Single photon sources (SPSs) based on quantum emitters hold promise in
quantum radiometry as metrology standard for photon fluxes at the low light
level. Ideally this requires control over the photon flux in a wide dynamic
range, sub-Poissonian photon statistics and narrow-band emission spectrum. In
this work, a monochromatic single-photon source based on an organic dye
molecule is presented, whose photon flux is traceably measured to be adjustable
between 144 000 and 1320 000 photons per second at a wavelength of (785.6 +/-
0.1) nm, corresponding to an optical radiant flux between 36.5 fW and 334 fW.
The high purity of the single-photon stream is verified, with a second-order
autocorrelation function at zero time delay below 0.1 throughout the whole
range. Featuring an appropriate combination of emission properties, the
molecular SPS shows here application in the calibration of a silicon
Single-Photon Avalanche Detector (SPAD) against a low-noise analog silicon
photodiode traceable to the primary standard for optical radiant flux (i.e. the
cryogenic radiometer). Due to the narrow bandwidth of the source, corrections
to the SPAD detection efficiency arising from the spectral power distribution
are negligible. With this major advantage, the developed device may finally
realize a low-photon-flux standard source for quantum radiometry
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
Quantifying backflash radiation to prevent zero-error attacks in quantum key distribution
Single-photon avalanche diodes (SPADs) are the most widespread commercial solution for single-photon counting in quantum
key distribution applications. However, the secondary photon emission that arises from the avalanche of charge carriers that
occurs during the detection of a photon may be exploited by an eavesdropper to gain information without inducing errors in the
transmission key. In this paper, we characterize such backflash light in gated InGaAs/InP SPADs and discuss its spectral and
temporal characterization for different detector models and different operating parameters. We qualitatively bound the maximum
information leakage due to backflash light and propose solutions for preventing such leakage
Quantifying backflash radiation to prevent zero-error attacks in quantum key distribution
Single-photon avalanche diodes (SPADs) are the most widespread commercial solution for single-photon counting in quantum key distribution applications. However, the secondary photon emission that arises from the avalanche of charge carriers that occurs during the detection of a photon may be exploited by an eavesdropper to gain information without inducing errors in the transmission key. In this paper, we characterize such backflash light in gated InGaAs/InP SPADs and discuss its spectral and temporal characterization for different detector models and different operating parameters. We qualitatively bound the maximum information leakage due to backflash light and propose solutions for preventing such leakage
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
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
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