Weak value measurements have been a real breakthrough in the quantum measurement framework. In particular, quantum measurements may take advantage by anomalous weak values, i.e. values out of the eigenvalues spectrum of the measured observable, both for implementing new measurement techniques and studying Quantum Mechanics foundations. In this report we show three experiments with single photons presenting anomalous weak values: the first one tests the incompatibility between quantum mechanics and noncontextual hidden variables theories, the second one is the first realization of a sequential weak value evaluation of two incompatible observables on the same photon, and the last one shows how sequential weak values can be used to test Leggett-Garg inequalities extended to multiple-measurements scenarios.
Quantum key distribution (QKD)1 is a quantum technology already present in the market. This technology will become an essential point to secure our communication systems and infrastructure when today’s public key cryptography will be broken by either a mathematical algorithm or by, eventually, the development of quantum computers. One of the main task of quantum metrology and standardization in the next future is ensuring that QKD apparatus works as expected, and appropriate countermeasures against quantum hacking are taken. In this paper, we discuss the security of one of the QKD most critical (and quantum-hackered) components, i.e., single photon detectors based on fiber-pigtailed InGaAs SPADs. We analyze their secondary photon emission (backflash light) that can be exploited by an eavesdropper (Eve) to gain information without introducing errors in the key. We observed a significant light leakage from the detection event of fiber-pigtailed InGaAs SPADs. This may represent a significant security threat in all QKD apparatus. We provide a method to quantify the amount of potential information leakage, and we propose a solution to fix this potential security bug in practical QKD apparatus.
In quantum mechanics, the eigenvalues and their corresponding probabilities specify the expectation value of a physical observable, which is known to be a statistical property related to large ensembles of particles. In contrast to this paradigm, we demonstrate a unique method allowing to extract the expectation value of a single particle, namely, the polarisation of a single protected photon, with a single experiment. This is the first realisation of quantum protective measurements.
Properties of quantum light represent a tool for overcoming limits of classical optics. Several experiments have demonstrated this advantage ranging from quantum enhanced imaging to quantum illumination. In this work, experimental demonstration of quantum-enhanced resolution in confocal fluorescence microscopy will be presented. This is achieved by exploiting the non-classical photon statistics of fluorescence emission of single nitrogen-vacancy (NV) color centers in diamond. By developing a general model of super-resolution based on the direct sampling of the kth-order autocorrelation function of the photoluminescence signal, we show the possibility to resolve, in principle, arbitrarily close emitting centers. Finally, possible applications of NV-based fluorescent nanodiamonds in biosensing and future developments will be presented.
We present two recent results achieved in INRIM laboratories paving the way for next future commercial use of quantum imaging techniques. The first exploits non-classical photon statistics of single nitrogen-vacancy color centers in diamond for realising super-resolution. A little more in detail we demonstrate that the measurement of high order correlation functions allows overcoming Abbe limit. The second exploits ghost imaging in a specific case of practical interest, i.e. in measuring magnetic structures in garnets.
In this proceeding we discuss as quantum correlations can enhance measurements capabilities [1,2,3,6], discussing few examples as target detection in a noisy environment and holometer. The first [2] is a quantum enhanced scheme preserving a strong advantage over classical counterparts even in presence of large amount of noise and losses. Our work, inspired by [3], has been performed exploiting only photon number correlations in twin beams. Thus, for its simplicity it can find widespread use. Even more important by challenging the common believe that real application of quantum technologies is limited by fragility to noise and losses, it paves the way to their real application. Then, we describe as the same kind of correlations can find application in a completely different area of physics, i.e. in testing quantum gravity. Indeed, recently, effects in interferometers connected to noncommutativity of position variables in different directions were considered in two coupled interferometers [5], the ``holometer'' [6]. We show that the use of quantum correlated light beams could lead to significant improvements.
KEYWORDS: Diamond, Surface plasmons, Near infrared, Quantum information, Single photon, Confocal microscopy, Polarization, Chemical vapor deposition, Color centers, Quantum optics
Single-photon sources (SPS) play a key-role in many applications, spanning from quantum metrology, to quantum information and to the foundations of quantum mechanics. Even if an ideal SPS (i. e. emitting indistinguishable, ”on-demand” single photons, at an arbitrarily fast repetition rate) is far to be realized due to real-world deviations from the ideality, much effort is currently devoted to improving the performance of real sources. With regards to the emission probability, it appears natural to employ sources that are in principle deterministic in the single- photon emission (single quantum emitters such as single atoms, ions, molecules, quantum dots, or color centers in diamond) as opposed to probabilistic ones (usually heralded SPS based on parametric down-conversion). We present an overview of our latest results concerning a work-in-progress NIR pulsed single photon source based on single quantum emitters (color centers in diamond) exploiting recently reported centers. They are particularly interesting because of the narrow emission line (tipically less than 5 nm), the shorter excited state lifetime with respect to NV centres (1 - 2 ns compared to 12 ns, allowing a ten-fold photon emission rate upon saturation) and the polarized emission.
We present some Quantum Tomography related results recently obtained in the Quantum Optics labs of the National Institute of Metrological Research (INRIM). Initially we describe the first experimental implementation of a new protocol for the reconstruction of a photon-number-resolving (PNR) detector’s POVM (Positive Operator-Valued Measure): such a protocol, exploiting the strong quantum correlations of an ancillary state, results more robust and efficient than its classical counterparts. The second part of the paper focuses on the quantum characterization of a transition-edge sensor (TES) based PNR detector, i.e. the experimental tomography of the POVM of a TES, with a method based on a quorum of coherent probes: we show the reconstruction of the POVM elements up to 11 detected photons and 100 incoming photons, demonstrating the linearity of such a device. Finally, we present a method for the experimental reconstruction of the modal structure of multimode optical fields exploiting a single measurement of higher-order photon number autocorrelation functions. We show our reconstructions of up to three different modes per optical state, demonstrating the excellent agreement with the theoretical predictions and the robustness of our method itself.
With the recent progresses in quantum technologies, single photon sources have gained a primary relevance. Here we present a heralded single photon source characterized by an extremely low level of noise photons, realized by exploiting low-jitter electronics and detectors and fast custom-made electronics used to control an optical shutter (a LiNbO3 waveguide optical switch) at the output of the source. This single photon source showed a second-order autocorrelation function g(2)(0) = 0:005(7), and an Output Noise Factor (defined as the ratio of noise photons to total photons at the source output) of 0:25(1)%, among the best ever achieved.
Quantum Key Distribution together with its intrinsic security represent the more promising technology to meet the challenging requests of novel generation communication protocols. Beyond its relevant commercial interests, QKD is currently and deeply investigated in research fields as quantum information and quantum mechanics foundations, in order to push over the limits of the actual resources needed to ensure the security of quantum communication. Aim of the paper is to contribute to this open debate presenting our last experimental implementations concerning two novel quantum cryptographic schemes which do not require some of the most widely accepted conditions for realizing QKD. The first is Goldenberg-Vaidman1,2 protocol, in which even if only orthogonal states (that in principle can be cloned without altering the quantum state) are used, any eavesdropping attempt is detectable. The second is N093 protocol which, being based on the quantum counterfactual effect, does not even require any actual photon transmission in the quantum channel between the parties for the communication. The good agreement between theoretical predictions and experimental results represent a proof of principle of the experimental feasibility of the novel protocols.
Optical correlations represent a resource for the development of technologies with very promising opportunities for future widespread applications.1–14 Here we will show, both theoretically and experimentally, the optical illusionist game, an innovative protocol that exploit correlations between thermal beams of light.15 In the game, an illusionist demonstrates that if two uncorrelated light beams excited in the same state are mixed in a beam splitter (BS), then no correlations arise between them. On the other hand, the presence of the BS can be identified by the illusionist when the public is asked to insert the BS behind the illusionist’s back. Here we unveil the trick and the physics that explain these counterintuitive correlations.
1
We present an experimental implementation of spectral properties engineering on biphoton light, emitted via ultrafast type II spontaneous parametric down conversion (SPDC), based on the shaping of the pump pulse spectrum and propagation of the emitted correlated photons through dispersive media. Spectral properties of
a biphoton state are fully characterized by the two-photon spectral amplitude (TPSA). Exploiting the group velocity dispersion (GVD) induced by the passage of optical fields through dispersive media, an energy to time two dimensional Fourier transform of the TPSA is operated: this returns a technique to reconstruct TPSA by means of a temporal measurement among the delay between the laser pulse emission (trigger) and the detection times of the two correlated photons. Exploiting this kind of measurement it is possible to deeply resolve the interference pattern in the shape of TPSA. In this research we report on the conditions under which subtle structure on TPSA spectra can be deliberately engineered via modulation of the pump beam spectrum.
Quantum properties of the optical field represent a resource of the utmost relevance for the development of quantum
technologies, allowing unprecedented results in disciplines ranging from quantum information and metrology to quantum
imaging. Spatial quantum correlations generated by of parametric down conversion (PDC) represents a tool for quantum
imaging because they are intrinsically multimode, a requirement for obtaining large degree of correlation over small
portions of the beams, allowing to register the spatial structure of an object. In particular a very interesting example is
provided by the detection of weak objects, a result that could have important practical applications. The principle of this
technique is to take advantage of the correlation in the noise of two conjugated branches of PDC emission: in fact,
subtracting the noise measured on one branch from the image of a weak object obtained in the other branch, the image of
the object, eventually previously hidden in the noise, could be restored.
Here, after a general summary of quantum imaging techniques, firstly we will show how we have reached a sub shot
noise regime and then improved this result up to reach a regime where it was possible to achieve the first experimental
realisation of sub shot noise imaging of a weak absorbing object.
Generation of two-photon light with given spectral and temporal properties is of great interest for quantum
communication and quantum metrology applications. In particular, preparation of biphotons with ultra-narrow
correlation time is a very important task. In a recent series of papers, our group analyzed the generation of twophoton
wavepackets, produced by Spontaneous Parametric Down Conversion, in crystals with linearly chirped
quasi-phase matching grating. Wavepackets present very broad spectra but a broad spectrum does not necessarily
imply small correlation times, although the inverse is true. Indeed, the spectrum broadening induced by the
grating is inhomogeneous; for this reason, the two-photon spectral amplitude present a phase (a frequency chirp)
that depend nonlinearly on the frequency. Hence, the two-photon wavepackets are not Fourier transform-limited.
As suggested in, the ideal way to make the wavepacket perfectly transform limited is to insert in the path of
the biphotons a proper optical medium that compensates the non-linear part of the phase factor present in the
spectral amplitude. In our work, we investigate the non-local temporal compression of the photons induced
by the insertion of a standard optical fibre in the path of one of the two photons. We present and discuss a
systematic study of this phenomenon and some optimal situation where the full numerical calculation shows an
effect that can be clearly observed with a realistic set-up. The study has open the way to the practical realization
of this idea.
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 [Phys. Rev. Lett. 75 (7), pp. 12391243, 1995] 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 which travel along separate
channels, i.e. two different paths inside a balanced Mach-Zehnder interferometer. Here we present an experiment
realizing this scheme.
Speckle structure1 of Parametric Down Conversion (PDC) has recently received a large attention due to the
relevance in view of applications to quantum imaging. The possibility of tailoring the speckle size by acting
on the pump intensity and dimensions is an interesting tool for the applications to quantum imaging and in
particular to the detection of weak objects under shot-noise limit. In this paper we report a systematic detailed
study of the speckle structure, in particular of the one in type II PDC, with attention to its variation with pump
beam characteristics (power and radius).
We discuss a scheme for a photon-counting detection system that overcomes the difficulties of photon-counting at high
rates at telecom wavelengths. Our method uses an array of N detectors and a 1-by-N optical switch with a control circuit
to direct input light to live detectors. We conclude that in addition to detection deadtime reduction, the multiplexed
switch also reduces so-called trigger deadtime, common to infrared photon counting detectors. By implementing the new
algorithm we obtain an overall deadtime reduction of a factor of 5 when using just N=2 multiplexed detectors. In
addition to deadtime reduction, our scheme reduces afterpulsing and background counts (such as dark counts). We
present experimental results showing the advantage of our system as compared to passive multi-detector detection
systems and our previous active multiplexing system that only reduced detection deadtime.
We discuss a scheme for a photon-counting detection system that overcomes the difficulties of photon-counting at
extremely high rates. Our method uses an array of N detectors and a 1-by-N optical switch with a control circuit to direct
input light to live detectors. Detector deadtime is significantly reduced by an active routing of single photons to the
detector that has had the most time to recover from its last firing. In addition to deadtime reduction, our scheme reduces
afterpulsing and background counts (such as dark counts). We present experimental results showing the advantageous
performance of our system as compared to passive multi-detector detection systems. We conclude that intelligent active
management of a group of N photon-counting detectors yields the highest photon counting rates, an important
technological challenge for fast developing quantum metrology and quantum key distribution applications. Also, we
report our experimental progress in developing an integrated device based on this scheme.
In this paper we review our recent works on the generation of different Bell states within the lineshape of
parametric down-conversion (SPDC) and their possible applications. Indeed, for polarization-entangled two-photon
states produced via SPDC, the frequency-angular lineshape allowed by phase matching is considered. It
is shown that there are always different Bell states generated for different mismatch values within the natural
bandwidth. Consideration is made for two different methods of polarization entanglement preparation, based on
type-II SPDC and on SPDC in two type-I crystals producing orthogonally polarized photon pairs. Different Bell
states can be filtered out by either frequency selection or angular selection, or both. Our theoretical calculations
are confirmed by a series of experiments, performed for the two above-mentioned ways of producing polarization-entangled
photon pairs and with two kinds of measurements: frequency-selective and angular-selective. Finally, we mention possible application to quantum communication with fibers.
Quantum communication in free space is the next challenge of telecommunications. Since we want to determine the outcome of quantum communication by means of single photons, we must understand how a single photon interacts with the atmosphere. In this brief article, some simulation results for realistic and generic atmospheric conditions are reported, a related experiment is considered and its results are described and discussed. Furthermore, the setup of a future experiment, currently under preparation, is described and analyzed.
In this paper we will present some experimental researches about Quantum Communication performed in "Carlo Novero" Quantum Optics laboratory at INRiM (former IEN). After a general review of our studies, we will describe our recent researches on propagation of polarization entangled photons in optical fibres focused on the investigation of the effect of two-photon interference in the second-order Glauber's correlation function and on the characterization of this quantum channel as a Complete Positive (CP) map. We will then describe an innovative method, based on detectors operating in Geiger mode (on/off), for reconstructing the photon statistics of quantum optical states, presenting experimental data collected to test the extension of this method to multi-partite states.
In this proceeding we present the most recent studies performed at IEN "Galileo Ferraris" on quantum information by using quantum optical states. After a general summary of the most recent studies, among them we will present in some details the results of two recent experiments. The first was addressed to tomographic reconstruction of a quantum state by using an innovative theoretical scheme based on a variable quantum efficiency of the detector. This scheme has been applied to Fock (PDC heralded photons), coherent (attenuated laser beam) and thermal states, for which experimental results will be presented. The second was pointed to experimentally investigate the effects of fibre propagation of PDC light produced in a type II crystal and, in particular, to the restoration of entanglement due to wave packet dispersion. Also in this case we will present and discuss our most recent data. Finally, we will shortly acknowledge of realisation of a heralded photon source with strong spectral selection.
We report the results of a new realization of Ghose, Home and Agarwal experiment to test wave-particle duality where some limitations of the former experiment, realized by Mizobuchi and Othake, are overcome. Our results, in agreement with quantum mechanics predictions, indicates that complementarity between wave and particle behavior must be interpreted in a weaker sense as a gradual disappearing of interference when "which-path" indications are obtained and not as a mutual exclusive aspects as in the original Bohr's statement.
Our last experimental results on the realization of a measurement-conditional unitary operation at single photon level are presented. This gate operates by rotating by 90° the polarization of a photon produced by means of Type-II Parametric Down Conversion conditional to a polarization measurement on the correlated photon. We then propose a new scheme for measuring the quantum efficiency of a single photon detection apparatus by using this set-up. We present experimental results obtained with this scheme compared with traditional biphoton calibration. Our results show the interesting potentiality of the suggested scheme.
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