The ability to generate and handle quantum correlations in twin-beam states is the foundation to overcome the imaging limitations imposed by classical light [1], relying on efficient detectors able to select temporal correlations among photons with maximized signal-to-noise ratio. Indeed, quantum information is contained in the interaction with the sample of just two photons, thus losing even one of them or mixing them up with background can completely compromise the experiment. To this end, assessing and evaluating the expected probability of measurement errors is of utmost importance to optimize the detector design. A typical experimental setup of a quantum imaging system based on Spontaneous Parametric Down Conversion (SPDC), the most effective way of producing temporally and spatially correlated entangled photons [2], involves a pump laser hitting a non-linear birefringent crystal producing a down-converted signal, some passive optical components to generate polarization-entangled photons and a pixelated photon detector that eventually reveals the position of coincident entangled photons. The overall detection efficiency of the measurement setup (i.e. heralding efficiency ηH) gathers all its possible losses and non-idealities of the mentioned setup components, namely the crystal conversion efficiency, the optical losses, the detector Photon Detection Efficiency (PDE) and fill-factor. In this study, we consider a detection based on a Single Photon Avalanche Diode (SPAD) array, able to detect coincidences on-chip and with event-driven architecture, which provides fast readout, low power consumption and optimized data throughput. However, related to every on-chip coincidence detection and event-driven based architecture, some operations require a certain amount of time to be performed that, combined with the generation of undesired single photons (both due to limited heralding efficiency and SPAD Dark Count Rate, DCR), yield to three measurement errors, here described with their related probabilities. Firstly, due to the finite duration of the coincidence window ∆tCOINC, there exist a probability Pfalseof wrongly detecting as coincident photons two single photons, and can be expressed as:\begin{equation*}{P_{false}} = \frac{{\left[ {2{\mu _{SOURCE{\text{ }}}}{\eta _H}\left( {1 - {\eta _H}} \right) + {\text{DCR}}} \right]\left( {1 - {e^{ - \left[ {2{\mu _{SOURCE}}{\eta _H}\left( {1 - {\eta _H}} \right) + {\text{DCR}}} \right]\Delta {t_{COINC}}}}} \right)}}{{{\mu _{SOURCE}}\eta _H^2 + \left[ {2{\mu _{SOURCE}}{\eta _H}\left( {1 - {\eta _H}} \right) + DCR} \right]\left( {1 - {e^{ - \left[ {2{\mu _{SOURCE}}{\eta _H}\left( {1 - {\eta _H}} \right) + {\text{DCR}}} \right]\Delta {t_{COINC}}}}} \right)}}\tag{1}\end{equation*}where µSOURCEis the number of photon pairs generated by an ideal non-linear crystal. Secondly, in the time needed for the array to "freeze" the pixels state after a coincidence event (∆tFREEZE) so that they can be readout, there is the probability Pspuriousof detecting a third spurious event after a coincidence has been detected, which is expressed as:\begin{equation*}{P_{spurious}} = 1 - {e^{ - \left[ {2{\mu _{SOURCE\eta }}_H\left( {1 - {\eta _H}} \right) + DCR} \right]\Delta {t_{FREEZE}}}}\tag{2}\end{equation*}
The ability to generate and handle quantum correlations in twin-beam states is the foundation to overcome the imaging limitations imposed by classical light [1], relying on efficient detectors able to select temporal correlations among photons with maximized signal-to-noise ratio. Indeed, quantum information is contained in the interaction with the sample of just two photons, thus losing even one of them or mixing them up with background can completely compromise the experiment. To this end, assessing and evaluating the expected probability of measurement errors is of utmost importance to optimize the detector design. A typical experimental setup of a quantum imaging system based on Spontaneous Parametric Down Conversion (SPDC), the most effective way of producing temporally and spatially correlated entangled photons [2], involves a pump laser hitting a non-linear birefringent crystal producing a down-converted signal, some passive optical components to generate polarization-entangled photons and a pixelated photon detector that eventually reveals the position of coincident entangled photons. The overall detection efficiency of the measurement setup (i.e. heralding efficiency ηH) gathers all its possible losses and non-idealities of the mentioned setup components, namely the crystal conversion efficiency, the optical losses, the detector Photon Detection Efficiency (PDE) and fill-factor. In this study, we consider a detection based on a Single Photon Avalanche Diode (SPAD) array, able to detect coincidences on-chip and with event-driven architecture, which provides fast readout, low power consumption and optimized data throughput. However, related to every on-chip coincidence detection and event-driven based architecture, some operations require a certain amount of time to be performed that, combined with the generation of undesired single photons (both due to limited heralding efficiency and SPAD Dark Count Rate, DCR), yield to three measurement errors, here described with their related probabilities. Firstly, due to the finite duration of the coincidence window ∆tCOINC, there exist a probability Pfalseof wrongly detecting as coincident photons two single photons, and can be expressed as:\begin{equation*}{P_{false}} = \frac{{\left[ {2{\mu _{SOURCE{\text{ }}}}{\eta _H}\left( {1 - {\eta _H}} \right) + {\text{DCR}}} \right]\left( {1 - {e^{ - \left[ {2{\mu _{SOURCE}}{\eta _H}\left( {1 - {\eta H}} \right) + {\text{DCR}}} \right]\Delta {t{COINC}}}}} \right)}}{{{\mu _{SOURCE}}\eta _H^2 + \left[ {2{\mu _{SOURCE}}{\eta _H}\left( {1 - {\eta _H}} \right) + DCR} \right]\left( {1 - {e^{ - \left[ {2{\mu _{SOURCE}}{\eta H}\left( {1 - {\eta H}} \right) + {\text{DCR}}} \right]\Delta {t{COINC}}}}} \right)}}\tag{1}\end{equation*}where µSOURCEis the number of photon pairs generated by an ideal non-linear crystal. Secondly, in the time needed for the array to "freeze" the pixels state after a coincidence event (∆tFREEZE) so that they can be readout, there is the probability Pspuriousof detecting a third spurious event after a coincidence has been detected, which is expressed as:\begin{equation*}{P{spurious}} = 1 - {e^{ - \left[ {2{\mu _{SOURCE\eta }}_H\left( {1 - {\eta H}} \right) + DCR} \right]\Delta {t{FREEZE}}}}\tag{2}\end{equation*}
Quantum technology promises improvements in imaging, computing, and communication, for example using the resource of entanglement between photons with spatial correlations. Detecting spatial correlations, or coincidences, between entangled photons scalably, efficiently, and affordably is therefore an essential capability. However, this task is non-trivial for existing camera technologies, which require low illumination intensities or low detection duty cycles to count coincidences at high signal-to-noise ratios, resulting in long acquisition time, or use expensive custom electronic components. Here, we present an entanglement imaging system based around a novel Single-Photon Avalanche Diode (SPAD) array camera, optimized for sparse illumination with correlated photon pairs. The system is capable of maintaining a duty cycle close to 100%, while simultaneously detecting spatially resolved coincidences with high SNR, enabling the acquisition of real-time entanglement videos at a ~Hz frame rate. We use our system to demonstrate real-time monitoring of entanglement interference visibility, optical system point spread function, as well as real-time widefield entanglement-enhanced phase imaging. Our results show that SPAD array cameras represent a natural choice for scalable entanglement detection and imaging applications.
Spatial correlations between two photons are the key resource in realising many quantum imaging schemes. Measurement of the bi-photon correlation map is typically performed using single-point scanning detectors or single-photon cameras based on CCD technology. However, both approaches are limited in speed due to the slow scanning and the low frame-rate of CCD-based cameras, resulting in data acquisition times on the order of many hours. Here we employ a high frame rate, single photon avalanche diode (SPAD) camera, to measure the spatial joint probability distribution of a bi-photon state produced by spontaneous parametric down-conversion, with statistics taken over 107 frames acquired in just 140 seconds. We verified the presence of spatial entanglement between our photon pairs through the violation of an Einstein-Podolsky-Rosen criterion, with a confidence level of 227 sigmas. Our work demonstrates the potential of SPAD cameras in the rapid characterisation of photon correlations, leading the way towards quantum imaging in real-time.
Single-photon avalanche diode (SPAD) arrays are essential tools in biophotonics, optical ranging and sensing, and quantum optics. However, their small number of pixels, low quantum efficiency, and small fill factor have so far hindered their use for practical imaging applications. Here, we demonstrate full-field entangled photon-pair correlation imaging using a 100-kpixel SPAD camera. By measuring photon coincidences between more than 500 million pairs of positions, we retrieve the full point spread function of the imaging system and subsequently high-resolution images of target objects illuminated by spatially entangled photon pairs. We show that our imaging approach is robust against stray light, enabling quantum imaging technologies to move beyond laboratory experiments towards real-world applications such as quantum light detection and ranging.
Photon correlated beams, generated by parametric down-conversion in nonlinear crystals (PDC), have been proved successfully for the measurement of quantum efficiency, of single photon detectors [1, 2, 3, 4] and can be used for measuring transmittance, as well. The main feature relies on the realization of two correlated quantum channels yielding coincident events. To reduce uncertainty [4, 5, 6] and determine its ultimate limit, contributions to statistical noise in a real experimental set-up are here investigated, and a general model is proposed to calculate the maximum likelihood best estimators and evaluate the probability of coincidence. In typical measurement scheme a channel is set as trigger (idler, i) and attention is paid to catch at least the same amount of correlated photons in the other channel (signal, s) [4]. Unwanted uncorrelated photons due to PDC itself result in a decreased degree of correlation, noise fluctuations in coincidence measurements and asymmetry between the two channels, whose mean photon rate can be quite different. We indicate by with the mean rate of correlated photons in s and i channels We address the total mean photon rates in both channels as where terms take into account also additive background photon rates due to straylight and detector noise. Actually statistical noise is also due to losses of correlated photons by optical elements, non ideal detectors and electronic devices in both channels. We distinguish, therefore, between optical losses ( transmittance of the correlated photon path), detection losses detectorquantum efficiency), electronic losses associated to the dead times time during which the detection system is unable to detect photons). It is usual to identify dead times as non-extending and extending: in the first case, all photons following a revealed photon within the fixed time interval are ignored, while in the second case any incoming photon produces or prolongs Eventually the finite duration of the time coincidence window, modifies the distribution probability of coincident events. Let us consider the distribution probability of single channel
Quantum communication is a fast-growing field that takes advantage of the quantum physics laws to protect and secure sensitive data. This work takes part of the European project UNIQORN (Affordable Quantum Communication for Everyone: Revolutionizing the Ecosystem from Fabrication to Application) whose aim is to develop a Quantum System on Chip (QSoC) for telecom application. The Integrated Circuit (IC) designed contributes in the QRNG block of the system, tailored to communicate with the integrated non-linear optics circuit. Such detector is a 32×1 linear array based on Single-Photon Avalanche-Diode (SPAD) detectors for the generation of a raw random number, by revealing the position on the array of the single photon impinging on it, realized in a BCD 0.16 μm technology. The linear array architecture consists of 32 pixels, pitched at 125 μm, each made by 4 SPADs with different diameter (5 μm, 10 μm, 20 μm, 50 μm). Two operation modes are implemented: Single-Hit Mode, needed to reveal the (5-bit) position of the pixel triggered by a single photon, representing a random number, in a time window synchronous with the laser emission. Multi Hit Mode, used to identify a coincidence of a certain number of photons(more than one, two, three or four) detected within a specified time window, thanks to a multi threshold coincidence detection logic.
Efficient Generation of Entangled Photon-Pairs The first experiments with correlated photons have been performed in the context of EPR-Bell experiments on the realistic and local properties of quantum mechanics. The source used there produced pairs of polarization entangled photons from a 2-photon decay of Calcium atoms. The technical requirements of these experiments were high (vacuum systems, stronge dye-lasers, etc.) whereas the efficiency of the source was quite low. An important step forward was the introduction of spontaneous parametric down conversion (SPDC), which has become the most common source in quantum optics for generating correlated or entangled photon pairs. In this process photons of an intense pump laser convert to photon pairs in an optical nonlinear crystal. Conservation of energy and momentum leads to strong correlations between the generated photons. With this kind of two-photon source it was possible to realize or improve many experiments on the foundations of quantum mechanics addressing the EPR-Paradoxon and in the new field of quantum information. But again, more advanced experiments and applications suffer from the limited ef- ficiency of the fluorescence process. Many photon pairs are lost by spatial and spectral filtering, which is necessary to achieve polarization entanglement and long coherence times. Different techniques have been implemented to increase the number of photon pairs using two-crystal arrangements, focusing techniques or periodically poled crystals. Most of these methods have the disadvantage that no entangled photons have been observed. It is the aim of this work to increase the yield and to improve the mode definition of entangled photon pairs generated by resonant enhancement of the pump mode and the fluorescence modes. As a first step a linear cavity for the pump mode was realized. Since the conversion probability is proportional to the pump power it was possible to increase the photon pair count rate by factor of 7 over the previous source. Besides the possibility of further improvement on already established pair correlation experiments, such an enhancement allows to build a compact source for photon pairs, in which an expensive argon-ion laser is replaced by a cheap diode laser. Among other applications such sources are of strong interest for quantum cryptography. 3 In many quantum information experiments optical fibers are use to carry the photons over long distance. Therefore, light from the parametric down-conversion source has to be efficiently coupled into fibers. In the second part we report on a new method to optimize collection efficiency by matching the angular distribution of the parametric fluorescence to the spatial mode of an optical fiber. By using this technique, we detected 366500 polarization-entangled photon pairs per second in the near-infrared region in single-mode optical fibers for 465 mW pump power (at 351.1 nm) with a 2 mm BBOcrystal. The entanglement of the photon pairs was verified by measuring polarization correlations of more than 96% in a HV-basis and in a ±45◦-basis. To our knowledge, such enormous count rates of highly entangled photon pairs have not been reached yet with any other technique. In the third part of this thesis we investigated the process of parametric downconversion in a cavity which is resonant to certain longitudinal down-conversion modes only. The idea of placing the parametric down-conversion source inside a cavity is not new. Such a device is usually referred to as a single or double resonant optical parametric oscillator (OPO) and is mainly used to generate squeezed quantum states. In that kind of application the system is operating close to but still under the threshold of oscillation. In our application the situation is quite different. The system is operating far below threshold so that mainly spontaneous emission occurs. In that mode correlations between single photons can still be observed. But bouncing the light back and forth inside the cavity increases the interaction length and hence enhances the signal levels of the down-conversion fields. Further, by resonating two certain modes only, the bandwidth is reduced by orders of magnitude and the coherence time is found to be inverse proportional to the bandwidth. A similar experiment has already been realized with a type-I parametric down-converter in the resonator. We have tried to realize a compact double resonant OPO far below threshold with a type-II parametric down-converter in a high-finesse cavity to realize a bright source of entangled photon pairs with extremely narrow bandwidth.