This PDF file contains the front matter associated with SPIE Proceedings Volume 11835, including the Title Page, Copyright information, and Table of Contents.

Continuous-variable quantum computing (CVQC) boasts, by way of quantum optics, one of the largest scalability potentials of all quantum computing platforms. In order to enable universal CVQC, i.e., exponential speedup as well as fault tolerance, one requires quantum resources (states and/or gates) with a non-Gaussian Wigner function. We present several state preparation techniques, using photon-number-resolving detection, that enable the generation of resource states such as GKP or binomial error encodings.

Random numbers which are unpredictable and definitely unknown by anyone before they are generated are now used in a large number of real-world applications ranging from authentication, gaming, many online activities to simulations and optimizations. The development of a trusted randomness source is thus a necessity. In this work we present a simple design of a certifiable quantum random number generation and its. In particular we show how real-time low-latency randomness can be generated from measurements on time-bin photonic states every 0.12s. We generate a block of 2^13 random bits certifiable against the most powerful quantum adversary with its error bounded by 2^-64. Further our device is suitable for continuous operation giving it a potential application as a quantum randomness beacon.

Single-Photon Avalanche-Diode (SPAD) arrays find extensive use in quantum imaging techniques that exploit entangledphotons states to overcome sensitivity limitations of classical imaging. Thanks to their compactness, low-voltage operation, single-photon sensitivity, absence of readout noise, and high frame-rate, SPAD arrays are particularly suited to detect temporally correlated photons over a scattered background. This work presents a scheme useful to model a generic quantum imaging measurement set-up, with its losses and non-idealities, and it provides the resulting calculations of pair rate (in case of quantum states made of two photons) and spurious single-photon rate at detector level. The computed rates are used to evaluate the performance in terms of signal-to-noise ratio of a possible SPAD array architecture based on an onchip photon coincidences detection, followed by an event-driven readout, which transfers only the addresses of those pixels involved in the coincidence event. Although bringing plenty of advantages in terms of power consumption, data storage, and readout time, especially as the pixels number increases, the intrinsic non-ideal operation timings of this architecture are linked to three possible cases of wrong detection. A detailed computation of these error probabilities is provided, together with a discussion about which design parameters most influence the detected signal quality. Since every on-chip coincidence detection and event-driven architecture is characterized by those same finite operation timings, the presented computation method can be considered a useful tool to optimize the design of detection systems used in quantum imaging and microscopy framework.

Visible-infrared photon pair sources working in broadband mid-infrared region are useful for applications such as heralded mid-infrared single photon sources. Here, we demonstrate experimentally a wavelength variable visible-infrared photon pair source in the mid-infrared region over a wide spectral range of 2 to 5 μm. By changing the angle of the nonlinear crystal in the source, the observed wavelengths of the signal photons change from 600 to 965 nm, corresponding to the idler wavelengths in 1186-4694 nm.

Quantum imaging and microscopy profit from entangled photons to surpass the boundaries of classical optics, thus improving image resolution. Thanks to their single-photon sensitivity, readout noise absence, low-voltage operation and high frame-rate, detectors based on Single-Photon Avalanche-Diodes (SPADs) are particularly suited for this application field. We discuss strengths and weaknesses of different SPAD based architectures (classified in SiPMs, SPAD arrays, or SiPM arrays), highlighting those to be exploited as quantum imagers. As just SPAD arrays are capable of spatial resolution at single-SPAD level and, through the possible implementation of quantum-specific on-chip processing, we identified them as the forefront detector type for quantum imaging and microscopy.

Optical quantum technologies such as quantum sensing, quantum cryptography and quantum computation all utilize properties of non-classical light, such as precise photon-number and entangled photon-pair states, to surpass technologies based on the classical light. A common route for obtaining heralded single photons is spontaneous four-wave mixing in optical fibers, allowing for a well-defined spatial mode, for high efficiency integration into optical fiber networks. These fibers are typically pumped using large, commercial, pulsed lasers requiring high-power (~10 W) pump lasers and are limited to ~MHz repetition rate. Here we propose a cost- efficient, compact and mobile alternative. Photon pairs at 660 nm and 960 nm will be created using four-wave mixing in commercial birefringent optical fiber, pumped using transform limited picosecond pulses with GHz repetition rates derived from a 785 nm CW laser diode using cavity-enhanced optical frequency comb generation. The pulses are predicted to have average power of 275 mW, a peak power of >40 W, and predicted photon yield of >2000 pairs detected per second. This design will be later utilized to implement a quantum illumination scheme based on a coincidence count between idler and signal photons -- instead of joint measurement between signal and idler. This will allow for quantum advantage over classic LIDAR without the requirement for maintaining an interferometric stability in free space.

Growing interest in quantum machine learning has resulted into very innovative algorithms and vigorous studies that demonstrate their power. These studies, although very useful, are often designed for fault-tolerant quantum computers that are far from reality of today's noise-prone quantum computers. While companies such as IBM have ushered in a new era of quantum computing by allowing public access to their quantum computers, quantum noise as well as decoherence are daunting obstacles that not only degrade the performance of quantum algorithms, but also make them infeasible for running on current-era quantum processors. We address the feasibility of a quantum machine learning algorithm on IBM quantum processors to shed light on their efficacy and weaknesses to design noise-aware algorithms that work around these limitations. We compare and discuss the results by implementing a quantum convolutional filter on a real quantum processor as well as a simulator.

Diamond optical emitters have been explored extensively in recent years for applications to quantum information and processing. Similarly, rare-earth dopant ions in ceramic crystals have also been explored for quantum applications, especially memories. Recently, both classes of these quantum emitters have been explored for bio-sensing. Each has unique advantages. In this talk I will discuss the possibility simultaneously using both of these material systems for advanced biosensing. I will also discuss some of our recent work to fabricate composite particles containing both systems, in an attempt to get the “best of both worlds.”

Classical sensing techniques such as range finding and clock synchronization use timing modulated light sources to provide the timing correlations needed for their implementation. Corresponding quantum schemes utilise spontaneous parametric down converted light sources to provide the timing correlations without the need for timing modulation, although entangled states are too fragile to be fully exploited presently. Here we demonstrate the use of thermal light as an alternative source for timing correlations, via the photon bunching generated from a laser operating below threshold, and showcase its practical viability by successful range finding measurements.

This paper illustrates the results of an experiment performed in the frame of the Asiago Observatory Stellar Intensity Interferometry program, aimed to exploit the quantum properties of the photon stream emitted from celestial objects. Data are acquired over two telescopes separated by approximately 4 km in photon-counting mode and analyzed in post-processing. The temporal and spatial correlation function g⁽²⁾ on the bright star Vega has been successfully measured at zero baseline and at a ~2 km baseline. The result is fully consistent with that expected for a source with the angular diameter of Vega (approximately 3.3 mas).

Quantum illumination is a technology that aims to improve the detection sensitivity of a target by utilizing quan- tum entangled state of light. We are studying quantum illumination using two-mode squeezed light. In quantum illumination, one light wave in a quantum entangled state is emitted toward a target during atmospheric disturbance. Therefore, it is extremely important to investigate the propagation characteristics of light waves under various atmospheric conditions toward the realization of quantum illumination. As an example of atmospheric disturbances, we have investigated the effect of uniform fog on light wave propagation in previous studies. It has been found that the effect of uniform fog on the propagation of laser light is mainly energy attenuation. Here we measure the propagation characteristics of single-mode squeezed light in a fog. We succeeded in observing the squeezed light that passed through the fog, and confirmed that the noise level of the squeezed light was degraded by the influence of the fog. It was found that the degradation of the noise level can be explained by considering that the influence of fog is mainly the attenuation of energy.

The concept of temporal imaging draws from the analogy between paraxial propagation of light in space and in dispersive media. So far, temporal imaging has been demonstrated with ultrafast light, corresponding to spectral precision in the THz range. In our quantum memory, we have implemented full spectral and temporal processing capabilities for ultra narrowband photons, which allow us to perform temporal imaging with MHz bandwidth and kHz precision. Implementation of this concept leads to many implications, which give rise to two distinct super-resolved spectroscopy schemes, inspired by recent developments in spatial super-resolved imaging. One scheme takes advantage of elaborate interference in the quantum memory, while the other uses optimized analysis of a homodyne detector traces. Overall, our approach not only brings temporal imaging to the previously untackled regime but also points to a variety of new schemes useful in quantum sensing.

Use of non-classical light in a quantum illumination scheme provides an advantage over classical illumination when used for LIDAR with a simple and realistic detection scheme based on Geiger-mode single photon detectors. Here we provide an analysis that accounts for the additional information gained when detectors do not fire that is typically neglected and show an improvement in performance of quantum illumination. Moreover, we provide a theoretical framework quantifying performance of both quantum and classical illumination for simple target detection, showing parameters for which a quantum advantage exists. Knowledge of the regimes that demonstrate a quantum advantage will inform where possible practical quantum LIDAR utilising non-classical light could be realised.

Quantum ghost imaging utilizes entangled photon pairs to enable an alternative image acquisition method. Information from either one of the photons does not allow for image reconstruction, however the image can be reconstructed by utilising the correlations that exist between the photon pair. Interestingly, these photon pairs can be either degenerate or non-degenerate in nature. Non-degenerate ghost imaging offers the ability to image with wavelength bandwidths where spatially resolving detectors are impractical, ineffective or expensive. Due to the scanning nature of spatially resolving detectors and the inherent low light levels of quantum experiments, imaging speeds are rather unsatisfactory. To overcome this limitation, we propose a two-step deep learning approach to establish an optimal early stopping point, tested on a non-degenerate system. In step one, we enhance the reconstructed image after each measurement by a deep convolution auto-encoder, followed by step two where a classifier is used to recognise the image. We achieved a recognition confidence of 75% at 20% of the image reconstruction time, hence reducing the image reconstruction time 5-fold while maintaining the image information. This, therefore, leads to a faster, more efficient image acquisition technique. Although tested on a non-degenerate system, our procedure can be extended to many such systems that are of quantum nature. We believe that this two-step deep learning approach will prove valuable to the community who are focusing their efforts on time-efficient ghost imaging.

We present novel methods to perform plenoptic imaging at the diffraction limit by measuring intensity correlations of light. The first method is oriented towards plenoptic microscopy, a promising technique which allows refocusing and depth-of-field enhancement, in post-processing, as well as scanning free 3D imaging. To overcome the limitations of standard plenoptic microscopes, we propose an adaptation of Correlation Plenoptic Imaging (CPI) to the working conditions of microscopy. We consider and compare different architectures of CPI microscopes, and discuss the improved robustness with respect to previous protocols against turbulence around the sample. The second method is based on measuring correlations between the images of two reference planes, arbitrarily chosen within the tridimensional scene of interest, providing an unprecedented combination of image resolution and depth of field. The results lead the way towards the realization of compact designs for CPI devices.

In this article, we review the proposed experiments for the Deep Space Quantum Link (DSQL) mission concept aiming to probe gravitational effects on quantum optical systems. Quantum theory and general relativity are the two most successful frameworks we have to describe the universe. These theories have been validated through experimental confirmations in their domains of application— the macroscopic domain for relativity, and the microscopic domain for quantum theory. To date, laboratory experiments conducted in a regime where both theories manifest measurable effects on photons are limited. Satellite platforms enable the transmission of quantum states of light between different inertial frames and over distances impossible to emulate in the laboratory. The DSQL concept proposes simultaneous tests of quantum mechanics and general relativity enabled by quantum optical links to one or more spacecrafts.

The private capacity of a lossy bosonic channel shows that there remains not much room to improve known point-to-point quantum key distribution (QKD) protocols further, in terms of the key rate versus distance. The current question in our field is how to overcome this fundamental limit with the help of a single station in the middle between communicators. In this talk, we explore recent trials to achieve this goal, such as adaptive measurement-device-independent QKD and twin-field QKD. These trials are good milestones towards the realization of quantum repeater networks.

In this paper, we propose the use of multi-photon quantum cryptography to provide higher data exchange rates and security in quantum networks. We investigate different routing strategies and their effects on key rates, path establishment probabilities, and security. Our simulation varies parameters such as the network size, topology, length of fiber channels, and probability of channel decoherence. Lastly, we also examine the effect of including a trusted nodes in the network and examine the key-rates under different routing strategies.

Network information theory establishes communication protocols among multiple senders and multiple receivers in the presence of correlations introduce noise over a network. We here present a framework of non-local network coding of multiple senders whereby network communication capacities improve over their classical counterparts. The framework exploits a Bell scenario and shows the usefulness of non-local and quantum resources in network information theory. Two-sender and two-receiver interference channels are considered in particular, for which network coding is characterized by two-input and four-outcome Bell scenarios. Resources for the network coding are classified It is shown that non-signaling (quantum) correlations lead to strictly higher channel capacities in general than quantum (local) correlations. It is also shown that, however, more non-locality does not necessarily imply a higher channel capacity. The framework can be generally applied to network communication protocols.

When packets of information go through a network, each packet can be transmitted on a different route. The aggregation of quantum network provides a potential for us to use communication channels more efficiently and may contribute robust transmissions of quantum information with less quantum resources. To evaluate this possibility, we use multiplexing of independent quantum channels to distribute quantum error correction over the network. In this talk we also apply spatial-temporal single photon multiplexing and observe a significant saving in the quantum communication resources, and further we discuss how such quantum networks differ from their classical counterparts.

This manuscript discusses the most relevant aspects of the practical implementation of a long-range Quantum Key Distribution (QKD) link with trusted nodes, achieving the highest possible secret key rate generation within the security and system-level constraints. To this purpose, it describes the implementation of an end-to-end QKD system, including implementation aspects from the physical transmission of photon states through a standard telecommunications grade optical fiber, to consideration of quantum metrology and information reconciliation protocols based on forward error correction codes. In addition, since there are circumstances when a fiber optical link may not be available, it examines the problems involved with the implementation of a Free Space Optics (FSO) QKD link. The manuscript also discusses the problem of information reconciliation in Continuous Variable (CV) QKD scenarios on FSO links, showing that in long distance links, since the sign of the received Gaussian samples contains the largest fraction of information, Unequal Error Protection (UEP) reverse reconciliation schemes can be designed. The presented results have been achieved within the NATO SPS project “Analysis, design and implementation of an end-to-end 400 km QKD link”.

Energy-time entanglement can enhance two photon absorption by combining a precise two photon resonance with temporal correlations between the arrival times of the two photons. In general, the optimal timing of photon arrival is determined by the dynamics of the system between the initial and the final excitations. In this presentation, we show that the optimal timing of photon arrival corresponds to a specific phase dispersion in the frequency difference of the two photons. The absorption cross-section of energy-time entangled photons can be enhanced beyond the rate observed for optimal photon coincidence by adjusting the phase dispersion of one of the photons to match the optimal characteristics. We determine the maximal enhancement factor as a function of the bandwidth of the intermediate states and their detuning relative to the average photon frequency. Significant additional enhancements can be achieved when the bandwidth of the intermediate states is very narrow.

Efficient analysis of the quantum states of photons are essential for photonic quantum technologies. In the first part, we report the experimental demonstration of direct and efficient verification of entanglement between two multimode-multiphoton systems (one photon in three modes and two photons in three modes) using just two sets of classical correlation tables with and without a discrete Fourier transformation of the optical modes, clearly demonstrating a dramatic reduction in the resources required for entanglement verification. In the second part, we report our experimental demonstration of adaptive quantum state estimation for quantum states of photons changing in time.