This PDF file contains the front matter associated with SPIE Proceedings Volume 11295, including the Title Page, Copyright Information, Table of Contents, Author and Conference Committee lists.

Quantum computing promises to transform almost every industry, and as such there is currently a huge industrial and academic effort to realize the world’s first useful quantum computer, requiring ~1,000,000 physical qubits. Of the various technological approaches being pursued, photons are particularly appealing due to their low noise properties, ease of manipulation and compatibility with existing and established CMOS manufacturing techniques – through silicon photonics. In this talk I will outline the photonic approach to quantum computing, introducing the integrated quantum photonic circuit and highlighting routes to a scalable quantum technology platform.

Quantum walks serve as novel tools for performing efficient quantum computation and simulation. In a recent experimental demonstration [1] we have realized photonic quantum walks for simulating cyclic quantum systems, such as hexagonal lattices or aromatic molecules like benzene. In that experiment we explored the wave function dynamics and the probability distribution of a quantum particle located on a six-site system (with periodic boundary conditions), alongside with simpler demonstration of three- and four-site systems, under various initial conditions. Localization and revival of the wave function were demonstrated. After revisiting that experiment we will theoretically analyze the case of noisy quantum walks by implementing the bit-phase flip channel. This will allow us to draw conclusions regarding the performance of our photonic quantum simulation in noisy environments. Finally, we will briefly outline some future directions.

Future quantum networks offer the potential for new communication and computation applications. These quantum networks will undoubtedly require the routing of quantum information between distant parties. In order to reliably achieve the transmission of entangled states over such a network, some entanglement distillation protocol can be implemented on an ensemble of entangled photon pairs. Here, we demonstrate such a protocol by recovering quantum information using local filters on each photon of a polarization-entangled state in the presence of a common source of decoherence in the telecom fiber infrastructure, polarization mode dispersion (PMD).

In this talk, I will discuss our pathway toward quantum simulations and computation with neutral atoms in arrays of optical tweezers cooled to their motional ground state. Initially, I will talk about two-atom interference experiments, e.g. the atomic analog to the Hong-Ou-Mandel effect, and how we create spin entangled states. I will then talk about how we scale our system to accommodate 80 single atoms, loaded into 100 traps while maintaining full single-atom control. For this efficient loading process, we control atomic collisions in the tweezers using grey molasses. When combined with ground-state cooling and atom-rearranging, this should allow us to form a micro-BEC with controlled atom number and entropy.

Quantum key distribution (QKD) allows users to generate shared encryption keys that are guaranteed to be theoretically secure by the laws of quantum mechanics. Most implementations use light pulses that are attenuated by the propagation medium. This leads to a fundamental rate-distance limit in QKD that was thought to be impossible to overcome with current technology. The recent proposal of "Twin-Field QKD", however, changed this belief and showed how to overcome this limit and perform long-distance QKD with present-day technology. In my talk I will review the most significant theoretical and experimental results in this emerging and rapidly growing research area.

The future global-scale quantum communication network will require free-space and satellite links able to work in daylight conditions and compatible with the telecom fiber infrastructure. Here we present a full prototype for daylight quantum key distribution at 1550 nm exploiting an integrated silicon-photonics chip as state encoder. We tested our prototype in the urban area of Padua (Italy) over a 145m-long free-space link, obtaining a quantum bit error rate around 0.5% and an averaged secret key rate of 30 kbps. The developed chip represents a cost-effective solution for portable free-space transmitters and a promising resource for future satellite missions.

We report on the first experimental implementation of a scalable M-ary quantum receiver with time-resolved photon detection and real-time adaptive displacement using off-the-shelf components. We implemented M=4, 6, 8, 12, and 16 coherent frequency shift keying communication links with optimal parameters for maximal energy sensitivity. Then, we compared the measured sensitivity with the best sensitivity that can be achieved with a perfect classical measurement (shot noise limit). We observe unconditional, below the shot-noise limit sensitivity and a favorable sensitivity scaling with the alphabet length M.

Quantum dots reaching the best performances as non-classical light emitters are based on the GaAs platform. The main drawback is their natural emission wavelength, generally around 800-900 nm, so not suitable for fibre-based experiments. In this talk we will discuss two approaches to reach the telecom C-band, within the maximum transmission window of silica fibers. On the one hand, the use of quantum frequency conversion will enable the use of non-linear processes to transfer the high quality NIR photons to any other wavelength, here 1550 nm [1]. On the other hand, growth engineering can be utilized to modify size and strain conditions of the quantum dot, enabling the emission at telecom O- and C-band with the same dot stoichiometry as for the NIR ones [2]. [1] J. H. Weber, et al., Nat. Nanotechnol. 14, 23 (2019). [2] C. Nawrath et al., Appl. Phys. Lett. 115, 023103 (2019).

"Quantum-repeater-enabled networks" was recorded at Photonics West 2020 in San Francisco, California.

Wavelength division multiplexing (WDM) improves key generation rates in quantum key distribution (QKD) and enables multi-user networks using shared quantum resources. The last years have seen a flurry of research on the implementation of WDM in fiber-based QKD where low-loss telecom components are widely available. However, WDM has not yet been demonstrated in free-space and satellite-based quantum communication in the wavelength range detectable by spacesuitable silicon single photon avalanche diodes. Here, we report a low-loss WDM at around 810 nm based on volume Bragg gratings. The total insertion loss for four wavelength channels matches state-of-the-art telecommunication WDM in the C-band. We characterize the performance in conjunction with a polarization-entangled photon source and report on preliminary assessments of background noise for a terrestrial free-space link in broad daylight.

In theory, quantum key distribution (QKD) offers information-theoretic secure communications. In practice, however, it does not because most security proofs of QKD use assumptions that are not satisfied by the implementations, and thus their security is seriously threatened by quantum hacking. This includes, for instance, the assumption that the legitimate users of the system have a precise mathematical model to describe the behaviour of their apparatuses, that the systems are located in perfectly shielded labs which prevent any unwanted information to be leaked to the eavesdropper, and that all quantum devices and classical post-processing units are honest and follow the prescriptions of the protocol. Here, we review recent results that significantly relax these unrealistic and hardly feasible requirements, and pave the way towards achieving practical QKD with implementation security and enhanced performance.

Quantum memories are a fundamental of any global-scale quantum Internet, since these units are necessary for guaranteeing an optimal performance in a high-performance quantum networking scenario. The main problem of quantum memories is the low retrieval efficiency of the systems from the registers of the quantum memory. Here, we define a novel quantum memory called high-retrieval-efficiency (HRE) quantum memory for the quantum Internet. An HRE quantum memory unit integrates local unitary operations on its hardware level for the optimization of the readout procedure and utilizes the advanced techniques of quantum machine learning. The local unitaries of an HRE quantum memory achieve the optimization of the readout procedure in an unsupervised manner without the use of any labeled data or training sequences. The HRE quantum memory is a particularly convenient unit for the construction of a powerful, global-scale quantum Internet.

High performance quantum networks require extensive utilization of multiplexing techniques, relaying on photonic spatial and temporal structure. Multimode quantum memories for light, where photons are stored in the form of collective atomic excitations, may constitute building blocks of such networks; however, a requirement arises to implement complex linear operations on the stored excitations. In our photonic processor we experimentally demonstrate how the off-resonant ac-Stark shift (ACS) may be employed to manipulate the spatial and temporal structure of photons stored in a highly multimode quantum memory as the collective atomic excitations – spin waves.

Memory is a precious commodity across many different areas. I will present two ways in which quantum devices can lower memory requirements. The first application is the simulation of stochastic processes, i.e. processes that exhibit some randomness. We have experimentally realized quantum simulations of classical stochastic processes. Our simulators have lower memory requirements than the optimal classical simulators. The second type of quantum device I will discuss is a quantum autoencoder, which autonomously learns how to compress quantum data. We have developed and experimentally realized a photonic quantum autoencoder that is trained based on sets of quantum states.

Diamond cavity optomechanical devices enable coherent conversion of information between optical and mechanical domains, thanks to a combination of low optical and mechanical loss, the ability to support intense optical fields without suffering from multi-photon optical absorption, and enhanced optomechanical coupling arising from nanoscale device geometry. In this talk we will review our progress in using these devices to create optical memories, wavelength conversion devices, and phase sensitive all-optical switches.

Quantum 2.0 applications such as gravitational sensing require narrow linewidth lasers at specific wavelength and significant optical power. For single photon lidar applications such as to image through scattering media, intense yet short optical pulses are required. These are requirements not readily provided by existing laser systems. We suggest the use of master oscillator power amplifier (MOPA) systems consisting of a seed source providing the required spectral and temporal optical characteristics combined with a semiconductor based tapered amplifier to amplify the seed power to levels adequate for the required quantum applications. Considerations of the construction of such systems are discussed. Furthermore, there operational specifications will be determined and the suitability for quantum applications will be discussed.

Identification of a suitable source of single photons via second-order autocorrelation function measurement within an array of thousand possible candidates is a routine, key step of any practical realization in quantum optics. Within this work, we have shown that machine learning algorithms enable high precision classification between “single” and “not single” quantum emitters based on sparse autocorrelation data measurement and require < 1 s acquisition time, while conventional methods demand > 1 min. Machine learning assisted classification, done on a sparse 1-s dataset, provides ~85% accuracy of “single”/“not single” emitter identification versus only 57% of the conventional Levenberg-Marquardt approach.

We report on pulsed-laser induced and solid-immersion lens (SIL) assisted generation of nitrogen-vacancy centers (NVs) in diamond. The NVs are characterized by a narrow distribution of the optical transition linewidth peaked around 50 MHz. This includes the effect of long-term spectral diffusion caused by a necessary repump green laser. Such high-quality NVs are excellent candidates for practical implementations employing two-photon quantum interference with separate NV centers. The perspective of implementation of these NVs in open microcavities will be discussed.

Entangled two-photon absorption (e2PA) employs advantages of classical two-photon absorption techniques while operating in a linear excitation regime at low fluxes and potentially having greatly enhanced absorption probabilities. A major challenge in measuring the e2PA cross section, σ_e2PA, is to discriminate e2PA from one- photon losses. Carefully designed transmittance measurements are required to distinguish the two mechanisms. For example, the e2PA signal should depend on the time delay between photons within an entangled pair (in contrast to one-photon losses). Here we present an experimental system implementing this characterization. We perform transmittance experiments for Zinc-tetraphenylporphyrin (ZnTPP) in toluene solution. We use entangled photons produced via spontaneous parametric downconversion at 810 nm wavelength as an excitation source. We show that the change in transmittance associated with e2PA in the sample is less than 1% in our experiment. From our measurements we conclude ZnTPP's σ_e2PA ≤1:7 x 10^-19 cm².

We report on the compressive near infrared single-pixel imaging using the sinusoidally gated InGaAs/InP single-photon avalanche diode (SG-SPAD). The SG-SPAD is one of the most efficient and practical single-photon detector in NIR region, and it enable us to realize an imaging with an extremely low number of photons. We successfully took 2- dimensional 64 × 64 images of a transparent object with an entire photon detection events of 10⁵.

We theoretically analyze the phase sensitivity of the Induced-Coherence Interferometer, including the case where the sensitivity is "boosted" into the bright input regime with coherent-light seeding. We find scaling which reaches below the shot noise limit, even when seeding the spatial mode which does not interact with the sample - or when seeding the undetected mode. Furthermore, the setup allows use of subtracted intensity measurements, instead of direct or homodyne measurements - a significant practical advantage. Bright, super-sensitive phase estimation of an object with different light fields for interaction and detection is possible.

We present a study of nonlocal polarization-mode dispersion (PMD) compensation in the framework of quantum information theory. We consider distribution of polarization-entangled photon pairs through optical fibers, where PMD acts as a decoherence mechanism. The use of additional controlled PMD in one of the two optical paths can restore the original degree of entanglement fully or in part, depending on the system configuration, in a nonlocal fashion. Using the quantum analog of the Shannon entropy, the Von Neumann entropy, we evaluate the quantum mutual information of propagated polarization-entangled photon pairs as a function of the fiber-channel PMD, and quantify the beneficial effect of nonlocal PMD compensation in terms of mutual quantum information restoration. All the relevant quantities can be extracted from the reduced density matrix characterizing the twophoton state polarization, which is obtained experimentally by means of customary polarization tomography.

Oil accidents largely occur due to mining and transport by tankers, but also due to the pipeline transport. The reason can be a leakage of the pipelines and their damage caused by their age or external influences. The leaks of oil into the soil present a high risk because of their inconspicuousness, especially in case of a slow leak, which may go unnoticed for a while. Petroleum products are soluble in water, which may lead to the contamination of the surrounding groundwater. Self-monitoring of these transport systems is an important part of their operation. This article deals with the measuring of the leak by petroleum products using optical fibers. During the measuring, we use a distributed temperature system (DTS). The DTS is based on a principle of an optical reflectometer, i.e. the light impulse is transmitted to the fiber, part of the impulse returns back to the detector due to optical scattering. The phenomenon of the returning part of the light impulse is called the Stimulated Raman Scattering. The DTS can measure temperature along the entire length of the fiber in real time. Basically, we can imagine a thousand of sensors along the measured track with the accuracy of 1 meter. In this case, we have an optical cable laid down along the testing polygon. This testing polygon is intended to simulate the transport of the oil products in the pipelines. Many different leaks are simulated here. The measuring is based on the volatility of petroleum products and their ability to evaporate. The surface of the object, from which the oil evaporates, decreases in temperature. Sudden change in temperature at a given point detects a possible leakage of petroleum products. This leak can be immediately pinpointed with the accuracy of 1 meter and necessary intervention can be performed.