Quantum Communications and Quantum Imaging XXII

M.Genovese, A.Avella, A.Meda, A.Paniate, I.Ruo Berchera, We present in detail two works realized in INRIM. The first consists of exploiting entanglement to enhance imaging of a pure phase object in a non-interferometric setting. This wide-field method, based on the "transport of intensity equation", provides the absolute value of the phase without prior knowledge of the object. It does not require spatial and temporal coherence of the incident light. Besides improving image quality at a fixed number of photons, we demonstrate clear reduction of the uncertainty in the quantitative phase estimation. This research also paves the way for applications at different wavelengths, e.g., X-ray imaging, where reducing the photon dose is of utmost importance. Then, we demonstrate a novel imaging technique, named Light Field Ghost Imaging, that exploits light correlations and light field imaging principles to enable going beyond the limitations of ghost imaging in a wide range of applications. Notably, our technique removes the requirement to have prior knowledge of the object distance allowing us to refocus in post-processing and to perform 3D imaging while retaining all the benefits of GI

We describe ghost displacement in which one of a pair of correlated beam is first displaced coherently before passing through an object and then being detected. When the detector does not fire, the beam that does not interact with the object receives a nonlocal displacement at transverse spatial locations where the object is transparent. This allows an image to be formed in correlation. The method has previously been used in single mode fiber to transfer amplitude and phase information nonlocally and covertly *. We introduce an experimental method for performing imaging via ghost displacement. We use a pulsed 842.2nm VCSEL to generate pseudo-thermal light via amplitude and phase modulation and obtain correlated twin beams using a beam splitter and we show the first experimental results from this system. * U. Zanforlin et al., Phys. Rev. A 107, 022619 (2023).

We demonstrate that the two - mode squeezed vacuum states of the radiation field achieve the same bound as the number states enabling us to make the quantum metrology of open systems practical. Further we show how joint photon counting distributions saturate the Cramer-Rao bounds and we bring out importance of the time reversed quantum metrology.

We investigate the optimization of direct measurement twin beam (signal-idler) quantum illumination. We focus on each of the three main components of such a system in turn: the detectors, the light beam parameters and the information processing protocol. Surprisingly, there can be an advantage to having a signal detector whose quantum efficiency is significantly less than perfect. This advantage does not vanish in the quantum lidar limits of low object reflectivities or signal strengths. We show that decreasing the pulse separation, while keeping the photon flux fixed to retain the same degree of covertness, can improve target detection performance dramatically. Finally we show that post-selecting on the idler detector firing is sub-optimal for target detection.

Super-resolution microscopy seeks to surpass the resolution limit dictated by the diffraction of light. Among the various super-resolution methods, far-field are of the particular interest due to their non-invasive nature. For instance, the substitution of uniform sample illumination with a structured light beam enables resolution enhancement without perturbing the sample. Recent breakthroughs in quantum metrology suggest an alternative path: replacing the conventional intensity measurement in the image plane with spatial mode demultiplexing, also known as Hermite-Gaussian imaging (HGI). In this study, we introduce a novel combined technique that takes advantage from improving both measurement and illumination by implementing HGI within the conventional Image Scanning Microscopy (ISM) approach. Our experimental results demonstrate a 2.5-fold improvement in lateral resolution compared to the generalised Rayleigh limit. The combined approach demonstrates superiority over both ISM and HGI individually, enhancing lateral resolution and minimizing the impact of the artefacts on the final image.

Optical Parametric Amplifiers (OPA) when configured as Phase-Sensitive Amplifiers (PSA) are student under various configurations including high pump powers, pump modes, and expand known literature on spatial mode-mixing and realize true image amplification in the PSA [1,2]. In this work we present imaging of unknown target exceeding gain bandwidth of any one pump-mode/power related PSA configuration by sweeping through the pump mode profiles by simulation. This method leverages our conceptual understanding of the PSA and applies the desaturation of detected modes by gain equalization in reconstructing the tomographic image.

Quantum-secured optical channels based on Quantum Key Distribution technology have generated a significant global interest. Although the maturity level of the short distance (less than 100 km) quantum-secured channels is at a deployment level, instituting such channels over long distance faces technological challenges, which is the subject of a world-wide research. In this presentation an industry approach towards the future deployment of long-distance quantum-secured optical channels in operational environments will be discussed, including the vision, requirements, and R&D activities in both terrestrial and satellite-based methodologies.

Quantum communication will be at the heart of many future quantum technologies providing the foundation to for them to coherently interact. Here we introduce the concepts of quantum multiplexing and aggregation for use in such quantum communication related tasks. Quantum multiplexing involves encoding multiple-qubits of information onto each transmitted photon and allows us to significantly reduce the effect channel losses have within those tasks. Next quantum aggregation allows quantum messages to be divided and transmitted in a coherent way over different paths when resources in given paths are limited. We will discuss potential implementations and the resources required.

A protocol for quantum key distribution (QKD) using continuous variables (CV), not relying on a shared phase reference, is proposed. The protocol is based on a single-quadrature encoding of the key bits using coherent states (the unidimensional CV QKD) and subsequent alignment of the measurement bases, conditioned on the variance minimization in the unmodulated quadrature. The protocol allows to circumvent the numerous known hacking attacks on the shared phase reference (local oscillator), at the same time not relying on the pilot tones for phase locking with the “local” local oscillator in the receiver station. Sufficiently high repetition rates and long coherence times are required to accumulate reliable statistics for the bases alignment but should be feasible with the current technology. Stability of the protocol against residual Gaussian-distributed phase noise is analysed to confirm its feasibility. The protocol paves the way to low-cost, efficient, and secure realizations of CV QKD.

In theory, quantum key distribution (QKD) provides unconditionally secure keys due to the laws of quantum mechanics. However, real-world QKD systems often use strongly attenuated lasers which enables the photon number splitting (PNS) attack. Decoy state protocols weaken an eavesdropper’s chance to perform the compromising PNS attack undetected. In small multi-user QKD networks the photons can be distributed to the users via passive beam splitters. This approach lowers the sender-to-receiver QKD channel efficiency (relative to a standard point-to-point QKD system) and reduces the mean photon number in the sender-to-receiver channel. For this contribution, we investigated the performance of decoy state protocols in a passive multi-user system. We compared this point-to-multipoint setup to a standard point-to-point system by checking the photon number dependent yields, hereby showing that with some restrictions such a system would be safe against the PNS attack.

In this paper, we focus on free-space quantum communication systems. Unlike classical channels where the effects of turbulent media can be predicted with existing theoretical models, these mechanisms cannot be directly applied to quantum states. In our approach that relies on photons with polarization entanglement, another realm of problems is encountered. Proper response of the detection system to non-classical features of light requires that photon pairs with proper polarization and correlation characteristics are received. Therefore, it is necessary to research a wide array of operating conditions for different levels of turbulence and properly replicate those on our laboratory testbed. In this paper, we present a system that integrates an atmospheric chamber with a quantum communication link and analysis instrumentation. A system is developed that allows scaling the experiments over different ranges and evaluation of integrity of the quantum states under practical operating conditions.

We expand the traditional two-photon Hong-Ou-Mandel (HOM) effect onto a higher-dimensional set of spatial modes. This enables a quantum network router that provides a controllable redistribution of entangled photon states over four spatial modes using a novel idea of directionally unbiased linear-optical Grover four-ports. The familiar Hong-Ou-Mandel (HOM) effect occurs when two indistinguishable photons impinge on adjacent ports of a 50:50 beam-splitter. Two-photon interference causes the photons to always emerge from the same output port in the same spatial mode. This traditional HOM method, observed on a beam-splitter with two input and two output ports, always has the two-photon state simultaneously occupying both output spatial modes, leaving no room to alter the propagation direction of outgoing states. The presented higher-dimensional HOM effect allows manipulation of quantum photon amplitudes in four spatial modes by using directionally unbiased linear-optical devices such as Grover coin optical multiports, beam splitters, and phase shifters. This could be used as a linear-optical switch /router for quantum networks.

Quantum circuits based on single photons and linear optical elements (PhQC) are an ideal candidate to enable quantum information processing at room temperature. Although PhQC cannot achieve universality without post-selection, boson sampling (a well-known sampling problem) has a non-trivial computational complexity related to #P-hard. The key question now is how to utilize that complexity for practical problems? Here we present a new hybrid quantum / classical neural network model for image reorganization that achieves a 96.6% testing accuracy only 4 photons and 16 modes without PhQC optimization. We also show it outperforms the situation where coherent light sources are used.

This work proposes a circuit implementation of the encoding-decoding circuit of a N x N grayscale-based Flexible Representation of Quantum Images (FRQI). The circuit implementation is carried out using Qiskit on IBM Quantum Lab Notebooks. The encoded FRQI is considered as an unsorted database with a single table, where the key of the table represents the pixel’s position (X,Y). The other column of the table stands for the grayscale in this position, it is encoded by an angle and implemented by a multi-controlled rotation gate. Every pixel of the image corresponds to an item in the table and is represented by a superposition of the basis states in the computational basis. Subsequently, the Grover’s algorithm is used for retrieving the position from the FRQI after performing a comparison with a given grayscale individual pixel encoded over a single qubit. The physical constraints associated with the used IBM quantum device are discussed, and the limitations of Grover’s algorithm for searching the pixel are addressed. Expanding this study to a more general search for a symbol based on multiple pixels in the FRQI is presented.

We present our recent works for development of practical optical non-Gaussian state suitable for large-scale optical quantum computation in time domain. Our talk will focus on demonstration of a single step of cat breeding protocol for generation of Gottesman-Kitaev-Preskill qubit and high-rate generation of non-Gaussian state to improve the current protocol

If a world-wide quantum network is established only with optical devices, it leads to a cost-efficient high-speed quantum internet in the future. It is natural to imagine that such an all-optical network is composed of various protocols specialized in intracity, intercity and intercontinental quantum communication. Here I will talk about recent rapid progress on this kind of all-photonic approach towards the quantum internet.

Quantum Key Distribution (QKD) is a method for securely distributing secret keys for cryptographic purposes between two trusted parties over a quantum channel. For reaching global-scale quantum key distribution, satellite-based approaches are most promising as already demonstrated by the Chinese MICIUS mission. In my presentation, I want to summarize our efforts in building a very compact QKD sender unit featuring polarization-encoded BB84 with weak coherent pulses. The unit is suitable for deployment in cube satellite space missions such as our missions QUBE and QUBE-II which aim to demonstrate affordable and scalable key distribution in the near future. I will present first results from ground-based stationary tests with our sender unit. Furthermore, I will give a prospect on the timeline of the missions, including the launch of QUBE, which is currently scheduled for summer 2024.

Satellite constellations are an attractive solution for achieving global quantum communication, an idea which is supported by several successful quantum key distribution (QKD) demonstrations via the Micius satellite. Whilst overcoming the exponential losses in optical fibres, free space optical communication via satellite faces several challenges. We model the channel capacity from a low earth orbit (LEO) satellite to an optical ground station (OGS) downlink, and include losses due to diffraction, turbulence, pointing errors, optical efficiencies, atmospheric extinction, visibility, and cloud cover. We present a simulation platform for estimating the secret key rate for various scenarios. We show that the losses can be greatly reduced by combining spatial, temporal, and wavelength filtering with advances in adaptive optics (AO) techniques and acquisition, tracking, and pointing (ATP) systems.

This talk will consider the future of metal halide perovskite (MHP) photovoltaic (PV) technologies as photovoltaic deployment reaches the terawatt scale. The requirements for significantly increasing PV deployment beyond current rates and what the implications are for technologies attempting to meet this challenge will be addressed. In particular how issues of CO2 impacts and sustainability inform near and longer-term research development and deployment goals for MHP enabled PV will be discussed. To facilitate this, an overview of current state of the art results for MHP based single junction, and multi-junctions in all-perovskite or hybrid configurations with other PV technologies will be presented. This will also include examination of performance of MHP-PVs along both efficiency and reliability axes for not only cells but also modules placed in context of the success of technologies that are currently widely deployed.

The recent advent of robust, refractory (having a high melting point and chemical stability at temperatures above 2000°C) photonic materials such as plasmonic ceramics, specifically, transition metal nitrides (TMNs), MXenes and transparent conducting oxides (TCOs) is currently driving the development of durable, compact, chip-compatible devices for sustainable energy, harsh-environment sensing, defense and intelligence, information technology, aerospace, chemical and oil & gas industries. These materials offer high-temperature and chemical stability, great tailorability of their optical properties, strong plasmonic behavior, optical nonlinearities, and high photothermal conversion efficiencies. This lecture will discuss advanced machine-learning-assisted photonic designs, materials optimization, and fabrication approaches for the development of efficient thermophotovoltaic (TPV) systems, lightsail spacecrafts, and high-T sensors utilizing TMN metasurfaces. We also explore the potential of TMNs (titanium nitride, zirconium nitride) and TCOs for switchable photonics, high-harmonic-based XUV generation, refractory metasurfaces for energy conversion, high-power applications, photodynamic therapy and photochemistry/photocatalysis. The development of environmentally-friendly, large-scale fabrication techniques will be discussed, and the emphasis will be put on novel machine-learning-driven design frameworks that leverage the emerging quantum solvers for meta-device optimization and bridge the areas of materials engineering, photonic design, and quantum technologies.

Multi-mode interference can be used to simulate arbitrarily complex quantum systems, providing us with an opportunity to study fundamental nonclassical aspects of quantum statistics. Here, we consider the effects of looking for a photon in the path of an interferometer when the photon is not detected in that path. It is shown that the effects of not detecting the photon can be explained by quantum contextuality and the associated negative values of quasiprobabilities relating different measurement contexts to each other.

We propose using thin-film lithium niobate on insulator (LNOI) doped with Erbium (Er3+) as a promising solution for implementing large-scale quantum memory. However, the transition from bulk crystals to thin films poses challenges, notably reduced optical depth, which is critical for broad atomic frequency comb memory. To address this, we plan to utilize impedance-matched cavities, increasing the effective optical depth. Furthermore, the cavity would boost the rate of spontaneous emission, increasing the efficiency of spectral hole burning. Our preliminary results reveal high-Q micro-ring resonators (Q ≈ 190k) on Er3+: LNOI, demonstrating a nearly 3.5-fold reduction in the optical lifetime due to cavity resonance. We expect our platform will improve the performance of telecom-compatible atomic frequency comb quantum memory.

A ring-laser-gyro (RLG) is a rotation sensor based on the Sagnac effect. Its ultimate sensitivity is given by the shot-noise. RLG are ring optical cavities where an in-cavity optically active laser volume emits two counter propagating beams that, due to the Sagnac effect, have different frequencies. This frequency difference is proportional to the rotation rate of the ring itself. Here we present noise floor measurement for a large ring laser showing that the reached sensitivity level is not consistent with an independent beam model. The measured sensitivity is, indeed, about one order of magnitude better than expected. This unexpected result, that paves the way to the use of large RLG in a wide range of measures in fundamental physics as well as to experimentally investigating quantum effects in non-inertial reference frames. It is most probably due to coupling of the phases of the two beams mediated by the laser medium and mirror back-scattering.

Light-matter interactions are often rewritten into a picture of hybrid light-matter excitations called polaritons. In a regime of strong optical nonlinearities, polariton number state selective interactions between modes emerge, which deterministically shape noise statistics and generate entanglement in macroscopic quantum states of light. To understand such systems, the Hamiltonian is separated into a polariton number density preserving part and a mode mixing part. The number preserving interactions lend themselves nicely to exact solutions which serve as a starting definition for polaritons with energy that is not quadratic in field strength. When mode mixing interactions are analyzed in the number density preserving interaction picture, photon number selective dynamics emerge as a clear consequence. We present a specific resonant cavity coupled multi-level system that supports such dynamics.

Multimode Squeezer based high-speed RNG Optical Parametric Amplifiers (OPA) when configured as Phase-Sensitive Amplifiers (PSA) are student under various configurations including high pump powers, pump modes, and expand known literature on spatial mode-mixing and realize true image amplification in the PSA [1,2]. In this work we present a configuration for generating several streams of uncorrelated detector currents which are used to form a high-speed random number generator (RNG). The number of such streams is proportional to eigenmodes with gain > 1; such modes are shown to be made programmable with pump mode in quantum theory. References: 1. Annamalai, M., Stelmakh, N., Vasilyev, M., & Kumar, P. (2011). Spatial modes of phase-sensitive parametric image amplifiers with circular and elliptical Gaussian pumps. Optics Express, 19(27), 26710-26724. 2. Annamalai, M. (2012). Mode Structure of a Noiseless Phase-sensitive Image Amplifier.

We present numerical results from simulations using deep reinforcement learning to control a measurement-based quantum processor—a time-multiplexed optical circuit sampled by photon- number-resolving detection—and find it generates squeezed cat states quasi-deterministically, with an average success rate of 98%, far outperforming all other proposals. Since squeezed cat states are deterministic precursors to the Gottesman-Kitaev-Preskill (GKP) bosonic error code, this is a key result for enabling fault tolerant photonic quantum computing. Informed by these simulations, we also discovered a one-step quantum circuit of constant parameters that can generate GKP states with high probability, though not deterministically.

Continuous-variable optical quantum computing demonstrates a unique and strong advantage in scalability by time-multiplexing. To overcome classical computers, however, it requires what is called non-Gaussian states which are generated at unpredictable timing and therefore pose challenges especially when combined with time-multiplexing. We report our recent progress on the development of dynamic control on photonic non-Gaussian states in time-multiplexing by synchronizing the optical processor to the heralding signal of the state generation. This achievement will enable large-scale continuous-variable quantum computing in a resource-efficient fashion.

Entanglement is a key resource in many quantum communication tasks such as quantum cryptography. Inevitable losses in quantum channels degrade entanglement; therefore, practically making entanglement not useful for these tasks. To mitigate loss, different techniques have been proposed, where a noiseless linear amplifier (NLA) is one of them. A crucial aspect of noiseless quantum amplifiers is their probability of success. Thus, optimizing optical noiseless amplifiers and enhancing their success probability within a designated amplification range is a significant objective. In this context, we demonstrate a pioneering experimental protocol, strategically crafted to improve the probability of success of existing NLA protocols.

By means of Quantum steering, one user is able to control the quantum state shared with an other distant user with superior ability than allowed by a local hidden state model. Certifying the presence of quantum steering is therefore crucial for the verification of quantum channels. Nevertheless, its connection to the metrological power of the quantum state, via the violation of the Cramér-Rao inequality, has been recently proved. However, such a direct assessment would demand operating in the asymptotic regime of a large number of repetitions. For this reason, in this work we extended the already existent protocol, explicitly accounting for the limited number of resources, and put it under test in a quantum optics experiment. Moreover, experimental imperfections in the setup required the adaptation of the original test to the multiparameter scenario. Our results provide then guidelines to apply such a metrological approach to the validation of quantum channels.

It is shown that a fixed measurement setting, e.g., a measurement in the computational basis, can detect all entangled states by preparing multipartite quantum states, called network states. We present network states for both cases to construct decomposable entanglement witnesses (EWs) equivalent to the partial transpose criteria and also non-decomposable EWs that detect undistillable entangled states beyond the partial transpose criteria. Entanglement detection by state preparation can be extended to multipartite states such as graph states, a resource for measurement-based quantum computing. Our results readily apply to a realistic scenario, for instance, an array of superconducting qubits. neutral atoms, or photons, in which the preparation of a multipartite state and a fixed measurement are experimentally feasible.

Quantum nonlocality arising from entangled particles cannot be explained by any classical notion of local causality. In the standard Bell scenario with a source and two measurement stations, entangled states can lose their ability to exhibit nonlocality due to noisy conditions. Given the fundamental and technological significance of nonlocality and the fact that these noisy entangled states can naturally arise in practical situations, it is of utmost importance to unveil their nonlocality. Here, we demonstrate that nonlocal correlations can be generated from quantum states that cannot show nonlocality in bipartite Bell scenarios. We show how to robustly characterize these states and experimentally activate their nonlocality when embedded in a photonic network, by violating classical constraints in this quantum network. These results have implications in foundations of quantum nonlocality and direct applications to quantum technologies by significantly increasing the robustness of nonlocal correlations to noise. Ref. L. Villegas-Aguilar, E. Polino, F. Ghafari, et al. “Nonlocality activation in a photonic quantum network”, preprint arXiv:2309.06501, (2023).

Over the past few years, remarkable advancements have occurred across various fronts in quantum computing. Despite the huge progress, the availability of fault-tolerant quantum computers remains out of reach for now or decades away. Hence, a crucial challenge is to effectively utilize current NISQ devices to attain a quantum advantage. In this vein, Quantum Approximate Optimization Algorithm (QAOA) was suggested in order to potentially demonstrate a computational advantage in combinatorial optimization problems on NISQ computers. On the other hand, quantum error mitigation (QEM), a recently developed method to cope with errors, has been developed, and their utility has been validated in practical problems with more than 100 qubits. Therefore, in this paper, we optimize the QAOA circuits and apply several error mitigation methods such as dynamic decoupling, Pauli-twirling, and zero noise extrapolation to scale the problem size on IBM quantum processors. In addition, we study the effect of the depth of the QAOA ansatz on IBM quantum processors and discuss optimal implementation for scalable QAOA. We test our implementations on Max-cut problems and compare them with previous works.

Photonic quantum technologies are noteworthy candidates in the achievement of quantum advantage for quantum information processing. Moreover, their capabilities for fast signal processing have attracted the interest of researchers in the field of quantum reservoir computing (QRC). In our research, we propose a scalable quantum photonic platform for QRC suitable for solving temporal tasks. In our platform, an optical pulse recirculating through an optical cavity creates a quantum memory, thus not needing external classical storage. A classical signal is sequentially encoded in the quantum field fluctuations of external optical pulses, which interact with the cavity pulse using a beam-splitter (BS). A nonlinear crystal is placed inside the cavity to generate non-trivial dynamics and create a quantum network of entangled modes. A homodyne detector is placed at one of the output paths of the BS for sequential data collecting. Our work focuses on the ability to process classical signals in real time and the noise robustness of our architecture.

RSA is considered the most commonly employed public key exchange algorithm for the cyber age, securing text messaging, email, and money transactions, among many other applications. The security of RSA largely depends on the fact that it is practically impossible to find the factors of a large integer N (thousands of digits long) using classical computing. The quantum algorithm of Shor permits optimal factorization of a large integer N in a polynomial time, and represents therefore a real threat to RSA and cybersecurity safeguards. Running this algorithm on a large N requires large number of qubits and reduced errors, which are not achievable with current quantum computers. This work presents a circuit implementation of the Shor’s algorithm on the IBM quantum lab Notebooks for the factorization of a small integer N encoded by four qubits. The circuit is based on the implementation of a Modular Exponentiation Function and the Quantum Fourier Transform. This circuit implementation consists of an optimized architecture with a reduced number of auxiliary qubits. We present physical constraints of the used IBM quantum hardware and discuss limitations of the circuit's depth and width.