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.

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.

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.

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.

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.

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.

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.

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.

Recent developments in quantum imaging have shown that resolution limits, previously thought to be fundamental to the specific optical system considered, are largely due to the suboptimal measurement scheme used. Moreover, surface roughness is a critical parameter in many disciplines, particularly in engineering fields, and its accurate estimation is of extreme importance. In this work, we develop a theoretical framework for describing rough surfaces and calculate bounds on the amount of information that can be gathered about the surface roughness using two different measurement schemes: Direct imaging, whereby the intensity of the field propagating from the surface is measured at the image plane; and Spatial mode demultiplexing, where the optical state is first decomposed into a spatial basis and the individual modes are then measured individually. We demonstrate in particular the superiority of mode sorting over direct imaging to extract information about relevant surface parameters in the regime of small roughness. This paves the way towards more accurate quantum-inspired optical techniques for quality inspection of manufactured surfaces in the sub-micrometre regime.