This PDF file contains the front matter associated with SPIE Proceedings Volume 11881, including the Title Page, Copyright information, and Table of Contents.

Since they were first introduced, quantum communications have evolved into a mature technology that is making its way to market. Quantum key distribution (QKD), in particular, allows the exchange of information with security that stems from the very laws of quantum mechanics and hence is not impaired by future technological advances. These same laws, however, restrict the QKD transmission range, as they forbid a straightforward amplification of quantum signals. This leads to a fundamental rate-distance bound known as the “repeaterless secret key capacity” (SKC0), which was thought to be impossible to overcome without a full-fledged quantum repeater. The recent proposal of “Twin-Field QKD” challenged this belief and showed how to overcome the SKC0 with an effective quantum repeater, realisable with present-day technology. This allows to perform QKD over long-distance and high-loss channels. In my talk, I will review the main concepts and most significant results behind this novel idea.

A source of bright and pure single photons is an essential tool for photonic quantum technologies, predicted to enable secure communication and networking. Self-assembled quantum dots are among the leading candidates to realise such a source. The currently highest-performing quantum dots emit at wavelengths unsuitable for fibre transmission, with telecom quantum dots lagging in performance. An intermediate step is to use quantum frequency conversion. Here we report to our knowledge the brightest quantum dot based source of telecom photons by frequency converting a near-infrared quantum dot embedded in a micropillar cavity to the telecom C-band. In a single-photon BB84 protocol, this source is capable of producing asymptotic keys rates of 1 kbps at over 150 km of optical fibre.

Quantum enhanced receivers are endowed with resources to achieve higher sensitivities than conventional technologies. For application in optical communications, they provide improved discriminatory capabilities for multiple non-orthogonal quantum states. In this work, we propose and experimentally demonstrate a new decoding scheme for quadrature phase-shift encoded signals. Our receiver surpasses the standard quantum limit and outperforms all previously known non-adaptive detectors at low input powers. Unlike existing approaches, the receiver only exploits linear optical elements and on-off photo-detection. This circumvents the requirement for challenging feed-forward operations that limit communication transmission rates and can be readily implemented with current technology.

Developing global quantum communication networks is integral to the realisation of the quantum internet, which is expected to impart a similar revolutionary impact on the technological landscape as the classical internet. Satellite-based quantum communications provides a practical route to global quantum networking. In this work, we model finite statistics to determine the finite secret key length generation in SatQKD systems that implement trusted-node downlink operation with weak coherent pulse sources. We optimise the finite key rate for different practical operations and determine the key generation footprints. Our work provides an essential guide for future satellite missions to establish performance benchmarks for both sources and detectors.

We present a miniaturised free-space quantum key distribution (QKD) system which allows key exchange between a handheld transmitter and a fixed terminal. The QKD system requires to be optically aligned emphasising the need of a beamsteering unit for later applications. To maintain within the size, weight and power restrictions, the active beamsteering hardware is exclusively located inside the receiver. Our target is consumer use so we present rigorous characterisation against a range of background light levels to show anticipated performance outside of a laboratory environment. Experimental results show a reduction in the raw count rate commensurate with the transmission of the added components (74.5%) and a small degradation of the error rate (0.5 percentage points) due to the worse signal-to-noise ratio. These combine to a 50% reduction in estimated secret key rate of the system with the additional components for beam steering.

Medium-range terrestrial free-space quantum key distribution systems enable widespread secure networked communications in dense urban environments, where it would be infeasible to install a large number of short optical fibre links. Such networks need to perform over a wide range of conditions and their design has to balance key rate maximisation versus robust key generation over the greatest range of circumstances. Practicalities, such as manufacturability and deployment, further constrain the design space. Here, we examine challenges in translating experiment into engineering reality and identify efficient BB84 weak coherent pulse-decoy state protocol parameter regimes suitable for medium-range QKD systems considering likely system performance and environmental conditions.

Applications of quantum key distribution are becoming more diverse due to the increase in interest in the secure key sharing protocol. Transmission through free-space channels has risen in popularity in recent years, primarily due to global coverage using satellite platforms. However, free-space channels come with challenges that need to be addressed, such as; diffraction loss, background noise, pointing-and-tracking, and atmospheric aberration. Novel design and use of state-of-the-art detector technologies in quantum receivers can help alleviate these difficulties. This paper presents and discusses the implementation of 2D single-photon sensor technology for free-space quantum key distribution. We present an experimental method that utilizes independent single-photon avalanche diode pixel read-out to reduce background noise contributions while simultaneously increasing optical field-of-view. Finally, we show and discuss single-photon level beaconing capabilities for pointing and tracking.

Optical interconnect technology has been adopted for over half a century. Today approximately one billion optical connectors and interconnects are deployed annually, and the growth is expected to increase as more technologies incorporate optical and photonic components. Although its basic function (the conveyance of photons over a channel) has remained the same, optical interconnect has been constantly evolving and innovating over the decades to push the boundaries of performance. In this paper we report on the evolution of optical interconnect technology from its initial applications in long haul telecommunications links to its future application to quantum networks.

Future quantum networks will provide multi-node entanglement enabling secure quantum communication on a global scale. Traditional two-party quantum key distribution (2QKD) consumes pairwise entanglement which is costly in constrained networks. Quantum conference key agreement (QCKA) leverages multipartite entanglement within networks to directly produce identical keys among N users, providing up to N-1 rate advantage over 2QKD. Here, we present a four-user QCKA protocol using photonic GHZ states distributed over fibre with combined lengths up to 50 km. Furthermore, we investigate a constrained network consisting of a 6-qubit photonic graph state which we apply network coding routines to distil suitable resource states.

Quantum networks have begun to connect many users together with Quantum Key Distribution links. We present a scalable, full mesh, polarisation entanglement-based quantum network without trusted nodes. We discuss our progress towards building a dynamic quantum network with more users, long distance (≈50 km) links and improved polarisation stability in the optical fibres. Lastly, minimising the resource overhead and optimising the network control based on end-user requirements are important features we are incorporating into our network.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible.

We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for quantum sensors such as optical clocks and cold-atom interferometry systems. This includes the [5s²S_1/2-5p²P_1/2] cooling transition in strontium⁺ ion optical clocks at 422 nm, the [5s₂ ¹S₀-5p¹P₁] cooling transition in neutral strontium clocks at 461 nm and the [5s²s_1/2 − 6p²P_3/2] transition in rubidium at 420 nm.

Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

Single-photon avalanche diode (SPAD) detectors are of significant interest for numerous applications, including light detection and ranging (LIDAR), and quantum technologies such as quantum-key distribution and quantum information processing. Here we present a record low noise-equivalent-power (NEP) for Ge-on-Si SPADs using a pseudo-planar design, showing high detection efficiency in the short-wave infrared; a spectral region which is key for quantum technologies and hugely beneficial for LIDAR. These devices can leverage the benefits of Si avalanche layers, with lower afterpulsing compared to InGaAs/InP, and reduced cost due to Si foundry compatibility. By scaling the SPAD pixels down to 26μm diameter, a step change in performance has been demonstrated, with significantly reduced dark count rates (DCRs), and low jitter (134ps). Ge-on-Si SPADs were fabricated using photolithography techniques and characterised using time-correlated single-photon counting. The DCR reaches as low as kilocount/s at 100K for excess bias up to ~5%. This reduction in DCR enables higher temperature operation; e.g. the DCR of a 26μm diameter pixel at 150 K is approximately equivalent to a 100 μm diameter pixel at 77 K (100s of kilocounts/s). These low values of DCR, coupled with the relatively temperature independent single photon detection efficiencies (SPDE) of ~29% (at 1310nm wavelength) leads to a record low NEP of 7.7×10⁻¹⁷WHz^−1/2. This is approximately 2 orders of magnitude lower than previous similarly sized mesa-geometry Ge-on-Si SPADs. This technology can potentially offer a lowcost, Si foundry compatible SPAD operating at short-wave infrared wavelengths, with potential applications in quantum technologies and autonomous vehicle LIDAR.

International standardisation will play a crucial role in accelerating the commercial adoption of quantum technologies. We report on the activities in mainstream international standards bodies to standardize different aspects of quantum technologies and identify where standards will be most relevant and will not impede future innovation.

The QSNET consortium is building a UK network of next-generation atomic and molecular clocks that will achieve unprecedented sensitivity in testing variations of the fine structure constant, α, and the electron-to-proton mass ratio, μ. This in turn will provide more stringent constraints on a wide range of fundamental and phenomenological theories beyond the Standard Model and on dark matter models.

The Atom Interferometer Observatory and Network (AION) is an experimental programme that uses quantum sensors based on cold strontium atoms to search for ultra-light dark matter, to explore mid-frequency gravitational waves and to probe other frontiers in fundamental physics. AION will complement other planned searches for dark matter and probe mergers involving intermediate-mass black holes and explore early-universe cosmology.

Seven leading UK institutions have embarked on a programme to construct a series of atom interferometers with baselines stretching from 10 m to 1 km. Building upon existing partnerships with the UK National Quantum Technology Hub in Sensors and Timing, the MAGIS Collaboration in the US and the Fermi National Accelerator Laboratory, the AION project will share many technical features with existing systems and, in particular, with the MAGIS experimental programme. In addition, AION will benefit from operating in a network with its US counterpart, as well as with other atom interferometers such as MIGA, ZAIGA and ELGAR as part of a new generation of experiments designed to search for new fundamental physics.

There is significant interest in potential experimental tests of macroscopic quantum effects, both to test potential modifications to quantum theory and to probe the quantum nature of gravity. A proposed platform with which to generate the required macroscopic quantum spatial superposition is a nanodiamond containing a negatively charged nitrogen vacancy (NV-) centre. In this review, methods to fabricate nanodiamonds containing NV­suitable for these quantum applications are discussed. The proposed probes of the macroscopic limits of quantum theory are presented along with the spin physics of the NV- centre relevant to those tests.

The sensitivity of atom interferometers depends on the fidelity of the light pulses used as beamsplitters and mirrors. Atom interferometers typically employ pulses that affect π/2 and π fractional Rabi oscillations, the fidelities of which are reduced when there are variations in atomic velocity and laser intensity. We have previously demonstrated the application of optimal control theory to design pulses more robust to such errors; however, if these variations exhibit a time dependence over periods on the order of the interferometer duration then phase shifts can be introduced in the final fringe that potentially reduce the sensitivity. In this paper, we explain why care must be taken when optimising interferometer pulse sequences to ensure that phase shifts arising from inter-pulse variations are not significantly increased. We show that these phase shifts can in fact be minimised by choosing an appropriate measure of individual pulse fidelity.

Alternatives to focal-plane detector arrays have emerged in recent years and are now being widely investigated. One of the most promising of these are single-pixel imaging techniques. Single-pixel techniques recreate a scene using the knowledge of projected patterns and the measured backscattered signals. This research investigates the implications of using idealised patterns, Fourier-transformed patterns and camera-captured patterns generated by plane wave decomposition methods. Hadamard patterns are projected into the far-field of the phased-array modulator source and used for robust reconstruction in a reflective arrangement. The choice of the optimal pattern sets from these sources are used for single-pixel imaging reconstruction and compressed sensing. The technique is robust to poor signal-to-noise conditions and is applicable in cases where a limited number of measurements are possible. Our technique and methodology can be further applied to any region of the electromagnetic spectrum where phased-array sources are available, such as in the radar regime.

A low-cost, mass-producible laser-cooling platform would have a transformative effect in the burgeoning field of quantum technologies and the wider research of atomic sensors. Recent advancements in the micro-fabrication of diffractive optics and vacuum apparatus have paved the way for a simple, stackable solution to the laser cooling of alkali atoms. In this paper we will highlight our recent investigations into a chip-scale, cold-atom platform, outlining our approach for on-chip wavelength referencing, examining a solution for imaging atoms in a planar stacked device, and finally discussing the limitations to passively pumped vacuum longevity. These results will be discussed in the context of an outlined road-map for the production and commercialisation of chip-scale, cold-atom sensors.

The accurate control of magnetic fields is a cornerstone of multiple emerging quantum technologies. These technologies often require passive high permeability magnetic shielding and internal active field-generating coils to create their own bespoke magnetic field landscape. However, magnetic fields generated by coils are distorted by high permeability shielding, preventing the accurate and efficient generation of the desired field environment. Here, we design a cylindrical four-layer magnetic shield with an interior hybrid active-passive coil system that is explicitly optimised to include the electromagnetic coupling between the active and passive components. We use a combination of analytical methods and numerical simulations to determine the shield parameters - geometry, thickness, and access hole positions - to maximise the passive shielding efficiency and minimise the shield-induced Johnson noise and weight. Then, we apply an analytical formulation of the magnetic field, which accounts for the interaction with the magnetic shield, to design nine orthogonal hybrid active-passive field-generating coils inside the shield. The coils will be manufactured on thin low-via flex-PCBs near the shield's interior surface and generate three uniform fields and six gradient fields that deviate by less than 0.4% and 2%, respectively, over an internal cylindrical region extending over half the diameter and length of the innermost shield layer. These hybrid active-passive coils can accurately remove deviations in the background field or generate various complex magnetic field landscapes. Consequently, the hybrid shield provides an ideal platform for miniaturising and commercialising quantum technologies that require precisely-controlled magnetic fields within a low-noise environment.

We demonstrate two-photon interference and polarization entanglement at 2090 nm, constituting a crucial leap towards free-space mid-infrared quantum communication systems in a spectral region with high atmospheric transparency and reduced solar background.

Optical activity is a macroscopic property of chiral molecules which manifests as a rotation of the plane of linear polarization when light passes through a sample. We have developed a compact Bell inequality experiment for quantum probing of chiral liquids, based on polarization measurements. In particular, we show that we can use a Bell-type inequality configuration to measure the optical activity of D-Limonene, a chiral molecule which is a major component in the oil of citrus fruit peels.

Travelling wave parametric amplifiers (TWPAs) made from highly nonlinear reactive superconducting thin films have been demonstrated to be a viable technology for various quantum applications, including fundamental physics experiments such as astronomy and axion dark matter searches, as well as commercial applications like quantum computational and communication systems. In this paper, we present the design of a kinetic inductance TWPA comprising a patterned titanium nitride film that can operate at 0.3K to demonstrate the feasibility of operation closer to 1K temperature, paving the way to achieve even higher bath temperature operation. We discuss in detail the design of our TWPA, along with the predicted gain-bandwidth product and other characteristics. We perform the preliminary experimental investigation of the thin film properties and compare that with the simulated results. We found that there are several discrepancies between the measured and the predicted behaviour of the thin film. We attribute these differences to the fact that the fabricated thin film has a different gap voltage, resistivity and thickness to what we expected. With a new set of estimated parameters, we successfully reproduce the measured transmission profile. We further show that by utilising bridges to ensure equipotential grounds for the CPW lines, we are able to reduce the rippling effect and achieve a higher gain with broader bandwidth. We expect that our TWPA can achieve higher than 20 dB gain from approximately 0–8 GHz.

Photonics integration is a key technology for realizing large-scale photonic quantum information processing. We demonstrate state-of-art reconfigurable photonic processors based on low-loss silicon nitride waveguide networks. We present the science behind such a processor, which consists of a large mesh of integrated reconfigurable Mach Zehnder interferometers. In this talk, we will present the newest results of the current generation of our programmable quantum photonic processors obtained by classical as well as quantum optical characterization. Furthermore, we show the challenges of scaling up quantum photonic processors and the range of potential applications of large-scale quantum information processing those will enable.

Broadband ultra-low noise amplification is important for many fundamental physics experiments. In our case, we aim to develop a quantum limited Josephson Travelling Wave Parametric Amplifier (JTWPA) for dark matter search experiments. In this paper, we focus on the development of the JTWPA, in particular to optimise the performance of the amplifier with a simplified fabrication prospect. We present our methodology for the optimisation process, focusing on three important aspects, namely utilising the minimal number of tunnel junction required, maximising the operational bandwidth and achieving a 50 Ω characteristic impedance. We first explore the relations between the important circuit parameters of the JTWPA and its performance indicators. Using the information obtained, we perform our optimisation process to search for the optimal design parameters. We then compare the gain bandwidth performance of the different optimised models, and present our findings with further analyses. We demonstrate that our optimised model requires 4 less tunnel junctions compared to the conventional model, and we are able to improve the operational bandwidth by 68% while maintaining the characteristic impedance of the device at 50 Ω.