This PDF file contains the front matter associated with SPIE Proceedings Volume 12335, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Optical clocks can now achieve a higher stability and lower systematic uncertainty than the highest performance microwave atomic clocks. For a Trapped Ion Space Optical Clock (TISOC) project funded by the European Space Agency (ESA) we are developing an optical clock based on a trapped laser-cooled strontium ion for future deployment in space. In a laboratory setting, a ⁸⁸Sr⁺ system has been shown to provide excellent performance and crucially has reduced size, mass, laser power and complexity compared to alternatives such as lattice clocks. Spaceborne optical atomic clocks will offer transformative capabilities for future science, navigation, and earth observation programmes. As a first step towards space deployment, the design used in our existing single ion clocks was employed as a baseline to develop a set of finite element models. These were used to simulate the response of the ion trap and accompanying vacuum chamber to vibration, shock and thermal conditions encountered during launch and space deployment. Additionally, an electrostatic model has been developed to investigate the relationship between the ion trap geometrical tolerances and the trapping efficiency. We present the results from these analyses and how they have helped design a more robust prototype for experimental testing.

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 [5s₂¹S₀-5p¹P₁] 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.

Interferometric sensors provide an excellent opportunity for studying novel aspects of quantum mechanics. In this paper, we present the updated design for a suspended, cryogenically cooled table-top interferometer that can operate at the standard quantum limit of sensitivity. In this mode of operation, we will be able to probe aspects of macroscopic entanglement, quantum correlations, and semi-classical and quantum gravity models. We present the up-to-date experimental progress as well as the results of ongoing investigations.

The experimental investigation of fundamental theories of Physics nowadays is more and more related to the development of new cutting-edge technologies. We are exploiting the most advanced techniques used in gravitational waves detectors to search for quantum fluctuations of space-time as predicted by some theories of quantum gravity. For this purpose, we are building twin co-located interferometers that are targeted to achieve unprecedented levels of sensitivity. Exploiting quantum technology, like squeezed states of light, will allow further reduction of the detector’s noise and improve its sensitivity not only to quantum gravity phenomena, but to dark matter and high-frequency gravitational waves as well.

Igniting Questions, Detecting Answers; Hamamatsu Photonics introduces the world’s first photon-number-resolving scientific camera Hamamatsu introduces their scientific camera – the ORCA-Quest®, with incredibly low noise of 0.27 electrons rms and a high pixel count of 9.4 megapixels. In quantitative imaging, the photoelectric noise generated when light is converted into electrical signals is the all-important factor that determines the lower detection limit of a camera. The ORCA-Quest is able to reduce this photoelectric noise to a level below the signals generated by photons, making it the world’s first camera to achieve 2D photon-number-resolving measurement, meaning that it accurately measures the number of photons within each pixel.

We present the design and status of a detector to search for axions and axion-like particles in the galactic halo using quantum-enhanced interferometry. The operating principle is related to previously reported ideas, but aims for axions in the mass range from 10⁻¹⁶ eV up to 10⁻⁸ eV. We also show how to apply squeezed states of light to enhance the sensitivity similar to the gravitational-wave detectors. This experiment has the potential to be further scaled up to a multi-kilometre long detector and to then set constraints of the axion-photon coupling coefficient of ∼ 10⁻¹⁸ GeV⁻¹ for axion masses of 10⁻¹⁶ eV, or detect a signal.

An inverse design in electromagnetic method based on the topological derivative approach is presented. Topological derivative method is used to measure the sensitivity of a given functional with respect to an infinitesimal perturbation in a domain. The topological derivative concept has been successfully applied in many relevant fields such as geophysics, multi-scale material design and inverse problems. In this work, to design the electromagnetic devices an objective function dependent on the scattering parameters is considered. Finally, numerical results are presented to illustrate the performance of the optimisation approach.

Satellite based Quantum Key Distribution (QKD) in Low Earth Orbit (LEO) is currently the only viable technology to span thousands of kilometres. Since the typical overhead pass of a satellite lasts for a few minutes, it is crucial to increase the the signal rate to maximise the secret key length. For the QUARC CubeSat mission due to be launched within two years, we are designing a dual wavelength, weak-coherent-pulse decoy-state Bennett- Brassard ’84 (WCP DS BB84) QKD source. The optical payload is designed in a 12×9×5cm³ bespoke aluminium casing. The Discrete Variable QKD Source consists of two symmetric sources operating at 785 nm and 808 nm. The laser diodes are fixed to produce Horizontal,Vertical, Diagonal, and Anti-diagonal (H,V,D,A) polarisation respectively, which are combined and attenuated to a mean photon number of 0.3 and 0.5 photons/pulse. We ensure that the source is secure against most side channel attacks by spatially mode filtering the output beam and characterising their spectral and temporal characterstics. The extinction ratio of the source contributes to the intrinsic Qubit Error Rate(QBER) with 0.817±0.001%. This source operates at 200MHz, which is enough to provide secure key rates of a few kilo bits per second despite 40 dB of estimated loss in the free space channel.¹

Quantum Key Distribution (QKD) has the potential to secure indoor optical wireless links. In a typical room scenario, an indoor optical wireless link will have a transmitter on the ceiling and a receiver on a desk. Ambient light from room (typically LED) lighting on the ceiling and sunlight coming through the windows present a challenging environment for free-space QKD links to operate, and a key challenge is to mitigate the noise induced by ambient light, particularly sunlight. A combination of spectral and spatial filtering can be used to reduce the effect of ambient light, with a narrowband optical filter typically used. Moreover, the wavelength of operation is key to further reduce the impact of ambient light. Wavelengths in ‘quiet’ regions of the solar spectrum, such as the atmospheric absorption bands, are promising candidates. We are currently working on a system that operates at 1370 nm, where water and carbon dioxide absorption band in the atmosphere attenuate the solar spectrum substantially. This paper reports the design and modelling of the system, with a series of validation measurements to characterise the effect of solar radiation on a typical photon-counting detector as would be used in a QKD system. The aim of this work is to show the feasibility of the wavelength region around 1370 nm as a necessary step towards a low noise QKD receiver for indoor optical wireless links in a practical environment.

Quantum Key Distribution (QKD), a technology for growing mathematically secure key encryption keys, is now on the verge of becoming widely commercially available. Due to lack of standardization, multiprotocol QKD receivers are particularly beneficial for satellite QKD, so that an optical ground station is not limited to a sub-set of satellites. Moreover, if both transmitter and receiver can operate with different protocols, they will be able to actively adapt to specific conditions by choosing the most suitable protocol. In this work, we present the design and performance of a multiprotocol reconfigurable free-space QKD receiver. The reconfigurability relies on polarization-based optical routing, which can also be used to optimize the performance of time-bin QKD protocols.

Quantum key distribution (QKD) is a quantum communications protocol which provides the growth of encryption keys under guaranteed security. Due to the single-photon nature of many QKD protocols, QKD systems can be optically jammed by overwhelming a receiver with many photons at wavelengths at which the single photon detectors are responsive, causing a prohibitively high quantum bit error rate (QBER). In satellite QKD (SatQKD), which relies on communication during brief satellite visual contact, short jamming periods could prevent access to secure communications for much longer periods of time. Optical jamming (OJ) can be achieved both from within line-of-sight by targeting the receiver with a light source, or, in the case of downlink SatQKD, from without line-of-sight by reflecting OJ light off the transmitting satellite.1 The latter attack can be effective 1000km from the ground station, which presents challenges to the deployment of SatQKD in mission-critical applications. In this work, we present two investigations for OJ attacks on SatQKD. Firstly, we present an experimental demonstration utilizing SPAD array technology to locate and mitigate within line-of-sight OJ at long range. Secondly, we present simulations quantifying the effectiveness of without line-of-sight OJ against SatQKD systems and outline mitigation techniques inspired by RF communications. Implementation of the mitigation techniques will be essential for defence applications.

Quantum key distribution (QKD) offers the highest possible levels of communication secrecy. Using the laws of quantum mechanics, QKD protocols allow two distant parties to establish symmetric encryption keys that can be proven information theoretically secure. In order to make this technology accessible to a wide range of sectors, it is essential to address the questions of cost, volume production and compatibility with standard Telecom/Datacom infrastructures. While over the last few years, a number of works were devoted to the demonstration of photonic integrated circuits for quantum communications, a practical solution to interface these chips in a complete system remained an elusive goal. We review our efforts in integrating the core optical functions of quantum key distribution onto quantum photonic chips and in demonstrating the first standalone photonic integrated QKD system. Our approach tackles various system integration challenges related to packaging, optoelectronic design and power consumption. The quantum hardware is assembled in pluggable interconnects that guarantee efficient thermal management and forward compatibility of a same host electronics with successive generations of chips. Autonomous operation and long-term stability are demonstrated in realistic operation conditions. Our work offers new pathways for practical implementations of QKD and its viable deployment at large scales.

Quantum networks have been shown to connect users with full-mesh topologies without trusted nodes. We present advancements on our scalable polarisation entanglement-based quantum network testbed, which has the ability to perform protocols beyond simple quantum key distribution. Our approach utilises wavelength multiplexing, which is ideal for quantum networks across local metropolitan areas due to the ease of connecting additional users to the network without increasing the resource requirements per user. We show a 10 user fully connected quantum network with metropolitan scale deployed fibre links, demonstrating polarisation stability and the ability to generate secret keys over a period of 10.8 days with a network wide average-effective secret key rate of 3.38 bps.

We demonstrate the feasibility of a network-oriented Quantum Key Distribution (QKD) from affordable components, in the presence of White Rabbit time synchronisation on the same optical fibre. This allows for an optimal usage of resources, where optical networks are tailored for fast and accurate time distribution and lighter QKD systems exploit the network timing for synchronising the operations between distant users.

Gaussian modulated coherent state continuous variable quantum key distribution protocol is proven to provide high security while being compatible with classical coherent communication equipment. Here, we demonstrate a high-speed Gaussian modulated coherent state continuous variable quantum key distribution with a symbol rate of 40 MHz. Our system is consisted of field deployable data acquisition and processing hardware. We have also developed a software tool chain for recording data and parameter monitoring in real time. The data is then post processed to distill secret keys. We have achieved a secret key rate of 2.3 Mbps after all the post processing stages, at a transmission distance of 25 km.

We report the field trial of a Gaussian Modulated Coherent State (GMCS) Continuous Variable Quantum Key Distribution (CVQKD) system over 49.82km of railway trackside fibre. We have examined the effect of a moving train on the QKD system and implemented compensation methods, mainly for polarisation stabilization. The system generated 4kbps secure key, in the asymptotic limit, under the collective attacks. The CVQKD transmitter and receiver were deployed at the Rail Innovation and Development Centre (RIDC) at Melton Mowbray, UK, and remotely monitored and operated from the University of York, at York, UK.

The performance of quantum key distribution (QKD) is heavily dependent on the physical properties of the channel over which it is executed. Propagation losses and perturbations in the encoded photons’ degrees of freedom, such as polarisation or phase, limit both the QKD range and key rate. The maintenance of phase coherence over optical fibres has lately received considerable attention as it enables QKD over long distances, e.g., through phase-based protocols like Twin-Field (TF) QKD. While optical single mode fibres (SMFs) are the current standard type of fibre, recent hollow core fibres (HCFs) could become a superior alternative in the future. Whereas the co-existence of quantum and classical signals in HCF has already been demonstrated, the phase noise resilience required for phase-based QKD protocols is yet to be established. This work explores the behaviour of HCF with respect to phase noise for the purpose of TF-QKD-like protocols. To achieve this, two experiments are performed. The first, is a set of concurrent measurements on 2 km of HCF and SMF in a double asymmetric Mach-Zehnder interferometer configuration. The second, uses a TF-QKD interferometer consisting of HCF and SMF channels. These initial results indicate that HCF is suitable for use in TF-QKD and other phase-based QKD protocols.

Laser-based addressing units are core components for trapped-ion quantum computers, in which ions are held in defined, separate, micron-scale position fields and are brought into different qubit states via an addressing beam with an ion-specific wavelength. The spin of the valence electrons of the atomic ions represents the qubits states. An addressing action causes a stimulated Raman transition, which in turn causes a change in electron spin and thus a qubit rotation. Our unit is designed for 10 ⁴⁰Ca+ ions kept in a linear microchip ion trap. The hardware platform utilizes waveguides with integrated collimating micro lenses, followed by a cascaded 4f optical setup, consisting of a focusing and a beam expander unit. An aspherical lens realizes a lateral shift tolerant focus inside the trap. The addressing unit achieves minimal foci beam waists of 1.1 μm, with a minimal ion separation of 5.31 μm. The operating addressing wavelength ranges from 395 nm to 405 nm. Fault relevant ion crosstalk ratio is less than 10⁻⁴. Addressing operations can be conducted in parallel on all captured ions. The specified laser-based addressing unit provides a stable and low-error solution for qubit operations with coherence times of several seconds. Further adaptions of the waveguide structures and the implementation of micro optical lenses offer the potential of fully integrating the system into the microchip ion trap. Such a quasi-monolithic trap integration constitutes a compact, and highly scalable addressing and detection system with the ability to address as many qubit registers as the ion trap capacity allows.

Nitrogen-vacancy (NV) pair in diamond has been widely investigated due to its profound quantum properties even at relatively higher temperature compared with those materials having to satisfy strict cryogenic conditions to display quantum states. Spin properties of NV center in diamond can be tuned by external electromagnetic radiations, which leads to its application in detecting microwave signals. Here in this work, density functional theory (DFT) method has been used to analyze electronic properties of diamond with NV centers with the aim of utilizing it for detecting very weak microwave radiations from electron cryotron radiations, to ultimately determine neutrino mass.

Recently both experimental and theoretical works have shown optically addressable molecular spins could have a great potential for quantum information processing. Experimental works such as spin qubit initialisation, coherence control, and readout suggest spin-bearing molecules can be a great candidate for quantum computing. Time-resolved electron spin resonance on molecular radicals at high temperature indicates molecular spins could be the cornerstones for high-temperature quantum gate operations, thus overcoming the low-temperature technical barrier for maintaining quantum circuits effectively. In this proceeding, we have discussed the potential of molecular materials, especially two dimensional molecular network, for optically driven quantum information processing, in combination with nanophotonic devices. Although this is only a theoretical proposal, we hope this can be inspiring for the future development of quantum computing. Obviously there are many difficulties on the way forward, such as single spin readout in molecules, optimal design of molecular networks and corresponding optical instruments, which are be solved in the future.