The results of a marine trial for a cold atom system for inertial navigation (HARLEQUIN) is presented. The system is a deployable atom interferometer demonstrator for navigation using inertial measurements. The interferometer is a grating based, Mach-Zehnder interferometer using rubidium-87 to make measurements of acceleration in a single axis. The output of the quantum sensor is used to condition classical sensors, in a quantum-classical hybrid measurement scheme. The subsystems of HARLEQUIN have been tested aboard a marine platform operated by the UK General Lighthouse Authority, to determine its performance in marine environments and informing an upgrade path to a fully deployable quantum sensor. Presented are results from the trial and upgrade path towards a fully deployable quantum sensor.

Optically pumped magnetometers (OPMs) have emerged as a powerful technique for high-resolution magnetic field imaging. However, achieving sub-millimeter spatial resolution at sub-picotesla sensitivities ( < 1 pT / Hz ) remains challenging, particularly under finite-field conditions. We present a high-resolution magnetic imaging system based on a free-induction-decay (FID) OPM integrated with a two-axis scanning micromirror for automated beam steering. The double-pass optical configuration allows millimeter-scale devices under test (DUTs) to be positioned directly behind the vapor cell. This enables a standoff distance of 2.7 mm between the magnetic source and the atomic vapor, improving practical imaging resolution by increasing the amplitude of near-field magnetic signals sampled within the sensitive volume. Spatial resolution is experimentally demonstrated by imaging a custom printed circuit board (PCB) containing antiparallel copper tracks spaced 2 mm apart, with measured field maps in close agreement with Biot-Savart predictions. The OPM achieves an optimal field sensitivity of 0.5 PT / Hz , demonstrating the system's capability for high-precision magnetic field measurements. The imaging system is further validated by resolving polarity-dependent asymmetries in a bridge rectifier integrated circuit (IC) and tracking current dynamics in a ceramic battery in situ. These results highlight the potential of OPM-based systems for noninvasive diagnostics of electronic circuits and batteries.

Laser-cooled atoms are increasingly being used to realise practical quantum devices, motivating the development of compact and robust atom sources. Grating magneto-optical traps (gMOTs) simplify the cold-atom source architecture but are typically vapour-loaded and provide limited atomic flux. Here we explore the loading of gMOTs from cold-atom beams. We numerically simulate loading to show that unbalanced diffracted beams deflect incoming atoms away from the trap centre, thereby strongly constraining radial loading. In contrast, axial loading injects atoms directly into the trapping volume and largely avoids these effects. We experimentally demonstrate rapid axial loading of a gMOT, achieving loading rates of 2.1 × 10^9 atoms s−1 using a moving optical molasses to transfer atoms from a 2D MOT into the gMOT. These results establish axial loading as a robust route to high-flux gMOT operation for portable cold-atom systems.

Increasing the resilience of positioning systems that currently rely on Global Navigation Satellite System (GNSS) signals can be achieved by incorporating stable and sensitive measurements of the permanent crustal anomalies in the Earth's magnetic field. We have realised this concept using an in-house-developed, wearable, Free-Induction-Decay Optically Pumped Magnetometer (FID-OPM) to carry out precise and stable measurements of the geomagnetic field in a walking trial. We present an end-to-end validation, including qualification of FID-OPM performance, alongside quantification of improvement in accuracy when data from this sensor is added to a dead-reckoning estimation of position. Using our wearable sensor system we achieve a Beckmann-distributed radial positioning error of 2.24 m over a route exceeding 500 m in length and spanning approximately 360 s.

Nanofabrication using focused ion beam and reactive ion etching techniques was conducted on single-crystal aluminium substrates with three crystallographic orientations: (111), (100), and (110). Among these, the (110) surface exhibited the highest etch rate of 15±2 nm/min and the lowest surface roughness of 2±0.5 nm using reactive ion etching. This enhanced understanding of how crystal orientation affects surface quality is expected to contribute significantly to the advancement of low-loss diffractive optical elements across the infrared to extreme ultraviolet spectral ranges. This is the first time that etching on single-crystal aluminium has been investigated with focused ion beam and reactive ion etching, providing references for potential optical device fabrication.

We present a method for absolute calibration of received radio frequency in the ultra-low frequency and very low frequency range. This is achieved with the use of a radio frequency optically pumped magnetometer. We describe a method using an optically pumped sample where the RF broadening of the Cs magnetic resonance allows the magnitude of the received field to be calibrated against the ground-state gyromagnetic ratio of the Cs atoms. This frequency-based calibration avoids the geometric and electrostatic response functions that affect inductive sensors, such as fluxgates, search coils, and SQUID magnetometers. We demonstrate the calibration of magnetic measurement using oscillating magnetic fields in the 300 Hz-20 kHz range and a sensor noise floor of 15 fT Hz-1/2. This radio frequency sensor may be used as a widely tunable narrowband receiver for communication, ranging, or penetrative conductivity imaging.

Abstract Zero-field optically pumped magnetometers (OPMs) represent a promising modality in biomagnetism research due to their ultra-sensitive capabilities contained within compact sensor packages that enable contactless biomagnetic measurements. In this study, we utilized a zero-field OPM to simultaneously measure three-axis components of biomagnetic fields produced by muscle activity in the human forearm caused through independent finger movements, with the goal of recognizing multiple gestures using only a single magnetic sensor. We recorded muscle activity caused by the opening and closing of each finger 60 times, triggered by an auditory tone, and trained a neural network to classify the different finger movements. We utilized the triaxial OPM and trained neural network in a live-testing mode to replicate the participant’s hand gestures onto a robotic-hand proxy, with $$\approx 1~$$ s delay, firstly for three fingers (little, middle, and thumb) and subsequently for all five fingers. Using a single OPM sensor, we captured, recognized, and replicated finger gestures from one participant in a controlled laboratory setting. In the three-gesture live test, all three finger movements were identified and replicated with ≥ 98% accuracy. In the five-gesture live test, four out of five fingers were identified and replicated with ≥ 90% accuracy, with the little finger being the most challenging to distinguish with a reduced 45% accuracy. These results demonstrate that a single zero-field OPM sensor in a triaxial measurement scheme can be effectively used for contactless recognition and replication of multiple finger movements, providing a proof-of-concept toward future wearable OPM-based magnetomyography systems.

Doppler-broadening thermometry (DBT) can be used as a calibration-free primary thermometer suitable for practical applications, e.g. reliably measuring temperatures over long periods of time in environments where sensor retrieval for recalibration is impractical. We report on our proof-of-concept investigations into DBT with alkali-metal-vapour cells, with a particular focus on both absorption and frequency accuracy during scans. We reach sub-kelvin temperature accuracy, and experimental absorption-fit residuals below 0.05%, in a simple set-up. The outlook for portable, practical devices is bright, with clear prospects for future improvement. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.

An optical atomic frequency-reference based on a vapour of atoms is a strong candidate for a creating a portable atomic clock with sufficient stability to be a reliable independent timing system for major infrastructure in the event of GNSS denial. We present the development of optical atomic frequency-standards within the University of Strathclyde, where MEMS vapour cells and lasers have been developed for a miniaturised rubidium frequency-reference. Existing chip-scale systems have been demonstrated by others using a microwave transition, however an optical clock offers a significantly higher frequency to enable a lower instability.

Ultracold atoms are crucial for unlocking truly precise and accurate quantum metrology and provide an essential platform for quantum computing, communication, and memories. One of the largest ongoing challenges is the miniaturization of these quantum devices. Here, we show that the typically macroscopic optical lattice architecture at the heart of many ultraprecise quantum technologies can be realized with a single-input laser beam on the same diffractive chip already used to create the ultracold atoms. Moreover, this inherently ultrastable platform enables access to a plethora of new lattice dimensionalities and geometries, ideally suited for the design of high-accuracy, portable quantum devices.

Electro-optic modulators are widely used for the generation of optical sidebands for various applications. Here, we report on a technique enabling control of the relative amplitudes of optical sidebands generated by electro-optic modulators. The technique makes use of a phase modulator and Mach-Zehnder amplitude modulator, connected in series to break the symmetry of the sideband amplitudes. The generated optical sideband spectrum can be controlled by the two radio frequency (RF) modulation amplitudes, the attenuation level of the amplitude modulator, and the relative RF phase of the two modulations. We demonstrate near-complete suppression of one first-order sideband and with simultaneously achieving equal amplitudes of the carrier and the other first-order sideband with > 94% purity. The technique can be utilised to produce a spectrum with effectively two frequency components. This enables application in atomic physics experiments such as the minimisation of off-resonant light shifts that can limit the performance of atomic clocks and interferometers. We demonstrate operation in the near-infrared, at 795 nm, using commercial-off-the-shelf components and circumventing the need for frequency doubling of lasers in the optical communications region.

Two emerging developments in photon based (quantum) thermometry are described, namely, active ring-resonator thermometry (ARRT) and small-scale practical Doppler broadening thermometry (pDBT). These developments are sensing methodologies that are frequency based and directly linked to the physics of the measurement approach. Through relying on the physics instead of external calibration to provide reliable thermometry direct traceability to the kelvin is obtained. The medium-term objective of this work is to develop practical primary thermometers that need no external calibration, having modest uncertainties (0.1–1 K) but able to provide reliable thermodynamic temperatures, during the lifetime of the required measurements. The long-term objective is to develop these (and possibly other) photon-based practical primary temperature measurement methods that can be widely deployed, supplanting conventional techniques but providing reliable permanent traceability to the kelvin in the measurement setting.

Diffraction gratings have simplified the optical implementation of magneto-optical traps (MOTs) to require only a single input beam, however reaching high atom number and fast loading has proven to be a challenge. We equipped an atom chip with a grating surface and paired it with a velocity-tunable 2D+-MOT as an atomic source to facilitate efficient loading together with magnetic trapping. Using uniform grating illumination, we magneto-optically trap 1.0(1)x109atoms within one second, cool them to 14.1(3)mu K, and transfer a quarter of them into the magnetic chip trap. This is a key step towards simple portable quantum sensors employing (ultra-)cold atoms.

A cold atom system for an inertial navigation system (INS) demonstrator utilising atom interferometry is presented. Laser-cooled rubidium-87 atoms in a grating magneto-optical trap (gMOT) are used to measure acceleration along a single axis. This system demonstrates a novel technique in quantum-enabled navigation which could offer significant improvement in precision and reduction in the drift present in classical INSs. Ruggedised lasers and control electronics allow potential deployment in maritime navigation in global navigation satellite system (GNSS) denied environments. Considerations are made towards a pathway for modular expansion and development of the system to 6-axis operation.

We present compact, robust cold-atom platforms based on grating MOTs, highlighting recent innovations in gMOT technology, their integration with vacuum systems, and how these simplified systems are being applied to quantum sensing applications.

We report initial research to develop a compact and practical primary thermometer based on Doppler broadening thermometry (DBT). The DBT sensor uses an intrinsic property of thermalized atoms, namely, the Doppler width of a spectral line characteristic of the atoms being probed. The DBT sensor, being founded on a primary thermometry approach, requires no calibration or reference, and so in principle could achieve reliable long-term in-situ thermodynamic temperature measurement. Here we describe our approach and report on initial proof-of-concept investigations with alkali metal vapour cells. Our focus is to develop long-term stable thermometers based on DBT that can be used to reliably measure temperatures for long periods and in environments where sensor retrieval for re-calibration is impractical such as in nuclear waste storage facilities.

Spatial variation in the intensity of magnetospheric and ionospheric fluctuations during solar storms creates ground-induced currents, of importance in both infrastructure engineering and geophysical science. This activity is presently measured using a network of ground-based magnetometers, typically consisting of extensive installations at established observatory sites. We show that this network can be enhanced by the addition of remote quantum magnetometers which combine high sensitivity with intrinsic calibration. These nodes utilize scalable hardware and run independently of wired communication and power networks. We demonstrate that optically pumped magnetometers, utilizing mass-produced and miniaturized components, offer a single scalable sensor with the sensitivity and stability required for space weather observation. We describe the development and deployment of an off-grid magnetic sensing node, powered by a solar panel, present observed data from periods of low and high geomagnetic activity, and compare it to existing geomagnetic observatories.

The microfabrication of cesium vapor cells for optically pumped magnetometry relies on optimisation of buffer gas pressure in order to maximise atomic coherence time and sensitivity to external magnetic signals. We demonstrate post-bond nitrogen buffer gas pressure tuning through localized heating of an integrated micro-pill dispenser. We characterize the variation in the intrinsic longitudinal relaxation rate, $\gamma_{10}$, and magnetic sensitivity, as a function of the resulting nitrogen buffer gas pressure. Measurements are conducted through employing an optically pumped magnetometer operating in a free-induction-decay configuration. $\gamma_{10}$ is extracted across a range of nitrogen pressures between $\sim$60-700Torr, measuring a minimum of 140Hz at 115Torr. Additionally, we achieve sensitivities as low as 130fT/$\sqrt{\text{Hz}}$ at a bias field amplitude of $\sim 50${\mu}$T. With the optimal nitrogen buffer gas pressure now quantified and achievable post-fabrication, these mass-producible cells can be tailored to suit a variety of sensing applications, ensuring peak magnetometer performance.