arXiv subjects

Erling Riis

Publications and source records attributed to Erling Riis.

At least 19 recordsLinked to original sources

arXiv2026

Field validation of GNSS-independent positioning enhancement using a wearable ultra-stable quantum magnetometer

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.

physics.atom-ph↗

arXiv2026

Rapid axial loading of a grating MOT with a cold-atom beam

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 $2.1 \times 10^9$ 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.

physics.atom-ph↗

arXiv2026

Intrinsic atomic calibration of oscillating magnetic fields in ULF and VLF bands

We present a method for absolute calibration of received radio-frequency in the ultra low frequency (ULF), and very low frequency (VLF) range. This is achieved with the use of a radio frequency optically pumped magnetometer (RF-OPM). 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 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.

physics.atom-ph↗

arXiv2025

High-Resolution Atomic Magnetometer-Based Imaging of Integrated Circuits and Batteries

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 ($\mathrm{< 1\,pT/\sqrt{Hz}}$) 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 $\mathrm{0.5\,pT/\sqrt{Hz}}$, 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.

physics.atom-ph↗

arXiv2025

Practical primary thermometry via alkali-metal-vapour Doppler broadening

Doppler-broadening thermometry (DBT) can be used as a calibration-free primary reference suitable for practical applications, e.g. reliably measuring temperatures over long periods of time in environments where sensor retrieval 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 setup. The outlook for portable, practical devices is bright, with clear prospects for future improvement.

physics.atom-ph↗

arXiv2024

A triaxial vectorization technique for a single-beam zero-field atomic magnetometer to suppress cross-axis projection error

Zero-field optically pumped magnetometers (OPMs) have emerged as an important technology for biomagnetism due to their ulta-sensitive performance, contained within a non-cryogenic small-scale sensor-head. The compactness of such OPMs is often achieved through simplified detection schemes, which typically provide only single-axis magnetic field information. However, multi-axis static magnetic fields on non-measurement axes cause a systematic error that manifests as amplitude and phase errors across the measurement axis. Here we present a triaxial operational technique for a compact zero-field OPM which suppresses multi-axis systematic errors through simultaneous measurement and closed-loop active control of the static magnetic fields across all axes. The demonstrated technique requires magnetic modulation across two axes while providing static field information for all three axes. We demonstrate this technique on a rubidium laboratory-based zero-field magnetometer, achieving a bandwidth of 380 Hz with sensitivities of $<25$ fT/$\sqrt{\rm{Hz}}$ across both transverse axes and $65$ fT/$\sqrt{\rm{Hz}}$ along the beam axis. Using the proposed triaxial technique, we demonstrate precise tracking of a 2 Hz triaxial vector test signal and suppression of systematic cross-axis projection errors over an extended period, $\simeq20$~min.

physics.atom-ph↗

arXiv2024

A high-flux cold-atom source utilising a grating atom chip

Bose-Einstein condensates (BECs) have been proposed for many applications in atom interferometry, as their coherence over long evolution times promises unprecedented sensitivity. To date, BECs can be efficiently created in devices using atom chips, but these are still complex and place high demands on size, weight and power. To further simplify these setups, we equipped an atom chip with a nano-structured diffraction-grating to derive all beams for the magneto-optical trap (MOT) from a single laser beam. Moreover, using a 2D$^+$-MOT as an atomic source and a beam with uniform intensity for the grating illumination, we capture $1\times10^9$ atoms in one second, cool them to $14\,\mu$K, and demonstrate magnetic trapping using the atom chip. This is a major step towards the simplification of portable BEC devices for quantum sensing on earth and in space.

cond-mat.quant-gas↗

arXiv2024

Single-beam grating-chip 3D and 1D optical lattices

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 ultra-precise 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 ultra-stable platform enables access to a plethora of new lattice dimensionalities and geometries, ideally suited for the design of high-accuracy, portable quantum devices.

cond-mat.quant-gas↗

arXiv2023

Optimal binary gratings for multi-wavelength magneto-optical traps

Grating magneto-optical traps are an enabling quantum technology for portable metrological devices with ultracold atoms. However, beam diffraction efficiency and angle are affected by wavelength, creating a single-optic design challenge for laser cooling in two stages at two distinct wavelengths - as commonly used for loading e.g. Sr or Yb atoms into optical lattice or tweezer clocks. Here, we optically characterize a wide variety of binary gratings at different wavelengths to find a simple empirical fit to experimental grating diffraction efficiency data in terms of dimensionless etch depth and period for various duty cycles. The model avoids complex 3D light-grating surface calculations, yet still yields results accurate to a few percent across a broad range of parameters. Gratings optimized for two (or more) wavelengths can now be designed in an informed manner suitable for a wide class of atomic species enabling advanced quantum technologies.

physics.atom-ph↗

arXiv2023

Optical pumping enhancement of a free-induction-decay magnetometer

Spin preparation prior to a free-induction-decay (FID) measurement can be adversely affected by transverse bias fields, particularly in the geophysical field range. A strategy that enhances the spin polarization accumulated before readout is demonstrated, by synchronizing optical pumping with a magnetic field pulse that supersedes any transverse fields by over two order of magnitude. The pulsed magnetic field is generated along the optical pumping axis using a compact electromagnetic coil pair encompassing a micro-electromechanical systems (MEMS) vapor cell. The coils also resistively heat the cesium (Cs) vapor to the optimal atomic density without spurious magnetic field contributions as they are rapidly demagnetized to approximately zero field during spin readout. The demagnetization process is analyzed electronically, and directly with a FID measurement, to confirm that the residual magnetic field is minimal during detection. The sensitivity performance of this technique is compared to existing optical pumping modalities across a wide magnetic field range. A noise floor sensitivity of $238\,\mathrm{fT/\surd{Hz}}$ was achieved in a field of approximately $\mathrm{50\,μ{T}}$, in close agreement with the Cramér-Rao lower bound (CRLB) predicted noise density of $258\,\mathrm{fT/\surd{Hz}}$.

physics.atom-ph↗

arXiv2023

Practical Doppler broadening thermometry

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.

physics.atom-ph↗

arXiv2023

A cold-atom Ramsey clock with a low volume physics package

We demonstrate a Ramsey-type microwave clock interrogating the 6.835~GHz ground-state transition in cold \textsuperscript{87}Rb atoms loaded from a grating magneto-optical trap (GMOT) enclosed in an additively manufactured loop-gap resonator microwave cavity. A short-term stability of $1.5 \times10^{-11} $~$τ^{-1/2}$ is demonstrated, in reasonable agreement with predictions from the signal-to-noise ratio of the measured Ramsey fringes. The cavity-grating package has a volume of $\approx$67~cm\textsuperscript{3}, ensuring an inherently compact system while the use of a GMOT drastically simplifies the optical requirements for laser cooled atoms. This work is another step towards the realisation of highly compact portable cold-atom frequency standards.

physics.atom-ph↗

arXiv2023

An additive-manufactured microwave cavity for a compact cold-atom clock

We present an additive-manufactured microwave cavity for a Ramsey-type, double resonance, compact cold-atom clock. Atoms can be laser cooled inside the cavity using a grating magneto-optic trap (GMOT) with the cavity providing an excellent TE011-like mode while maintaining sufficient optical access for atomic detection. The cavity features a low Q-factor of 360 which conveniently reduces the cavity-pulling of the future clock. Despite the potential porosity of the additive-manufacturing process, we demonstrate that the cavity is well-suited for vacuum. A preliminary clock setup using cold atoms allows for measuring the Zeeman spectrum and Rabi oscillations in the cavity which enables us to infer excellent field uniformity and homogeneity respectively, across the volume accessed by the cold atoms. Ramsey spectroscopy is demonstrated, indicating the cavity is suitable for clock applications. Finally, we discuss the limitations of the future clock.

physics.atom-ph↗

arXiv2022

Cold-atom shaping with MEMS scanning mirrors

We demonstrate the integration of micro-electro-mechanical-systems (MEMS) scanning mirrors as active elements for the local optical pumping of ultra-cold atoms in a magneto-optical trap. A pair of MEMS mirrors steer a focused resonant beam through a cloud of trapped atoms shelved in the \textit{F}=1 ground-state of \textsuperscript{87}Rb for spatially-selective fluorescence of the atom cloud. Two-dimensional control is demonstrated by forming geometrical patterns along the imaging axis of the cold atom ensemble. Such control of the atomic ensemble with a microfabricated mirror pair could find applications in single atom selection, local optical pumping and arbitrary cloud shaping. This approach has significant potential for miniaturisation and in creating portable control systems for quantum optic experiments.

physics.atom-ph↗

arXiv2022

A grating-chip atomic fountain

Cold atom fountain clocks provide exceptional long term stability as they increase interrogation time at the expense of a larger size. We present a compact cold atom fountain using a grating magneto-optical trap (GMOT) to laser cool and launch the atoms in a simplified optical setup. The fountain is evaluated using coherent population trapping and demonstrates improved single-shot stability from the launch. Ramsey times up to 100 ms were measured with a corresponding fringe linewidth of 5 Hz. This technique could improve both short- and long-term stability of cold atom clocks whilst remaining compact for portable applications.

physics.atom-ph↗

arXiv2022

A Digital Alkali Spin Maser

Self-oscillating atomic magnetometers, in which the precession of atomic spins in a magnetic field is driven by resonant modulation, offer high sensitivity and dynamic range. Phase-coherent feedback from the detected signal to the applied modulation creates a resonant spin maser system, highly responsive to changes in the background magnetic field. Here we show a system in which the phase condition for resonant precession is met by digital signal processing integrated into the maser feedback loop. This system uses a modest chip-scale laser and mass-produced dual-pass caesium vapour cell and operates in a 50 microtesla field, making it a suitable technology for portable measurements of the geophysical magnetic field. We demonstrate a Cramer-Rao lower bound-limited resolution of 50 fT at 1 s sampling cadence, and a sensor bandwidth of 10 kHz. This device also represents an important class of atomic system in which low-latency digital processing forms an integral part of a coherently-driven quantum system.

physics.atom-ph↗

arXiv2021

Stand-alone vacuum cell for compact ultracold quantum technologies

Compact vacuum systems are key enabling components for cold atom technologies, facilitating extremely accurate sensing applications. There has been important progress towards a truly portable compact vacuum system, however size, weight and power consumption can be prohibitively large, optical access may be limited, and active pumping is often required. Here, we present a centilitre-scale ceramic vacuum chamber with He-impermeable viewports and an integrated diffractive optic, enabling robust laser cooling with light from a single polarization-maintaining fibre. A cold atom demonstrator based on the vacuum cell delivers $10^7$ laser-cooled $^{87}$Rb atoms per second, using minimal electrical power. With continuous Rb gas emission active pumping yields a $10^{-7}\,$mbar equilibrium pressure, and passive pumping stabilises to $3\times 10^{-6}\,$mbar, with a $17\,$day time constant. A vacuum cell, with no Rb dispensing and only passive pumping, has currently kept a similar pressure for more than \ch{500 days}. The passive-pumping vacuum lifetime is several years, estimated from short-term He throughput, with many foreseeable improvements. This technology enables wide-ranging mobilization of ultracold quantum metrology.

physics.atom-ph↗

arXiv2021

Gouy phase-matched angular and radial mode conversion in four-wave mixing

Studying the conversion between transverse light modes via four-wave mixing in a heated rubidium vapour, we demonstrate and explain a transfer between azimuthal and radial mode numbers. These relate to orthogonal modal dimensions, which one would not normally expect to interact. While angular momentum conservation in this nonlinear process dictates the selection rules for the angular mode number, the role of the radial mode number is more esoteric. We demonstrate systematically that the Gouy phase is the key to understanding this conversion, leading to strikingly different conversion behaviour in the thick and thin medium regime. Our experimental investigation of the transition between these regimes bridges the gap between previous experiments in atomic thick media and work in nonlinear crystals. Our work sets a clear starting point to explore new territory in the thick medium regime, allowing efficient radial-to-azimuthal and radial-to-radial mode conversion.

physics.atom-ph↗