arXiv subjects

T. M. Fromhold

Publications and source records attributed to T. M. Fromhold.

At least 19 recordsLinked to original sources

arXiv2025

Additive Manufacturing for Advanced Quantum Technologies

The development of quantum technology has opened up exciting opportunities to revolutionize computing and communication, timing and navigation systems, enable non-invasive imaging of the human body, and probe fundamental physics with unprecedented precision. Alongside these advancements has come an increase in experimental complexity and a correspondingly greater dependence on compact, efficient and reliable hardware. The drive to move quantum technologies from laboratory prototypes to portable, real-world instruments has incentivized miniaturization of experimental systems relating to a strong demand for smaller, more robust and less power-hungry quantum hardware and for increasingly specialized and intricate components. Additive manufacturing, already heralded as game-changing for many manufacturing sectors, is especially well-suited to this task owing to the comparatively large amount of design freedom it enables and its ability to produce intricate three-dimensional forms and specialized components. Herein we review work conducted to date on the application of additive manufacturing to quantum technologies, discuss the current state of the art in additive manufacturing in optics, optomechanics, magnetic components and vacuum equipment, and consider pathways for future advancement. We also give an overview of the research and application areas most likely to be impacted by the deployment of additive manufacturing techniques within the quantum technology sector.

physics.app-ph↗

arXiv2023

Bi-planar magnetic stabilisation coils for an inertial sensor based on atom interferometry

Inertial sensors that measure the acceleration of ultracold atoms promise unrivalled accuracy compared to classical equivalents. However, atomic systems are sensitive to various perturbations, including magnetic fields, which can introduce measurement inaccuracies. To address this challenge, we have designed, manufactured, and validated a magnetic field stabilisation system for a quantum sensor based on atom interferometry. We solve for the magnetic field generated by surface currents in-between a pair of rectangular coils and approximate the surface current using discrete wires. The wires are wound by-hand onto machined panels which are retrofitted onto the existing mounting structure of the sensor without interfering with any experimental components. Along the central $60$ mm of the $y$-axis, which aligns with the trajectory of the atoms during interferometry, the coils are measured to generate an independent uniform axial magnetic field with a strength of $B_z=\left(22.81\pm0.01\right)$ $μ$T/A [$\mathrm{mean}\pm2σ\mathrm{std. error}$] and an independent linear axial field gradient of strength $\mathrm{d}B_z/\mathrm{d}y=\left(10.6\pm0.1\right)$ $μ$T/Am. The uniform $B_z$ field is measured to deviate by a maximum value of $1.3$% in the same region, which is a factor of three times more uniform than the previously-used on-sensor rectangular $B_z$ compensation set.

physics.atom-ph↗

arXiv2023

Benchtop magnetic shielding for benchmarking atomic magnetometers

Here, a benchtop hybrid magnetic shield containing four mumetal cylinders and nine internal flexible printed circuit boards is designed, constructed, tested, and operated. The shield is designed specifically as a test-bed for building and operating ultra-sensitive quantum magnetometers. The geometry and spacing of the mumetal cylinders are optimized to maximize shielding efficiency while maintaining Johnson noise $<15$ fT/$\sqrt{}$Hz. Experimental measurements at the shield's center show passive shielding efficiency of $\left(1.0\pm0.1\right){\times}10^6$ for a $0.2$ Hz oscillating field applied along the shield's axis. The nine flexible printed circuit boards generate three uniform fields, which all deviate from perfect uniformity by ${\leq}0.5$% along $50$% of the inner shield axis, and five linear field gradients and one second-order gradient, which all deviate by ${\leq}4$% from perfect linearity and curvature, respectively, over measured target regions. Together, the target field amplitudes are adjusted to minimize the remnant static field along $40$% of the inner shield axis, as mapped using an atomic magnetometer. In this region, the active null reduces the norm of the magnitudes of the three uniform fields and six gradients by factors of $19.5$ and $19.8$, respectively, thereby reducing the total static field from $1.68$ nT to $0.23$ nT.

physics.ins-det↗

arXiv2022

Bespoke magnetic field design for a magnetically shielded cold atom interferometer

Quantum sensors based on cold atoms are being developed which produce measurements of unprecedented accuracy. Due to shifts in atomic energy levels, quantum sensors often have stringent requirements on their internal magnetic field environment. Typically, background magnetic fields are attenuated using high permeability magnetic shielding, with the cancelling of residual and introduction of quantisation fields implemented with coils inside the shield. The high permeability shield, however, distorts all magnetic fields, including those generated inside the sensor. Here, we demonstrate a solution by designing multiple coils overlaid on a 3D-printed former to generate three uniform and three constant linear gradient magnetic fields inside the capped cylindrical magnetic shield of a cold atom interferometer. The fields are characterised in-situ and match their desired forms to high accuracy. For example, the uniform transverse field, $B_x$, deviates by less than $0.2$% over more than $40$% of the length of the shield. We also map the field directly using the cold atoms and investigate the potential of the coil system to reduce bias from the quadratic Zeeman effect. This coil design technology enables targeted field compensation over large spatial volumes and has the potential to reduce systematic shifts and noise in numerous cold atom systems.

physics.app-ph↗

arXiv2022

Magnetic Field Design in a Cylindrical High-Permeability Shield: The Combination of Simple Building Blocks and a Genetic Algorithm

Magnetically-sensitive experiments and newly-developed quantum technologies with integrated high-permeability magnetic shields require increasing control of their magnetic field environment and reductions in size, weight, power and cost. However, magnetic fields generated by active components are distorted by high-permeability magnetic shielding, particularly when they are close to the shield's surface. Here, we present an efficient design methodology for creating desired static magnetic field profiles by using discrete coils electromagnetically-coupled to a cylindrical passive magnetic shield. We utilize a modified Green's function solution that accounts for the interior boundary conditions on a closed finite-length high-permeability cylindrical magnetic shield, and determine simplified expressions when a cylindrical coil approaches the interior surface of the shield. We use an analytic formulation of simple discrete building blocks to provide a complete discrete coil basis to generate any physically-attainable magnetic field inside the shield. We then use a genetic algorithm to find optimized discrete coil structures composed of this basis. We use our methodology to generate an improved linear axial gradient field, $\mathrm{d}B_z/\mathrm{d}z$, and transverse bias field, $B_x$. These optimized structures increase, by a factor of seven and three compared to the standard configurations, the volume in which the desired and achieved fields agree within $1\%$ accuracy, respectively. This coil design method can be used to optimize active--passive magnetic field shaping systems that are compact and simple to manufacture, enabling accurate magnetic field control in spatially-confined experiments at low cost.

physics.app-ph↗

arXiv2021

Planar Coil Optimization in a Magnetically Shielded Cylinder

Hybrid magnetic shields with both active field generating components and high-permeability magnetic shielding are increasingly needed for a variety of technologies and experiments that require precision-controlled magnetic field environments. However, the fields generated by the active components interact with the passive magnetic shield, distorting the desired field profiles. Consequently, optimization of the active components needed to generate user-specified target fields must include coupling to the high-permeability passive components. Here, we consider the optimization of planar active systems, on which an arbitrary static current flows, coupled to a closed high-permeability cylindrical shield. We modify the Green's function for the magnetic vector potential to match boundary conditions on the shield's interior surface, enabling us to construct an inverse optimization problem to design planar coils that generate user-specified magnetic fields inside high-permeability shields. We validate our methodology by designing two bi-planar hybrid active--passive systems, which generate a constant transverse field, $\mathbf{B}=\mathbf{\hat{x}}$, and a linear field gradient, $\mathbf{B}=(-x~\mathbf{\hat{x}}-y~\mathbf{\hat{y}}+2z~\mathbf{\hat{z}})$, respectively. For both systems, the inverse-optimized magnetic field profiles agree well with forward numerical simulations. Our design methodology is accurate and flexible, facilitating the miniaturization of high-performance hybrid magnetic field generating technologies with strict design constraints and spatial limitations.

physics.app-ph↗

arXiv2021

Using graphene conductors to enhance the functionality of atom-chips

We show that the performance and functionality of atom-chips can be transformed by using graphene-based van der Waals heterostructures to overcome present limitations on the lifetime of the trapped atom cloud and on its proximity to the chip surface. Our analysis involves Green-function calculations of the thermal (Johnson) noise and Casimir-Polder atom-surface attraction produced by the atom-chip. This enables us to determine the lifetime limitations produced by spin-flip, tunneling and three-body collisional losses. Compared with atom-chips that use thick metallic conductors and substrates, atom-chip structures based on two-dimensional materials reduce the minimum attainable atom-surface separation to a few 100 nm and increase the lifetimes of the trapped atom clouds by orders of magnitude so that they are limited only by the quality of the background vacuum. We predict that atom-chips with two-dimensional conductors will also reduce spatial fluctuations in the trapping potential originating from imperfections in the conductor patterns. These advantages will enhance the performance of atom-chips for quantum sensing applications and for fundamental studies of complex quantum systems.

quant-ph↗

arXiv2020

Emergence and Control of Complex Behaviours in Driven Systems of Interacting Qubits with Dissipation

Progress in the creation of large scale, artificial quantum coherent structures demands the investigation of their nonequilibrium dynamics when strong interactions, even between remote parts, are non-perturbative. Analysis of multiparticle quantum correlations in a large system in the presence of decoherence and external driving is especially topical. Still, scaling behaviour of dynamics and related emergent phenomena are not yet well understood. We investigate how the dynamics of a driven system of several quantum elements (e.g., qubits or Rydberg atoms) changes with increasing number of elements. Surprisingly, a two-element system exhibits chaotic behaviours. For larger system sizes a highly stochastic, far from equilibrium, {\em hyperchaotic} regime emerges. Its complexity systematically scales with the size of the system, proportionally to the number of elements. Finally, we demonstrate that these chaotic dynamics can be efficiently controlled by a periodic driving field. The insights provided by our results indicate the possibility of a reduced description for the behaviour of a large quantum system in terms of the transitions between its qualitatively different dynamical regimes, which are controlled by a relatively small number of parameters, and may prove useful in the design, characterization and control of large artificial quantum structures.

quant-ph↗

arXiv2020

Optimal Inverse Design of Magnetic Field Profiles in a Magnetically Shielded Cylinder

Magnetic shields that use both active and passive components to enable the generation of a tailored low-field environment are required for many applications in science, engineering, and medical imaging. Until now, accurate field nulling, or field generation, has only been possible over a small fraction of the overall volume of the shield. This is due to the interaction between the active field-generating components and the surrounding high-permeability passive shielding material. In this paper, we formulate the interaction between an arbitrary static current flow on a cylinder and an exterior closed high-permeability cylinder. We modify the Green's function for the magnetic vector potential and match boundary conditions on the shield's interior surface to calculate the total magnetic field generated by the system. We cast this formulation into an inverse optimization problem to design active--passive magnetic field shaping systems that accurately generate any physical static magnetic field in the interior of a closed cylindrical passive shield. We illustrate this method by designing hybrid systems that generate a range of magnetic field profiles to high accuracy over large interior volumes, and simulate them in real-world shields whose passive components have finite permeability, thickness, and axial entry holes. Our optimization procedure can be adapted to design active--passive magnetic field shaping systems that accurately generate any physical user-specified static magnetic field in the interior of a closed cylindrical shield of any length, enabling the development and miniaturization of systems that require accurate magnetic shielding and control.

physics.app-ph↗

arXiv2020

Ultrafast strain-induced charge transport in semiconductor superlattices

We investigate the effect of hypersonic (> 1 GHz) acoustic phonon wavepackets on electron transport in a semiconductor superlattice. Our quantum mechanical simulations demonstrate that a GHz train of picosecond deformation strain pulses propagating through a superlattice can generate current oscillations whose frequency is several times higher than that of the strain pulse train. The shape and polarity of the calculated current pulses agree well with experimentally measured electric signals. The calculations also explain and accurately reproduce the measured variation of the induced current pulse magnitude with the strain pulse amplitude and applied bias voltage. Our results open a route to developing acoustically-driven semiconductor superlattices as sources of millimetre and sub-millimetre electromagnetic waves.

physics.app-ph↗

arXiv2019

Additively manufactured ultra-high vacuum chamber below $10^{-10}$ mbar

Metal-based additive manufacturing (AM) represents a paradigm change in engineering and production methods across multiple industries and sectors. AM methods enable mass reduction and performance optimisation well beyond that achievable via conventional manufacturing, thereby impacting significantly on aerospace and space technologies. Technologies relying on high and ultra-high vacuum (UHV), such as x-ray photo-electron spectroscopy, photo-sensors, cameras and cryostats, could also benefit greatly from AM. Despite recent advances in AM processing of metals, additively manufactured UHV chambers have so far not been achieved. Reducing the mass of UHV equipment is particularly critical for the development of portable cold atom systems, which are expected to underpin the next generation of sensing and timekeeping technologies and to allow novel space-based sensors for fundamental research. We demonstrate here an additively manufactured UHV chamber reaching a pressure below $10^{-10}$ mbar, enabling a cloud of cold $^{85}$Rb atoms to be trapped - the starting point for many precision timekeeping and sensing devices. The chamber is manufactured from aluminium alloy AlSi10Mg by laser powder bed fusion and has a mass of less than a third of a commercially-available equivalent. Outgassing analysis based on mass spectrometry was performed and it was demonstrated that even without active pumping the system remains in the $10^{-9}$ mbar regime for up to 48 hours.

quant-ph↗

arXiv2018

Tunnel spectroscopy of localised electronic states in hexagonal boron nitride

Hexagonal boron nitride (hBN) is a large band gap layered crystal, frequently incorporated in van der Waals (vdW) heterostructures as an insulating or tunnel barrier. Localised states with energies within its band gap can emit visible light, relevant to applications in nanophotonics and quantum information processing. However, they also give rise to conducting channels, which can induce electrical breakdown when a large voltage is applied. Here we use gated tunnel transistors to study resonant electron tunnelling through the localised states in few atomic-layer hBN barriers sandwiched between two monolayer graphene electrodes. The measurements are used to determine the energy, linewidth, tunnelling transmission probability, and depth within the barrier of more than 50 distinct localised states. A three-step process of electron percolation through two spatially separated localised states is also investigated.

cond-mat.mes-hall↗

arXiv2017

Effects of classical stochastic webs on the quantum dynamics of cold atomic gases in a moving optical lattice

We introduce and investigate a system that uses temporal resonance-induced phase space pathways to create strong coupling between an atomic Bose-Einstein condensate and a traveling optical lattice potential. We show that these pathways thread both the classical and quantum phase space of the atom cloud, even when the optical lattice potential is arbitrarily weak. The topology of the pathways, which form web-like patterns, can by controled by changing the amplitude and period of the optical lattice. In turn, this control can be used to increase and limit the BEC's center-of-mass kinetic energy to pre-specified values. Surprisingly, the strength of the atom-lattice interaction and resulting BEC heating of the center-of-mass motion is enhanced by the repulsive inter-atomic interactions.

cond-mat.quant-gas↗

arXiv2017

Atom chips with free-standing two-dimensional electron gases: advantages and challenges

In this work we consider the advantages and challenges of using free-standing two-dimensional electron gases (2DEG) as active components in atom chips for manipulating ultracold ensembles of alkali atoms. We calculate trapping parameters achievable with typical high-mobility 2DEGs in an atom chip configuration, and identify advantages of this system for trapping atoms at sub-micron distances from the atom chip. We show how the sensitivity of atomic gases to magnetic field inhomogeneity can be exploited for controlling the atoms with quantum electronic devices and, conversely, using the atoms to probe the structural and transport properties of semiconductor devices.

cond-mat.quant-gas↗

arXiv2017

3D-printed components for quantum devices

Recent advances in the preparation, control and measurement of atomic gases have led to new insights into the quantum world and unprecedented metrological sensitivities, e.g. in measuring gravitational forces and magnetic fields. The full potential of applying such capabilities to areas as diverse as biomedical imaging, non-invasive underground mapping, and GPS-free navigation can only be realised with the scalable production of efficient, robust and portable devices. Here we introduce additive manufacturing as a production technique of quantum device components with unrivalled design freedom, providing a step change in efficiency, compactness and facilitating systems integration. As a demonstrator we present a compact ultracold atom source using less than ten milliwatts power to produce large samples of cold rubidium gases in an ultrahigh vacuum environment. This disruptive technology opens the door to drastically improved integrated structures, which will further reduce power consumption, size and assembly complexity in scalable series manufacture of bespoke quantum devices.

quant-ph↗

arXiv2016

Graphene-hexagonal boron nitride resonant tunneling diodes as high-frequency oscillators

We assess the potential of two-terminal graphene-hBN-graphene resonant tunneling diodes as high-frequency oscillators, using self-consistent quantum transport and electrostatic simulations to determine the time-dependent response of the diodes in a resonant circuit. We quantify how the frequency and power of the current oscillations depend on the diode and circuit parameters including the doping of the graphene electrodes, device geometry, alignment of the graphene lattices, and the circuit impedances. Our results indicate that current oscillations with frequencies of up to several hundred GHz should be achievable.

cond-mat.mes-hall↗

arXiv2015

Resonant tunnelling between the chiral Landau states of twisted graphene lattices

A new class of multilayered functional materials has recently emerged in which the component atomic layers are held together by weak van der Waals forces that preserve the structural integrity and physical properties of each layer. An exemplar of such a structure is a transistor device in which relativistic Dirac Fermions can resonantly tunnel through a boron nitride barrier, a few atomic layers thick, sandwiched between two graphene electrodes. An applied magnetic field quantises graphene's gapless conduction and valence band states into discrete Landau levels, allowing us to resolve individual inter-Landau level transitions and thereby demonstrate that the energy, momentum and chiral properties of the electrons are conserved in the tunnelling process. We also demonstrate that the change in the semiclassical cyclotron trajectories, following a tunnelling event, is a form of Klein tunnelling for inter-layer transitions.

cond-mat.mes-hall↗

arXiv2015

Superfluid flow above the critical velocity

Superfluidity and superconductivity have been studied widely since the last century in many different contexts ranging from nuclear matter to atomic quantum gases. The rigidity of these systems with respect to external perturbations results in frictionless motion for superfluids and resistance-free electric current in superconductors. This peculiar behaviour is lost when external perturbations overcome a critical threshold, i.e. above a critical magnetic field or a critical current for superconductors. In superfluids, such as liquid helium or ultracold gases, the corresponding quantities are critical rotation rate and critical velocity, respectively. Enhancing the critical values is of great fundamental and practical value. Here we demonstrate that superfluidity can be achieved for flow above the critical velocity through quantum interference induced resonances. This has far reaching consequences for the fundamental understanding of superfluidity and superconductivity and opens up new application possibilities in quantum metrology, e.g. in rotation sensing.

cond-mat.quant-gas↗