arXiv subjects

Robert P. Smith

Publications and source records attributed to Robert P. Smith.

At least 19 recordsLinked to original sources

arXiv2025

Magnetic Field-dependent Isotope Effect Supports Radical Pair Mechanism in Tubulin Polymerization

Weak magnetic fields and isotopes have been shown to influence biological processes; however, the underlying mechanisms remain poorly understood, particularly because the corresponding interaction energies are far below thermal energies, making classical explanations challenging or impossible. Microtubules, dynamic cytoskeletal fibers, offer an ideal system to test weak magnetic field effects due to their self-assembling capabilities, sensitivity to magnetic fields, and their central role in cellular processes. In this study, we use a combination of experiments and simulations to explore how nuclear spin dynamics affect microtubule polymerization by examining interactions between magnesium isotope substitution and weak magnetic fields. Our experiments reveal an isotope-dependent effect, which can be explained via a radical pair mechanism, explicitly arising from nuclear spin properties rather than isotopic mass differences. This nuclear spin-driven isotope effect is notably enhanced under an applied weak magnetic field of approximately 3 mT. Our theoretical model based on radical pairs achieves quantitative agreement with our experimental observations. These results establish a direct connection between quantum spin dynamics and microtubule assembly, providing new insights into how weak magnetic fields influence cellular and biomolecular functions.

physics.bio-ph↗

arXiv2025

Shift of the Bose-Einstein condensation temperature due to dipolar interactions

We report the first measurements of the BEC critical temperature shift due to dipolar interactions, employing samples of ultracold erbium atoms which feature significant (magnetic) dipole-dipole interactions in addition to tuneable contact interactions. Using a highly prolate harmonic trapping potential, we observe a clear dependence of the critical temperature on the orientation of the dipoles relative to the trap axis. Our results are in good agreement with mean-field theory for a range of contact interaction strengths. This work opens the door for further investigations into beyond-mean-field effects and the finite-temperature phase diagram in the more strongly dipolar regime where supersolid and droplet states emerge.

cond-mat.quant-gas↗

arXiv2024

Characterisation of three-body loss in ${}^{166}$Er and optimised production of large Bose-Einstein condensates

Ultracold gases of highly magnetic lanthanide atoms have enabled the realisation of dipolar quantum droplets and supersolids. However, future studies could be limited by the achievable atom numbers and hindered by high three-body loss rates. Here we study density-dependent atom loss in an ultracold gas of ${}^{166}$Er for magnetic fields below 4 G, identifying six previously unreported, strongly temperature-dependent features. We find that their positions and widths show a linear temperature dependence up to at least $15\,μ\textrm{K}$. In addition, we observe a weak, polarisation-dependent shift of the loss features with the intensity of the light used to optically trap the atoms. This detailed knowledge of the loss landscape allows us to optimise the production of dipolar BECs with more than $2 \times 10^5$ atoms and points towards optimal strategies for the study of large-atom-number dipolar gases in the droplet and supersolid regimes.

cond-mat.quant-gas↗

arXiv2023

Universal equation of state for wave turbulence in a quantum gas

Boyle's 1662 observation that the volume of a gas is, at constant temperature, inversely proportional to pressure, offered a prototypical example of how an equation of state (EoS) can succinctly capture key properties of a many-particle system. Such relations are now cornerstones of equilibrium thermodynamics. Extending thermodynamic concepts to far-from-equilibrium systems is of great interest in various contexts including glasses, active matter, and turbulence, but is in general an open problem. Here, using a homogeneous ultracold atomic Bose gas, we experimentally construct an EoS for a turbulent cascade of matter waves. Under continuous forcing at a large length scale and dissipation at a small one, the gas exhibits a non-thermal, but stationary state, which is characterised by a power-law momentum distribution sustained by a scale-invariant momentum-space energy flux. We establish the amplitude of the momentum distribution and the underlying energy flux as equilibrium-like state variables, related by an EoS that does not depend on the details of the energy injection or dissipation, or the history of the system. Moreover, we show that the equations of state for a wide range of interaction strengths and gas densities can be empirically scaled onto each other. This results in a universal dimensionless EoS that sets benchmarks for the theory and should also be relevant for other turbulent systems.

cond-mat.quant-gas↗

arXiv2022

Interacting Bose-condensed gases

We provide an overview of the effects of interactions in Bose-condensed gases. We focus on phenomena that have been explored in ultracold atom experiments, covering both tuneable contact interactions and dipolar interactions. Our discussion includes: modifications to the ground state and excitation spectrum, critical behaviour near the Bose--Einstein condensation temperature, the unitary regime where the interactions are as strong as allowed by quantum mechanics, quantum droplets in mixtures, and supersolids in dipolar gases.

cond-mat.quant-gas↗

arXiv2022

JAXFit: Trust Region Method for Nonlinear Least-Squares Curve Fitting on the GPU

We implement a trust region method on the GPU for nonlinear least squares curve fitting problems using a new deep learning Python library called JAX. Our open source package, JAXFit, works for both unconstrained and constrained curve fitting problems and allows the fit functions to be defined in Python alone -- without any specialized knowledge of either the GPU or CUDA programming. Since JAXFit runs on the GPU, it is much faster than CPU based libraries and even other GPU based libraries, despite being very easy to use. Additionally, due to JAX's deep learning foundations, the Jacobian in JAXFit's trust region algorithm is calculated with automatic differentiation, rather than than using derivative approximations or requiring the user to define the fit function's partial derivatives.

cs.LG↗

arXiv2022

How to realise a homogeneous dipolar Bose gas in the roton regime

Homogeneous quantum gases open up new possibilities for studying many-body phenomena and have now been realised for a variety of systems. For gases with short-range interactions the way to make the cloud homogeneous is, predictably, to trap it in an ideal (homogeneous) box potential. We show that creating a close to homogeneous dipolar gas in the roton regime, when long-range interactions are important, actually requires trapping particles in soft-walled (inhomogeneous) box-like potentials. In particular, we numerically explore a dipolar gas confined in a pancake trap which is harmonic along the polarisation axis and a cylindrically symmetric power-law potential $r^p$ radially. We find that intermediate $p$'s maximise the proportion of the sample that can be brought close to the critical density required to reach the roton regime, whereas higher $p$'s trigger density oscillations near the wall even when the bulk of the system is not in the roton regime. We characterise how the optimum density distribution depends on the shape of the trapping potential and find it is controlled by the trap wall steepness.

cond-mat.quant-gas↗

arXiv2022

Measuring Laser Beams with a Neural Network

A deep neural network (NN) is used to simultaneously detect laser beams in images and measure their center coordinates, radii and angular orientations. A dataset of images containing simulated laser beams and a dataset of images with experimental laser beams, generated using a spatial light modulator, are used to train and evaluate the NN. After training on the simulated dataset the NN achieves beam parameter rootmean-square-errors (RMSEs) of less than 3.4% on the experimental dataset. Subsequent training on the experimental dataset causes the RMSEs to fall below 1.1%. The NN method can be used as a stand-alone measurement of the beam parameters or can compliment other beam profiling methods by providing an accurate region-of-interest.

physics.optics↗

arXiv2021

First and second sound in a compressible 3D Bose fluid

The two-fluid model is fundamental for the description of superfluidity. In the nearly-incompressible-liquid regime, it successfully describes first and second sound, corresponding, respectively, to density and entropy waves, in both liquid helium and unitary Fermi gases. Here, we study the two sounds in the opposite regime of a highly compressible fluid, using an ultracold $^{39}$K Bose gas in a three-dimensional box trap. We excite the longest-wavelength mode of our homogeneous gas, and observe two distinct resonant oscillations below the critical temperature, of which only one persists above it. In a microscopic mode-structure analysis, we find agreement with the hydrodynamic theory, where first and second sound involve density oscillations dominated by, respectively, thermal and condensed atoms. Varying the interaction strength, we explore the crossover from hydrodynamic to collisionless behavior in a normal gas.

cond-mat.quant-gas↗

arXiv2021

Quantum Gases in Optical Boxes

Advances in light shaping for optical trapping of neutral particles have led to the development of box traps for ultracold atoms and molecules. These traps have allowed the creation of homogeneous quantum gases and opened new possibilities for studies of many-body physics. They simplify the interpretation of experimental results, provide more direct connections with theory, and in some cases allow qualitatively new, hitherto impossible experiments. Here we review progress in this emerging field.

cond-mat.quant-gas↗

arXiv2020

Atom Cloud Detection Using a Deep Neural Network

We use a deep neural network to detect and place region-of-interest boxes around ultracold atom clouds in absorption and fluorescence images---with the ability to identify and bound multiple clouds within a single image. The neural network also outputs segmentation masks that identify the size, shape and orientation of each cloud from which we extract the clouds' Gaussian parameters. This allows 2D Gaussian fits to be reliably seeded thereby enabling fully automatic image processing.

cond-mat.quant-gas↗

arXiv2020

Bidirectional dynamic scaling in an isolated Bose gas far from equilibrium

Understanding and classifying nonequilibrium many-body phenomena, analogous to the classification of equilibrium states of matter into universality classes, is an outstanding problem in physics. Any many-body system, from stellar matter to financial markets, can be out of equilibrium in a myriad of ways; since many are also difficult to experiment on, it is a major goal to establish universal principles that apply to different phenomena and physical systems. At the heart of the classification of equilibrium states is the universality seen in the self-similar spatial scaling of systems close to phase transitions. Recent theoretical work, and first experimental evidence, suggest that isolated many-body systems far from equilibrium generically exhibit dynamic (spatiotemporal) self-similar scaling, akin to turbulent cascades and the Family-Vicsek scaling in classical surface growth. Here we observe bidirectional dynamic scaling in an isolated quench-cooled atomic Bose gas; as the gas thermalises and undergoes Bose-Einstein condensation, it shows self-similar net flows of particles towards the infrared (smaller momenta) and energy towards the ultraviolet (smaller lengthscales). For both infrared (IR) and ultraviolet (UV) dynamics we find that the scaling exponents are independent of the strength of the interparticle interactions that drive the thermalisation.

cond-mat.quant-gas↗

arXiv2019

From single-particle excitations to sound waves in a box-trapped atomic Bose-Einstein condensate

We experimentally and theoretically investigate the lowest-lying axial excitation of an atomic Bose-Einstein condensate in a cylindrical box trap. By tuning the atomic density, we observe how the nature of the mode changes from a single-particle excitation (in the low-density limit) to a sound wave (in the high-density limit). Throughout this crossover the measured mode frequency agrees with Bogoliubov theory. Using approximate low-energy models we show that the evolution of the mode frequency is directly related to the interaction-induced shape changes of the condensate and the excitation. Finally, if we create a large-amplitude excitation, and then let the system evolve freely, we observe that the mode amplitude decays non-exponentially in time; this nonlinear behaviour is indicative of interactions between the elementary excitations, but remains to be quantitatively understood.

cond-mat.quant-gas↗

arXiv2019

Can three-body recombination purify a quantum gas?

Three-body recombination in quantum gases is traditionally associated with heating, but it was recently found that it can also cool the gas. We show that in a partially condensed three-dimensional homogeneous Bose gas three-body loss could even purify the sample, that is, reduce the entropy per particle and increase the condensed fraction $η$. We predict that the evolution of $η$ under continuous three-body loss can, depending on small changes in the initial conditions, exhibit two qualitatively different behaviours - if it is initially above a certain critical value, $η$ increases further, whereas clouds with lower initial $η$ evolve towards a thermal gas. These dynamical effects should be observable under realistic experimental conditions.

cond-mat.quant-gas↗

arXiv2018

Synthetic dissipation and cascade fluxes in a turbulent quantum gas

Scale-invariant fluxes are the defining property of turbulent cascades, but their direct measurement is a notorious problem. Here we perform such a measurement for a direct energy cascade in a turbulent quantum gas. Using a time-periodic force, we inject energy at a large lengthscale and generate a cascade in a uniformly-trapped Bose gas. The adjustable trap depth provides a high-momentum cutoff $k_{\textrm{D}}$, which realises a synthetic dissipation scale. This gives us direct access to the particle flux across a momentum shell of radius $k_{\textrm{D}}$, and the tunability of $k_{\textrm{D}}$ allows for a clear demonstration of the zeroth law of turbulence: we observe that for fixed forcing the particle flux vanishes as $k_{\textrm{D}}^{-2}$ in the dissipationless limit $k_{\textrm{D}}\rightarrow \infty$, while the energy flux is independent of $k_{\textrm{D}}$. Moreover, our time-resolved measurements give unique access to the pre-steady-state dynamics, when the cascade front propagates in momentum space.

cond-mat.quant-gas↗

arXiv2018

Universal Prethermal Dynamics of Bose Gases Quenched to Unitarity

Understanding strongly correlated phases of matter, from the quark-gluon plasma to neutron stars, and in particular the dynamics of such systems, $e.g.$ following a Hamiltonian quench, poses a fundamental challenge in modern physics. Ultracold atomic gases are excellent quantum simulators for these problems, thanks to tuneable interparticle interactions and experimentally resolvable intrinsic timescales. In particular, they give access to the unitary regime where the interactions are as strong as allowed by quantum mechanics. Following years of experiments on unitary Fermi gases, unitary Bose gases have recently emerged as a new experimental frontier. They promise exciting new possibilities, including universal physics solely controlled by the gas density and novel forms of superfluidity. Here, through momentum- and time-resolved studies, we explore both degenerate and thermal homogeneous Bose gases quenched to unitarity. In degenerate samples we observe universal post-quench dynamics in agreement with the emergence of a prethermal state with a universal nonzero condensed fraction. In thermal gases, dynamic and thermodynamic properties generically depend on both the gas density $n$ and temperature $T$, but we find that they can still be expressed in terms of universal dimensionless functions. Surprisingly, the total quench-induced correlation energy is independent of the gas temperature. Our measurements provide quantitative benchmarks and new challenges for theoretical understanding.

cond-mat.quant-gas↗

arXiv2018

Elliptic flow in a strongly-interacting normal Bose gas

We study the anisotropic, elliptic expansion of a thermal atomic Bose gas released from an anisotropic trapping potential, for a wide range of interaction strengths across a Feshbach resonance. We show that in our system this hydrodynamic phenomenon is for all interaction strengths fully described by a microscopic kinetic model with no free parameters. The success of this description crucially relies on taking into account the reduced thermalising power of elastic collisions in a strongly interacting gas, for which we derive an analytical theory. We also perform time-resolved measurements that directly reveal the dynamics of the energy transfer between the different expansion axes.

cond-mat.quant-gas↗

arXiv2017

Universal Scaling Laws in the Dynamics of a Homogeneous Unitary Bose Gas

We study the dynamics of an initially degenerate homogeneous Bose gas after an interaction quench to the unitary regime at a magnetic Feshbach resonance. As the cloud decays and heats, it exhibits a crossover from degenerate- to thermal-gas behaviour, both of which are characterised by universal scaling laws linking the particle-loss rate to the total atom number $N$. In the degenerate and thermal regimes the per-particle loss rate is $\propto N^{2/3}$ and $N^{26/9}$, respectively. The crossover occurs at a universal kinetic energy per particle and at a universal time after the quench, in units of energy and time set by the gas density. By slowly sweeping the magnetic field away from the resonance and creating a mixture of atoms and molecules, we also map out the dynamics of correlations in the unitary gas, which display a universal temporal scaling with the gas density, and reach a steady state while the gas is still degenerate.

cond-mat.quant-gas↗