arXiv subjects

Michael McNeil Forbes

Publications and source records attributed to Michael McNeil Forbes.

At least 19 recordsLinked to original sources

arXiv2026

Local Quantum Cooling for Large Fermi Systems with Pairing

We present a framework for local quantum cooling that can be efficiently applied to large-scale Fermi systems. The method introduces local Hermitian operators as a cooling potential while strictly preserving the unitarity of time evolution. Our formulation scales favorably with system size and can be seamlessly integrated into time-dependent density-functional theory frameworks. We demonstrate that energy cooling arises from the damping of particle currents and pairing-field fluctuations. Furthermore, we develop a variant of the scheme that allows the particle number to vary in time, enabling controlled density scans. The method is generic and versatile, as illustrated by applications to spin-imbalanced unitary Fermi gases and to nuclear matter in the neutron-star crust. The framework can be naturally extended to include stochastic noise, providing a foundation for studying thermalization in strongly interacting Fermi superfluids.

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arXiv2026

W-SLDA Toolkit: A simulation platform for ultracold Fermi gases

We present the W-SLDA Toolkit, a general-purpose software package for simulating ultracold Fermi gases within the framework of density functional theory and its time-dependent extensions. The toolkit enables fully microscopic studies of interacting superfluid systems across the BCS-BEC crossover, including spin-imbalanced configurations and arbitrary external geometries. It provides both static and time-dependent solvers capable of describing a broad range of phenomena in one-, two-, and three-dimensional settings. In addition, the toolkit incorporates functionality for solving the standard Bogoliubov-de Gennes equations for fermions, extending its applicability to other physical systems such as superconductors. The code is implemented in C with GPU acceleration and is optimized for hybrid CPU/GPU execution on modern high-performance computing platforms. It ensures scalability on leadership-class supercomputers, enabling fully three-dimensional simulations with large atomic numbers, and allows for direct benchmarks of ultracold-atom experimental setups. Its modular architecture facilitates straightforward extensions, user customization, and seamless interoperability with other scientific software frameworks. Furthermore, an extensive collection of practical usage examples is provided through the integrated reproducibility packs functionality, ensuring transparency and reproducibility of computational results.

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arXiv2024

Framework for Multi-messenger Inference from Neutron Stars: Combining Nuclear Theory Priors

We construct an efficient parameterization of the pure neutron-matter equation of state (EoS) that incorporates the uncertainties from both chiral effective field theory ($χ$EFT) and phenomenological potential calculations. This parameterization yields a family of EoSs including and extending the forms based purely on these two calculations. In combination with an agnostic inner core EoS, this parameterization is used in a Bayesian inference pipeline to obtain constraints on the e os parameters using multi-messenger observations of neutron stars. We specifically considered observations of the massive pulsar J0740+6620, the binary neutron star coalescence GW170817, and the NICER pulsar J0030+0451. Constraints on neutron star mass-radius relations are obtained and compared. The Bayes factors for the different EoS models are also computed. While current constraints do not reveal any significant preference among these models, the framework developed here may enable future observations with more sensitive detectors to discriminate them.

astro-ph.HE↗

arXiv2024

Fermionic Quantum Turbulence: Pushing the Limits of High-Performance Computing

Ultracold atoms provide a platform for analog quantum computer capable of simulating the quantum turbulence that underlies puzzling phenomena like pulsar glitches in rapidly spinning neutron stars. Unlike other platforms like liquid helium, ultracold atoms have a viable theoretical framework for dynamics, but simulations push the edge of current classical computers. We present the largest simulations of fermionic quantum turbulence to date and explain the computing technology needed, especially improvements in the ELPA library that enable us to diagonalize matrices of record size (millions by millions). We quantify how dissipation and thermalization proceed in fermionic quantum turbulence by using the internal structure of vortices as a new probe of the local effective temperature. All simulation data and source codes are made available to facilitate rapid scientific progress in the field of ultracold Fermi gases.

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arXiv2023

Atom Interferometric Imaging of Differential Potentials Using an Atom Laser

Interferometry is a prime technique for modern precision measurements. Atoms, unlike light, have significant interactions with electric, magnetic, and gravitational fields, making their use in interferometric applications particularly versatile. Here, we demonstrate atom interferometry to image optical and magnetic potential landscapes over an area exceeding $240 μm \times 600 μm$. The differential potentials employed in our experiments generate phase imprints in an atom laser that are made visible through a Ramsey pulse sequence. We further demonstrate how advanced pulse sequences can enhance desired imaging features, e.g. to image steep potential gradients. A theoretical discussion is presented that provides a semiclassical analysis and matching numerics.

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arXiv2023

Gravitational Caustics in an Atom Laser

Typically discussed in the context of optics, caustics are envelopes of classical trajectories (rays) where the density of states diverges, resulting in pronounced observable features such as bright points, curves, and extended networks of patterns. Here, we generate caustics in the matter waves of an atom laser, providing a striking experimental example of catastrophe theory applied to atom optics in an accelerated (gravitational) reference frame. We showcase caustics formed by individual attractive and repulsive potentials, and present an example of a network generated by multiple potentials. Exploiting internal atomic states, we demonstrate fluid-flow tracing as another tool of this flexible experimental platform. The effective gravity experienced by the atoms can be tuned with magnetic gradients, forming caustics analogous to those produced by gravitational lensing. From a more applied point of view, atom optics affords perspectives for metrology, atom interferometry, and nanofabrication. Caustics in this context may lead to quantum innovations as they are an inherently robust way of manipulating matter waves.

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arXiv2022

Rotating quantum turbulence in the unitary Fermi gas

Quantized vortices carry the angular momentum in rotating superfluids, and are key to the phenomenon of quantum turbulence. Advances in ultra-cold atom technology enable quantum turbulence to be studied in regimes with both experimental and theoretical control, unlike the original contexts of superfluid helium experiments. While much work has been performed with bosonic systems, detailed studies of fermionic quantum turbulence are nascent, despite wide applicability to other contexts such as rotating neutron stars. In this paper, we present the first large-scale study of quantum turbulence in rotating fermionic superfluids using an accurate orbital based time-dependent density functional theory (DFT) called the superfluid local density approximation (SLDA). We identify two different modes of turbulent decay in the dynamical equilibration of a rotating fermionic superfluid, and contrast these results with a computationally simpler orbital-free DFT, which we find can qualitatively reproduce these decay mechanisms if dissipation is explicitly included. These results demonstrate that one-body dissipation mechanisms intrinsic to fermionic superfluids play a key role differentiating fermionic from bosonic turbulence, but also suggest that simpler orbital-free theories may be corrected so that these more efficient techniques can be used to model extended physical systems such as neutron superfluids in neutron stars.

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arXiv2021

Detecting Entrainment in Fermi-Bose Mixtures

We propose an experimental protocol to directly detect the Andreev-Bashkin effect (entrainment) in the bulk mixture of a bosonic and fermionic superfluid using a ring geometry. Our protocol involves the interferometric detection of the entrainment-induced phase gradient across a superfluid due to the flow of another in which it is immersed. The choice of ring geometry eliminates variations in the stronger mean-field interaction which can thwart the detection of entrainment in other geometries. A significant enhancement of the entrainment phase shift signal is possible, if the dimer-boson scattering length turns out to be large, which can be measured by tuning the interaction to the limit of miscibility of the two superfluids. With suggested improvements and careful design implementation, one may achieve $\approx 67$% shift in the interferometer fringes.

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arXiv2020

Shock Waves in a Superfluid with Higher-Order Dispersion

Higher-order dispersion can lead to intriguing dynamics that are becoming a focus of modern hydrodynamics research. Such systems occur naturally, for example in shallow water waves and nonlinear optics, for which several types of novel dispersive shocks structures have been identified. Here we introduce ultracold atoms as a tunable quantum simulations platform for higher-order systems. Degenerate quantum gases are well controlled model systems for the experimental study of dispersive hydrodynamics in superfluids and have been used to investigate phenomena such as vortices, solitons, dispersive shock waves and quantum turbulence. With the advent of Raman-induced spin-orbit coupling, the dispersion of a dilute gas Bose-Einstein condensate can be modified in a flexible way, allowing for detailed investigations of higher-order dispersion dynamics. Here we present a combined experimental and theoretical study of shock structures generated in such a system. The breaking of Galilean invariance by the spin-orbit coupling allows two different types of shock structures to emerge simultaneously in a single system. Numerical simulations suggest that the behavior of these shock structures is affected by interactions with vortices in a manner reminiscent of emerging viscous hydrodynamics due to an underlying quantum turbulence in the system. This result suggests that spin-orbit coupling can be used as a powerful means to tun the effective viscosity in cold-atom experiments serving as quantum simulators of turbulent hydrodynamics, with applications from condensed matter and optics to quantum simulations of neutron stars.

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arXiv2019

Constraining the neutron-matter equation of state with gravitational waves

We show how observations of gravitational waves from binary neutron star (BNS) mergers over the next few years can be combined with insights from nuclear physics to obtain useful constraints on the equation of state (EoS) of dense matter, in particular, constraining the neutron-matter EoS to within 20% between one and two times the nuclear saturation density $n_0\approx 0.16\ {\text{fm}^{-3}}$. Using Fisher information methods, we combine observational constraints from simulated BNS merger events drawn from various population models with independent measurements of the neutron star radii expected from x-ray astronomy (the Neutron Star Interior Composition Explorer (NICER) observations in particular) to directly constrain nuclear physics parameters. To parameterize the nuclear EoS, we use a different approach, expanding from pure nuclear matter rather than from symmetric nuclear matter to make use of recent quantum Monte Carlo (QMC) calculations. This method eschews the need to invoke the so-called parabolic approximation to extrapolate from symmetric nuclear matter, allowing us to directly constrain the neutron-matter EoS. Using a principal component analysis, we identify the combination of parameters most tightly constrained by observational data. We discuss sensitivity to various effects such as different component masses through population-model sensitivity, phase transitions in the core EoS, and large deviations from the central parameter values.

astro-ph.HE↗

arXiv2018

A Minimal Nuclear Energy Density Functional

We present a minimal nuclear energy density functional (NEDF) called "SeaLL1" that has the smallest number of possible phenomenological parameters to date. SeaLL1 is defined by 7 significant phenomenological parameters, each related to a specific nuclear property. It describes the nuclear masses of even-even nuclei with a mean energy error of 0.97 MeV and a standard deviation 1.46 MeV, two-neutron and two-proton separation energies with r.m.s. errors of 0.69 MeV and 0.59 MeV respectively, and the charge radii of 345 even-even nuclei with a mean error $ε_r=$0.022 fm and a standard deviation $σ_r=$0.025 fm. SeaLL1 incorporates constraints on the EoS of pure neutron matter from quantum Monte Carlo calculations with chiral effective field theory two-body (NN) interactions at N3LO level and three-body (NNN) interactions at the N2LO level. Two of the seven parameters are related to the saturation density and the energy per particle of the homogeneous symmetric nuclear matter, one is related to the nuclear surface tension, two are related to the symmetry energy and its density dependence, one is related to the strength of the spin-orbit interaction, and one is the coupling constant of the pairing interaction. We identify additional phenomenological parameters that have little effect on ground-state properties, but can be used to fine-tune features such as the Thomas-Reiche-Kuhn sum rule, the excitation energy of the giant dipole and Gamow-Teller resonances, the static dipole electric polarizability, and the neutron skin thickness.

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arXiv2017

Negative mass hydrodynamics in a Spin-Orbit--Coupled Bose-Einstein Condensate

A negative effective mass can be realized in quantum systems by engineering the dispersion relation. A powerful method is provided by spin-orbit coupling, which is currently at the center of intense research efforts. Here we measure an expanding spin-orbit coupled Bose-Einstein condensate whose dispersion features a region of negative effective mass. We observe a range of dynamical phenomena, including the breaking of parity and of Galilean covariance, dynamical instabilities, and self-trapping. The experimental findings are reproduced by a single-band Gross-Pitaevskii simulation, demonstrating that the emerging features - shockwaves, soliton trains, self-trapping, etc. - originate from a modified dispersion. Our work also sheds new light on related phenomena in optical lattices, where the underlying periodic structure often complicates their interpretation.

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arXiv2016

Towards Quantum Turbulence in Cold Atomic Fermionic Superfluids

Fermionic superfluids provide a new realization of quantum turbulence, accessible to both experiment and theory, yet relevant to phenomena from both cold atoms to nuclear astrophysics. In particular, the strongly interacting Fermi gas realized in cold-atom experiments is closely related to dilute neutron matter in neutron star crusts. Unlike the liquid superfluids 4He (bosons) and 3He (fermions) where quantum turbulence has been studied in the laboratory, superfluid Fermi gases stand apart for a number of reasons. They admit a reliable theoretical description based on a DFT called the TDSLDA that describes both static and dynamic phenomena. Cold atom experiments demonstrate exquisite control over particle number, spin polarization, density, temperature, and interaction strength. Topological defects such as domain walls and quantized vortices, which lie at the heart of quantum turbulence, can be created and manipulated with time-dependent external potentials, and agree with the time-dependent theoretical techniques. While similar experimental and theoretical control exists for weakly interacting Bose gases, the unitary Fermi gas is strongly interacting. The resulting vortex line density is extremely high, and quantum turbulence may thus be realized in small systems where classical turbulence is suppressed. Fermi gases also permit the study of exotic superfluid phenomena such as a 3D LOFF supersolid, and a finite temperature pseudo-gap in the regime of classical turbulence. The dynamics associated with these phenomena has only started to be explored. Finally, superfluid mixtures have recently been realized, providing experimental access to phenomena like Andreev-Bashkin entrainment. Superfluid Fermi gases thus provide a rich forum for addressing phenomena related to quantum turbulence with applications ranging from terrestrial superfluidity to astrophysical dynamics in neutron stars.

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arXiv2016

Vortex pinning and dynamics in the neutron star crust

The nature of the interaction between superfluid vortices and the neutron star crust, conjectured by Anderson and Itoh in 1975 to be at the heart vortex creep and the cause of glitches, has been a long-standing question in astrophysics. Using a qualitatively new approach, we follow the dynamics as superfluid vortices move in response to the presence of "nuclei" (nuclear defects in the crust). The resulting motion is perpendicular to the force, similar to the motion of a spinning top when pushed. We show that nuclei repel vortices in the neutron star crust, and characterize the force as a function of the vortex-nucleus separation.

astro-ph.HE↗

arXiv2016

Microscopic Calculations of Vortex-Nucleus Interaction in the Neutron Star Crust

We investigate the dynamics of a quantized vortex and a nuclear impurity immersed in a neutron superfluid within a fully microscopic time-dependent three-dimensional approach. The magnitude and even the sign of the force between the quantized vortex and the nuclear impurity have been a matter of debate for over four decades. We determine that the vortex and the impurity repel at neutron densities, 0.014 fm$^{-3}$ and 0.031 fm$^{-3}$, which are relevant to the neutron star crust and the origin of glitches, while previous calculations have concluded that the force changes its sign between these two densities and predicted contradictory signs. The magnitude of the force increases with the density of neutron superfluid, while the magnitude of the pairing gap decreases in this density range.

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arXiv2015

Nuclear Energy Density Functionals: What do we really know?

We present the simplest nuclear energy density functional (NEDF) to date, determined by only 4 significant phenomenological parameters, yet capable of fitting measured nuclear masses with better accuracy than the Bethe-Weizsäcker mass formula, while also describing density structures (charge radii, neutron skins etc.) and time-dependent phenomena (induced fission, giant resonances, low energy nuclear collisions, etc.). The 4 significant parameters are necessary to describe bulk nuclear properties (binding energies and charge radii); an additional 2 to 3 parameters have little influence on the bulk nuclear properties, but allow independent control of the density dependence of the symmetry energy and isovector excitations, in particular the Thomas-Reiche-Kuhn sum rule. This Hohenberg-Kohn-style of density functional theory successfully realizes Weizsäcker's ideas and provides a computationally tractable model for a variety of static nuclear properties and dynamics, from finite nuclei to neutron stars, where it will also provide a new insight into the physics of the r-process, nucleosynthesis, and neutron star crust structure. This new NEDF clearly separates the bulk geometric properties - volume, surface, symmetry, and Coulomb energies which amount to 8MeV per nucleon or up to 2000MeV per nucleus for heavy nuclei - from finer details related to shell effects, pairing, isospin breaking, etc. which contribute at most a few MeV for the entire nucleus. Thus it provides a systematic framework for organizing various contributions to the NEDF. Measured and calculated physical observables - symmetry and saturation properties, the neutron matter equation of state, and the frequency of giant dipole resonances - lead directly to new terms not considered in current NEDF parameterizations.

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arXiv2014

Life Cycle of Superfluid Vortices and Quantum Turbulence in the Unitary Fermi Gas

The unitary Fermi gas (UFG) offers an unique opportunity to study quantum turbulence both experimentally and theoretically in a strongly interacting fermionic superfluid. It yields to accurate and controlled experiments, and admits the only dynamical microscopic description via time-dependent density functional theory (DFT) - apart from dilute bosonic gases - of the crossing and reconnection of superfluid vortex lines conjectured by Feynman in 1955 to be at the origin of quantum turbulence in superfluids at zero temperature. We demonstrate how various vortex configurations can be generated by using well established experimental techniques: laser stirring and phase imprinting. New imagining techniques demonstrated by the MIT group [Ku et al. arXiv:1402.7052] should be able to directly visualize these crossings and reconnections in greater detail than performed so far in liquid helium. We demonstrate the critical role played by the geometry of the trap in the formation and dynamics of a vortex in the UFG and how laser stirring and phase imprint can be used to create vortex tangles with clear signatures of the onset of quantum turbulence.

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arXiv2014

Quantized Superfluid Vortex Rings in the Unitary Fermi Gas

In a recent article, Yefsah et al. [Nature 499, 426 (2013)] report the observation of an unusual excitation in an elongated harmonically trapped unitary Fermi gas. After phase imprinting a domain wall, they observe oscillations almost an order of magnitude slower than predicted by any theory of domain walls which they interpret as a "heavy soliton" of inertial mass some 200 times larger than the free fermion mass or 50 times larger than expected for a domain wall. We present compelling evidence that this "soliton" is instead a quantized vortex ring by showing that the main aspects of the experiment can be naturally explained within the framework of time-dependent superfluid DFT.

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