arXiv subjects

D. J. M. Ahmed-Braun

Publications and source records attributed to D. J. M. Ahmed-Braun.

9 recordsLinked to original sources

arXiv2026

Coupled-channels method for the scattering hypervolume in ultracold atomic three-body collisions

We introduce a novel coupled-channels method for elastic three-body scattering in systems of identical bosonic alkali-metal atoms. The approach relies on the numerically exact two-body off-the-energy-shell transition matrix, constructed from realistic multichannel molecular interaction potentials that support many bound states. By rigorously accounting for this off-shell structure, the method captures both the short-range physics as well as multichannel couplings characteristic of alkali-metal potentials without resorting to model pseudopotentials. The central output is the complex three-body scattering hypervolume -- the three-body analogue of the two-body scattering length -- which we obtain with controlled and verifiable numerical accuracy. As a realistic benchmark, we apply our framework to spin-polarized potassium-39, performing full coupled-channels three-body scattering calculations and extracting the hypervolume over experimentally relevant conditions. The method is general and transferable to other atomic species and interaction models featuring deep molecular potentials with an arbitrarily large number of bound states.

cond-mat.quant-gas↗

arXiv2025

Probing non-Gaussian correlations through entanglement generation in a many-body quantum system

In understanding strongly correlated quantum systems, quantifying the non-Gaussian nature of interparticle correlations is invaluable. We show that, for a uniform quantum gas, there exists a natural connection between non-Gaussian correlations and the generation of momentum-space entanglement. Furthermore, this entanglement can be directly measured in an experiment using time-of-flight techniques. To prototype our method, we numerically study entanglement generation in a degenerate Bose gas following a quench to the unitary regime, where Gaussian and non-Gaussian correlations are generated sequentially in time.

cond-mat.quant-gas↗

arXiv2024

Non-equilibrium dynamics and atom-pair coherence in strongly interacting Bose-Fermi mixtures

Theoretical treatments of non-equilibrium dynamics in strongly interacting Bose-Fermi mixtures are complicated by the inherent non-Gaussian nature of the vacuum two-body physics, invalidating the typical Hartree-Fock-Bogoliubov approximation. Here, we apply the cumulant expansion to study non-equilibrium Bose-Fermi mixtures, which allows us to explicitly include the missing non-Gaussian quantum correlations, leading to a consistent dynamical theory of a Bose-Fermi mixture near an interspecies Feshbach resonance. We first apply our theory to a study of atom-pair coherence in the gas, which is significantly enhanced by the competing influences of the Fermi sea and Bose-Einstein condensation, in agreement with analytical calculations. Then, we study the depletion of a degenerate Bose-Fermi mixture following a quench to the unitary regime, characterizing the resulting depletion of the Bose-Einstein condensate, the deformation of the Fermi surface, and the production of molecules. We find that at early times, the population dynamics scale quadratically with the hold time, and define an associated characteristic timescale set by the parameters of the mixture and the width of the Feshbach resonance.

cond-mat.quant-gas↗

arXiv2024

Emergent inflation of the Efimov spectrum under three-body spin-exchange interactions

We resolve the unexpected and long-standing disagreement between experiment and theory in the Efimovian three-body spectrum of Li-7, commonly referred to as the lithium few-body puzzle. Our results show that the discrepancy arises out of the presence of strong non-universal three-body spin-exchange interactions, which enact an effective inflation of the universal Efimov spectrum. This conclusion is obtained from a thorough numerical solution of the quantum mechanical three-body problem, including precise interatomic interactions and all spin degrees of freedom for three alkali-metal atoms. Our results show excellent agreement with the experimental data regarding both the Efimov spectrum and the absolute rate constants of three-body recombination, and in addition reveal a general product propensity for such triatomic reactions in the Paschen-Back regime, stemming from Wigner's spin conservation rule.

cond-mat.quant-gas↗

arXiv2024

Resonance triplet dynamics in the quenched unitary Bose gas

The quenched unitary Bose gas is a paradigmatic example of a strongly interacting out-of-equilibrium quantum system, whose dynamics become difficult to describe theoretically due to the growth of non-Gaussian quantum correlations. We develop a conserving many-body theory capable of capturing these effects, allowing us to model the post-quench dynamics in the previously inaccessible time regime where the gas departs from the universal prethermal stage. Our results show that this departure is driven by the growth of strong lossless three-body correlations, rather than atomic losses, thus framing the heating of the gas in this regime as a fully coherent phenomenon. We uncover the specific few-body scattering processes that affect this heating, and show that the expected connection between the two-body and three-body contacts and the tail of the momentum distribution is obscured following the prethermal stage, explaining the absence of this connection in experiments. Our general framework, which reframes the dynamics of unitary quantum systems in terms of explicit connections to microscopic physics, can be broadly applied to any quantum system containing strong few-body correlations.

cond-mat.quant-gas↗

arXiv2024

Higgs oscillations in a unitary Fermi superfluid

Symmetry-breaking phase transitions are central to our understanding of states of matter. When a continuous symmetry is spontaneously broken, new excitations appear that are tied to fluctuations of the order parameter. In superconductors and fermionic superfluids, the phase and amplitude can fluctuate independently, giving rise to two distinct collective branches. However, amplitude fluctuations are difficult to both generate and measure, as they do not couple directly to the density of fermions and have only been observed indirectly to date. Here, we excite amplitude oscillations in an atomic Fermi gas with resonant interactions by an interaction quench. Exploiting the sensitivity of Bragg spectroscopy to the amplitude of the order parameter, we measure the time-resolved response of the atom cloud, directly revealing amplitude oscillations at twice the frequency of the gap. The magnitude of the oscillatory response shows a strong temperature dependence, and the oscillations appear to decay faster than predicted by time-dependent BCS theory applied to our experimental setup.

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arXiv2023

Efimovian three-body potential from broad to narrow Feshbach resonances

We analyse the change in the hyperradial Efimovian three-body potential as the two-body interaction is tuned from the broad to narrow Feshbach resonance regime. Here, it is known from both theory and experiment that the three-body dissociation scattering length $a_-$ shifts away from the universal value of $-9.7 \ r_{\mathrm{vdW}}$, with $r_{\mathrm{vdW}} = \frac{1}{2} \left(m C_6/\hbar^2 \right)^{1/4}$ the two-body van der Waals range. We model the three-body system using a separable two-body interaction that takes into account the full zero-energy behaviour of the multichannel wave function. We find that the short-range repulsive barrier in the three-body potential characteristic for single-channel models remains universal for narrow resonances, whilst the change in the three-body parameter originates from a strong decrease in the potential depth. From an analysis of the underlying spin structure we further attribute this behavior to the dominance of the two-body interaction in the resonant channel compared to other background interactions.

cond-mat.quant-gas↗

arXiv2022

Evolution of the unitary Bose gas for broad to narrow Feshbach resonances

We study the post-quench dynamics of unitary Bose gases using a two-channel model, focusing on the effect of variations in the width of the Feshbach resonance due to density changes. We generally find that increasing the density leads to a corresponding increase in the production of closed channel molecules, a decrease in the build up of quantum depletion and a transition from linear to quadratic early-time growth of the two-body contact as well as the condensed pair fraction. Motivated by the presence of closed-channel molecules in the unitary regime, we study the embedded two-body problem finding a transition from open to closed-channel dominated dimers due to many-body effects.

cond-mat.quant-gas↗

arXiv2020

Multichannel effects in the Efimov regime from broad to narrow Feshbach resonances

We study Efimov physics of three identical bosons with pairwise multichannel interactions for Feshbach resonances of adjustable width. We find that the two-body multichannel nature of the interaction can affect the universal three-body spectrum, especially for resonances of intermediate width. The shifts in this universal spectrum are caused by trimer states in the closed interaction channels that couple to the universal Efimov states. However, in the narrow resonance limit we find that the Efimov spectrum is set by the resonance width parameter $r^*$ only independent of the interaction potential considered. In the broad resonance limit all excited Efimov trimer energies approach the ones from the corresponding single-channel system for the scenarios investigated.

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