arXiv subjects

John L. Helm

Publications and source records attributed to John L. Helm.

4 recordsLinked to original sources

arXiv2020

Observation of two-dimensional Anderson localisation of ultracold atoms

Anderson localisation -- the inhibition of wave propagation in disordered media -- is a surprising interference phenomenon which is particularly intriguing in two-dimensional (2D) systems. While an ideal, non-interacting 2D system of infinite size is always localised, the localisation length-scale may be too large to be unambiguously observed in an experiment. In this sense, 2D is a marginal dimension between one-dimension, where all states are strongly localised, and three-dimensions, where a well-defined phase transition between localisation and delocalisation exists as the energy is increased. Here we report the results of an experiment measuring the 2D transport of ultracold atoms between two reservoirs, which are connected by a channel containing pointlike disorder. The design overcomes many of the technical challenges that have hampered observation of localisation in previous works. We experimentally observe exponential localisation in a 2D ultracold atom system.

cond-mat.quant-gas↗

arXiv2019

Splitting and recombination of bright-solitary-matter waves

Solitons are long-lived wavepackets that propagate without dispersion and exist in a wide range of one-dimensional (1D) nonlinear systems. A Bose-Einstein condensate trapped in a quasi-1D waveguide can support bright-solitary-matter waves (3D analogues of solitons) when interatomic interactions are sufficiently attractive that they cancel dispersion. Solitary-matter waves are excellent candidates for a new generation of highly sensitive interferometers, as their non-dispersive nature allows them to acquire phase shifts for longer times than conventional matter-waves interferometers. However, such an interferometer is yet to be realised experimentally. In this work, we demonstrate the splitting and recombination of a bright-solitary-matter wave on a narrow repulsive barrier, which brings together the fundamental components of an interferometer. We show that both interference-mediated recombination and classical velocity filtering effects are important, but for a sufficiently narrow barrier interference-mediated recombination can dominate. We reveal the extreme sensitivity of interference-mediated recombination to the experimental parameters, highlighting the potential of soliton interferometry.

cond-mat.quant-gas↗

arXiv2017

Spin-orbit coupled interferometry with ring-trapped Bose--Einstein condensates

We propose a method of atom-interferometry using a spinor Bose-Einstein condensate (BEC) with a time-varying magnetic field acting as a coherent beam-splitter. Our protocol creates long-lived superpositional counterflow states, which are of fundamental interest and can be made sensitive to both the Sagnac effect and magnetic fields on the sub-micro-G scale. We split a ring-trapped condensate, initially in the $m_f=0$ hyperfine state, into superpositions of internal $m_f=\pm1$ states and condensate superflow, which are spin-orbit coupled. After interrogation, relative phase accumulation can be inferred from a population transfer to the $m_f=\pm1$ states. The counterflow generation protocol is adiabatically deterministic and does not rely on coupling to additional optical fields or mechanical stirring techniques. Our protocol can maximise the classical Fisher information for any rotation, magnetic field, or interrigation time, and so has the maximum sensitivity available to uncorrelated particles. Precision can increase with the interrogation time, and so is limited only by the lifetime of the condensate.

cond-mat.quant-gas↗

arXiv2012

Bright matter-wave soliton collisions at narrow barriers

We study fast-moving bright solitons in the focusing nonlinear Schrödinger equation perturbed by a narrow Gaussian potential barrier. In particular, we present a general and comprehensive analysis of the case where two fast-moving bright solitons collide at the location of the barrier. In the limiting case of a delta-function barrier, we use a quasi-analytic method to show that the relative norms of the outgoing waves depends sinusoidally on the relative phase of the incoming waves, and to determine whether one, or both, of the outgoing waves are bright solitons. We show using numerical simulations that this quasi-analytic result is valid in the high velocity limit: outside this limit nonlinear effects introduce a skew to the phase-dependence, which we quantify. Finally, we numerically explore the effects of introducing a finite-width Gaussian barrier. Our results are particularly relevant, as they can be used to describe a range of interferometry experiments using bright solitary matter-waves.

cond-mat.quant-gas↗