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Atomic and Laser Physics
Credit: Jack Hobhouse

Professor Christopher Foot

Professor of Physics

Research theme

  • Quantum optics & ultra-cold matter

Sub department

  • Atomic and Laser Physics

Research groups

  • Ultracold quantum matter
  • AION/Magis
Christopher.Foot@physics.ox.ac.uk
Telephone: 01865 (2)72256
Clarendon Laboratory, room 161
  • About
  • Publications

Observation of a bilayer superfluid with interlayer coherence

Nature Communications Nature Research 16:1 (2025) 7201

Authors:

Erik Rydow, Vijay Pal Singh, Abel Beregi, En Chang, Ludwig Mathey, Christopher J Foot, Shinichi Sunami

Abstract:

Controlling the coupling between different degrees of freedom in many-body systems is a powerful technique for engineering novel phases of matter. We create a bilayer system of two-dimensional (2D) ultracold Bose gases and demonstrate the controlled generation of bulk coherence through tunable interlayer Josephson coupling. We probe the resulting correlation properties of both phase modes of the bilayer system: the symmetric phase mode is studied via a noise-correlation method, while the antisymmetric phase fluctuations are directly captured by matter-wave interferometry. The measured correlation functions for both of these modes exhibit a crossover from short-range to quasi-long-range order above a coupling-dependent critical point, thus providing direct evidence of bilayer superfluidity mediated by interlayer coupling. We map out the phase diagram and interpret it with renormalization-group theory and Monte Carlo simulations. Additionally, we elucidate the underlying mechanism through the observation of suppressed vortex excitations in the antisymmetric mode.
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Parametric resonance with linear damping: a general formula for the excitation threshold for high orders

Physica Scripta IOP Publishing 100:7 (2025) 075257

Authors:

Christopher J Foot, Dimitrios Trypogeorgos

Abstract:

We derive a general formula for the excitation threshold of parametric resonances of an oscillator with linear damping from consideration of the asymptotic properties of the Mathieu equation. This provides a good approximation for resonances of order m ≥ 2, and it is especially useful for high-order resonances in systems with light damping for which other approaches are cumbersome. Parametric resonance is ubiquitous in mechanical and electrical systems and its threshold is an important consideration, e.g., for systems that would be damaged by a high amplitude of resonantly excited motion. We present the expressions in a form useful for understanding systems with high quality factors such as trapped atomic ions, micro-mechanical devices and other oscillators, especially those with low dissipation in vacuum. High-order parametric resonances are extremely narrow making direct numerical simulation computationally intensive as well as less insightful.
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Detecting Phase Coherence of 2D Bose Gases via Noise Correlations

Physical Review Letters American Physical Society (APS) 134:18 (2025) 183407

Authors:

Shinichi Sunami, Vijay P Singh, Erik Rydow, Abel Beregi, En Chang, Ludwig Mathey, Christopher J Foot
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Terrestrial Very-Long-Baseline Atom Interferometry: summary of the second workshop

EPJ Quantum Technology SpringerOpen 12:1 (2025) 42

Authors:

Adam Abdalla, Mahiro Abe, Sven Abend, Mouine Abidi, Monika Aidelsburger, Antun Balaž, Hannah Banks, Rachel L Barcklay, Michele Barone, Michele Barsanti, Mark G Bason, Angelo Bassi, Aleksandar Belić, Shayne Bennetts, Daniela Bortoletto, Oliver Buchmueller

Abstract:

This summary of the second Terrestrial Very-Long-Baseline Atom Interferometry (TVLBAI) Workshop provides a comprehensive overview of our meeting held in London in April 2024 (Second Terrestrial Very-Long-Baseline Atom Interferometry Workshop, Imperial College, April 2024), building on the initial discussions during the inaugural workshop held at CERN in March 2023 (First Terrestrial Very-Long-Baseline Atom Interferometry Workshop, CERN, March 2023). Like the summary of the first workshop (Abend et al. in AVS Quantum Sci. 6:024701, 2024), this document records a critical milestone for the international atom interferometry community. It documents our concerted efforts to evaluate progress, address emerging challenges, and refine strategic directions for future large-scale atom interferometry projects. Our commitment to collaboration is manifested by the integration of diverse expertise and the coordination of international resources, all aimed at advancing the frontiers of atom interferometry physics and technology, as set out in a Memorandum of Understanding signed by over 50 institutions (Memorandum of Understanding for the Terrestrial Very Long Baseline Atom Interferometer Study).
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CNN-Based Vortex Detection in Atomic 2D Bose Gases in the Presence of a Phononic Background

Machine Learning: Science and Technology IOP Publishing (2025)

Authors:

Magnus Sesodia, Shinichi Sunami, En Chang, Erik Rydow, Christopher Foot, Abel Beregi

Abstract:

<jats:title>Abstract</jats:title> <jats:p>Quantum vortices play a crucial role in both equilibrium and dynamical phenomena in two-dimensional (2D) superfluid systems. Experimental detection of these excitations in 2D ultracold atomic gases typically involves examining density depletions in absorption images, however the presence of a significant phononic background renders the problem challenging, beyond the capability of simple algorithms or the human eye. Here, we utilize a convolutional neural network (CNN) to detect vortices in the presence of strong long- and intermediate-length scale density modulations in finite-temperature 2D Bose gases. We train the model on datasets obtained from ab initio Monte Carlo simulations using the classical-field method for density and phase fluctuations, and Gross-Pitaevskii simulation of realistic expansion dynamics. We use the model to analyze experimental images and benchmark its performance by comparing the results to the matter-wave interferometric detection of vortices, confirming the observed scaling of vortex density across the Berezinskii-Kosterlitz-Thouless (BKT) critical point. The combination of a relevant simulation pipeline with machine-learning methods is a key development towards the comprehensive understanding of complex vortex-phonon dynamics in out-of-equilibrium 2D quantum systems.</jats:p>
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