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Journal of Physics B Atomic Molecular and Optical Physics

Parametric resonance of an interface in coherently coupled Bose–Einstein condensates and possible implications for pilot-wave theories

Vicky Howse, Raymond J. Spiteri

Published Aug 4, 2026• 0 citations
Publisher

Abstract

Abstract Faraday or parametric instability has been observed in various contexts in Bose–Einstein condensates. In Rabi-coupled condensates, the instability can arise without explicit modulation of external parameters. We study the dynamics of an interface in a two-component immiscible condensate near the miscible-immiscible threshold, with Rabi coupling between components. We show that a small initial perturbation leads to oscillations that grow in amplitude with increasing Rabi frequency until the interface produces droplet-like structures. We find that the interface oscillations are driven by parametric resonance with density oscillations in the bulk. This parametric instability of the interface provides a link to the walking droplet pilot wave system, which is governed by the Faraday instability. Because walking droplets simulate fundamentally quantum behaviors, and ‘vortex molecules,’ which are hosted by a Rabi-coupled condensate, simulate quark confinement, this observation suggests a mechanism-level connection between these systems and provides a possible bridge between distinct analog models of fundamental particles.

DOI
10.1088/1361-6455/ae94c7

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Abstract Faraday or parametric instability has been observed in various contexts in Bose–Einstein condensates. In Rabi-coupled condensates, the instability can arise without explicit modulation of external parameters. We study the dynamics of an interface in a two-component immiscible condensate near the miscible-immiscible threshold, with Rabi coupling between components. We show that a small initial perturbation leads to oscillations that grow in amplitude with increasing Rabi frequency until the interface produces droplet-like structures. We find that the interface oscillations are driven by parametric resonance with density oscillations in the bulk. This parametric instability of the interface provides a link to the walking droplet pilot wave system, which is governed by the Faraday instability. Because walking droplets simulate fundamentally quantum behaviors, and ‘vortex molecules,’ which are hosted by a Rabi-coupled condensate, simulate quark confinement, this observation suggests a mechanism-level connection between these systems and provides a possible bridge between distinct analog models of fundamental particles.
We study the parametric resonance of capillary waves on the interface between two immiscible Bose-Einstein condensates pushed towards each other by an oscillating force. Guided by analytical models, we solve numerically the coupled Gross-Pitaevskii equations for a two-component Bose-Einstein condensate at zero temperature. We show that, at moderate amplitudes of the driving force, the instability is stabilized due to nonlinear modifications of the oscillation frequency. When the amplitude of the driving force is large enough, we observe a detachment of droplets from the Bose-Einstein condensates, resulting in the generation of quantum vortices (skyrmions). We analytically investigate the vortex dynamics, and conditions of quantized vortex generation.
A Faraday-wave-like parametric instability is investigated via mean-field and Floquet analysis in immiscible binary Bose-Einstein condensates. The condensates form a so-called ball-shell structure in a two-dimensional harmonic trap. To trigger the dynamics, the scattering length of the core condensate is periodically modulated in time. We reveal that in the dynamics the interface becomes unstable towards the formation of oscillating patterns. The interface oscillates subharmonically, exhibiting an mmm-fold rotational symmetry that can be controlled by maneuvering the amplitude and the frequency of the modulation. Using Floquet analysis we are able to predict the generated interfacial tension of the mixture and derive a dispersion relation for the natural frequencies of the emergent patterns. A heteronuclear system composed of $^{87}\mathrm{Rb}\ensuremath{-}^{85}\mathrm{Rb}$ atoms can be used for the experimental realization of the phenomenon, yet our results are independent of the specifics of the employed atomic species and of the parameters such as scattering lengths and trap strengths at which the driving is applied.
By studying the dynamic stability of Bose-Einstein-condensed binary mixtures trapped on the surface of an ideal two-dimensional spherical bubble, we show how the Rabi coupling between the species can modulate the interactions leading to parametric resonances. In this spherical geometry, the discrete unstable angular modes drive both phase separations and spatial patterns, with Faraday waves emerging and coexisting with an immiscible phase. Noticeable is the fact that, in the context of discrete kinetic energy spectrum, the only parameters to drive the emergence of Faraday waves are the $s\text{\ensuremath{-}}wave$ contact interactions and the Rabi coupling. Once analytical solutions for population dynamics are obtained, the stability of homogeneous miscible species is investigated through Bogoliubov--de Gennes and Floquet methods, with predictions being analyzed by full numerical solutions applied to the corresponding time-dependent coupled formalism.
We investigate interfacial Faraday instabilities in a quasi-two-dimensional, phase-separated binary Bose-Einstein condensate driven by periodic modulation of the interspecies scattering length. In the strongly immiscible regime, the interspecies interaction energy is concentrated near the internal interface, linking the modulation to interfacial tension and the capillary restoring force. We quantify the first-order tension response through its logarithmic sensitivity and account for nonlinear tension variations at finite modulation amplitudes. A sharp-interface capillary model and Floquet analysis yield discrete azimuthal-mode frequencies and primary instability tongues that shift toward higher driving frequencies with increasing mode order. Gross-Pitaevskii simulations reveal star-shaped interface patterns with threefold to eightfold rotational symmetry. The eightfold mode exhibits subharmonic oscillations and approximately exponential growth of its period-averaged amplitude over a small-deformation interval. These results provide a route to frequency-selective excitation of interfacial capillary modes through interspecies-interaction control.
We investigate parametric excitation and pattern formation in a harmonically trapped two-component Bose-Einstein condensate. We assume the condensate to be in the miscible phase, but near the miscible-immiscible phase transition, where total-density and spin-density excitations are decoupled. By periodically modulating the atomic scattering lengths, Faraday patterns can be generated in both density and spin channels. In an elongated condensate, the pattern in the spin channel corresponds to a one-dimensional standing wave with the two components exhibiting an out-of-phase density oscillation, where the modulation frequency and the oscillation period are related to the velocity of the spin sound. After the spin pattern is fully developed, the system quickly enters a nonlinear destabilization regime. For a pancake-shaped condensate, a two-dimensional Faraday pattern is generated with an interesting lll-fold rotational symmetry. The number of nodes along the radial and angular directions increases with larger modulation frequencies. We also compare the growth rates of spin Faraday patterns generated with different modulation protocols, which are accessible to current experiments.
The onset of pattern formation in a spatially homogeneous system subjected to external driving is an important topic in various scientific fields. A celebrated classical example is the Faraday instability, where a vertically oscillated fluid surface undergoes a parametric resonance, giving rise to standing waves that self-organize into regular spatial patterns. Bose-Einstein condensates (BECs) provide an ideal quantum-mechanical platform for studying pattern-forming mechanisms due to their exceptional degree of experimental control. As a compressible state of quantum matter, a condensate responds sensitively to external perturbations, including time-periodic modulation of interactions, trapping potentials, or external fields. These features make BECs particularly well suited for exploring driven nonequilibrium phases and pattern formation. In this chapter, we review the remarkable progress achieved in this field over the past two decades. We begin with the first theoretical proposal predicting parametric instabilities and emergent Faraday waves in driven condensates. We then discuss key experimental and theoretical breakthroughs that confirmed these predictions and refined the understanding of the underlying mechanisms. This line of research has culminated in the recent observation of a stabilized square lattice pattern in a periodically driven BEC confined in a two-dimensional geometry. This driven superfluid state with superposed density modulation was shown to exhibit some features of a supersolid state.