Geng et al., Sci. Adv. 11, eadw9752 (2025) 27 August 2025
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RESEARCH ARTICLE
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PHYSICS
The Rayleigh- Taylor instability in a binary
quantum fluid
Yanda Geng
1
†, Junheng Tao
1
†, Mingshu Zhao
1
, Shouvik Mukherjee
1
, Stephen P. Eckel
2
*,
Gretchen K. Campbell
1
*, Ian B. Spielman
1
*
Instabilities, where small uctuations seed the formation of large- scale structures, govern dynamics in a variety of
uid systems. The Rayleigh- Taylor instability (RTI), present from tabletop to astronomical scales, is an iconic ex-
ample characterized by mushroom- shaped incursions appearing when immiscible uids are forced together. De-
spite its ubiquity, RTI experiments are challenging; here, we report the observation of the RTI in an immiscible
binary superuid consisting of a two- component Bose- Einstein condensate. We force these components together
to initiate the instability, and observe the growth of mushroom- like structures. The interface can also be stabi-
lized, allowing us to spectroscopically measure the “ripplon” interface modes. Last, we use matter- wave interfer-
ometry to transform the superuid velocity eld at the interface into a vortex chain. These results—in agreement
with our theory—demonstrate the close connection between the RTI in classical and quantum uids.
INTRODUCTION
Fluid systems are replete with instabilities with wide- ranging im-
pact: be it droplet formation via the Plateau- Rayleigh instability
(1,2), destabilization of fusion reactions in tokamaks via magneto-
hydrodynamic instabilities (3), or galactic structure formation via
uid- gravitational instabilities (4). More specically, the iconic
Rayleigh- Taylor instability (RTI) (5,6) is crucial in classical uids
across scales, from laboratory experiments to astronomical phe-
nomena (7–10). e RTI is driven by buoyancy forces that press im-
miscible uids together, such as when a uid of higher density is
placed above a lower density uid in a gravitational potential. Under
these conditions, innitesimal uctuations at the horizontal inter-
face grow exponentially: At even a minuscule local elevation in-
crease the lighter uid upwells, and at local depressions the denser
uid sinks.Figure1 illustrates how this process unfolds: A nearly
at interface rst develops sinusoidal modulations via the RTI,
which, in conjunction with the Kelvin- Helmholtz instability, evolve
into bubble- , spike- , and mushroom- like structures that nally dis-
solve into a turbulent mixture. e ubiquitous presence of the RTI
in classical uids raises natural questions: Does it have analogs in
quantum uids, and, if so, how do the two relate?
Two- component Bose- Einstein condensates (BECs) with ferro-
magnetic interactions are ideal RTI candidates (11–13) as strong
repulsion between atoms in dierent internal states drives phase
separation (14) and magnetic gradient forces can rapidly switch be-
tween stable and metastable congurations. In both congurations,
interface waves with small height uctuations
cos
are
well described by a linearized model (12, 15), which predicts the
dispersion relation
that connects the wave vector k to the angular frequency
: the same
as for classical uid interfaces. Here,
is the dierential force be-
tween the layers; m is the uid particle mass;
is the average number
density of the uids; and
is the interfacial tension, which is analo-
gous to surface tension at a liquid- gas interface. In the stable con-
figuration,
, the frequency
is always real (blue curve in
Fig.2A). Consequently, initial uctuations at the interface travel as
waves known as gravity- capillary waves in classical uids, or rip-
plons in superuids. As a type of quantum excitations, ripplons are
of particular theoretical interest (16–18) as their energy is a fractional
power of momentum: neither quadratic (like usual nonrelativistic
particles) nor linear (like relativistic particles and Nambu- Goldstone
bosons in quantum eld theory). In the unstable conguration (or-
ange and green inFig.2A),
,
is real (solid curves) only for
wave vectors larger than
c
=
− ρ∕σ
(arrows) and becomes
imaginary for smaller wave vectors (dashed curves). e latter de-
scribes the RTI where excitations grow exponentially in time. Here,
we describe the experimental observation of the RTI as well as
ripplon excitations in a ferromagnetically interacting spinor BEC
of
23
Na atoms. We use matter- wave interference between our two
atomic states to transform information regarding the tangential ve-
locity dierence across the uid interface (i.e., counterow) into
an observable pearl- necklace- like chain of vortices. is interface-
velocimetry complements direct measurements of atomic density as
the RTI drives rapid growth of uid counterow (Fig.1).
RESULTS
Our experiments begin with homogeneous quasi–two- dimensional
(2D) BECs in a square conning potential, which enable the formation
of well- dened interfaces at y=0 with minimal symmetry breaking
(Fig.1). e BEC’s width W=114(3) μm greatly exceeds the interface
width, given by the spin healing length ξ
s
≈2 μm (see the Supple-
mentary Materials) for
23
Na atoms in the
≡
and
≡
hyperne states. We apply a magnetic eld
x, y
along
and precisely align the interface in the
plane by
introducing a small gradient [
≡
B
=
μT/cm] to our other-
wise uniform bias eld. is gradient exerts oppositely directed Stern-
Gerlach forces with dierence
∕h =
μ
−μ
B
∕h = 9.3
(
5
Hz/μm
2
=
1
2m
k +
σ
k
3
(1)
1
Joint Quantum Institute, University of Maryland and National Institute of Stan-
dards and Technology, College Park, MD 20742, USA.
2
National Institute of Stan-
dards and Technology, Gaithersburg, MD 20899, USA.
*Corresponding author. Email: stephen. eckel@ nist. gov (S.P.E.); gretchen. camp-
bell@ nist. gov (G.K.C.); ian. spielman@ nist. gov (I.B.S.)
†These authors contributed equally to this work.
Copyright © 2025 The
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reserved; exclusive
licensee American
Association for the
Advancement of
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original U.S.
Government Works.
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