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Observation of Bose–Einstein condensates in an Earth-orbiting research lab

Sérgio Sacani
Sérgio Sacani
Sérgio SacaniSupervisor de Geologia na Halliburton at Halliburton

Quantum mechanics governs the microscopic world, where low mass and momentum reveal a natural wave–particle duality. Magnifying quantum behaviour to macroscopic scales is a major strength of the technique of cooling and trapping atomic gases, in which low momentum is engineered through extremely low temperatures. Advances in this feld have achieved such precise control over atomic systems that gravity, often negligible when considering individual atoms, has emerged as a substantial obstacle. In particular, although weaker trapping felds would allow access to lower temperatures1,2 , gravity empties atom traps that are too weak. Additionally, inertial sensors based on cold atoms could reach better sensitivities if the free-fall time of the atoms after release from the trap could be made longer3 . Planetary orbit, specifcally the condition of perpetual free-fall, ofers to lift cold-atom studies beyond such terrestrial limitations. Here we report production of rubidium Bose–Einstein condensates (BECs) in an Earth-orbiting research laboratory, the Cold Atom Lab. We observe subnanokelvin BECs in weak trapping potentials with free-expansion times extending beyond one second, providing an initial demonstration of the advantages ofered by a microgravity environment for cold-atom experiments and verifying the successful operation of this facility. With routine BEC production, continuing operations will support long-term investigations of trap topologies unique to microgravity4,5 , atom-laser sources6 , few-body physics7,8 and pathfnding techniques for atom-wave interferometry9–12

Observation of Bose–Einstein condensates in an Earth-orbiting research lab

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Nature | Vol 582 | 11 June 2020 | 193
Article
ObservationofBose–Einsteincondensates
inanEarth-orbitingresearchlab
David C. Aveline1,2 ✉, Jason R. Williams1,2
, Ethan R. Elliott1,2
, Chelsea Dutenhoffer1
,
James R. Kellogg1
, James M. Kohel1
, Norman E. Lay1
, Kamal Oudrhiri1
, Robert F. Shotwell1
,
Nan Yu1
& Robert J. Thompson1 ✉
Quantummechanicsgovernsthemicroscopicworld,wherelowmassandmomentum
revealanaturalwave–particleduality.Magnifyingquantumbehaviourto
macroscopicscalesisamajorstrengthofthetechniqueofcoolingandtrapping
atomicgases,inwhichlowmomentumisengineeredthroughextremelylow
temperatures.Advancesinthisfieldhaveachievedsuchprecisecontroloveratomic
systemsthatgravity,oftennegligiblewhenconsideringindividualatoms,has
emergedasasubstantialobstacle.Inparticular,althoughweakertrappingfields
wouldallowaccesstolowertemperatures1,2
,gravityemptiesatomtrapsthataretoo
weak.Additionally,inertialsensorsbasedoncoldatomscouldreachbetter
sensitivitiesifthefree-falltimeoftheatomsafterreleasefromthetrapcouldbemade
longer3
.Planetaryorbit,specificallytheconditionofperpetualfree-fall,offerstolift
cold-atomstudiesbeyondsuchterrestriallimitations.Herewereportproductionof
rubidiumBose–Einsteincondensates(BECs)inanEarth-orbitingresearchlaboratory,
theColdAtomLab.WeobservesubnanokelvinBECsinweaktrappingpotentialswith
free-expansiontimesextendingbeyondonesecond,providinganinitial
demonstrationoftheadvantagesofferedbyamicrogravityenvironmentfor
cold-atomexperimentsandverifyingthesuccessfuloperationofthisfacility.With
routineBECproduction,continuingoperationswillsupportlong-terminvestigations
oftraptopologiesuniquetomicrogravity4,5
,atom-lasersources6
,few-bodyphysics7,8
andpathfindingtechniquesforatom-waveinterferometry9–12
.
With the launch and operation of the Cold Atom Lab (CAL), NASA has
establishedthesustainedstudyanddevelopmentofquantumtechnolo-
gies in orbit. This versatile, multi-user research facility has travelled
over400 millionkilometresonboardtheInternationalSpaceStation
(ISS) since June 2018, under remote operation from the Jet Propul-
sion Laboratory. Exploiting the microgravity environment of space,
researcherscanutilizethefullsensitivityofultracoldmatter-wavesto
explorefundamentalphysicsandtheorganizingprinciplesofcomplex
systemsfromwhichstructureanddynamicsemerge—executingmajor
‘thrusts’oftheNationalResearchCouncil’sDecadalSurveythatdefine
the frontier of space-based fundamental physical science13
.
Understanding quantum mechanics has made possible the
now-ubiquitous technologies of lasers, semiconductors and
transistors,butremainingelusiveisitsrelationshipwithgeneralrelativ-
ity, the physics of gravity, which is well understood at macroscale to
astronomic scales. Scaling quantum mechanics to macroscopic sizes
isaprimarygoalofcoolingatomicgasestowardsabsolutezero,where
wave-like behaviour markedly increases as temperature drops. With
enoughcooling,eachatom’swavelengthapproachestheinterparticle
spacingandthesystemexhibitsthemacroscopicquantumbehaviour
ofsuperfluidity.Foradilutegasofbosons—thetypeofatomscontained
in CAL—this phenomenon is known as Bose–Einstein condensation.
These quantum gases, and particularly BECs, have been studied for
theirintrinsicproperties,usedasanaloguesofmoreinaccessiblesys-
tems,orappliedasinertialsensingmatterwaves.Thestate-of-the-art
technology has advanced to such a degree that additional cooling
techniquesarestifledbygravity.Forexample,theconfiningpotentials
that trap the atoms can be adiabatically decompressed to decrease
temperature, but only until the local potential minimum is collapsed
by gravity’s asymmetric pull. Furthermore, following release from an
atom trap, gravity-induced centre-of-mass motion greatly limits the
system’sutilityasaninertialsensor.Theidealconditionsforengineer-
ing macroscopic atom-waves thus becomes ultracold temperatures
combined with reduced gravity.
Pioneering cold-atom experiments have mitigated the effects of
gravity through a variety of methods. Ground-based levitation tech-
niques accomplish a localized counter-balance to gravity in decom-
pressionexperiments1
,buttheyintroducemass-andstate-dependent
forcesthatbroadlylimitatom-waveinterferometryandtestsbetween
multiple atomic species. Because cold-atom experiments require
an ultrahigh vacuum (UHV) to thermally isolate the atoms from the
ambientenvironment,theprimarysolutionsareeithertoenlargethe
sizeoftheUHVchamberorincreasethefree-falltimeoftheentireappa-
ratus. Ground-based interferometers have achieved state-of-the-art
https://doi.org/10.1038/s41586-020-2346-1
Received: 30 October 2019
Accepted: 26 March 2020
Published online: 11 June 2020
Check for updates
1
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. 2
These authors contributed equally: David C. Aveline, Jason R. Williams, Ethan R. Elliott.
✉e-mail: David.C.Aveline@jpl.nasa.gov; Robert.J.Thompson@jpl.nasa.gov
194 | Nature | Vol 582 | 11 June 2020
Article
interrogationtimesandtemperaturesbylaunchingatomsinastation-
ary10-m-tallUHVchamber14,15
.Incontrast,portablecold-atomsystems
that are compact and ruggedized have realized temporary free-fall
through drop towers16–18
, Einstein elevators19
, zero-g aircraft20,21
and
suborbital launch vehicles22–26
. Notably, six minutes of microgravity
achievedinasoundingrocketinparabolicflightabovetheKármánline
(thealtitudeof100kmabovesealevel,whichisconsideredthebound-
ary of space) demonstrated production and manipulation of BECs26
.
For these portable instruments to advance in scope and capability,
theiroperationtimesandrepetitionratesmustincrease.Asthenatural
next step, new generations of quantum experiments seek to operate
aversatileultracoldatomicphysicslabinthepersistentmicrogravity
conditionsoflowEarthorbit3
.Spacemissionsenablelargevariations
of velocity and gravitational potential compared to ground-based
laboratories, inspiring many proposed missions. These concepts
include tests of Einstein’s equivalence principle with quantum proof
masses27,28
, gravitational wave detection that could complement the
LaserInterferometerSpaceAntenna(LISA)29–32
,opticalatomicclocks
for a ‘world-clock’33
, direct detection of dark matter and dark energy
candidates27,34
,geodesy,Earthandplanetarysciences35–37
andadvanced
navigation capabilities37–39
.
Therealizationofanoff-worldcold-atominstrumentpresentsnew
technicalchallenges.Itnotonlycallsforrobusthardwareadheringto
the stringent size, weight and power requirements of a spaceborne
instrument,butmustalsoreliablyandautonomouslyoperateforyears
without manual intervention. In addition to technological matura-
tion(uniqueaspectsandchallengesindevelopingthisEarth-orbiting
cold-atom facility are elaborated in Methods), long-term operation
allowsadjustmentandadaptationofthepreciseproceduresandpro-
tocolsneededtocontrolatomsinmicrogravity.Thecapabilitiesofthe
CALinstrumentsenablemultipleteamsofscientiststostudyquantum
gasesatunprecedentedlowenergiesanddensitiesproducedinweakly
confiningtraps.The87
RbBECswereportherewillspecificallysupport
investigationsofatomlasersources6,9
,bubble-shelltopologiesunique
to microgravity4,5
and advanced techniques for Earth-orbiting atom
interferometry including delta kick cooling9
and novel decompres-
sion protocols10,11
.
Followingitslaunchon21May2018ontheOA-9Antares230rocket,
theCALhardwarewasinstalledandfullypoweredonboardtheISSby
June 2018, commencing daily operation as the first ultracold atom
system in orbit. As described in ref. 40
, the instrument is composed of
three primary subsystems: the science module, the laser and optics
system, and the electronics, supported by thermal, mechanical and
software control, all designed for modular integration into a stand-
ardized ISS equipment rack (see Fig. 1). Re-configurable traps that
hold and manipulate cold atoms are created by the current-carrying
wiresofan‘atomchip’thatformsthetopwallofarectangularglasscell
underUHV41
.Wire-loopemitterspositionedneartheambientsurface
of the chip transmit radio frequency (RF) or microwave radiation for
evaporativecoolingandstatepreparation.Furtherinstrumentdetails
can be found in Methods and Extended Data Fig. 1.
Implementing the same stages of laser cooling and magnetic
trappingof87
Rbinthestate|F = 2,mF = 2⟩,asexecutedontheground40
,
we find that RF-induced evaporative cooling (see Methods) reveals
markedlydifferentresultsinmicrogravity.Figure 2showsaterrestrial
BEC produced before the launch of CAL compared to an evaporated
atom cloud on the ISS. We observe an on-orbit increase in the atom
numberofnearlythreefold.Throughtheapplicationofvariedmagnetic
field gradients, we confirm that approximately half of the atoms are
in the magnetically insensitive state |2, 0⟩, forming a halo-like cloud
around the location of the magnetic trap. On Earth, the dominating
force acting on atoms in this |2, 0⟩ state is gravity. In microgravity,
however, the dominant force acting on these |2, 0⟩ atoms is due to
the quadratic Zeeman (QZ) effect, given by42
h• (287.58 Hz G−2
)•B2
.
Quad-locker exploded view Science module
Quad-locker installation on ISS
b
d e
f
Science cell
Source cell
Atom chip
Vacuum chamber
z
x
y
dw = 2 mmId
Iz
Imaging (z)
Imaging (y)
zim ≈ 4 mm
ISS orbit
a
z
x
y
EXPRESS rack 7
inside Destiny
Single locker
Quad-locker
z
x
y
c
Fig.1|CALhardwareconfiguration.a,TheCALpayloadresidesinan‘EXpedite
thePRocessingofExperimentstotheSpaceStation’(EXPRESS)rackwithinthe
USLabDestinyModule,locatedwhereshownintheISS.Thex,y,zcoordinates
areillustrated.b,Thepayloadwasinstalledbyastronautsduringa6-hcrew
operationunderguidancefromJPL(imagesource:NASA).c,ViewofEXPRESS
rack7.CALispackagedwithinthisrackinasinglelockerandaquad-locker,
requiringoneandfourpayloadslots,respectively.Thesinglelockerhousesthe
powerelectronics,whilethequad-lockercontainsthecomputer,lasersources,
opticalcontrolnetwork,controlelectronicsandthemagneticallyshielded
sciencemodule.d,Anexplodedviewofthequad-locker,illustratingitsmodular
design,includingdraweraccesstolasersandelectronicsfromthefront,andthe
sciencemoduletotheright.e,Aninsideviewofthesciencemodule,withoutthe
fronthalf-shellofthemagneticshields.f,Attheheartofthesciencemodule,an
aluminiumopticalbenchholdsadual-cellvacuumchamber.Formingthetop
walloftheUHVsciencecellisanatomchip,whichcangeneratere-configurable
magnetictrapswithitscurrent-carryingwires.Theprimarytrapiscentrally
locatedonawindowofdiameterdw = 2 mm,formedbychipcurrentsIz andId.This
opticalaccessallowsabsorptionimagingalongthezaxis,whileCAL’sprimary
imagingisaccomplishedalongtheyaxiscentredatzim ≈ 4 mmbelowthechip
(purplearrows).SeeMethodsandExtendedDataFig. 1formoredetailsaboutthe
opticalbeams.
Nature | Vol 582 | 11 June 2020 | 195
Increasingtheappliedfieldgradienttoapproximately∇B = 43 G cm−1
,
we demonstrate motion of the |2, 0⟩ halo-cloud due to this QZ effect
(Fig. 2g). Further incidental QZ forces acting on the |2, 0⟩ halo-cloud
throughoutthestandardoperationofourchip-basedtrap(owingtoits
largemagneticfieldsandgradients)arediscussedbelow.Bycontrolled
manipulation and isolation of the atoms in the |2, 0⟩ state, we readily
confirm production of 87
Rb |2, 2⟩ BECs in orbit. Figure 3 illustrates the
onsetofcondensationascloudtemperatureisreducedbelowtheBEC
transition temperature. Tell-tale signatures of condensation include
thesuddenonsetofabimodaldensitydistributionbelowthetheoreti-
cally predicted critical temperature at long time of flight (TOF), and a
denseellipticalcore(anisotropicexpansion)surroundedbyathermal
gas background (isotropic expansion).
Taking advantage of the intrinsic advantages of microgravity,
we demonstrate a combination of low temperatures and extended
free-space expansion times with only minor readjustments to our
pre-launchprocedures.Beginningfromanominal87
RbBEC,wedecom-
press the trap, transfer the atoms to a magnetically insensitive state,
and release the atoms with parameters empirically adjusted in orbit
to minimize the temperature and drift velocity of the atom cloud in
free-fall. We benefit from the relatively high repetition rate (once per
minute)andextendedexperimentaltimeavailableontheISStomake
theseadjustments,receivereal-timefeedback,andmodifyaccordingly.
Decompression is carried out by two stages of linear reduction to the
bias fields applied by the external coils, thereby decreasing the trap
frequency and translating the trap centre approximately 1 mm away
from the chip (see Methods).
Images of atoms, along with the respective cloud positions and
widths, during free expansion up to 1.118 s are shown in Fig. 4, with
the residual expansion of the BEC corresponding to a kinetic energy
temperatureequivalentof231(9) pK(720(79) pK)inthedirectionparal-
lel (normal) to the chip surface. In the earliest expansion time shown
inFig. 4a,thehalo-cloudof|2,0⟩producedduringevaporationisalso
present,coastingawayfromthechipwiththemotionimpartedbythe
QZ effect when the location of the trap bottom shifts during decom-
pression. Producing this combination of low temperatures and long
expansion times using a simple adjustment to our ground procedure
demonstrates not only the natural benefit of microgravity, but also
theabilitytorelyonempiricallyderivedoptimizationoverthecourse
of days, as well as the potential for further improvements. As CAL sci-
enceoperationscontinue,investigatorswilltakefurtheradvantageof
microgravity by introducing more complex decompression curves,
allowing greater adiabaticity, less residual centre-of-mass motion,
lower temperatures and subhertz trapping frequencies.
The next generation of ultracold gas experiments using weak traps
mayalsobenefitfromfurtherstudiesoftheobservedhalo-cloudof|2,
0⟩ atoms. We note that the halo-cloud is shaped by a combination of
factors:theinitialtemperatureanddensitydistributionofthetrapped
atoms, the momentum distribution of |2, 0⟩ as they are generated by
in situ RF coupling, and finally the dominating forces due to the QZ
effectinthepresenceofthechip’sstrongmagneticfieldsandgradients.
Anatomthatundergoesaforced-evaporationtransitionintothe|2,0⟩
state by application of resonant RF radiation (‘RF knife’) would lose
potentialenergyequaltothemagnetictrapdepthandpropagatewith
kinetic energy proportional to kBT plus a new, lower potential energy
determined by the QZ effect. In the present work, the magnetic fields
during evaporation form an attractive QZ potential for atoms in the
|2, 0⟩ state with 18 μK depth and aspect ratio ≤0.2 (see Methods). The
timescales on which we currently observe the halo-clouds are con-
sistent with confinement and subsequent release from such a trap in
Rotated larger view
of the in-orbit result
–150 150–100 100–50 500
–150 150–100 100–50 500
Decompression to
10% bias field
Applied B-field gradient
15 G cm–1 for 19 ms
Applied B-field gradient
7 G cm–1 for 19 ms
Applied B-field gradient
43 G cm–1 for 44 ms
In-orbit
Ground
a
b
c d e f g
z
x
|2,2〉
|2,1〉
|2,0〉|2,0〉
|2,2〉
|2,1〉
|2,0〉
•
B
x
z B B B
Fig.2|BECproductioninCALontheEarthandinorbit.a,Afalse-colour
absorptionimageshowsaBECproducedinCALonthegroundat22 msTOF,
totalling4.5 × 104
atomsandaBECfractionof15%.b,Likewise,aBECat22 ms
TOFproducedinCALonboardtheISSusingthesameparametersasonthe
ground,totalling1.3 × 105
atoms,showsthatalargeportionoftheseatomshave
adensityprofiledistinctlydifferentfromtheexpectedbimodaldistribution.
Plotsadjacenttotheimagesaandbshowthedata(green)ofcolumndensity
alongpixelsofthexaxis.Fitsindicatethecondensate(blue),thermalcloud
(red),andresidualhalo-cloud(orange)of|2,0⟩atoms.c,Alargerregionof
interestthanshowninb,usingthesameproductionsequencesexceptfor
changestothereleaseprocedureasfollows(d–g).d,Releasedatomswiththe
locationofthetrapbottommoved0.65 mmawayfromthesurfaceofthechip.
Amagneticallyinsensitivehalo-cloudof|2,0⟩atomsisleftbehindduringthis
re-positioningoftheprimarytrapbottom.e, f,Thesamedecompression
procedureasshownind,butfollowedbyapplyingatransversegradientof
7 G cm−1
(e)orof15 G cm−1
(f)forthefirst19 msofa22-msTOF.Thehalo-cloud
remainsunmoved.Afaintcloudof|2,1⟩atomsthatwasweaklyconfinedinthe
trapexperienceslessaccelerationandtrailshalfwaybehindthe|2,2⟩atoms.
g,Thesamedecompressionprocedureasshownind,butfollowedby
applicationofastrongertransversegradientof43 G cm−1
forthefirst44 msofa
50-msTOF,whichacceleratesthe|2,0⟩cloudduetotheQZeffect42
.Anarrow
beloweachpaneld–gillustratesthedirectionalforceappliedbyincremental
changesinmagneticfields,dynamically,B˙,orspatially,∇B.
196 | Nature | Vol 582 | 11 June 2020
Article
microgravity,whichisnormallyoverpoweredbygravitationalsaginour
measurementsonEarth.Wenotethatalow-frequencyanharmonictrap
naturally formed and populated with large numbers of magnetically
insensitiveatomsthroughouttheevaporationprocessisanintriguing
alternative to decompression and adiabatic rapid passage for future
microgravityexperiments.Wearecurrentlyexploringtheviabilityof
directly producing a BEC in such a trap. A BEC in a three-dimensional
quartic trap (as given near the minimum of the QZ potential and in
the absence of gravity) is expected to have a substantially flatter con-
densate density profile near the centre as compared to that of a har-
monicallytrappedcondensate43
,presentingopportunitiestoobserve
transitionsandphasesofquantummatterinarelativelyhomogeneous
system.Also,werecognizethattheabsenceofthefirst-orderZeeman
shift for atoms in magnetically insensitive states makes these gases
385 µm
550µm
PSD = 1
Tc = 130 nK
PSD = 0.7PSD = 0.6
0
1
2
3
4
OD
Fig.3|OnsetofBose–Einsteincondensationinlow
Earthorbit. False-colourabsorptionimagesof87
Rb
atomiccloudsevaporatedandreleasedfromCAL’s
magneticatomchiptraparedisplayedassurfaceplots
(top)andcorrespondingarrays(bottom).OD
indicatesopticaldensity.Fromlefttoright,thefinal
frequencyofaforcedevaporationrampusinganRF
knifeinamagneticatomchiptrapislowered,
increasingthephasespacedensity(PSD)andforming
aBECmarkedbyasignaturespikeinthecentral
density.Attheconclusionoftheevaporationramp,
themagnetictrappotentialisdecompressedand
extinguishedbeforeimagingtheatoms22 mslater.
TheBECcriticaltemperature(Tc)is130 nK
(correspondingto500 nKintheoriginaltrap).The
rightmostimageshowsanatomcloudwith4.9 × 104
totalatoms,26%condensedinaBECatatemperature
of17 nKmeasuredfromthedistributionofthe
surroundingthermalcloud.
zCoM = 3.1 mm s–1
Cloud position
f
CoMposition(mm)
2
3
4
0
1
200
Cloud size
g
Halo-
cloud
a
0.023
BEC
0
TOF =
18 ms 318 ms
b c d e
718 ms 1,018 ms518 ms
r.m.s.width(mm)
50
100
150
0
0 200 400
Time of flight (ms)
600 800 1,000 1,200
•
xCoM = 40 µm s–1•
zrms = 262(14) µm s–1
xrms = 148(3) µm s–1
z
x
Fig.4|In-orbitfreeexpansionofultracoldatoms.a–e,Eachpanelshowsa
densityprofile(left)andanabsorptionimageof87
Rbatoms(right)after
increasingfreeexpansiontimes(TOFinms:a,18;b,318;c,518;d,718;e,1,018)in
persistentmicrogravity.Ina,abimodaldistributionofcondensedatoms(aBEC)
isseenintheabsorptionimage,andaresidualcloudof|2,0⟩atoms(‘halo-cloud’)
isdriftingupandawayfromtheBECatmorethan50 mm s−1
.Apost-processing
methodofarrayreductionthroughbicubicinterpolationincreasesthecontrast
oftheatomsignaltorevealatomswithTOF > 1 s(seeMethods).Thedensityscale
isconstantacrosstheseabsorptionimages,selectedsothattheatomsignalis
stilldistinguishablefromtheimagebackgroundfortheatomcloudsthathave
undergonethelongestfreeexpansiontime.Anadjusteddensityscaleforeach
expansiontimeisshowntotheleftofeachimage,plottingthehorizontalsumsof
thepixelswithnoappliedinterpolation.f,Plotofcentre-of-mass(CoM)position
(‘Cloudposition’)versustimeofflightyieldstheresidualcentre-of-mass
velocities, x˙ = 40 μm sCoM
−1
 (red)and z˙ = 3.1 mm sCoM
−1
 (blue).g,Plotofr.m.s.
width(‘Cloudsize’)versustimeofflightshowstheresidualexpansionoftheBEC,
correspondingtokineticenergiesof231(9) pKand720(79) pK(alongxandz,
respectively),derivedfromthecloudexpansionrates,vxrms = 148(3) μm s−1
and
vzrms = 262(14) μm s−1
,measuredbetween168 msto768 msTOF.Errorbars,±1s.d.
withn ≥ 3(seeMethods).
Nature | Vol 582 | 11 June 2020 | 197
highly immune to frequency shifts and broadening of spectral lines
fromstraymagneticfieldsandgradientsrespectively,andsouniquely
suited for high-precision spectroscopy and applications requiring
longatomic-statecoherencetimes44,45
.Theprospectofconfiningand
manipulatingatomsalreadypreparedinmagneticallyinsensitivestates
without the need for an optical trap therefore presents a more subtle
butstillenablingadvantageofmicrogravityfornext-generationpreci-
sion metrology with quantum gases.
We have used the baseline capabilities of CAL in low Earth orbit to
demonstrate immediate and fundamental benefits of microgravity
for ultracold atom experiments, including demonstration of novel
evaporationregimesandby-products,free-spaceBECexpansiontimes
overonesecondinduration,anddecompression-cooledcondensates
with picokelvin effective temperatures. These experiments form the
start of potentially years of science operations, with additional capa-
bilitiesoftheinstrumenttobeemployedovertime.Thefirstseriesof
experiments is currently underway, using 87
Rb to study bubble-shell
geometries and investigate applications of atom interferometry for
future precision measurements in the areas of Earth observation and
fundamentalphysics4–6,9–11
.Theon-orbitproductionof87
RbBECusing
RFevaporationreportedheresetsthestageforproductionof87
RbBECs
viamicrowaveevaporation,andthesubsequentsympatheticcooling
of potassium40
. With this capability comes additional prospects for
interferometry9
,theutilizationofFeshbachresonancestocontroldif-
ferentialcentre-of-massdistributionsofdual-speciesquantumgasmix-
tures8,12
,andfew-bodysystemsinnewtemperatureanddensityregimes
thatareprerequisitesforthenextgenerationofEfimovexperiments7,8
.
Meanwhile,futuremodularupgradesfortheCALinstrumentareavail-
ableforextendedmissionstudies,includingasciencemodulebuiltby
JPLfeaturinganatom-waveinterferometer.Additionally,payloadsfor
follow-onmissionsareinproposalanddevelopmentstages46
,assuring
the continued presence and application of ultracold atoms in orbit.
Onlinecontent
Anymethods,additionalreferences,NatureResearchreportingsum-
maries, source data, extended data, supplementary information,
acknowledgements, peer review information; details of author con-
tributions and competing interests; and statements of data and code
availabilityareavailableathttps://doi.org/10.1038/s41586-020-2346-1.
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Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
© The Author(s), under exclusive licence to Springer Nature Limited 2020
Article
Methods
CALutilizestwoimagingsystemsthatcollectdataontheatomdensity
for rubidium and potassium. While mainly used for absorption imag-
ing,theyarealsoabletocollectfluorescenceimagesinordertoassess
themagneto-opticaltrap(MOT)statusandmonitorlaserpowers.The
primarysystemimagesalongthesurfaceofthechip(yaxis)byapply-
ingtwopulsesoflaserlightseparatedbynominally54 mswithapulse
duration of 40 μs. Circularly polarized imaging light, ‘Imaging (y)’ in
Fig. 1, is directed along the y axis together with an applied magnetic
field.AllthedatapresentedinthisArticlewerecollectedbythisprimary
system.Anorthogonalangleofviewisavailable,labelled‘Imaging(z)’in
Fig. 1,andiscollectedthroughtheatomchipwindow.Forthepurpose
of displaying images of the atom signal at long expansion times (as in
Fig. 4),contrastbetweentheatomcloudandtheimagebackgroundis
increasedbyreducingthenumberofpixelsintheimageusingbicubic
interpolation and anti-aliasing. Specific details of this process can be
found in the kernel for MATLAB’s ‘imresize’ function. The data taken
at free expansion times of 918 ms, 1,018 ms and 1,118 ms are first pro-
cessed with these methods, and then the centre of mass extracted
(depicted as triangles in Fig. 4f). The rest of the data’s error bars
representstandarddeviationsfrommultipleshotswithn ≥ 3.Widthsof
theatomiccloud,whichareusedtodeterminemomentum,arefound
fromtwo-dimensionalGaussianfitstoimagesthatundergonoresizing
(data with free expansion times of 168 ms to 768 ms).
The dual-cell vacuum chamber is based on ColdQuanta’s commer-
cially offered RuBECi41
, which is augmented for CAL-specific science
objectives and space flight compatibility. The source cell contains
rubidium and potassium alkali metal dispensers with natural abun-
dance of the isotopes. The MOT is formed in the UHV science cell
approximately 15 mm below the chip surface using three incident
beamsthatareretro-reflectedtocreateasix-beamthree-dimensional
MOT.ExtendedDataFig. 1aillustratestheMOTlocationinthescience
cell, and the four optical beams that are incident upon atoms in the
y–z plane. More details about the hardware and configuration can
be found in the referenced material40,41
. From the MOT, the atoms are
gathered in a quadrupole magnetic trap formed by external coils and
then transported along the z axis to the atom chip.
Thefirst-ordermagneticpotentialfor87
RbatomsintheF = 2hyper-
fine manifold is proportional to the product of the field modulus and
themagneticquantumnumberoftheatom,mF.Dependingonthesign
ofmF,thecorrespondingZeemanstatewillbeconfinedtowards(+)or
repelledfrom(−)afieldminimuminthechip-trap.AnappliedRFfield
(theRFknife)thencouplesthetrappedatomsinthe|F = 2,mF = 2⟩state
to the less confined |2, 1⟩, the magnetically insensitive |2, 0⟩, or the
anti-trapped |2, −1⟩ and |2, −2⟩ states.
During CAL’s evaporation stages, the magnetic trap is formed by
applying bias fields along the x and y axes (Fig. 1f) that combine with
the atom chip fields to yield relatively large trap oscillation frequen-
cies for 87
Rb atoms in the |2, 2⟩ state, which, near the approximately
harmonictrapbottomof7 G,correspondtotraposcillationfrequencies
(ωx,ωy,ωz) = 2π × (216,1,000,1,080) Hz.Withsuchtighttraps,collisions
among atoms are sufficiently high to cool atoms to BEC in just over a
secondofevaporationtime.Forthe|2,0⟩state,thesesamefieldsform
anattractiveQZpotentialforthe|2,0⟩statewithan18 μKdepth.Similar
tothe|2,2⟩trap,withatrapaspectratioof x x ω ω/ = / = 0.2z x x z
2 2 2 2
,
any|2,0⟩atomsconfinedbytheQZforceandabletoexchangeenergy
between dimensions should produce an aspect ratio of ≤0.2, which
is lower than that of the original harmonic trap due to the additional
quartic terms.
Followingforcedevaporationweapplytwostagesofdecompression
in orbit, during which we apply a linear decrease to the magnetic bias
fields down to 12% of their evaporation values over the first 100 ms,
then further decrease the bias fields to 8% with a second stage, var-
ied from 100 ms to 200 ms in duration. These two decompression
stages lower the trap oscillation frequencies for the |2, 2⟩ state to
(ωx, ωy, ωz) = 2π × (8.2, 30, 46) Hz and (ωx, ωy, ωz) = 2π × (11, 20, 15) Hz,
respectively.Chipcurrentsareheldconstantduringalldecompression
protocols reported in the current work, and then all chip currents are
extinguishedwithin20 μsinordertoreleaseatomsforfreeexpansion
and imaging after varied time of flight. The second decompression
ramptimeof181 mswasempiricallydeterminedtolimitcentre-of-mass
motion to a drift velocity of 3.1 mm s−1
. We then perform 3 ms of adi-
abaticrapidpassageintothe|2,0⟩statetominimizeforcesfromstray
magnetic gradients.
Stringent requirements on the design and implementation of CAL
were required to not only satisfy the size, weight, power and thermal
management requirements of the ISS, but also to assure robust and
reproducible operation over its lifetime in the variable ISS environ-
ment.CALincorporatesrelativelyhighrepetitionrateswithlong-term
operation (running hundreds of times each day for over 1.5 years).
These features provide large and repeatable datasets with iterative
feedbackcapabilitiesnecessaryforadvancingthescientificandtech-
nical maturation of microgravity-enabled atomic gas experiments.
The total volume (0.4 m3
) and mass (233 kg) were optimized to allow
CAL to occupy one single locker and one quad-locker of an EXPRESS
rack (as illustrated in Fig. 1). The average daily power draw for all CAL
subsystemsis510 W.Forthermalmanagementofthelaserandelectron-
ics subsystems, as well as for the vacuum chamber inside the science
module,airandwatercoolingareusedatsupplytemperaturesof18 °C.
Incontrasttoanalogousterrestrialandmicrogravitycold-atomsys-
tems demonstrated to date, CAL is designed to operate for years in a
highly dynamic environment without regular human intervention.
Access to this orbital environment, relatively high duty cycles, and
mitigation of external perturbations allows investigators to pursue
longer microgravity-enabled experimental campaigns and demon-
strates advances in technology towards exploring fundamental sci-
encethatisinaccessibleontheground(includinggeodesywithatomic
test masses and high-precision tests of the underlying principles of
Einstein’s general relativity). Magnetic shielding inside the science
moduleisdesignedtomitigatetheeffectsoftheEarth’smagneticfield,
which varies in both direction and amplitude over each 90-min orbit.
In parallel, the excessive radiation environment (for example, during
passesthroughtheSouthAtlanticAnomaly),localperturbationsfrom
neighbouring experiments on the ISS and from astronaut activities,
thehighrotationrateoftheISS,andtheintensevibrationandacoustic
environmentsduringlaunchwereallaccountedforinthedesignofCAL.
Dataavailability
Source data for Fig. 4 are provided with the paper. The datasets gen-
erated and analysed during the current study are available from the
corresponding authors on reasonable request.
Acknowledgements We gratefully acknowledge the contributions of current and former
members of CAL’s management and technical teams, T. Winn, K. Muse, L. Clonts, J. Lam, J. Liu,
C. Tran, J. Tarsala, T. Tran, S. Haque, M. McKee, J. Trager, J. Mota, G. Miles, D. Strekalov, I. Li,
S. Javidnia, A. Sengupta, D. Conroy, A. Croonquist, E. Burt, M. Krutzik, S. Kulas and V. Schkolnik,
and the ColdQuanta team, including E. Salim, L. Czaia, J. Ramirez-Serrano, J. Duggan, and
D. Anderson. We recognize the continuing support of JPL’s Astronomy, Physics and Space
Technology Directorate, L. Livesay, T. Gaier, D. Coulter, C. Lawrence and U. Israelsson. We thank
CAL’s principal investigators and science team members, N. Bigelow, N. Lundblad, C. Sackett,
E. Cornell, P. Engels and M. Mossman, for their guidance, along with CAL’s Science Review
Board, including B. DeMarco and R. Walsworth. We also recognize the steadfast support from
NASA’s Division of Space, Life, and Physical Sciences Research and Applications (SLPSRA),
C. Kundrot, D. Malarik, M. Lee, B. Carpenter and D. Griffin. This work was funded by NASA’s
SLPSRA programme office, and operated by the Jet Propulsion Laboratory, California Institute
of Technology, under contract with NASA. US Government sponsorship is acknowledged.
Author contributions D.C.A., J.R.W. and E.R.E. optimized and operated the instrument during
the CAL commissioning phase, collected and analysed the associated data, and prepared this
manuscript. D.C.A., J.R.W. and E.R.E. were responsible for instrument hardware integration,
experimental operation, optimization and data acquisition during the CAL integration and test
phase. D.C.A. led CAL’s ground testbed and the integration and testing of the science module

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Observation of Bose–Einstein condensates in an Earth-orbiting research lab

  • 1. Nature | Vol 582 | 11 June 2020 | 193 Article ObservationofBose–Einsteincondensates inanEarth-orbitingresearchlab David C. Aveline1,2 ✉, Jason R. Williams1,2 , Ethan R. Elliott1,2 , Chelsea Dutenhoffer1 , James R. Kellogg1 , James M. Kohel1 , Norman E. Lay1 , Kamal Oudrhiri1 , Robert F. Shotwell1 , Nan Yu1 & Robert J. Thompson1 ✉ Quantummechanicsgovernsthemicroscopicworld,wherelowmassandmomentum revealanaturalwave–particleduality.Magnifyingquantumbehaviourto macroscopicscalesisamajorstrengthofthetechniqueofcoolingandtrapping atomicgases,inwhichlowmomentumisengineeredthroughextremelylow temperatures.Advancesinthisfieldhaveachievedsuchprecisecontroloveratomic systemsthatgravity,oftennegligiblewhenconsideringindividualatoms,has emergedasasubstantialobstacle.Inparticular,althoughweakertrappingfields wouldallowaccesstolowertemperatures1,2 ,gravityemptiesatomtrapsthataretoo weak.Additionally,inertialsensorsbasedoncoldatomscouldreachbetter sensitivitiesifthefree-falltimeoftheatomsafterreleasefromthetrapcouldbemade longer3 .Planetaryorbit,specificallytheconditionofperpetualfree-fall,offerstolift cold-atomstudiesbeyondsuchterrestriallimitations.Herewereportproductionof rubidiumBose–Einsteincondensates(BECs)inanEarth-orbitingresearchlaboratory, theColdAtomLab.WeobservesubnanokelvinBECsinweaktrappingpotentialswith free-expansiontimesextendingbeyondonesecond,providinganinitial demonstrationoftheadvantagesofferedbyamicrogravityenvironmentfor cold-atomexperimentsandverifyingthesuccessfuloperationofthisfacility.With routineBECproduction,continuingoperationswillsupportlong-terminvestigations oftraptopologiesuniquetomicrogravity4,5 ,atom-lasersources6 ,few-bodyphysics7,8 andpathfindingtechniquesforatom-waveinterferometry9–12 . With the launch and operation of the Cold Atom Lab (CAL), NASA has establishedthesustainedstudyanddevelopmentofquantumtechnolo- gies in orbit. This versatile, multi-user research facility has travelled over400 millionkilometresonboardtheInternationalSpaceStation (ISS) since June 2018, under remote operation from the Jet Propul- sion Laboratory. Exploiting the microgravity environment of space, researcherscanutilizethefullsensitivityofultracoldmatter-wavesto explorefundamentalphysicsandtheorganizingprinciplesofcomplex systemsfromwhichstructureanddynamicsemerge—executingmajor ‘thrusts’oftheNationalResearchCouncil’sDecadalSurveythatdefine the frontier of space-based fundamental physical science13 . Understanding quantum mechanics has made possible the now-ubiquitous technologies of lasers, semiconductors and transistors,butremainingelusiveisitsrelationshipwithgeneralrelativ- ity, the physics of gravity, which is well understood at macroscale to astronomic scales. Scaling quantum mechanics to macroscopic sizes isaprimarygoalofcoolingatomicgasestowardsabsolutezero,where wave-like behaviour markedly increases as temperature drops. With enoughcooling,eachatom’swavelengthapproachestheinterparticle spacingandthesystemexhibitsthemacroscopicquantumbehaviour ofsuperfluidity.Foradilutegasofbosons—thetypeofatomscontained in CAL—this phenomenon is known as Bose–Einstein condensation. These quantum gases, and particularly BECs, have been studied for theirintrinsicproperties,usedasanaloguesofmoreinaccessiblesys- tems,orappliedasinertialsensingmatterwaves.Thestate-of-the-art technology has advanced to such a degree that additional cooling techniquesarestifledbygravity.Forexample,theconfiningpotentials that trap the atoms can be adiabatically decompressed to decrease temperature, but only until the local potential minimum is collapsed by gravity’s asymmetric pull. Furthermore, following release from an atom trap, gravity-induced centre-of-mass motion greatly limits the system’sutilityasaninertialsensor.Theidealconditionsforengineer- ing macroscopic atom-waves thus becomes ultracold temperatures combined with reduced gravity. Pioneering cold-atom experiments have mitigated the effects of gravity through a variety of methods. Ground-based levitation tech- niques accomplish a localized counter-balance to gravity in decom- pressionexperiments1 ,buttheyintroducemass-andstate-dependent forcesthatbroadlylimitatom-waveinterferometryandtestsbetween multiple atomic species. Because cold-atom experiments require an ultrahigh vacuum (UHV) to thermally isolate the atoms from the ambientenvironment,theprimarysolutionsareeithertoenlargethe sizeoftheUHVchamberorincreasethefree-falltimeoftheentireappa- ratus. Ground-based interferometers have achieved state-of-the-art https://doi.org/10.1038/s41586-020-2346-1 Received: 30 October 2019 Accepted: 26 March 2020 Published online: 11 June 2020 Check for updates 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. 2 These authors contributed equally: David C. Aveline, Jason R. Williams, Ethan R. Elliott. ✉e-mail: David.C.Aveline@jpl.nasa.gov; Robert.J.Thompson@jpl.nasa.gov
  • 2. 194 | Nature | Vol 582 | 11 June 2020 Article interrogationtimesandtemperaturesbylaunchingatomsinastation- ary10-m-tallUHVchamber14,15 .Incontrast,portablecold-atomsystems that are compact and ruggedized have realized temporary free-fall through drop towers16–18 , Einstein elevators19 , zero-g aircraft20,21 and suborbital launch vehicles22–26 . Notably, six minutes of microgravity achievedinasoundingrocketinparabolicflightabovetheKármánline (thealtitudeof100kmabovesealevel,whichisconsideredthebound- ary of space) demonstrated production and manipulation of BECs26 . For these portable instruments to advance in scope and capability, theiroperationtimesandrepetitionratesmustincrease.Asthenatural next step, new generations of quantum experiments seek to operate aversatileultracoldatomicphysicslabinthepersistentmicrogravity conditionsoflowEarthorbit3 .Spacemissionsenablelargevariations of velocity and gravitational potential compared to ground-based laboratories, inspiring many proposed missions. These concepts include tests of Einstein’s equivalence principle with quantum proof masses27,28 , gravitational wave detection that could complement the LaserInterferometerSpaceAntenna(LISA)29–32 ,opticalatomicclocks for a ‘world-clock’33 , direct detection of dark matter and dark energy candidates27,34 ,geodesy,Earthandplanetarysciences35–37 andadvanced navigation capabilities37–39 . Therealizationofanoff-worldcold-atominstrumentpresentsnew technicalchallenges.Itnotonlycallsforrobusthardwareadheringto the stringent size, weight and power requirements of a spaceborne instrument,butmustalsoreliablyandautonomouslyoperateforyears without manual intervention. In addition to technological matura- tion(uniqueaspectsandchallengesindevelopingthisEarth-orbiting cold-atom facility are elaborated in Methods), long-term operation allowsadjustmentandadaptationofthepreciseproceduresandpro- tocolsneededtocontrolatomsinmicrogravity.Thecapabilitiesofthe CALinstrumentsenablemultipleteamsofscientiststostudyquantum gasesatunprecedentedlowenergiesanddensitiesproducedinweakly confiningtraps.The87 RbBECswereportherewillspecificallysupport investigationsofatomlasersources6,9 ,bubble-shelltopologiesunique to microgravity4,5 and advanced techniques for Earth-orbiting atom interferometry including delta kick cooling9 and novel decompres- sion protocols10,11 . Followingitslaunchon21May2018ontheOA-9Antares230rocket, theCALhardwarewasinstalledandfullypoweredonboardtheISSby June 2018, commencing daily operation as the first ultracold atom system in orbit. As described in ref. 40 , the instrument is composed of three primary subsystems: the science module, the laser and optics system, and the electronics, supported by thermal, mechanical and software control, all designed for modular integration into a stand- ardized ISS equipment rack (see Fig. 1). Re-configurable traps that hold and manipulate cold atoms are created by the current-carrying wiresofan‘atomchip’thatformsthetopwallofarectangularglasscell underUHV41 .Wire-loopemitterspositionedneartheambientsurface of the chip transmit radio frequency (RF) or microwave radiation for evaporativecoolingandstatepreparation.Furtherinstrumentdetails can be found in Methods and Extended Data Fig. 1. Implementing the same stages of laser cooling and magnetic trappingof87 Rbinthestate|F = 2,mF = 2⟩,asexecutedontheground40 , we find that RF-induced evaporative cooling (see Methods) reveals markedlydifferentresultsinmicrogravity.Figure 2showsaterrestrial BEC produced before the launch of CAL compared to an evaporated atom cloud on the ISS. We observe an on-orbit increase in the atom numberofnearlythreefold.Throughtheapplicationofvariedmagnetic field gradients, we confirm that approximately half of the atoms are in the magnetically insensitive state |2, 0⟩, forming a halo-like cloud around the location of the magnetic trap. On Earth, the dominating force acting on atoms in this |2, 0⟩ state is gravity. In microgravity, however, the dominant force acting on these |2, 0⟩ atoms is due to the quadratic Zeeman (QZ) effect, given by42 h• (287.58 Hz G−2 )•B2 . Quad-locker exploded view Science module Quad-locker installation on ISS b d e f Science cell Source cell Atom chip Vacuum chamber z x y dw = 2 mmId Iz Imaging (z) Imaging (y) zim ≈ 4 mm ISS orbit a z x y EXPRESS rack 7 inside Destiny Single locker Quad-locker z x y c Fig.1|CALhardwareconfiguration.a,TheCALpayloadresidesinan‘EXpedite thePRocessingofExperimentstotheSpaceStation’(EXPRESS)rackwithinthe USLabDestinyModule,locatedwhereshownintheISS.Thex,y,zcoordinates areillustrated.b,Thepayloadwasinstalledbyastronautsduringa6-hcrew operationunderguidancefromJPL(imagesource:NASA).c,ViewofEXPRESS rack7.CALispackagedwithinthisrackinasinglelockerandaquad-locker, requiringoneandfourpayloadslots,respectively.Thesinglelockerhousesthe powerelectronics,whilethequad-lockercontainsthecomputer,lasersources, opticalcontrolnetwork,controlelectronicsandthemagneticallyshielded sciencemodule.d,Anexplodedviewofthequad-locker,illustratingitsmodular design,includingdraweraccesstolasersandelectronicsfromthefront,andthe sciencemoduletotheright.e,Aninsideviewofthesciencemodule,withoutthe fronthalf-shellofthemagneticshields.f,Attheheartofthesciencemodule,an aluminiumopticalbenchholdsadual-cellvacuumchamber.Formingthetop walloftheUHVsciencecellisanatomchip,whichcangeneratere-configurable magnetictrapswithitscurrent-carryingwires.Theprimarytrapiscentrally locatedonawindowofdiameterdw = 2 mm,formedbychipcurrentsIz andId.This opticalaccessallowsabsorptionimagingalongthezaxis,whileCAL’sprimary imagingisaccomplishedalongtheyaxiscentredatzim ≈ 4 mmbelowthechip (purplearrows).SeeMethodsandExtendedDataFig. 1formoredetailsaboutthe opticalbeams.
  • 3. Nature | Vol 582 | 11 June 2020 | 195 Increasingtheappliedfieldgradienttoapproximately∇B = 43 G cm−1 , we demonstrate motion of the |2, 0⟩ halo-cloud due to this QZ effect (Fig. 2g). Further incidental QZ forces acting on the |2, 0⟩ halo-cloud throughoutthestandardoperationofourchip-basedtrap(owingtoits largemagneticfieldsandgradients)arediscussedbelow.Bycontrolled manipulation and isolation of the atoms in the |2, 0⟩ state, we readily confirm production of 87 Rb |2, 2⟩ BECs in orbit. Figure 3 illustrates the onsetofcondensationascloudtemperatureisreducedbelowtheBEC transition temperature. Tell-tale signatures of condensation include thesuddenonsetofabimodaldensitydistributionbelowthetheoreti- cally predicted critical temperature at long time of flight (TOF), and a denseellipticalcore(anisotropicexpansion)surroundedbyathermal gas background (isotropic expansion). Taking advantage of the intrinsic advantages of microgravity, we demonstrate a combination of low temperatures and extended free-space expansion times with only minor readjustments to our pre-launchprocedures.Beginningfromanominal87 RbBEC,wedecom- press the trap, transfer the atoms to a magnetically insensitive state, and release the atoms with parameters empirically adjusted in orbit to minimize the temperature and drift velocity of the atom cloud in free-fall. We benefit from the relatively high repetition rate (once per minute)andextendedexperimentaltimeavailableontheISStomake theseadjustments,receivereal-timefeedback,andmodifyaccordingly. Decompression is carried out by two stages of linear reduction to the bias fields applied by the external coils, thereby decreasing the trap frequency and translating the trap centre approximately 1 mm away from the chip (see Methods). Images of atoms, along with the respective cloud positions and widths, during free expansion up to 1.118 s are shown in Fig. 4, with the residual expansion of the BEC corresponding to a kinetic energy temperatureequivalentof231(9) pK(720(79) pK)inthedirectionparal- lel (normal) to the chip surface. In the earliest expansion time shown inFig. 4a,thehalo-cloudof|2,0⟩producedduringevaporationisalso present,coastingawayfromthechipwiththemotionimpartedbythe QZ effect when the location of the trap bottom shifts during decom- pression. Producing this combination of low temperatures and long expansion times using a simple adjustment to our ground procedure demonstrates not only the natural benefit of microgravity, but also theabilitytorelyonempiricallyderivedoptimizationoverthecourse of days, as well as the potential for further improvements. As CAL sci- enceoperationscontinue,investigatorswilltakefurtheradvantageof microgravity by introducing more complex decompression curves, allowing greater adiabaticity, less residual centre-of-mass motion, lower temperatures and subhertz trapping frequencies. The next generation of ultracold gas experiments using weak traps mayalsobenefitfromfurtherstudiesoftheobservedhalo-cloudof|2, 0⟩ atoms. We note that the halo-cloud is shaped by a combination of factors:theinitialtemperatureanddensitydistributionofthetrapped atoms, the momentum distribution of |2, 0⟩ as they are generated by in situ RF coupling, and finally the dominating forces due to the QZ effectinthepresenceofthechip’sstrongmagneticfieldsandgradients. Anatomthatundergoesaforced-evaporationtransitionintothe|2,0⟩ state by application of resonant RF radiation (‘RF knife’) would lose potentialenergyequaltothemagnetictrapdepthandpropagatewith kinetic energy proportional to kBT plus a new, lower potential energy determined by the QZ effect. In the present work, the magnetic fields during evaporation form an attractive QZ potential for atoms in the |2, 0⟩ state with 18 μK depth and aspect ratio ≤0.2 (see Methods). The timescales on which we currently observe the halo-clouds are con- sistent with confinement and subsequent release from such a trap in Rotated larger view of the in-orbit result –150 150–100 100–50 500 –150 150–100 100–50 500 Decompression to 10% bias field Applied B-field gradient 15 G cm–1 for 19 ms Applied B-field gradient 7 G cm–1 for 19 ms Applied B-field gradient 43 G cm–1 for 44 ms In-orbit Ground a b c d e f g z x |2,2〉 |2,1〉 |2,0〉|2,0〉 |2,2〉 |2,1〉 |2,0〉 • B x z B B B Fig.2|BECproductioninCALontheEarthandinorbit.a,Afalse-colour absorptionimageshowsaBECproducedinCALonthegroundat22 msTOF, totalling4.5 × 104 atomsandaBECfractionof15%.b,Likewise,aBECat22 ms TOFproducedinCALonboardtheISSusingthesameparametersasonthe ground,totalling1.3 × 105 atoms,showsthatalargeportionoftheseatomshave adensityprofiledistinctlydifferentfromtheexpectedbimodaldistribution. Plotsadjacenttotheimagesaandbshowthedata(green)ofcolumndensity alongpixelsofthexaxis.Fitsindicatethecondensate(blue),thermalcloud (red),andresidualhalo-cloud(orange)of|2,0⟩atoms.c,Alargerregionof interestthanshowninb,usingthesameproductionsequencesexceptfor changestothereleaseprocedureasfollows(d–g).d,Releasedatomswiththe locationofthetrapbottommoved0.65 mmawayfromthesurfaceofthechip. Amagneticallyinsensitivehalo-cloudof|2,0⟩atomsisleftbehindduringthis re-positioningoftheprimarytrapbottom.e, f,Thesamedecompression procedureasshownind,butfollowedbyapplyingatransversegradientof 7 G cm−1 (e)orof15 G cm−1 (f)forthefirst19 msofa22-msTOF.Thehalo-cloud remainsunmoved.Afaintcloudof|2,1⟩atomsthatwasweaklyconfinedinthe trapexperienceslessaccelerationandtrailshalfwaybehindthe|2,2⟩atoms. g,Thesamedecompressionprocedureasshownind,butfollowedby applicationofastrongertransversegradientof43 G cm−1 forthefirst44 msofa 50-msTOF,whichacceleratesthe|2,0⟩cloudduetotheQZeffect42 .Anarrow beloweachpaneld–gillustratesthedirectionalforceappliedbyincremental changesinmagneticfields,dynamically,B˙,orspatially,∇B.
  • 4. 196 | Nature | Vol 582 | 11 June 2020 Article microgravity,whichisnormallyoverpoweredbygravitationalsaginour measurementsonEarth.Wenotethatalow-frequencyanharmonictrap naturally formed and populated with large numbers of magnetically insensitiveatomsthroughouttheevaporationprocessisanintriguing alternative to decompression and adiabatic rapid passage for future microgravityexperiments.Wearecurrentlyexploringtheviabilityof directly producing a BEC in such a trap. A BEC in a three-dimensional quartic trap (as given near the minimum of the QZ potential and in the absence of gravity) is expected to have a substantially flatter con- densate density profile near the centre as compared to that of a har- monicallytrappedcondensate43 ,presentingopportunitiestoobserve transitionsandphasesofquantummatterinarelativelyhomogeneous system.Also,werecognizethattheabsenceofthefirst-orderZeeman shift for atoms in magnetically insensitive states makes these gases 385 µm 550µm PSD = 1 Tc = 130 nK PSD = 0.7PSD = 0.6 0 1 2 3 4 OD Fig.3|OnsetofBose–Einsteincondensationinlow Earthorbit. False-colourabsorptionimagesof87 Rb atomiccloudsevaporatedandreleasedfromCAL’s magneticatomchiptraparedisplayedassurfaceplots (top)andcorrespondingarrays(bottom).OD indicatesopticaldensity.Fromlefttoright,thefinal frequencyofaforcedevaporationrampusinganRF knifeinamagneticatomchiptrapislowered, increasingthephasespacedensity(PSD)andforming aBECmarkedbyasignaturespikeinthecentral density.Attheconclusionoftheevaporationramp, themagnetictrappotentialisdecompressedand extinguishedbeforeimagingtheatoms22 mslater. TheBECcriticaltemperature(Tc)is130 nK (correspondingto500 nKintheoriginaltrap).The rightmostimageshowsanatomcloudwith4.9 × 104 totalatoms,26%condensedinaBECatatemperature of17 nKmeasuredfromthedistributionofthe surroundingthermalcloud. zCoM = 3.1 mm s–1 Cloud position f CoMposition(mm) 2 3 4 0 1 200 Cloud size g Halo- cloud a 0.023 BEC 0 TOF = 18 ms 318 ms b c d e 718 ms 1,018 ms518 ms r.m.s.width(mm) 50 100 150 0 0 200 400 Time of flight (ms) 600 800 1,000 1,200 • xCoM = 40 µm s–1• zrms = 262(14) µm s–1 xrms = 148(3) µm s–1 z x Fig.4|In-orbitfreeexpansionofultracoldatoms.a–e,Eachpanelshowsa densityprofile(left)andanabsorptionimageof87 Rbatoms(right)after increasingfreeexpansiontimes(TOFinms:a,18;b,318;c,518;d,718;e,1,018)in persistentmicrogravity.Ina,abimodaldistributionofcondensedatoms(aBEC) isseenintheabsorptionimage,andaresidualcloudof|2,0⟩atoms(‘halo-cloud’) isdriftingupandawayfromtheBECatmorethan50 mm s−1 .Apost-processing methodofarrayreductionthroughbicubicinterpolationincreasesthecontrast oftheatomsignaltorevealatomswithTOF > 1 s(seeMethods).Thedensityscale isconstantacrosstheseabsorptionimages,selectedsothattheatomsignalis stilldistinguishablefromtheimagebackgroundfortheatomcloudsthathave undergonethelongestfreeexpansiontime.Anadjusteddensityscaleforeach expansiontimeisshowntotheleftofeachimage,plottingthehorizontalsumsof thepixelswithnoappliedinterpolation.f,Plotofcentre-of-mass(CoM)position (‘Cloudposition’)versustimeofflightyieldstheresidualcentre-of-mass velocities, x˙ = 40 μm sCoM −1  (red)and z˙ = 3.1 mm sCoM −1  (blue).g,Plotofr.m.s. width(‘Cloudsize’)versustimeofflightshowstheresidualexpansionoftheBEC, correspondingtokineticenergiesof231(9) pKand720(79) pK(alongxandz, respectively),derivedfromthecloudexpansionrates,vxrms = 148(3) μm s−1 and vzrms = 262(14) μm s−1 ,measuredbetween168 msto768 msTOF.Errorbars,±1s.d. withn ≥ 3(seeMethods).
  • 5. Nature | Vol 582 | 11 June 2020 | 197 highly immune to frequency shifts and broadening of spectral lines fromstraymagneticfieldsandgradientsrespectively,andsouniquely suited for high-precision spectroscopy and applications requiring longatomic-statecoherencetimes44,45 .Theprospectofconfiningand manipulatingatomsalreadypreparedinmagneticallyinsensitivestates without the need for an optical trap therefore presents a more subtle butstillenablingadvantageofmicrogravityfornext-generationpreci- sion metrology with quantum gases. We have used the baseline capabilities of CAL in low Earth orbit to demonstrate immediate and fundamental benefits of microgravity for ultracold atom experiments, including demonstration of novel evaporationregimesandby-products,free-spaceBECexpansiontimes overonesecondinduration,anddecompression-cooledcondensates with picokelvin effective temperatures. These experiments form the start of potentially years of science operations, with additional capa- bilitiesoftheinstrumenttobeemployedovertime.Thefirstseriesof experiments is currently underway, using 87 Rb to study bubble-shell geometries and investigate applications of atom interferometry for future precision measurements in the areas of Earth observation and fundamentalphysics4–6,9–11 .Theon-orbitproductionof87 RbBECusing RFevaporationreportedheresetsthestageforproductionof87 RbBECs viamicrowaveevaporation,andthesubsequentsympatheticcooling of potassium40 . With this capability comes additional prospects for interferometry9 ,theutilizationofFeshbachresonancestocontroldif- ferentialcentre-of-massdistributionsofdual-speciesquantumgasmix- tures8,12 ,andfew-bodysystemsinnewtemperatureanddensityregimes thatareprerequisitesforthenextgenerationofEfimovexperiments7,8 . Meanwhile,futuremodularupgradesfortheCALinstrumentareavail- ableforextendedmissionstudies,includingasciencemodulebuiltby JPLfeaturinganatom-waveinterferometer.Additionally,payloadsfor follow-onmissionsareinproposalanddevelopmentstages46 ,assuring the continued presence and application of ultracold atoms in orbit. Onlinecontent Anymethods,additionalreferences,NatureResearchreportingsum- maries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author con- tributions and competing interests; and statements of data and code availabilityareavailableathttps://doi.org/10.1038/s41586-020-2346-1. 1. Leanhardt, A. E. et al. Adiabatic and evaporative cooling of Bose–Einstein condensates below 500 picokelvin. Science 301, 1513–1515 (2003). 2. Ammann, H. & Christensen, N. Delta kick cooling: a new method for cooling atoms. Phys. Rev. Lett. 78, 2088–2091 (1997). 3. Safronova, M. et al. Search for new physics with atoms and molecules. Rev. 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  • 6. Article Methods CALutilizestwoimagingsystemsthatcollectdataontheatomdensity for rubidium and potassium. While mainly used for absorption imag- ing,theyarealsoabletocollectfluorescenceimagesinordertoassess themagneto-opticaltrap(MOT)statusandmonitorlaserpowers.The primarysystemimagesalongthesurfaceofthechip(yaxis)byapply- ingtwopulsesoflaserlightseparatedbynominally54 mswithapulse duration of 40 μs. Circularly polarized imaging light, ‘Imaging (y)’ in Fig. 1, is directed along the y axis together with an applied magnetic field.AllthedatapresentedinthisArticlewerecollectedbythisprimary system.Anorthogonalangleofviewisavailable,labelled‘Imaging(z)’in Fig. 1,andiscollectedthroughtheatomchipwindow.Forthepurpose of displaying images of the atom signal at long expansion times (as in Fig. 4),contrastbetweentheatomcloudandtheimagebackgroundis increasedbyreducingthenumberofpixelsintheimageusingbicubic interpolation and anti-aliasing. Specific details of this process can be found in the kernel for MATLAB’s ‘imresize’ function. The data taken at free expansion times of 918 ms, 1,018 ms and 1,118 ms are first pro- cessed with these methods, and then the centre of mass extracted (depicted as triangles in Fig. 4f). The rest of the data’s error bars representstandarddeviationsfrommultipleshotswithn ≥ 3.Widthsof theatomiccloud,whichareusedtodeterminemomentum,arefound fromtwo-dimensionalGaussianfitstoimagesthatundergonoresizing (data with free expansion times of 168 ms to 768 ms). The dual-cell vacuum chamber is based on ColdQuanta’s commer- cially offered RuBECi41 , which is augmented for CAL-specific science objectives and space flight compatibility. The source cell contains rubidium and potassium alkali metal dispensers with natural abun- dance of the isotopes. The MOT is formed in the UHV science cell approximately 15 mm below the chip surface using three incident beamsthatareretro-reflectedtocreateasix-beamthree-dimensional MOT.ExtendedDataFig. 1aillustratestheMOTlocationinthescience cell, and the four optical beams that are incident upon atoms in the y–z plane. More details about the hardware and configuration can be found in the referenced material40,41 . From the MOT, the atoms are gathered in a quadrupole magnetic trap formed by external coils and then transported along the z axis to the atom chip. Thefirst-ordermagneticpotentialfor87 RbatomsintheF = 2hyper- fine manifold is proportional to the product of the field modulus and themagneticquantumnumberoftheatom,mF.Dependingonthesign ofmF,thecorrespondingZeemanstatewillbeconfinedtowards(+)or repelledfrom(−)afieldminimuminthechip-trap.AnappliedRFfield (theRFknife)thencouplesthetrappedatomsinthe|F = 2,mF = 2⟩state to the less confined |2, 1⟩, the magnetically insensitive |2, 0⟩, or the anti-trapped |2, −1⟩ and |2, −2⟩ states. During CAL’s evaporation stages, the magnetic trap is formed by applying bias fields along the x and y axes (Fig. 1f) that combine with the atom chip fields to yield relatively large trap oscillation frequen- cies for 87 Rb atoms in the |2, 2⟩ state, which, near the approximately harmonictrapbottomof7 G,correspondtotraposcillationfrequencies (ωx,ωy,ωz) = 2π × (216,1,000,1,080) Hz.Withsuchtighttraps,collisions among atoms are sufficiently high to cool atoms to BEC in just over a secondofevaporationtime.Forthe|2,0⟩state,thesesamefieldsform anattractiveQZpotentialforthe|2,0⟩statewithan18 μKdepth.Similar tothe|2,2⟩trap,withatrapaspectratioof x x ω ω/ = / = 0.2z x x z 2 2 2 2 , any|2,0⟩atomsconfinedbytheQZforceandabletoexchangeenergy between dimensions should produce an aspect ratio of ≤0.2, which is lower than that of the original harmonic trap due to the additional quartic terms. Followingforcedevaporationweapplytwostagesofdecompression in orbit, during which we apply a linear decrease to the magnetic bias fields down to 12% of their evaporation values over the first 100 ms, then further decrease the bias fields to 8% with a second stage, var- ied from 100 ms to 200 ms in duration. These two decompression stages lower the trap oscillation frequencies for the |2, 2⟩ state to (ωx, ωy, ωz) = 2π × (8.2, 30, 46) Hz and (ωx, ωy, ωz) = 2π × (11, 20, 15) Hz, respectively.Chipcurrentsareheldconstantduringalldecompression protocols reported in the current work, and then all chip currents are extinguishedwithin20 μsinordertoreleaseatomsforfreeexpansion and imaging after varied time of flight. The second decompression ramptimeof181 mswasempiricallydeterminedtolimitcentre-of-mass motion to a drift velocity of 3.1 mm s−1 . We then perform 3 ms of adi- abaticrapidpassageintothe|2,0⟩statetominimizeforcesfromstray magnetic gradients. Stringent requirements on the design and implementation of CAL were required to not only satisfy the size, weight, power and thermal management requirements of the ISS, but also to assure robust and reproducible operation over its lifetime in the variable ISS environ- ment.CALincorporatesrelativelyhighrepetitionrateswithlong-term operation (running hundreds of times each day for over 1.5 years). These features provide large and repeatable datasets with iterative feedbackcapabilitiesnecessaryforadvancingthescientificandtech- nical maturation of microgravity-enabled atomic gas experiments. The total volume (0.4 m3 ) and mass (233 kg) were optimized to allow CAL to occupy one single locker and one quad-locker of an EXPRESS rack (as illustrated in Fig. 1). The average daily power draw for all CAL subsystemsis510 W.Forthermalmanagementofthelaserandelectron- ics subsystems, as well as for the vacuum chamber inside the science module,airandwatercoolingareusedatsupplytemperaturesof18 °C. Incontrasttoanalogousterrestrialandmicrogravitycold-atomsys- tems demonstrated to date, CAL is designed to operate for years in a highly dynamic environment without regular human intervention. Access to this orbital environment, relatively high duty cycles, and mitigation of external perturbations allows investigators to pursue longer microgravity-enabled experimental campaigns and demon- strates advances in technology towards exploring fundamental sci- encethatisinaccessibleontheground(includinggeodesywithatomic test masses and high-precision tests of the underlying principles of Einstein’s general relativity). Magnetic shielding inside the science moduleisdesignedtomitigatetheeffectsoftheEarth’smagneticfield, which varies in both direction and amplitude over each 90-min orbit. In parallel, the excessive radiation environment (for example, during passesthroughtheSouthAtlanticAnomaly),localperturbationsfrom neighbouring experiments on the ISS and from astronaut activities, thehighrotationrateoftheISS,andtheintensevibrationandacoustic environmentsduringlaunchwereallaccountedforinthedesignofCAL. Dataavailability Source data for Fig. 4 are provided with the paper. The datasets gen- erated and analysed during the current study are available from the corresponding authors on reasonable request. Acknowledgements We gratefully acknowledge the contributions of current and former members of CAL’s management and technical teams, T. Winn, K. Muse, L. Clonts, J. Lam, J. Liu, C. Tran, J. Tarsala, T. Tran, S. Haque, M. McKee, J. Trager, J. Mota, G. Miles, D. Strekalov, I. Li, S. Javidnia, A. Sengupta, D. Conroy, A. Croonquist, E. Burt, M. Krutzik, S. Kulas and V. Schkolnik, and the ColdQuanta team, including E. Salim, L. Czaia, J. Ramirez-Serrano, J. Duggan, and D. Anderson. We recognize the continuing support of JPL’s Astronomy, Physics and Space Technology Directorate, L. Livesay, T. Gaier, D. Coulter, C. Lawrence and U. Israelsson. We thank CAL’s principal investigators and science team members, N. Bigelow, N. Lundblad, C. Sackett, E. Cornell, P. Engels and M. Mossman, for their guidance, along with CAL’s Science Review Board, including B. DeMarco and R. Walsworth. We also recognize the steadfast support from NASA’s Division of Space, Life, and Physical Sciences Research and Applications (SLPSRA), C. Kundrot, D. Malarik, M. Lee, B. Carpenter and D. Griffin. This work was funded by NASA’s SLPSRA programme office, and operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. US Government sponsorship is acknowledged. Author contributions D.C.A., J.R.W. and E.R.E. optimized and operated the instrument during the CAL commissioning phase, collected and analysed the associated data, and prepared this manuscript. D.C.A., J.R.W. and E.R.E. were responsible for instrument hardware integration, experimental operation, optimization and data acquisition during the CAL integration and test phase. D.C.A. led CAL’s ground testbed and the integration and testing of the science module
  • 7. hardware. J.R.W. led flight instrument operation and atom-interferometer-related tests. E.R.E. led integration and operation of CAL’s engineering model testbed. C.D. established and led the mission operations and ground data systems during commissioning. J.R.K. prepared and coordinated ISS installation procedures and operations. J.R.K. and J.M.K. led the laser and optics subsystems and operated the system post-commissioning. R.F.S., K.O., N.Y. and N.E.L. led technical planning and provided guidance across multiple subsystems during the integration and test phase. R.J.T. proposed the instrument, gave scientific guidance and coordinated with principal investigators as CAL project scientist. All authors read, edited and approved the final manuscript. Competing interests The authors declare no competing interests. Additional information Correspondence and requests for materials should be addressed to D.C.A. or R.J.T. Peer review information Nature thanks A. Roura and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Reprints and permissions information is available at http://www.nature.com/reprints.
  • 8. Article ExtendedDataFig.1|Sciencemoduleopticalbeams.a,Anillustrative cross-sectionofthesciencemoduleinthey–zplane,showingtheopticalbeam pathsforlasercoolingandimaging.Collimators(depictedinblue)eachaccept opticalfibreinputsanddirectfree-spacebeamsintothevacuumchamber.Inthe sourcecell,ellipticalbeamcollimatorscreateatwo-dimensional(2D)MOT(the x-axiscollimatorisnotshown).Acoldatomicbeamisdirected(itsfluxenhanced bythe2DMOTpushbeam)uptotheUHVsciencecell,where>109 atomsare collectedinathree-dimensionalMOT.Inthisregion,lasercoolingbeamslabelled ‘MOT(a)’and‘MOT(b)’aredirectedalongthey–zplane,whileathirdcollimator (notshown)sendsits11-mm-diameterbeam(dottedcircle)alongthex-axisto completetheMOT.Eachbeamisretro-reflectedbyamirrortocreateafull six-beamMOTlocatedabout15 mmbelowtheatomchip,whichformsthe topmostwalloftheUHVchamber.CAL’sprimaryimagingbeam(also11 mmin diameter)isdirectedparalleltothechipsurfacealongtheyaxisjustunderthe chip,labelledas‘Imaging(y)’.Fluorescenceandabsorptionimagesalongthisaxis arecollectedonthe‘CMOS(y)’camera.Alternatively,through-chipimaging alongthezaxiscanbecollectedbythe‘CMOS(z)’camera,withanabsorption imagingbeamprovidedbythe‘Imaging(z)’lightthatpassesthrougha 0.75-mm-diameterapertureofthesourcecell.Thisapertureprovidesdifferential pumpingtomaintainUHVconditionsinthesciencecellwhilethesourcecellruns athigherpressuresofRbandK.ThebackgroundpressureofRbandKis controlledbyrunningindependentcurrentthrougheachoftwoalkalimetal dispensers:onecontainingRb,andtheotherK.AtomsarecollectedintheMOT beforeundergoingmolassescoolingandbecomingconfinedbythe coil-generatedmagnetictrap.Thetrappedcloudisthentransportedup15 mm byasecondpairofcoils,andthenloadedintotheatomchiptrap.b,A photographofasciencemodulewithoutitsfrontclam-shellofmagneticshields, showingthemechanicalstructurethatrigidlysupportsthevacuumandoptical hardware,aswellassomeofthethermalmanagementcomponents.Moredetails ofthesciencemoduleandcontrolhardwarecanbefoundinref.40 .