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I t occurs in objects as diverse as supercon-ductors, atomic nuclei and neutron stars.Several research groups are in a race to re-

create it in the laboratory in microscopicspecks of ultracold gas. If they succeed, it willenable experimental studies of processes thathave heretofore been the domain of theorists.“It” is a superfluid state of matter predictedto occur when quantum particles that nor-mally shun one another pair up and behaveen masse as a single body of fluid.

This superfluid state involves a broad classof quantum particles called fermions. Ac-cording to quantum mechanics, all particlesin nature are either bosons or fermions. Thedistinct characters of these two classes becomemost accentuated at very low temperatures:Bosons sociably gather all in a single quantumstate, forming a Bose-Einstein condensate.Fermions, in contrast, act as individualists, notwo occupying the same quantum state. Asthings cool, fermions increasingly occupy thelowest energy states, but they stack up one toa state, like people crowded onto a narrowflight of stairs. This state, in which most of thelowest energy states are occupied by one fer-mion each, is called a degenerate fermi gas.

In 1999 Deborah S. Jin and Brian De-Marco of JILA in Boulder, Colo., producedthe first degenerate fermi gas of atoms in a tinycloud of potassium atoms in a magnetic trap.But such a degenerate gas is only half the sto-ry. In similar degenerate systems that occur inliquid helium 3 and among electrons in su-perconductors, something new happens—

some of the fermions form up in pairs calledCooper pairs. These pairs, which are boson-ic, then form a superfluid state very similar toa Bose-Einstein condensate: in helium 3 it isresponsible for the liquid’s superfluid proper-ties; in a superconductor it allows the resis-tanceless flow of electricity.

Can such a superfluid state be made in the

gaseous fermion systems? Theory predictsthat atomic Cooper pairs usually will formonly at a temperature much colder than thatrequired for degeneracy, a temperature thatseems beyond the reach of experiment at themoment. Recently, however, an alternativemethod was suggested, based on the fact thatCooper pairing depends on not just the tem-perature but also the interaction between theatoms. So instead of making the gas colder,why not increase the interaction? Nature hasfortuitously provided a convenient way to ad-just the interaction—by applying a magneticfield of just the right strength to create what iscalled a Feshbach resonance, which generatesa powerful attractive or repulsive interactionbetween the atoms. (An attractive one is need-ed for Cooper pairs to form.)

In late 2002 a group led by John E.Thomas of Duke University used these tech-niques with lithium 6 atoms to produce re-sults highly suggestive of superfluidity. Thetrapped gas formed a thin cylinder, and whenthe trapping laser beams were turned off, thegas expanded radially to form a disk shape—

very little expansion took place along the axisof the cylinder. Such anisotropic expansionhad previously been predicted to be a hall-mark of the superfluid state.

As the Duke group pointed out, however,other effects can also generate such anisotrop-ic expansion. Indeed, experiments conductedearlier this year by Jin’s group and by Chris-tophe Salomon and his co-workers at theÉcole Normale Supérieure in Paris have ex-hibited similar anisotropic expansions in sit-uations where a superfluid cannot be present.

A technique for directly detecting theCooper pairs or the superfluid is needed. Jin,as well as Wolfgang Ketterle’s group at theMassachusetts Institute of Technology, re-cently reported using radio waves to studythe precise states of the atoms in the trapped

The Next Big ChillPHYSICISTS CLOSE IN ON A NEW STATE OF MATTER BY GRAHAM P. COLLINS

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A degenerate gas of fermionsoccurs in diverse situations,

as described below:

■ Superconductors: The electrons are degenerate and

form loosely correlated Cooperpairs, which produce the

superconductivity. Somethingsimilar must happen in high-

temperature superconductors, butthat process remains a mystery.

■ Neutron stars: The refusal ofneutrons (which are fermions) tooccupy identical quantum states

generates a repulsion thatprevents the star from collapsing

under its own immense weight. A similar repulsion stabilizes the

laboratory-made degenerate fermigases against collapse.

■ Quark-gluon plasma: As created at the Relativistic

Heavy Ion Collider at BrookhavenNational Laboratory, the exploding

cloud of free quarks (which arefermions) and gluons has

properties similar to a gas offermionic atoms released from the

confines of a trap.

A BUNCH OFDEGENERATES

SUPERFLUID? An ultracold gas oflithium 6, initially compressed in a

thin cylinder, expands radially whenreleased—a result that is suggestiveof superfluidity but is not conclusive.

The sequence runs from 0.1 to 2.0milliseconds after the release.

COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC.

The stress injury called dystoniaappears to originate in the brain,

not the muscles. “If you do an MRIon someone with focal dystonia,”says Edgar E. Coons of the Center

for Neural Science at New YorkUniversity, “you see a change in

the parts of the brain that receivetouch and motor feedback for each

finger. Those zones are normallyphysically separated in the brain.

But in people with dystonia, theregions merge.” Curiously, the

cramping and rigidity of focaldystonia often disappear during

everyday activities but resurface assoon as the musician starts to play.

IT GETS STUCKIN YOUR HEAD

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My left forearm twinges as I sit downat Kathleen M. Riley’s piano. Anhour of scribbling notes and two days

of working on a laptop computer have in-flamed my repetitive stress injury, an ailmentcommon among journalists—and musicians.In fact, hardworking musicians can developa much more severe condition, known as fo-cal dystonia, which cramps the hands so bad-ly that it often ends a promising career. In-jections of botulinum toxin can relieve dys-tonia for some, but the effect lasts only acouple of months.

I never had a career at the piano. But Ihave played Haydn’s Sonata No. 50 morethan 100 times over the past 20 years and atone point had even committed much of it tomemory. What is unnerving me, in part, is thecomputer attached to the Yamaha Disklavier

piano that will record justhow I touch each key.Also unsettling are Riley’srefereelike gaze and thevideo camera trained onmy left hand. But mainlymy trepidation is fed by agloomy certainty that thatsore hand will lag throughthe opening bars. As in-deed it does: what shouldbe a quiet, perfectly evenmotif of sixteenth notescomes out as a skewed,off-tempo jangle.

Riley, a music technologist and dystoniatherapist at New York University, can help.By linking the instrumented instrument withsoftware and a precisely synchronized videorecording, she has turned the piano into a

medical machine. The system captures thetime and velocity at which each note is struckand released. Even more important, it cap-tures the position of the performer’s hands,arms and body. Bad habits—slouching, an-gled wrists, rigid forearms, raised elbows—

can over years of playing contribute to focaldystonia.

“Athletes are coached about how to holdand move their bodies,” Riley says. “But mu-sicians rarely get that kind of instruction fromtheir teachers. And unlike athletes, musicianstend not to warm up before practicing, takebreaks to rest, or stretch out afterward.” Ri-ley, who so far has helped five musicians easetheir dystonia, uses the computer’s “pianoroll” display of a performance to detect whichfingers are cramping in certain passages. Thesynchronized video reveals unhealthy pos-tures and overly tense muscles. Riley thencoaches the musician to play in ways that al-lay the cramps.

After a minute of the Haydn sonata, forexample, she stops me and rewinds the video.As the Disklavier replays my performance—

the keys moving themselves, ghostly—shepoints to the monitor. “See how your leftwrist drops?” It is half an inch lower than myright wrist, forcing the left hand to cock up-ward and its fingers to flatten. “Also, you aresitting much too close,” she says. “Your leftelbow and wrist are locked, so your forearmis full of tension.”

Riley has me move the bench six inches,arch my back to shift forward my center ofgravity, and straighten and raise my wrists sothat the piano keys, rather than my sorejoints, bear the weight of my arms. I play thepassage again, and she brings up both record-

Musical MedicineA HIGH-TECH PIANO TREATS A REPETITIVE STRESS DISORDER BY W. WAYT GIBBS

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degenerate gas; if Cooper pairs were present,the binding energy of the pairs should showup clearly. Neither group saw such signs ofCooper pairs, but both uncovered useful newdetails of how fermionic atoms interact neara Feshbach resonance.

Several teams have recently studied the for-mation of loosely bound two-atom molecules

in their gases. “We hope that we can turn [themolecules] into Cooper pairs,” Ketterle says.And in August theorist Yvan Castin and hisco-workers at the École Normale Supérieuresuggested just how that might be done: first letthe molecules Bose-condense, then adjust theFeshbach resonance. If that is true, experi-menters are just two steps from their goal.

PIANO ROLL of author’s performance shows notes that are inconsistent in spacing and duration (top) and subsequentimprovement ( bottom).

COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC.