Lar cooling and trapping of neutral atoms*
William D.Phillips
National Institute of Standards and Technology,Physics Laboratory,
Atomic Physics Division,Gaithersburg,Maryland20899
高冷伤感网名[S0034-6861(98)00603-5]
INTRODUCTION
In1978,while I was a postdoctoral fellow at MIT,I read a paper by Art Ashkin(1978)in which he de-scribed how one might slow down an atomic beam of sodium using the radiation pressure of a lar beam tuned to an atomic resonance.After being slowed,the atoms would be captured in a trap consisting of focud lar beams,with the atomic motion being damped until the temperature of the atoms reached the microkelvin range.That paper was myfirst introduction to lar cooling,although the idea of lar cooling(the reduction of random thermal velocities using radiative forces)had been propod three years earlier in independent papers by Ha¨nsch and Schawlow(1975)and Wineland and De-hmelt(1975).Although the treatment in Ashkin’s paper was necessarily over-simplified,it provide
d one of the important inspirations for what I tried to accomplish for about the next decade.Another inspiration appeared later that same year:Wineland,Drullinger and Walls (1978)published thefirst lar cooling experiment,in which they cooled a cloud of Mg ions held in a Penning trap.At esntially the same time,Neuhaur,Hohen-statt,Toschek and Dehmelt(1978)also reported lar cooling of trapped Baϩions.
Tho lar cooling experiments of1978were a dra-matic demonstration of the mechanical effects of light, but such effects have a much longer history.The under-standing that electromagnetic radiation exerts a force became quantitative only with Maxwell’s theory of elec-tromagnetism,even though such a force had been con-jectured much earlier,partly in respon to the obrva-tion that comet tails point away from the sun.It was not until the turn of the century,however,that experiments by Lebedev(1901)and Nichols and Hull(1901,1903) gave a laboratory demonstration and quantitative mea-surement of radiation pressure on macroscopic objects. In1933Frisch made thefirst demonstration of light pressure on atoms,deflecting an atomic sodium beam with resonance radiation from a lamp.With the advent of the lar,Ashkin(1970)recognized the potential of inten,narrow-band light for manipulating atoms and in1972thefirst‘‘modern’’experiments demonstrated the deflection of atomic beams with lars(Picque´and Vialle,1972;Schieder et al.,197
2).All of this t the stage for the lar cooling proposals of1975and for the demonstrations in1978with ions.
Comet tails,deflection of atomic beams and the lar cooling propod in1975are all manifestations of the radiative force that Ashkin has called the‘‘scattering force,’’becau it results when light strikes an object and is scattered in random directions.Another radiative force,the dipole force,can be thought of as arising from the interaction between an induced dipole moment and the gradient of the incident lightfield.The dipole force was recognized at least as early as1962by Askar’yan, and in1968,Letokhov propod using it to trap atoms—even before the idea of lar cooling!The trap propod by Ashkin in1978relied on this‘‘dipole’’or‘‘gradient’’force as well.Nevertheless,in1978,lar cooling,the reduction of random velocities,was understood to in-volve only the scattering force.Lar trapping,confine-ment in a potential created by light,which was still only a dream,involved both dipole and scattering forces. Within10years,however,the dipole force was en to have a major impact on lar cooling as well.
Without understanding very much about what diffi-culties lay in store for me,or even appreciating the ex-citing possibilities of what one might do with lar cooled atoms,I decided to try to do for neutral atoms what the groups in Boulder and Heidelberg had done for ions:trap them and cool them.There
was,however, a significant difficulty:we could notfirst trap and then cool neutral atoms.Ion traps were deep enough to easily trap ions having temperatures well above room tem-perature,but none of the propod neutral atom traps had depths of more than a few kelvin.Significant cooling was required before trapping would be possible,as Ash-kin had outlined in his paper(1978),and it was with this idea that I began.
Before describing thefirst experiments on the decel-eration of atomic beams,let me digress slightly and dis-cuss why lar cooling is so exciting and why it has at-tracted so much attention in the scientific community: When one studies atoms in a gas,they are typically mov-ing very rapidly.The molecules and atoms in air at room temperature are moving with speeds on the order of300 m/s,the speed of sound.This thermal velocity can be reduced by refrigerating the gas,with the velocity vary-ing as the square root of the temperature,but even at77 K,the temperature at which N2condens into a liquid, the nitrogen molecules are moving at about150m/s.At 4K,the condensation temperature of helium,the He atoms have90m/s speeds.At temperatures for which atomic thermal velocities would be below1m/s,any gas in equilibrium(other than spin-polarized atomic hydro-
*The1997Nobel Prize in Physics was shared by Steven Chu,
Claude N.Cohen-Tannoudji,and William D.Phillips.This text
is bad on Dr.Phillips’s address on the occasion of the award.
721 Reviews of Modern Physics,Vol.70,No.3,July19980034-6861/98/70(3)/721(21)/$19.20©1998The Nobel Foundation
gen)would be condend,with a vapor pressure so low that esntially no atoms would be in the gas pha.As a result,all studies of free atoms were done with fast at-oms.The high speed of the atoms makes measurements difficult.The Doppler shift and the relativistic time dila-tion cau displacement and broadening of the spectral lines of thermal atoms,which have a wide spread of ve-locities.Furthermore,the high atomic velocities limit the obrvation time(and thus the spectral resolution)in any reasonably-sized apparatus.Atoms at300m/s pass through a meter-long apparatus in just3ms.The ef-fects are a major limitation,for example,to the perfor-mance of conventional atomic clocks.
The desire to reduce motional effects in spectroscopy and atomic clocks was and remains a major motivation for the cooling of both neutral atoms and ions.In addi-tion,some remarkable new phenomena appear when at-
oms are sufficiently cold.The wave,or quantum nature
of particles with momentum p becomes apparent only
when the de Broglie wavelength,given bydBϭh/p,be-comes large,on the order of relevant distance scales like
the atom-atom interaction distances,atom-atom para-
tions,or the scale of confinement.Lar cooled atoms
have allowed studies of collisions and of quantum col-
lective behavior in regimes hitherto unattainable.
Among the new phenomena en with neutral atoms is
Bo-Einstein condensation of an atomic gas(Anderson
et al.,1995;Davis,Mewes,Andrews,et al.,1995),which has been hailed as a new state of matter,and is already becoming a major newfield of investigation.Equally im-pressive and exciting are the quantum phenomena en with trapped ions,for example,quantum jumps (Bergquist et al.,1986;Nagourney et
al.,1986;Sauter et al.,1986),Schro¨dinger cats(Monroe et al.,1996),and quantum logic gates(Monroe et al.,1995).
LASER COOLING OF ATOMIC BEAMS
In1978I had only vague notions about the excitement that lay ahead with lar cooled atoms,but I concluded that slowing down an atomic beam was thefirst step. The atomic beam was to be slowed using the transfer of momentum that occurs when an atom absorbs a photon. Figure1shows the basic process underlying the‘‘scat-tering force’’that results.An atomic beam with velocity v is irradiated by an opposing lar beam.For each pho-ton that a ground-state atom absorbs,it is slowed by v recϭបk/m.In order to absorb again the atom must re-turn to the ground state by emitting a photon.Photons are emitted in random directions,but with a symmetric average distribution,so their contribution to the atom’s momentum averages to zero.The randomness results in a‘‘heating’’of the atom,discusd below.
For sodium atoms interacting with the familiar yellow resonance light,v recϭ3cm/s,while a typical beam veloc-ity is about105cm/s,so the absorption-emission process must occur about3ϫ104times to bring the Na atom to rest.In principle,an atom could radiate and absorb pho-tons at half the radiative
decay rate of the excited state (a2-level atom in steady state can spend at most half of its time in the excited state).For Na,this implies that a
photon could be radiated every32ns on average,bring-
ing the atoms to rest in about1ms.Two problems,op-
tical pumping and Doppler shifts,can prevent this from
happening.I had an early indication of the difficulty of
decelerating an atomic beam shortly after reading Ash-
kin’s1978paper.I was then working with a sodium
atomic beam at MIT,using tunable dye lars to study
the scattering properties of optically excited sodium.I
tuned a lar to be resonant with the Na transition from
3S1/2→3P3/2,the D2line,and directed its beam oppo-site to the atomic beam.I saw that the atoms near the老三电网院理论
beam source werefluorescing brightly as they absorbed
the lar light,while further away from the source,the
atoms were relatively dim.The problem,I concluded,
was optical pumping,illustrated in Fig.2.
Sodium is not a two-level atom,but has two ground
hyperfine levels(Fϭ1and Fϭ2in Fig.2),each of which
consists of veral,normally degenerate,states.Lar
excitation out of one of the hyperfine levels to the ex-
cited state can result in the atom radiating to the other
hyperfine level.This optical pumping esntially shuts
off the absorption of lar light,becau the linewidths
of the transition and of the lar are much smaller than
the paration between the ground state hyperfine
com-FIG.1.(a)An atom with velocity v encounters a photon with momentumបkϭh/;(b)after absorbing the photon,the atom is slowed byបk/m;(c)after re-radiation in a random direc-tion,on average the atom is slower than in
(a).
FIG.2.(a)The optical pumping process preventing cycling transitions in alkalis like Na;(b)u of a repumping lar to allow many absorption-emission cycles.
722William D.Phillips:Lar cooling and trapping of neutral atoms Rev.Mod.Phys.,Vol.70,No.3,July1998
ponents.Even for atoms excited on the3S1/2(Fϭ2)→3P3/2(FЈϭ3)transition,where the only allowed de-cay channel is to Fϭ2,off-resonant excitation of FЈϭ2 (the linewidth of the transition is10MHz,while the paration between FЈϭ2and FЈϭ3is60MHz)leads to optical pumping into Fϭ1after only about a hundred absorptions.This optical pumping made the atoms ‘‘dark’’to my lar after they traveled only a short dis-tance from the source.
An obvious solution[Fig.2(b)]is to u a cond lar frequency,called a repumper,to excite the atoms out of the‘‘wrong’’(Fϭ1)hyperfine state so that they can de-cay to the‘‘right’’state(Fϭ2)where they can continue to cool.Given the repumper,another problem becomes apparent:the Doppler shift.In order for the lar light to be resonantly absorbed by a counterpropagating atom moving with velocity v,the frequencyof the light must be k v lower than the resonant frequency for an atom at re
st.As the atom repeatedly absorbs photons, slowing down as desired,the Doppler shift changes and the atom goes out of resonance with the light.The natu-ral linewidth⌫/2of the optical transition in Na is10 MHz(full width at half maximum).A change in velocity of6m/s gives a Doppler shift this large,so after absorb-ing only200photons,the atom is far enough off reso-nance that the rate of absorption is significantly reduced. The result is that only atoms with the‘‘proper’’velocity
to be resonant with the lar are slowed,and they are
only slowed by a small amount.
Nevertheless,this process of atoms being slowed and
pushed out of resonance results in a cooling or narrow-
ing of the velocity distribution.In an atomic beam,there
is typically a wide spread of velocities around v th ϭ3k B T/m.Tho atoms with the proper velocity will absorb rapidly and decelerate.Tho that are too fast
will absorb more slowly,then more rapidly as they come
201钢
into resonance,andfinally more slowly as they continue
to decelerate.Atoms that are too slow to begin with will
absorb little and decelerate little.Thus atoms from a
range of velocities around the resonant velocity are
pushed into a narrower range centered on a lower veloc-
ity.This process was studied theoretically by Minogin
(1980)and in1981,at Moscow’s Institute for Spectros-
copy,was ud in thefirst experiment clearly demon-
strating lar cooling of neutral atoms(Andreev et al.,
1981).
Figure3shows the velocity distribution after such
cooling of an atomic beam.The data was taken in our
laboratory,but is equivalent to what had been done in
Moscow.The characteristic of this kind of beam cooling
is that only a small part of the total velocity distribution
(the part near resonance with the lar beam)is slowed
by only a small amount(until the atoms are no longer
resonant).The narrow peak,while it reprents true
cooling in that its velocity distribution is narrow,consists
轻伤鉴定of rather fast atoms.
One solution to this problem had already been out-
lined in1976by Letokhov,Minogin,and Pavlik.They
suggested a general method of changing the frequency
(chirping)of the cooling lar so as to interact with all the atoms in a wide distribution and to stay in resonance with the atoms as they are cooled.The Moscow group applied the technique to decelerating an atomic beam (Balykin et al.,1979)but without clear success(Balykin, 1980).[Later,in1983,John Prodan and I obtained the first clear deceleration and cooling of an atomic beam with this‘‘chirp-cooling’’technique(Phillips and Prodan,1983,1984;Phillips,Prodan,and Metcalf,1983a; Prodan and Phillips,1984).Thofirst attempts failed to bring the atoms to rest,something that wasfinally achieved by Ertmer,Blatt,Hall and Zhu(1985).]The chirp-cooling technique is now one of the two standard methods for decelerating beams.The other is‘‘Zeeman cooling.’’
By late1978,I had moved to the National Bureau of Standards(NBS),later named the National Institute of Standards and Technology(NIST),in Gaithersburg.I was considering how to slow an atomic beam,realizing that the optical pumping and Doppler shift problems would both need to be addresd.I understood how things would work using the Moscow chirp-cooling tech-nique and a repumper.I also considered using a broad-band lar,so that there would be light in resonance with the atoms,regardless of their velocity.[This idea was refined by Hoffnagle(1988)and demonstrated by Hall’s group(Zhu,Oates,and Hall,1991).]Finally I considered that instead of changing the frequency o
f the lar to stay in resonance with the atoms(chirping),one could u a magneticfield to change the energy level paration in the atoms so as to keep them in resonance with thefixed-frequency lar(Zeeman cooling).All of the ideas for cooling an atomic beam,along with
vari-FIG.3.Cooling an atomic beam with afixed frequency lar. The dotted curve is the velocity distribution before cooling, and the solid curve is after cooling.Atoms from a narrow ve-locity range are transferred to a slightly narrower range cen-tered on a lower velocity.
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William D.Phillips:Lar cooling and trapping of neutral atoms Rev.Mod.Phys.,Vol.70,No.3,July1998
ous schemes for avoiding optical pumping,were con-tained in a proposal (Phillips,1979)that I submitted to the Office of Naval Rearch in 1979.Around this time Hal Metcalf,from the State University of New York at Stony Brook,joined me in Gaithersburg and we began to consider what would be the best way to proceed.Hal contended that all the methods looked reasonable,but we should work on the Zeeman cooler becau it would be the most fun!Not only was Hal right about the fun we would have,but his suggestion led us to develop a technique with particularly advantageous properties.The idea is illustrated in Fig.4.
The atomic beam source directs atoms,which have a wide range of velocities,along the axis (z direction)of a tapered solenoid.This magnet has more windings at its entrance end,near the source,so the field is higher at that end.The lar is tuned so that,given the field-induced Zeeman shi
ft and the velocity-induced Doppler shift of the atomic transition frequency,atoms with ve-locity v 0are resonant with the lar when they reach the point where the field is maximum.Tho atoms then absorb light and begin to slow down.As their velocity changes,their Doppler shift changes,but is compensated by the change in Zeeman shift as the atoms move to a point where the field is weaker.At this point,atoms with initial velocities slightly lower than v 0come into reso-nance and begin to slow down.The process continues with the initially fast atoms decelerating and staying in resonance while initially slower atoms come into reso-nance and begin to be slowed as they move further down the solenoid.Eventually all the atoms with veloci-ties lower than v 0are brought to a final velocity that depends on the details of the magnetic field and lar tuning.
The first tapered solenoids that Hal Metcalf and I ud for Zeeman cooling of atomic beams had only a few ctions of windings and had to be cooled with air blown by fans or with wet towels wrapped around the coils.Shortly after our initial success in getting some substantial deceleration,we were joined by my first post-doc,John Prodan.We developed more sophisticated so-lenoids,wound with wires in many layers of different lengths,so as to produce a smoothly varying field that would allow the atoms to slow down to a stop while remaining in resonance with the cooling lar.
The later solenoids were cooled with water flowing over the coils.To improve the heat transfer,we fi
lled the spaces between the wires with various heat-conducting substances.One was a white silicone grea that we put onto the wires with our hands as we wound the coil on a lathe.The grea was about the same color and consis-tency as the diaper rash ointment I was then using on my baby daughters,so there was a period of time when,whether at home or at work,I emed to be up to my elbows in white grea.
The grea-covered,water-cooled solenoids had the annoying habit of burning out as electrolytic action at-tacked the wires during operation.Sometimes it emed that we no sooner obtained some data than the solenoid would burn out and we were winding a new one.
On the bright side,the frequent burn-outs provided the opportunity for refinement and redesign.Soon we were embedding the coils in a black,rubbery resin.While it was suppod to be impervious to water,it did not have good adhesion properties (except to clothing and human flesh)and the solenoids continued to burn out.Eventually,an epoxy coating aled the solenoid against the water that allowed the electrolysis,and in more recent times we replaced water with a fluorocar-bon liquid that does not conduct electricity or support electrolysis.Along the way to a reliable solenoid,we learned how to slow and stop atoms efficiently (Phillips and Metcalf,1982;Prodan,Phillips,and Metcalf,1982;Phillips,Prodan,and Metcalf,1983a,1983b,1984a,1984b,1985;Metcalf and Phillips,1985)
.
The velocity distribution after deceleration is mea-sured in a detection region some distance from the exit end of the solenoid.Here a parate detection lar beam produces fluorescence from atoms having the cor-rect velocity to be resonant.By scanning the frequency of the detection lar,we were able to determine the velocity distribution in the atomic beam.Obrvations with the detection lar were made just after turning off the cooling lar,so as to avoid any difficulties with hav-ing both lars on at the same time.Figure 5shows the velocity distribution resulting from Zeeman cooling:a large fraction of the initial distribution has been swept down into a narrow final velocity group.
One of the advantages of the Zeeman cooling tech-nique is the ea with which the optical pumping prob-lem is avoided.Becau the atoms are always in a strong axial magnetic field (that is the reason for the ‘‘bias’’windings in Fig.4),there is a well-defined axis of quan-tization that allowed us to make u of the lection rules for radiative transitions and to avoid the undesir-able optical pumping.Figure 6shows the energy levels of Na in a magnetic field.Atoms in the 3S 1/2(m F ϭ2)state,irradiated with circularly polarized ϩlight,must increa their m F by one unit,and so can go only to the 3P 3/2(m F Јϭ3)state.This state in turn can decay only to 3S 1/2(m F ϭ2),and the excitation process can be re-peated indefinitely.Of cour,the circular polarization is not perfect,so ot
her excitations are possible,and the may lead to decay to other states.Fortunately,in a high magnetic field,such transitions are highly
unlikely
FIG.4.Upper:Schematic reprentation of a Zeeman slower.Lower:Variation of the axial field with position.
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Rev.Mod.Phys.,Vol.70,No.3,July 1998
(Phillips and Metcalf,1982):either they involve a change in the nuclear spin projection m I ,which is for-bidden in the high field limit,or they are far from reso-nance.The features,combined with high purity of the circular polarization,allowed us to achieve,without a ‘‘wrong transition,’’the 3ϫ104excitations required to七夕来历
stop the atoms.Furthermore,the circular polarization produced some ‘‘good’’optical pumping:atoms not ini-tially in the 3S 1/2(m F ϭ2)state were pumped into this state,the ‘‘stretched’’state of maximum projection of angular momentum,as they absorbed the angular mo-mentum of the light.The various aspects of optical lection rules and optical pumping allowed the process of Zeeman cooling to be very efficient,decelerating a large fraction of the atoms in the beam.
十枣汤的配方
In 1983we discusd a number of the aspects of lar deceleration,including our early chirp-cooling re-sults,at a two-day workshop on ‘‘Lar-Cooled and Trapped Atoms’’held at NBS in Gaithersburg (Phillips,1983).I view this as an important meeting in that it and its proceedings stimulated interest in lar cooling.In early 1984,Stig Stenholm,then of the University of Hel-sinki,organized an international meeting on lar cool-ing in Tva
¨rminne,a remote peninsula in Finland.Figure 7shows the small group attending (I was the photogra-pher),and in that group,only some of the participants were even active in lar cooling at the time.Among the were Stig Stenholm [who had done pioneering work in the theory of lar cooling and the mechanical effects of light on atoms (Stenholm,1978a,1978b,1985,1986;Javanainen and Stenholm,1980a,1980b,1980c,1981a,1981b)]along with some of his young colleagues;Victor Balykin and Vladimir Minogin from the Moscow group;and Claude Cohen-Tannoudji and Jean Dalibard赣州一日游必去景点
无花果作用from Ecole Normale Supe
´rieure (ENS)in Paris,who had begun working on the theory of lar cooling and
trapping.Also prent were Ju
¨rgen Mlynek and Wolf-gang Ertmer,both of whom now lead major rearch groups pursuing lar cooling and atom optics.At that time,however,only our group and the Moscow group had published any experiments on cooling of neutral at-oms.
Much of the discussion at the Tva
¨rminne meeting in-volved the techniques of beam deceleration and the problems with optical pumping.I took a light-hearted attitude toward our trials and tribulations with optical pumping,often joking that any unexplained features in our data could certainly be attributed to optical pump-ing.Of cour,at the Ecole Normale,optical pumping had a long and distinguished history.Having been pio-neered by Alfred Kastler and Jean Brosl,optical pumping had been the backbone of many experiments in the Laboratoire de Spectroscopie Hertzienne (now the Laboratoire Kastler-Brosl).After one discussion in which I had joked about optical pumping,Jean Dali-bard privately mentioned to me,‘‘You know,Bill,at the Ecole Normale,optical pumping is not a joke.’’His gentle note of caution calmed me down a bit,but it turned out to be strangely prophetic as well.As we saw a few years later,optical pumping had an important,beautiful,and totally unanticipated role to play in lar cooling,and it was surely no joke.
STOPPING ATOMS
As successful as Zeeman cooling had been in produc-ing large numbers of decelerated atoms as in Fig.5,
we
FIG.5.Velocity distribution before (dashed)and after (solid)Zeeman cooling.The arrow indicates the highest velocity reso-nant with the slowing lar.(The extra bump at 1700m/s is from F ϭ1atoms,which are optically pumped into F ϭ2dur-ing the cooling
process.)
FIG.6.Energy levels of Na in a magnetic field.The cycling transition ud for lar cooling is shown as a solid arrow,and one of the nearly forbidden excitation channels leading to un-desirable optical pumping is shown dashed.
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William D.Phillips:Lar cooling and trapping of neutral atoms
Rev.Mod.Phys.,Vol.70,No.3,July 1998
