COHERENT SPECTROSCOPY ON SINGLE ATOMIC SYSTEM AT REST IN FREE SPACE II

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COHERENT SPECTROSCOPY ON SINGLE ATOMIC SYSTEM AT REST IN FREE SPACE II

H. Dehmelt

To cite this version:

H. Dehmelt. COHERENT SPECTROSCOPY ON SINGLE ATOMIC SYSTEM AT REST IN FREE SPACE II. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-299-C8-305.

�10.1051/jphyscol:1981837�. �jpa-00221733�

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Co Zloque C8, supp Zgment au n012, Tome 42, &cembre 1982 page C8-299

COHERENT SPECTROSCOPY ON S I N G L E ATOMIC SYSTEM A T REST TN FREE SPACE I 1

H. Dehmelt

Department of Physics, University of Washington, Seattle, KA 98195, U.S.A.

Abstract.-An individual atomic i o n localized i n the center of a small Paul r f quadrupole trap i s a promising object for ultimate resolution Zaser spectroscopy because, broadly speaking, the ion may be brought t o "A State of Complete Rest i n Free Space" by side band cooling. As a consequence, a l l Doppler s h i f t s vanish.

"Free Space" i s approxfmated insofar as the e l e c t r i c trapping f i e l d vanishes i n the center of the trap and there i s no Stark e f f e c t . Reither need there be a Zeeman E f f e c t as magnetic f i e l d s may be controlled d m t o the micro-Gauss range. IVaturaZ-

Zy, there i s no t r a n s i t time broadening. Minute laser powers provided by harmonic generators s u f f i c e for saturation of optical transitions as well focused beams may be used. MilZionfold atomic amplification o f the single-ion fluorescence from a metastable l e v e l may bring resolutions of 1 part i n 1018 within reach. Furthermore,

the close localization of an atomic particle i n space i s one of the most fundarnentali problems i n physics and worthy of study on i t s own merit.

Introduction.-Remarkable p r o g r e s s h a s occured s i n c e my f i r s t p r e s e n t a t i o n under t h e same t i t l e a t t h e l a s t Symposium on Frequency S t a n d a r d s and Metrology i n 1976 a t Copper Mountain. Important p a r t s of t h e program sketched i n t h i s p r e s e n t a t i o n have been r e a l i z e d . I n experiments a t Heidelberg an i n d i v i d u a l Ba+ i o n h a s been v i s u a l l y o b s e r v e d [ l , 2 ] . By o p t i c a l s i d e band c o o l i n g i t had been l o c a l i z e d t o <20002, a r e g i o n a p p r e c i a b l y s m a l l e r t h a n t h e wave-length o f t h e l i g h t . I n Boulder an i n d i - v i d u a l ~ g + h a s been l o c a l i z e d i n a d i f f e r e n t t r a p t o < 15pm[3]. While a " c a r r i e r - only" 10 Hz wide, one-photon o p t i c a l spectrum of a mono-ion o s c i l l a t o r h a s n o t y e t been observed, s i n g l e - i o n s p e c t r a o n l y % 3 0 MHz wider t h a n t h e n a t u r a l l i n e w i d t h have been seen131 by means of a n e x t e n s i o n o f t h e "shelved o p t i c a l e l e c t r o n " atomic a m p l i f i c a t i o n scheme[4]. N e v e r t h e l e s s , i n r e f e r e n c e t o t h e u l t i m a t e r e s o l u t i o n l a s e r spectroscopy p r o j e c t e d i n 1973151 a l l p r e v i o u s mono-ion o s c i l l a t o r experiments must b e c o n s i d e r e d a s s t r i c t l y p r e l i m i n a r y s t u d i e s , and even many more such s t u d i e s a r e l i k e l y t o b e n e c e s s a r y i n t h e f u t u r e .

blono-Ion O s c i l l a t o r For U l t i m a t e R e s o l u t i o n Laser Spectroscopy.-The goal. o f o u r c u r r e n t p r e l i m i n a r y experiment under p r e p a r a t i o n i s t h e o b s e r v a t i o n of a very narrow e l e c t r o n t r a n s i t i o n a t wo on an i n d i v i d u a l i o n s t o r e d a t t h e bottom o f t h e p o t e n t i a l w e l l formed by a s m a l l Paul r f quadrupole t r a p . The i o n s of t h e Group I I I A e l e - ments, namely TI+, 1n+, ~ a + , ~ 1 + , B+ a r e e x c e l l e n t c a n d i d a t e s f o r such work [4,5].

T h e i r m e t a s t a b l e lowest 3 ~ 0 l e v e l s have e x t r a o r d i n a r y long l i f e times; e - g . , t h e corresponding n a t u r a l l i n e w i d t h of t h e 3 3 ~ 0 l e v e l of ~ 1 ' i s e s t i m a t e d a s % 1 0 uHz.

"Shelved O p t i c a l ~ l e c t r o n " Atomic A m p l i f i c a t i o n Scheme.-However, i n a mono-ion o s c i l - l a t o r such a narrow w i d t h can only b e e x p l o i t e d when a powerful a m p l i f i c a t i o n -

mechanism f o r t h e correspondingly low s c a t t e r i n g i s d e v i s e d t o overcome t h e l a r g e l o s s e s i n t h e c o l l e c t i o n and counting of t h e s c a t t e r e d photons. Monitoring of t h e absence of t h e o p t i c a l e l e c t r o n from t h e ground s t a t e v i a l a s e r f l u o r e s c e n c e a t t h e s t r o n g - 3 ~ 1 i n t e r c o m b i n a t i o n l i n e of frequency wg o r a t t h e l s O - ^'plresonance l i n e p r o v i d e s such an atomic a m p l i f i c a t i o n mechanism141. For ~ 1 + , s e e F i g . 1, where t h e e s t i m a t e d l i f e t i m e of t h e 3 ~ 0 l e v e l i s %50 m s a b s o r p t i o n of a s i n g l e photon a t t h e f o r b i d d e n 'SO - 3 ~ 0 w - t r a n s i t i o n a t 20228, s u p p r e s s e s wn-laser

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981837

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Fia. I . The 4 lowest e l e c t r o n i c l e v e l s o f the 2 0 5 ~ 1 + i o n . I n the proposed double resonance scheme the forbidden w trans- s i t i o n a t 2022.8 of i n t e r e s t t o tRe 3 ~ o

/ ⁶ ³ ^~l e v e l o f 50 m n a t u r a ~ l i f e time i s ob- served i n d i r e c t l y : during t h e temporary she Zving o f the electron i n the nretastable

3po l e v e l it i s obviously impossibze t o

G 3 e observe the strong fluorescence from t h e short-lived 3 ~ 1 l e v e l when excited by

636 i n t e n s e o 2 laser radiation a t i2 = 19093.

I n t h i s m y t h e absorption o f one 20221 photon i s made t o gate t h e s c a t t e r i n g o f

6 ^IS, about a m i l l i o n 1909A photons e f f e c t i n g an impressive atomic amplification. The ions l n f , &+, A l f , B f have similar l e v e l

s t r u c t u r e s with the width o f 3 3 ~ o f ~ ~ f )

estimated a s % 10 pHz! From 151.

fluorescence a t 1909a f o r about 50 m s i n t h i s "Shelved O p t i c a l E l e c t r o n " scheme, d u r i n g which i n t e r v a l o t h e r w i s e % l o 6 photons might have been s c a t t e r e d . Frequency s h i f t s o f t h e s h a r p 20222 l i n e due t o t h e 19092 e x c i t a t i o n a r e o b v l a t e d by appropri- a t e p u l s i n g schemes. Centering of t h e i o n a t t h e bottom of t h e t r a p i s achieved v i a s i d e band c o o l l n g by t u n i n g t h e 19098 l a s e r a few megahertz below t h e 'so - 3 ~ 1 resonance a t w2 t h u s t h a t i t s D o p p l e r s i d e band a t w2 - wv i s p r e f e r e n t i a l l y e x c i t e d . The v i b r a t i o n frequency i n t h e p a r a b o l i c t r a p i s denoted by w,. Three-dimensional c o o l i n g t o < 1 mK i s achieved by means of a s i n g l e s t r a t e g i c a l l y d i r e c t e d l a s e r beam [ 6 ] .

Apparatus.-The h e a r t of t h e a p p a r a t u s LS t h e s m a l l P a u l r f t r a p c o n t a i n i n g t h e i o n . The d e s i g n used i n t h e Heidelberg ~ a + experiments i s w e l l s u i t e d f o r t h e f u t u r e work on TI+, e t c . and w i l l t h e r e f o r e b e sketched h e r e . The t r a p [ l , 2 ] , s e e F i g . 2, i s formed by s p h e r i c a l l y ground w i r e s t u b s , t h e "caps" and a w i r e " r i n g . " A p p l i c a t i o n

Fig. 2. Apparatus for trapping i n ultra-high vacuwn, coo l i n g , and v i s u a l l y observing an individual Baf i o n (schematic). Ba oven and electron gun were a c t u a l l y not i n t h e xz plane. Vacuwn envelope, pwnp, e t c . are not shown, from [ I ] .

of a n r f v o l t a g e V o ^c200 V o l t a t R = 21r 18 MHz c r e a t e d a 3-dimensional w e l l o f a x i a l depth ^%10 v o l t ( f o r ~ a + ) i n which t h e i o n o s c i l l a t e d a t an a x i a l frequency w,,

-

^21r ^.2.4 MHz and p e r p e n d i c u l a r f r e q u e n c i e s wvx*,wvy* = 271 ' 1 . 2 MHz w i t h

l w v x ~ - wvy* 1 ' 2" 10 KHz. I d e a l l y , t h e c o o l i n g l a s e r beam i s d i r e c t e d along t h e body d i a g o n a l [ 6 ] of t h e x*,y*,z* = z c o o r d i n a t e system formed by t h e p r i n c i p a l a x e s of t h e e f f e c t i v e t r a p p i n g p o t e n t i a l . It s u f f i c e s t o approximate t h e s e c o n d i t i o n s by s l i g h t l y deforming t h e r i n g e l e c t r o d e e l l i p t ~ c a l l y and a p p r o p r i a t e l y d i r e c t i n g t h e l a s e r beam through t h e gaps between r i n g and cap electrodes'!. The resonance

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t r a p was f i l l e d by c r o s s i n g and i o n i z i n g a very weak Ba atomic beam w i t h a very weak e l e c t r o n beam i n t h e c e n t e r of t h e t r a p .

Obtaining t h e u l t r a v i o l e t power necessary f o r work w i t h any of t h e TI+ l i k e i o n s i s s t i l l a s e r i o u s experimental problem*. For 1n+ and ~ 1 ' i t should b e p o s s i b l e t o o b t a i n s u f f i c i e n t l a s e r power t o s a t u r a t e t h e somewhat allowed w2 t r a n s i t i o n by frequency doubling t h e o u t p u t of a s t a b i l i z e d commercial r i n g dye l a s e r . However, s c a t t e r i n g r a t e s f o r t h e s e i o n s a r e much lower t h a n f o r ~1'. The weak l a s e r power s u f f i c i e n t f o r t h e f o r b i d d e n wo t r a n s i t i o n may probably b e o b t a i n e d by t r i p l i n g t h e frequency of a s o l i d s t a t e l a s e r . This l a t t e r approach may become f e a s i b l e i n t h e f u t u r e even f o r t h e w2 l a s e r when r e s o n a n t harmonic g e n e r a t i o n schemes have been developed.

Cooling.-The following d e s c r i p t i o n [1,7] b r i n g s o u t t h e e s s e n t i a l f e a t u r e s of o p t i - c a l c o o l i n g . Due t o t h e Doppler e f f e c t , when moving a t v e l o c i t y v o p p o s i t e t o t h e p r o p a g a t i o n of a p l a n e l i g h t wave of frequency w , a n atom i n i t s r e s t frame s e e s a n e x c i t a t i o n a t t h e frequency w ' = w ( l + v/c) near an atomic resonance a t w Then,

0 . .

a g a i n i n i t s r e s t frame, t h e atom re-emits photons a t w ' i n a l l d i r e c t i o n s I n ac- cordance w i t h i t s c h a r a c t e r i s t i c r a d i a t i o n p a t t e r n . Now t o a n o b s e r v e r i n t h e labo- r a t o r y due t o t h e Doppler e f f e c t t h e s e r e - e m i t t e d p h o t o n s w i l l e x h i b i t v a r i o u s frequency s h i f t s depending on t h e a n g l e between t h e d i r e c t i o n s of re-emission and motion. N e v e r t h e l e s s , t h e average energy of t h e re-emitted photons w i l l be+%' = -hw + ?iw(v/c). The average energy e x c e s s i n t h e energy o f t h e r e - e m i t t e d over t h a t o f t h e absorbed photons h a s t o come from t h e k i n e t i c energy i n t h e t r a n s l a t i o n a l motion which i s thereby cooled.

This p i c t u r e i s e a s i l y adapted t o t h e c a s e of one-dimensional p e r i o d i c motion a t frequency wv p a r a l l e l t o t h e l i g h t beam. Now due t o t h e modulated Doppler s h i f t t h e atom of resonance frequency a,, i n i t s r e s t frame, when i r r a d i a t e d by a l a b o r a - t o r y l i g h t s o u r c e a t t h e s h a r p frequency w s e e s a whole e x c i t a t i o n spectrum a t w; = w + nw,, n = 0,1,2. . . . For a s h a r p s p e c t r a l l i n e of width Aw, << w, and

wk = w + muv = w,, m>O, t h e e f f e c t of t h e m-th s i d e band predominates. Again, t h e average energy of t h e photons re-emitted i n a l l d i r e c t i o n s i s flw + *wv and t h e energy excess mTiwv has t o b e provided by t h e o s c i l l a t o r y motion. F o r t h e o s c i l - l a t i o n amplitude zo << Ao/2a ( i n t h e LAMB-DICKE regime) t h e spectrum s e e n by t h e atom c o n t a i n s only t h e c a r r i e r a t w of power E p l u s two weak symmetric s i d e bands a t w + ^wv o f Power Ec << E ( . z , / ~ A , ) ~ and t h e problem remains manageable even when t h e requirement Aw, << wv i s dropped, s e e F i g . 3. The t o p s e c t i o n of F i g . 3 shows t h e a b s o r p t i o n p r o f i l e o f t h e s t o r e d i o n when t h e l a s e r frequency w i s tuned through t h e e l e c t r o n i c resonance a t w,. The m u l t i p l e e x c i t a t i o n s o f t h e i o n i n i t s r e s t frame a t w - wv, w , w + wv according t o t h e p o s i t i o n of t h e s e s p e c t r a l components with r e s p e c t t o t h e Lorentz p r o f i l e c e n t e r e d a b o u t w,, s e e middle s e c t i o n of F i g . 3 , r e s u l t i n t h e re-emission of t h e s e same t h r e e components. When viewed from t h e l a b frame each of t h e 3 r e - e m i t t e d components w i l l develop i t s own m o Doppler s i d e bands, s e e bottom s e c t i o n of F i g . 3 , w i t h o u t changing t h e average photon energy.

Comparing t o p and bottom p a r t s of F i g . 3 showing a b s o r p t i o n and re-emission a s viewed from t h e l a b frame i l l u s t r a t e s t h e c o o l i n g p r o c e s s : e x c i t a t i o n by an upper/

lower o r c o o l i n g / h e a t i n g s i d e band photon e x t r a c t s / a d d s t h e energy i l w v from/to t h e v i b r a t i o n a l motion. F o r t h e c a s e d e p i c t e d w h i l e photons of energy hw a r e absorbed by t h e i o n t h o s e r e - e m i t t e d have a l a r g e r average energy ^%h(w + w,) f o r t h e t u n i n g w 2 wO - wV shown.

This s e m i c l a s s i c a l p i c t u r e i n d i c a t e s s t r o n g c o o l i n g and an e x p o n e n t i a l approach i n time of a b s o l u t e z e r o temperature. puant& e f f e c t s e s t a b l i s h a d e f i n i t e minimum temperature, however. Obviously, when t h e i o n i s i n t h e lowest v i b r a t i o n a l s t a t e v = 0 , i t cannot b e cooled any f u r t h e r and t h e power i n the c o o l i n g s i d e band must v a n i s h w h i l e t h a t i n t h e h e a t i n g s i d e band remains f i n i t e . This asymmetry i n s i d e band s t r e n g t h does n o t b e g i n w i t h v = 0 b u t i s always p r e s e n t , becoming more and more pronounced a s v d e c r e a s e s . A s i m i l a r asymmetry i s p r e s e n t i n t h e re-emission s i d e bands. The a t t a i n a b l e minimum v i b r a t i o n a l temperature and t h e corresponding v i b r a t i o n a l quantum number <vzmin a r e determined by t h e requirement t h a t t h e h e a t i n g

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JOURNAL DE PHYSIQUE

Lab frame absorption

o f t l w

ion rest frame 8 0 8:

absorption of h ( ~ , w?w,I

C

W-W, W

lab frame re-emission

of 'h ( w + - n w , l n = O.?1,?2

Fig. 3. Spectral components e f f e c t i v e i n side band cooling

(schematic).

As shown i n middle section, t h e i o n v i b r a t i o n a t wv i n t h e parabolic trap parallel t o the propagation d i r e c t i o n of t h e e x c i t i n g electromagnetic wave a t w sees spectral components

(of power E-, E, E+) a t w - wVJ W, w f wv. The case o f o s c i l l a t i o n amplitude

<< c/w and E+ << E, t h e L a d - Dicke regime, i s asswned here.

To an observer i n t h e Zab- frame the i o n appears t o have t h e response spectruni (e.g., for power absorption) shown i n t o p section. I n t h e i o n r e s t frame each o f t h e three ex- c i t i n g components i s re- emitted w i t h unshifted frequency and a strength determined by t h e Lorentzian response profiZe. When seen from the lab frame (compare bottom s e c t i o n ) each r e - emitted component developes i t s own two Doppler side bands.

Comparing t o p and bottom sections shows c2earZy for t h e tuning w + ^wv ⁼^wo asswned t h a t , while photons o f energy

% are absorbed by the ion, photons o f larger average

energy ^.Yd(w + u v ) are emitted by it. The balance ^2.Amv has t o come from t h e o s c i l l a t o r y motion, which i s thereby cooled. A f t e r 'Coherent Spec- troscopy on Single Atomic System o f Rest i n Free Space L"

e f f e c t of t h e s t r o n g e r b u t more o f f - r e s o n a n t s p e c t r a l components a t w - wv and w i s balanced by t h e c o o l i n g e f f e c t due t o t h e weaker, more r e s o n a n t s i d e band w + wv.

I n my 1976 Copper Mountain t a l k , using o n l y f a m i l i a r elementary quantum mechanics of atomic and molecular s p e c t r a I d i s c u s s e d t h e problem and a p p l i e d it t o t h e TI+ mono- i o n o s c i l l a t o r o b t a i n i n g a minimum temperature of % 0.1mK f o r c o o l i n g w i t h a h < 19098 l a s e r beam. I n t h i s c h a p t e r wo r e f e r s t o t h e allowed t r a n s i t i o n used f o r c o o l i n g .

The d i s c u s s i o n i s e a s i l y extended t o t h e r e a l i s t i c 3-dimensional c a s e of an i n d i v i d u a l s t o r e d i o n [ 2 ] . This r e q u i r e s i r r a d i a t i o n along +;*+_3*+_fi* where E*,j*,li*

r e f e r t o t h e p r i n c i p a l axes of t h e e l l i p s o i d a l t r a p p i n g p o t e n t i a l [ 6 ] . For t h e minimal v i b r a t i o n quantum number we o b t a i n e d = (g,, + 9 - ) / ( g + - 9-1 which a p p l i e d t o v i b r a t i o n a l o n g a l l t h r e e p r i n c i p a l axes. Here g,, g+, g- denote t h e v a l u e s of t h e Lorentz p r o f i l e a t w , w + wv and w - wv. Our r e s u l t s were l a t e r confirmed by more e l a b o r a t e t r e a t m e n t s [ 8 , 9 ] .

S h i f t s and Broadening o f t h e Resonance Line.-Crucial e s t i m a t e s made i n 1973 f o r ~ 1 + showed t h a t even f o r a t r a p s t r o n g enough t o allow e a s y confinement t o l e s s t h a n t h e o p t i c a l wave l e n g t h t h e S t a r k s h i f t posed no s e r i o u s problem[5]. For an e s t i m a t e d

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H z o r 6.JvO 1< 1 0 - l 9 a t 0 . 1 mK. For s m a l l , deep t r a p s i t i s a l s o necessary t o d i s - c u s s a n e l e c t r i c " t r a p s h i f t " p r o p o r t i o n a l t o t h e a p p l i e d t r a p v o l t a g e which should o c c u r when ground o r e x c i t e d s t a t e s have a n a p p r e c i a b l e e l e c t r o n i c e l e c t r i c quad- r u p o l e moment eQ. For heavy atoms i n a P s t a t e w i t h HFS quantum number F

,

¹^{eQ may}

b e a s l a r g e a s 3 eg2. The o r d e r of magnitude of t h e corresponding mF-dependent s h i f t 6Q is ^eQ$^.,^$,, r e f e r r i n g t o t h e e l e c t r i c t r a p p i n g f i e l d . For t h e ~ a + r f t r a p one f l n d s f e e +?,I s 30 KHz. However, s i n c e it i s an AC s h i f t , i t averages o u t t o zero. A neighboring i o n a t t h e e q u i l i b r i u m d i s t a n c e i n t h e we11[2] of Q 5 5m would cause a n analogous quadrupole s h i f t o f Q 200 Hz which would n o t b e averaged o u t by t h e motion o f t h e i o n s . For t h e Penning t r a p of [ 3 ] one f i n d s a DC s h i f t *

6Q = 2 H Z . Our Group I I I A mono-ion o s c i l l a t o r experiments a r e immune t o s u c h s h i f t s ? One might t h i n k t h e AmF = 0 t r a n s i t i o n s f o r o u r wo l i n e would show no ( n u c l e a r ) Zeeman e f f e c t . However, f o r t h e i s o e l e c t r o n i c 9 9 ~ g i s o t o p e a " n u c l e a r " g - f a c t o r f o r t h e 3 ~ 0 s t a t e about twice a s l a r g e a s f o r t h e 'so ground has been measured.

This i s explained by a s m a l l admixture of 3 ~ 1 t o t h e 3p0 s t a t e v i a t h e s t r o n g HFS i n t e r a c t i o n . Estimated Zeeman s p l i t t i n g s f o r m o u n t t o % 2KHz/Gauss r e q u i r i n g f a i r l y e l a b o r a t e s h i e l d i n g t o b r i n g t h i s 6z s h i f t down t o t h e l e v e l of 6S and 6D.

Outside t h e Lamb-Dicke dominant c a r r i e r regime t h e amplitude of t h e c a r r i e r i s s t r o n g l y dependent on t h e v i b r a t i o n amplitude. Thus, v a r i a t i o n s i n t h e v i b r a t i o n amplitude can broaden t h e c a r r i e r [ 5 ] . So f a r i t h a s been t a c i t l y assumed t h a t l a s e r s o u r c e s of a r b i t r a r i l y h i g h s h a r p n e s s and s h o r t - t e r m s t a b i l i t y a r e a v a i l a b l e . How- e v e r , t h e r e c o r d i n minimal l a s e r s p e c t r a l width s t i l l a p p e a r s t o b e a few Hertz.

T h i s i s s t i l l f a r above fundamental l i m i t a t i o n s which l i e around % Hz.

Fig. 4. Micro-photographic images o f 1, 2 and 3 trapped ~a' ions. The large photo- graph shows the ^c2 um t h i c k image (white arrow) of a singla i o n i n s i d e t h e r f quadrupole t r a p a s viewed through the gap be-hJeen the ring and the l e f t cap- electrodes ( t r a p structure illwninated by scattered Laser Zightl. ^Asketch o f t h e whole trap structure seen from the same angle i.s i n s e r t e d . The three small photos, going from top t o bottom, show, 10-fold enlarged, the central trap region, contuin- ing 1, 2 and 3 ions, from [2].

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C8-304 JOURNAL DE PHYSIQUE

R e s u l t s of P r e p a r a t o r y Experiments.-The most important r e s u l t s o f a r o b t a i n e d i n our mono-ion o s c i l l a t o r work121 is t h e 3-dimensional l o c a l i z a t i o n i n f r e e space t o a r e g i o n of 'L- 2000fi diameter of an i n d i v i d u a l ~ a + i o n , s e e F i g . 4 . The photographic image of t h e i o n has a d i f f r a c t i o n l ~ m i t e d diameter of 2 Um. I t i s p o s s i b l e t o de- convolute t h e much s m a l l e r l o c a l i z a t i o n r a n g e by comparing t h e 1-ion image w i t h a 3-ion image which, o b t a i n e d under o t h e r w i s e i d e n t i c a l c o n d i t i o n s , h a s a '1. 20 times l a r g e r a r e a and a 'L- 30 times lower peak b r i g h t n e s s . This i m p l i e s z,

-

^1,/211 ^and

t h a t t h e boundary of t h e LAMB-DICK3 regime of t h e dominant c a r r i e r h a s been reached.

The b e s t s p e c t r a l r e s o l u t i o n o b t a i n e d s o f a r by us[lO] i s a line-width of 'L- 40 MHz f o r t h e 6 2 ~ 1 / 2 - 6 2 ~ 1 / 2 - 5 2 ~ 3 / 2 two-photon t r a n s i t i o n [ l l ] t o t h e h i g h l y m e t a s t a b l e

5 ' ~ 3 / ? s t a t e of ~ a + which has a l i f e t i m e of 17 s e c , s e e F i g . 5 . This two-photon transition is obviously s t i l l s t r o n g l y power-broadened.

I n S e a t t l e , i n work r e c e n t l y made p o s s i b l e by s u p p o r t from t h e U.S. O f f i c e of Naval Research resonance f l u o r e s c e n c e from % 300 ~ g + i o n s s t o r e d i n a n r f t r a p u s i n g a 'L- Torr He b u f f e r and i n high vacuum h a s been s e e n . I t has n o t been p o s s i b l e s o f a r t o demonstrate c o o l i n g w i t h t h e weak i 1 UW l a s e r power c u r r e n t l y a v a i l a l e . This i s t o b e e x p e c t e d from t h e Heidelberg ~ a + work[l,2] where f o r 5 50 i o n s even w i t h l a s e r powers of 'L- 1 mW only i o n temperatures o f ¹^. 100 K could be r e a l i z e d . The presumed cause, r f h e a t i n g v i a ion-ion c o l l i s i o n s between d i f f e r e n t Ng+ i s o t o p e s o r w i t h f o r e i g n i o n s , however, i s expected t o d i s a p p e a r f o r a s i n g l e i o n i n t h e t r a p . The l a t t e r e x p e c t a t i o n i s c o r r o b o r a t e d by t h e o b s e r v a t i o n of s t o r a g e times a s l o n g a s 'L- 30 s e c f o r a s i n g l e uncooled i o n [ 2 ] . Thus i t s t i l l nay be p o s s i b l e t o keep a s i n g l e ~ g + i o n c o l d and confined once one has managed t h e n o t s o mean f e a t of g e t t i n g i t i n t o t h e bottom of t h e p a r a b o l i c w e l l with t h e low l a s e r power c u r r e n t l y a v a i l a b l e t o u s from o u r somewhat m a d e q u a t e equipment.

Conclusion.-After a f a i r l y d i f f i c u l t s t a r t , mono-ion o s c i l l a t o r spectroscopy now appears t o b e o f f and running, w i t h a t l e a s t f i v e groups a l l over t h e world a c t i v e l y p a r t i c i p a t i n g i n such s t u d i e s . The demonstration of e l e c t r o n i c l i n e w i d t h s and long- term frequency s t a b i l i t i e s i n such p a s s i v e d e v i c e s of 1 Hz and l e s s i s l i k e l y t o a c t a s a s t r o n g s t i m u l u s f o r the f u t u r e development of l a s e r s o u r c e s of improved s h o r t - term s t a b i l i t y , much c l o s e r t o t h e fundamental l i m i t n e a r % l o e 4 Hz and a l s o of new t y p e s of l a s e r harmonic g e n e r a t o r s . Thus t h e c u r r e n t promise o f a n atomic l i n e s p e c t r a l r e s o l u t i o n o f about 1 p a r t i n 1018 may b e r e a l i z e d i n t h e n o t t o o f a r f u t u r e . The b e n e f i t s t o p h y s i c s , geology, cosmology and technology r e s u l t i n g from such an advance i n t h e accuracy o f measurement o f o b s e r v a b l e s a s fundamental a s

Fig. 5. Resonance fluorescence near 493 nm from an i n d i v i d u a l bariwn i o n us. scanned frequency wa o f 650 nm l a s e r . The 4 9 3 nm

laser frequency w was adjusted t o

- w = 2n 280 MHz t o e f f e c t szde band cooling. The frequency coordinate c a l i b r a t i o n i s given for wa - wpd/2+. The sharp two- photon t r a n s i t i o n 6 2 ~ 1 / 2 -

62P1/2 - 5 2 ~ 3 / 2 t o t h e metastable 5 2 ~ 3 / 2 ZeveZ occuring for w - wa =

- w d i s v i s i b l e a t wa - osp - -

:% ²^% MHz. Here w S P , wpd are t h e r e s p e c t i v e one-photon

resonance frequencies, from [lo].

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i d e n t i t y of t h e atoms of t h e same element.

REFERENCES

[ I ] NEUHAUSER (W.) , HOHENSTA'IT (M.) , TOSCHEK (P.) and DEHMELT ( H . ) , Phys. Rev.

L e t t . , 1978, s, ^233.

[2] NEUHAUSER (W. ) , HOHENSTA'IT (M. ) , TOSCHEK (P. E . ) and DEHMELT (H. ) , Phys. R e v . ^J 1980, x, ^1137.

[3] WINELAND (D. J.) , ITANO (w. ^{M . ) ,}P h y s i c s L e t t e r s , 1981, E, ^75.

[4] DEHMELT (H. G.), BUZZ. Am. Phys. SOC., 1975, 0, 60.

[5] DEHMELT (H. G . ) , BUZZ. Am. Phys. S o c . , 1973, 18, 1571.

[6] WINELAND (D.) and DEHMELT (H. ) , BUZZ. Am. Phys. Soc., 1975, 20, 637.

[7] DEHMELT (H.), N a t u r e , 1976, 262, 777; BUZZ. Am. Phys. Soc., 1979, 24, 634.

[8] WINELAND (D. J . ) and ITAXO (W. M-), P h y s . Rev., 1979, e, ^1521-

[9] JAVANAINEN (J.) , AppZ. Phys., 1980, z, ^175.

[ l o ] NEUHAUSER ^(W.) , BOHENSTAm (M. ) , TOSCHEK (P. E. ) , and DEHMELT (H. G.) , ⁱⁿ

" S p e c t r a Z L i n e S h a p e s," 1981, WENDE (B.), e d i t o r . Walter de Gruyter & Co., B e r l i n , New York.

[ I l l DEHMELT (H. ) and TOSCHEK (P .) , B u Z Z e t i n APS, 1975, 2, 61.

[12] NAGOURNEY (W.) and DEHMELT ( H . ) , BUZZ. Am. P h y s . SO^., 1981, 26, 797 and 805.

FOOTNOTES

? D i r e c t i n g t h e l a s e r beam t h u s and t h e r e f o r e a l o n g an e q u i p o t e n t i a l s u r f a c e of t h e t r a p p i n g r f f i e l d a l s o p r a c t i c a l l y e l i m i n a t e s t h e 1. o r d e r Doppler e f f e c t due t o t h e f o r c e d R-micro-motion.

'TO demonstrate t h e "shelved o p t i c a l e l e c t r o n " a m p l i f i c a t i o n of any s h a r p e l e c t r o n i c t r a n s i t i o n , one may b e w i l l i n g t o a c c e p t t e m p o r a r i l y t h e u s e of t h r e e e n g i n e e r e d , s t a b i l i z e d , o p t i c a l , o r i n f r a r e d l a s e r s , given t h e c u r r e n t s t a t e o f l a s e r technology. Thus, t h e Heidelberg experiment c o u l d be supplemented by a 1.762 pm t u n a b l e s o l i d - s t a t e l a s e r t o e x c i t e t h e very s h a r p 6 2 ~ 1 2 -f ~ ~ 5 quadrupole t r a n s i t i o n / 2

t o t h e second m e t a s t a b l e ( s h e l v i n g ) l e v e l of l3'Ba+. The r e s t 00 t h e a p p a r a t u s , i n v o l v i n g a 493.4 ^nmdye l a s e r f o r t h e 6 2 ~ 1 / 2 - 6 2 ~ 1 2 t r a n s i t i o n used f o r c o o l i n g and d e t e c t i o n v i a f l u o r e s c e n c e and an a u x i l i a r y 649.6 nm l a s e r f o r c l e a n i n g o u t t h e f i r s t m e t a s t a b l e 5 2 ~ 3 / 2 l e v e l , would b e l e f t unchanged. The sr+ i o n o f f e r s s i m i l a r p o s s i b i l i t i e s .

;The i d e a l o f t o t a l decoupling from t h e surroundings i s c l o s e l y approached f o r t h e w o - t r a n s i t i o n i n ~ 1 + - l i k e i o n s s i n c e t h e v a n i s h i n g e l e c t r o n i c a n g u l a r momenta of t h e 6 l s o and 6 3 ~ o s t a t e s guarantee minimal e l e c t r i c and magnetic moments.