laser cooling and trapping

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1. Introduction Ewr! time an a t o m interchanges energy Lvith 21 resonant radiation the tield. momentum o f the absorbed light must b e o m p e n s a t d c for b> the
Stig Stenholm Research , Helsinki,.Finland
Laser cooling and trapping
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Stig Stenholm
Research Institute for Theoretical Physics. Siltavuorenpenger 00170 Helsinki. Finland Received 1 0 February 1988. in final form 12 April 198s
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Laser cooling and trapping
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1988 Eur. J. Phys. 9 242 (/0143-0807/9/4/001) View the table of contents for this issue, or go to the journal homepage for more
canberepresentedasalight-inducedforce.The quantum nature of the process imposes unavoidable fluctuations.andtheselead to a spreadinginthe interaction: process this can be result of the described a s a diffusion. Recently there has been much interest devoted to the mechanical manifestations of the atom-light interactions. theoretical The situation been has reviewedextensively [ 1-41 andthe livelyexperiI5-61. More mental progress been has discussed detailed references to earlier work may be looked up in these overviews. t o deflect. The light-induced forces can be used slow down stop or atoms. It is also possible t o attempt keep to atoms trapped by these forces entirely on the basis of their interaction with radiation. This would form an important complement to othermethodsusedtotrapatomicparticles,For charged particles. ions. trapping be i.e. the can achieved via ordinary electromagnetic forces. The trapping of neutral particles has. however. proved a very difficult task. which has been successfully performed in only the 1980s. The successful tirst method [ 7 ] was basedonamagneticconfinement method. but itssuccessdependedcruciallyonthe utilisation of terexperiments [S] have obtained trapping based entirely on lightinduced forces. But\vh> d o we want to trap atoms? One motiLationcomesfromthequestfor still bettertime standards higher and resolution in spectroscopic measurements.Withnearlystationaryatomsboth the second-order Doppler andline effect the broadeningduetofiniteinteractiontimescanbe eliminated. I n this w a y the trapping may be the first stage in achieving ;I relativespectralresolution of 1 0 - l'. \\hich might in turn give time standards with;I reIati\e stability o f IO-'-. T h e s l o ~ . a t o mrnay be used to investigate ultras lo\\-energy collisions. would appliwhich have cations in chemistry and physics. surface By isolatingsingleatoms in traps. one can investigate quantum effects directly on an individual system?. Thusonecanbuilduptheensembleaverage b! collecting the full quantum distribution event b> event. New quantum phenomena such a s interferenceandtunnellingcanbeinvestigated.Eventhe catching investigation and of rare species rnay become possible. For the ultra-cold atoms in a trap. the tie Broglie Ivavelengthbecomeslongandcollectivequantum effectsmaybecomeobservable. In particularthe achievement of Bose-Einstein condensation has been subject the of much work. densities The requiredforthisare still farfrombeingrealised
Advances in laser technology have made it possible to investigate and utilise novel phenomena connected with strong optical fields. This cluster of papers presents some areas of recent interest. They all realise physical features not earlier accessible to experimental investigations, and in addition to their fundamental interest they suggest important applications in the future. The momentum carried by the radiation has been used to cool and trap atomic particles. The phenomenon couples the atomic motion to the internal energy levels through the recoil of the scattered photons. The main effect is classical, but the quantum features appear as fluctuations that add stochasticity to the momentum exchange. Light forces provide an inspiring new tool for atomic manipulations and ultra-high-resolution spectroscopy. When strong light interacts with atomic energy levels, the interplay between coherent exchange of energy and irreversible spontaneous decay offers a way to inquire into the detailed statistics of quantum processes. This phenomenon of quantum jumps has led to new insight into the interpretation of quantum mechanical observations. The experiments have become possible because single trapped particles can be isolated and observed, and they provide an ultra-sensitive detection method for extremely weak atomic resonances. It is commonly supposed that only weak radiation fields display quantum features. The phenomenon of squeezing proves that even arbitrarily strong fields can carry non-classical quantum noise. This theoretically interesting feature has been observed experimentally, and it may find important applications in communication techniques and the detection of extremely feeble signals. When strong laser light impinges on atoms, electrons are ripped loose and enough energy is absorbed to ionise the atom in a multi-photon process. This is a fundamental manifestation of the interaction between matter and strong fields. Recently experiments have shown that, even when no longer bound, the electrons can still absorb energy quanta. This phenomenon is called above threshold ionisation.