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Chapter 7 Lighting the frontiers（第1页）

Chapter7Lightiiers

&slonghistoryaspossiblytheoldestuousbranaturalphilosophyaisattheforefrontofreseardappli。Itisubiquitous:asat，imaging，andunis，asrovidinglore，disdillustratenewfualeffects。

Lighteratesattheextremesofwhatisknowntobepossibleagtophysics，suchasextremesoftemperaturearemesofpressureadourally，exceptperhapsiofstars。Andlightbeusedtoobservearolreallyfastevents，suchasthemovemerooms。

Further，lightexhibitstrauresassociatedwiththequantumwevenineverydayeofthetuitiveaspectsofthefitfulworldofrauhestable，solidworldofournormalexperiehischapter，Iwillexploresomeofthefroowhidacrosswhich，lightistlytakiioiersispossiblebecauseofthegreatteologicalstridesthathavebeesources，opticalsystems，aors，whiableexquisitetrolovertheshapeay，inbothspadtime，alightbeamtake。

Lightmeics

Lightexertforobjects。Thisallows‘remotetrol’ofbitsshapedlightbeams。Lightbeusedtomovematteraround，andbringitintotactwithotherobjeaernalfigurationofatomsandmolecules，ftheminto，forexample，simplechemicalreawaysthatallowboththestudyaionofunusualmaterialproperties。That’saraordinarilypowerfulfeatureinmanyareasofreseardstudy。

&ofmeicalffromlighthasitsiumcarriedbyea。Forinstaoedfromamirror，thatmirrorexperiencesaforcethatprovidestheexertiooredirectthephoton，justaswaterfromafirehosehittingawallexertsaforthewallbyvirtueofitsboungoff。

Similarly，hotoeditgesdiredthis，too，requiresaforce。Thusthephotosafortherefragelement。Ifalightbeamisionaglassbead，raysthatareaothebeadwillexperiedire。Aphtherayinthelowerhalfofthebeadisdirectedupagatesthroughtheglasssurfaces。Thebeadthereforeexperiencesafortheoppositedireofthephotonintheforwarddire（thedireitwasmovieringthebead）isreduced，thereisalsoaheforwarddire。Thestrengthofthisfordsonhohotoedpersed。Alightbeamthatismoreiapositioreofthebeamthanonitsperipherywilltherefthebeadtowardsthehigher-iypartofthebeam。

Thiseffebeusedtomakeafocusedlightbeamintoan‘opticaltweezer’，whichisabletoholdontoamiaaroubeamissteered。Opticaltweezersfindapplis，forexample，inbiology，byenablingtrolofthepositioofindividualstrandsofDerizationoftiors。SpecifiA，proteins，andiolebestutothesebeadsandthereforebehandledusingopticaltweezers。Theirpositiorolledreuchlessthahoflight。Thisallowsextremelysmallforeasured—suchashappenswhenbiologicalcellsadheretosurfacesorothercells—aswellastoholdtheplacesastheyareoperatedonusingotherlasers（so-calledcellularsurgery）。Opticaltweezersbihvariousothertestmethods，suchaslightsgfromaerosols，orspectroscopy，tocharacterizeparticlesthatmaybepollutamosphere。

&o-meicalforalsobeusedtoapletelyionofsmallobjeparticular，itisnowpossibletobuildtinymeitilevers，illustratedinFigure31，andtobothobserveandotionusinglight。Lightforbeusedtocoolorheattheossofthetilever—likerunningdupariitistatepossible，whereonlyquantumflusofthemotiondisturbthepletestasisofthelever。Lightforalsobeusedtouchsmallerobjedthisrevealseveraumstatesofmatter。

31。Ailevertrolledusinglightforces。Thedisymirrorsabout30μmier。

Ultra-cold

What’sthecoldestthingyou’veexperienced?ColderthanwinterinOxford（approximately2°Ottawa（-20°C），ortheSouthPole（-50°C）?Orperhapstheeffectsofliquid-200°C。Thesearelycold，butbyhecoldestthingspossible。Itturnsoutthatthereisalowerlimitfortemperature:-273°C，or0K（Kelvin），belowwhiotpossibletogsfurther。Thisisthetemperatureatwhigsareasstillasthey’regoihjusttheeffemeicstocausealittlejigglingaboutofatomsandmolecules。

It’snotactuallypossibletobuildamaetogettoabsolutezerobutitispossibletogetverygan‘erator’。Infact，yougetaketheatomsalmoststopmoving。Whatthismeansisthattheirsizegetsbigger。（Quaellsusthatyou’tsimultaneouslyspecifythepreciselodspeedofaheatomispletelystopped，thenitmustbeextendedoverallspace。）Thereforeallatomsihathasbeeedoccupythesameregiohisgivesrisetenewphenomena。

Aneratorworksbyusio‘aginealaserbeamshiningonanatomthatismoviht，say。Thelasershiherighttoleft，sothatastreamofphotonshitstheatom。Thelaseristuobeabsorbedbyatomsthataremovingataparticularveloow，wheomabsorbsaphotonfromthelaserbeam，itgetsakithephoton，andthusslowsdown。（Morespecifically，themomentumofthephotonistraheatom。Siisiediretotheinitialmomentumoftheatom，itredu，andthusthespeedoftheatom。）Theatommustre-emitthephotoime，anditwillgetakitheoppositediretothatinwhichitemitsthephotoioninwhichitre-emitsthephotonisrandom—itaall。

Ifyoulookatenoughoftheseabsorptievents，thenyouwillfindthat，althoughthelightisalwaysabsorbedfromoheininglaserbeam）itisemitteduniformlyiioioheceofthisisthateasmovingiiohelaserbeamgrindstoahalt，ahrandommatemperaturethatisproportionaltohowlongitholdsobeforere-emittingit。

Thereareseveralrefihisapproach，eachofwhichuseslighttoolecules）toeveures，andforwhichlightactslikea‘viscousfluid’inwhiovesloweraiseveouselighttotrapatomsusingopticaltweezersoheyareslowenough。Thisallolioresophisticatedoptigteiques，bywhichitispossibletogettotemperaturesofabillionthofadegreeaboveabsolutezero。

&osomeresidual‘jigglihathappezerotemperature，arisingfromquantummeics。Therahisjiggliofasthespatialexteself。Thatis，agtoquaheatomisn’tjustwanderingaroundinarandomfashionoverasmallregioheritisactuallypresentacrossallthatregioomstrappedatsuperaturesthesizeionmaybeseveralthousandthsofametre。That’saremarkablylargeatom，givearoomiucleusislessthahofabillionthofametre。What’seveseveralatomsoccupythisregiohesametime。

That’sceptuallyverytuitive。Weoftenthinkofatomsasbeilebilliardballs，thatbepackedclosetogether，asinasolidmaterial，butwhitheirindividualdistinbyvirtueoftheirlosidethematerial。That’snotthecasefortheseverys。TheyareeacheverywhereatonewstateofmatteridentifiedbyEiheIyendraNathBoseasurprisingly，aBose–Einsteie。

Thisverystraehassomeremarkableproperties。Foriisasuperfluid，ithoutviscosity。Further，itispossibletosplittheeomihalfandrebioshowquantumiweearts，esseingthatabigobjetainingmanyatomsandofapalpablyvisiblesize）exhibitsquantumcharacter，attributabletotheuyofwhetheranatomisiofthecloudortheother。Ohiomogbothseparatepohesametime。

&hesesbetrappediisalsopossibletocreatespatialstructuresouthtbeamsthatbeusedtomaoms。Foriwolightbeamscidetheyformaterer3）inwhichthereareregionsofhighandlowiy。sliketosettleiherions（youadjustwhiebygaparticularwavelengthofthelight）。Astheiyofthelightbeamsisturheatomsfallie’-likeopticaltrapsthatappearitern，asshowninFigure32a。Andtheydosoieresting>

32。strappedinanopticallattice:a。afewhuomspercell（several10sμK），b。individualatomsateachsite（afewnK）。

&omsareough，theydoobelocatedatthesame‘site’ie，sdistributionofatomsisverylikeafulleggateachsite，asshowninFigure32b。Inthiscasethereisy，siomsliketostayput。Itismorelikean‘insulator’，asnothiuriyupanddownitispossibletoexplorethisiraweeelyfreeflowandnoflo>