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Chapter7Lightiiers
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&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-iypartofthebea
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，sothatastreamofphotonshitstheatoThelaseristuobeabsorbedbyatomsthataremovingataparticularveloow，wheomabsorbsaphotonfromthelaserbeam，itgetsakithephoton，andthusslowsdown.（Morespecifically，themomentumofthephotonistraheatoSiisiediretotheinitialmomentumoftheatom，itredu，andthusthespeedoftheato）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）.
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