Quantum Chance: nonlocality, teleportation and other quantum marvels

122 Pages·2014·2.135 MB·English

by  Gisin, Nicolas

QuantumChance
Nicolas Gisin
Quantum Chance
Nonlocality, Teleportation and Other Quantum Marvels
NicolasGisin
DepartmentofPhysics
UniversityofGeneva
Geneva
Switzerland
ISBN978-3-319-05472-8 ISBN978-3-319-05473-5(eBook)
DOI10.1007/978-3-319-05473-5
SpringerChamHeidelbergNewYorkDordrechtLondon
LibraryofCongressControlNumber:2014944813
TranslatedbyStephenLyle
L’impensableHasard.Non-localité,téléportationetautresmerveillesquantiques
OriginalFrencheditionpublishedby© ODILEJACOB,Paris,2012
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Foreword
“Love at first sight!” was how Nicolas Gisin described his emotion when he
learned about Bell’s theorem. When I heard this, I relived an autumn day of
1974 when I was immersed in study of John Bell’s paper, little known at the
time,andunderstoodthatitwaspossibletorenderanexperimentalverdicton
thedebatebetweenBohrandEinsteinontheinterpretationofquantumme-
chanics.EventhoughafewphysicistsknewoftheproblemraisedbyEinstein,
Podolsky,andRosen(EPR),notmanyhadheardofBell’sinequalities,andfew
werethosewhoconsideredquestionsrelatingtothefundamentalconceptsof
quantum mechanics worthy of serious attention. The EPR paper, published
in1935inPhysicalReview,wasreadilyavailableinuniversitylibraries,butthe
samecouldnotbesaidforthepaperbyBell,publishedinanobscurenewjour-
nalthatwasdestinedtodisappearafteronlyfourissues.Inthosepre-internet
days, papers not published in the major journals had to rely on photocopies
for their dissemination. I had got my own copy from a file put together by
ChristianImbert,ayoungprofessorattheInstitutd’Optique,ontheoccasion
ofavisitbyAbnerShimony,invitedtoOrsaybyBernardd’Espagnat.Butonce
under the spell of Bell’s ideas, I decided that my doctoral thesis would deal
with experimental tests of Bell’s inequalities, and Imbert accepted to take me
underhiswing.
In Bell’s (impressively clear) paper, I was able to identify the crucial chal-
lengeforexperimentalists:alteringtheorientationsofthepolarizationdetectors
while entangled particles were still propagating from their source into the
measurement regions. The point was to preclude influence of the polarizer
orientationseitherontheemissionmechanismoronthemeasurement,byap-
plicationoftheprincipleofrelativisticcausality,whichforbidsphysicaleffects
frompropagatingfasterthanthespeedoflight.Suchanexperimentwouldbe
abletoscrutinisetheessenceoftheconflictbetweenquantummechanicsonthe
onehandandtheworldviewheldbyEinsteinontheother.Einsteindefended
localrealism,whichcombinestwoprinciples.First,thatthereexistsaphysical
reality of a system. Second, that a system cannot be influenced (the locality
assumption) by anything that happens to another system separated from the
first by a spacelike interval of spacetime, since those two systems would have
v
vi QuantumChance
tocommunicatewithinfluencespropagatingfasterthanlight.Eventually,our
experimentsconfirmedthepredictionsofquantummechanics,forcingphysi-
cists to give up local realism, the view of the world defended so convincingly
byEinstein.Butshouldwegiveuprealismorgiveuplocality?
TheideathatoneshouldgiveupthenotionofphysicalrealityisnotoneI
findconvincing,becauseitseemstomethattheroleofaphysicistisprecisely
todescribetherealityoftheworld,andnotjusttobeabletopredicttheresults
that show up on our measurement devices. But then, if quantum mechanics
is confirmed on this count—as indeed seems unavoidable today—does that
mean we must accept the existence of nonlocal interactions, in apparent vio-
lation of Einstein’s principle of relativistic causality? And is there any hope of
exploitingthisquantumnonlocalitytotransmitausablesignal,e.g.,toswitch
onalamporplaceanorderatthestockexchange,thatwouldtravelfasterthan
light? But this is where another characteristic feature of quantum mechanics
comesintoplay,namelytheexistenceoffundamentalquantumindeterminism.
This amounts to the absolute impossibility of influencing the actual result of
any specific experiment whenever quantum mechanics predicts that several
results are possible. It is true that quantum mechanics can be used to make
very accurate calculations of the probabilities of the various possible results,
but these probabilities have only a statistical meaning when the same experi-
mentisrepeatedmanytimes,andtheytellusnothingabouttheresultofany
specificexperiment.Itisthisfundamentalquantumrandomnesswhichforbids
thepossibilityoffaster-than-lightcommunication.
Amongthemanypopularaccountsofrecentprogressinquantumphysics,
the present book by Nicolas Gisin takes a definite line here, stressing the
key role played by this fundamental quantum randomness, without which
we might dream one day of designing a superluminal telegraph system. If it
everbecameareality,thismythicalinventionofsciencefictionwouldrequire
a radical revision of physics as we know it today. Naturally, the aim here is
nottosuggestthattheremightbeuntouchableandimmutablephysicallaws,
beyond any form of revision—quite the contrary, I am personally convinced
that every physical theory will one day be superseded by one of wider scope.
Butsomeofourtheoriesaresofundamentalthattheirrevisionwouldinvolve
atrulyfar-reachingconceptualrevolution.Andalthoughweallknowofafew
examples of such revolutions through human history, they are nevertheless
so exceptional that they should not be envisaged lightly. In this context, the
explanation as to why quantum nonlocality, no matter how extraordinary it
may be, cannot overthrow the principle of relativistic causality which forbids
superluminalcommunicationseemstomeaparticularlyimportantfeatureof
NicolasGisin’sbook.
Foreword vii
The fact that this book adopts a particular stance on this issue, in contrast
tootherpopularaccounts,shouldcomeasnosurprise,sinceNicolasGisinhas
beenoneofthekeyplayersinthenewquantumrevolutionthattookplacein
thelastquarterofthetwentiethcentury.Thefirstquantumrevolution,atthe
beginningofthetwentiethcentury,wasbasedonthediscoveryofwave-particle
duality. It provided a way of describing with great accuracy the statistical
behaviourofatomsthatmakeupmatter,ofthecloudsofelectronsthatconduct
theelectriccurrentinametalorsemiconductor,andofthebillionsandbillions
of photons in a beam of light. It has also provided tools to understand the
mechanicalpropertiesofsolids,whereasclassicalphysicswasunabletoexplain
whymatter,madeupofpositiveandnegativechargesthatattractoneanother,
doesnotsimplycollapse.Quantummechanicshasgivenaprecisequantitative
description of the electrical and optical properties of materials, and offers
the conceptual framework needed to describe phenomena as surprising as
superconductivity and the strange properties of certain elementary particles.
Itwasinthecontextofthisfirstquantumrevolutionthatphysicistsinvented
new devices like the transistor, the laser, and integrated circuits, which have
brought us today into the age of the information society. But then towards
the 1960s, physicists began to ask new questions that had been pushed aside
duringthefirstquantumrevolution:
• How can we apply quantum physics with its purely statistical predictions
tosinglemicroscopicobjects?
• Do the astonishing properties of entangled pairs of quantum objects, as de-
scribedinthe1935EPRpaperbutneveractuallyobserved,reallycorrespond
to the way nature behaves, or have we reached the limits of quantum
mechanicswiththisissue?
Itwastheanswerstothesequestions,firstgivenbyexperimentalists,thenfur-
therrefinedbytheoreticians,thatlaunchedthesecondandongoingquantum
revolution.1
Thebehaviourofindividualquantumobjectshasbeenthesubjectoflively
debate between physicists. For a long time, the majority of the physics com-
munity thought that the question itself made little sense, and that it was in
any case of no importance since it seemed inconceivable that one could ever
1See,forexample,A.Aspect:JohnBellandthesecondquantumrevolution,forewordofJ.Bell:Speak-
ableandUnspeakableinQuantumMechanics:CollectedPapersonQuantumPhilosophy,Cambridge
UniversityPress(2004);J.DowlingandG.Milburn:Quantumtechnology:thesecondquantumrevo-
lution,PhilosophicalTransactionsoftheRoyalSocietyofLondon.SeriesA:Mathematical,Physicaland
EngineeringSciences361,1809,pp.1655–1674(2003).
viii QuantumChance
observe a single quantum object, let alone control and manipulate it. In the
wordsofErwinSchrödinger:2
Itisfairtostatethatwearenotexperimentingwithsingleparticles,anymore
thanwecanraiseIchthyosauriainthezoo.
Butsincethe1970s,experimentalistshavedevelopedwaystoobserve,manipu-
late,andcontrolsinglemicroscopicobjectssuchaselectrons,atoms,andions.
I still recall the enthusiasm at the international conference on atomic physics
in Boston in 1980 when Peter Toschek presented the first image of a single
trapped ion, observed directly by the fluorescence photons it emitted under
laserillumination.Experimentalprogresshassinceledtodirectobservationof
quantumjumps,thusendingdecadesofcontroversy.Ithasalsodemonstrated
thatthequantumformalismisperfectlycapableofdescribingthebehaviourof
single quantum objects, provided that one interprets the probabilistic results
ofthecalculationsintherightway.Asforthesecondquestion,concerningthe
propertiesofentanglement,thequantumpredictionswerefirsttestedonpairs
of photons, in a series of experiments that is gradually converging upon the
ideal conditions dreamt of by theoreticians like John Bell. And these experi-
mentshaveconsistentlyvalidatedthequantumpredictions,howeversurprising
theymayseem.
Having put together an applied physics group working on optical fibres
inthe1980s,andhavingalwayshadapersonalandtheoreticalinterestinthe
foundationsofquantummechanics(althoughsecretly,oratleastdiscreetlywith
regard to his employer, since in those days raising questions of this kind was
notnecessarilyconsideredaworthwhileoccupation),itwasquitenaturalthat
NicolasGisinshouldhavebeenamongthefirsttotestquantumentanglement
on photon pairs injected into optical fibres. With his detailed knowledge of
opticalfibretechnology,hewasabletousethecommercialtelecommunications
networkaroundGenevatodemonstratethatentanglementismaintainedeven
atseparationsofseveraltensofkilometers,tothesurpriseoftheexperimenters
themselves!Heusedseveralconceptuallysimpleteststobringouttheabsolutely
astoundingfeaturesofentanglementbetweenremoteevents,andtoimplement
a quantum teleportation’ protocol. Combining his skills as a theoretician on
quantum foundations and expert in optical fibre applications, he was among
thefirsttodevelopapplicationsofentanglementsuchasquantumcryptography
ortheproductionoftrulyrandomnumbers.
2E.Schrödinger:Aretherequantumjumps?BritishJournalforthePhilosophyofSciences,Vol.III,p.
240.
Foreword ix
This combination of talents is evident throughout this fascinating book,
which succeeds in presenting the subtle issues of quantum physics in a lan-
guagethatremainsaccessibletothegeneralnon-scientificpublic,andwithout
recourse to mathematical formalism. He explains entanglement, quantum
nonlocality,andquantumrandomness,anddescribesanumberoftheirappli-
cations.Butthisismorethanjustapopularaccount,andquantumspecialists
will find deep discussions of these phenomena, the true nature and conse-
quences of which still remain largely beyond our grasp, as the author points
out. Concerning the question as to whether the experimental refutation of
localrealismcompelsustoabandonthenotionofphysicalrealityortheidea
of locality,3 I take the same line as Nicolas Gisin: even if the concept of local
realism may have been consistent and intellectually satisfying, cutting it into
twopiecesandkeepingonlyoneofthemisdistinctlylessso.Howshouldone
define the autonomous physical reality of a system that is localised in space-
time if this system is affected by what happens to another system separated
from it by a spacelike interval? This book suggests a less brutal solution by
showingthat,ifonetakesintoaccounttheexistenceoffundamentalquantum
randomness, a nonlocal physical reality can be allowed to coexist in a more
peaceablewaywiththerelativisticcausalitysodeartoEinstein.Inthismanner,
even those physicists who know of these issues will find matter for reflection
in Nicolas Gisin’s book. And as regards the non-specialist reader, discovering
here the mysteries of entanglement and quantum nonlocality, she or he will
be carried straight to the heart of the problem and learn of all its subtleties,
explainedwithenlighteningclaritybyoneoftheworld’sleadingexperts.4
Palaiseau,May2012 AlainAspect
3Wemayforgetthedesperatesolutionthatconsistsinrejectingthenotionoffreewill,astepthatwould
makehumanbeingsintomerepuppetsunderthedirectionofgoodnessknowswhatkindofLaplacian
determinism.
4In2009, NicolasGisinwasthefirstwinneroftheprestigiousJohnStewartBellprize, attributedfor
researchonfoundationalproblemsofquantummechanicsandtheirapplications.
Preface
Had you lived at the time of the Newtonian revolution, would you have
wishedtounderstandwhatwasgoingon?Today,quantumphysicsgivesusthe
opportunitytolivethroughaconceptualrevolutionofsimilarimportance.This
book aims to help you understand what is happening, without mathematics,
but also without trying to conceal the conceptual difficulties. Indeed, while
physicsneedsmathematicstoexploretheconsequencesofitshypothesesandto
preciselycalculatesomeofitspredictions,mathematicsisnotneededtotellthe
greatstoryofphysics.Forwhatisinterestinginphysicsisnotthemathematics
but the concepts. So my purpose here is not to manipulate equations, but to
understand.
Certain parts of the book will demand a genuine intellectual effort on the
part of the reader. Everyone will understand something and no one will un-
derstandeverything!Inthisfield,theverynotionofunderstandingisblurred.
But I claim nevertheless that everyone can understand at least a part of the
conceptual revolution that is under way, and take pleasure from that under-
standing.Toachievethis,onemustsimplyacceptthatnoteverythingisgoing
to be transparent, and certainly not start out with the opinion all too often
voicedthatunderstandingphysicsisahopelesstask.
Ifpartofthediscussionseemstoodifficult,justreadon.Whatcomesnext
may throw light on the matter. Or sometimes you will realise that it was just
somesubtlepointslippedinformyphysicistcolleagues,fortheytoomaytake
pleasureinreadingthisbook.Andifnecessary,gobacklaterandrereadthose
passagesthatcausedyouproblems.Theimportantthingisnottounderstand
everything,buttoacquireanoverview.Attheendoftheday,youwillfindthat
onereallycanunderstandquitealotofquantumphysicswithouttheneedfor
mathematics!
Quantumphysicshasoftenbeenthesubjectofverboseinterpretationsand
fuzzyphilosophicaldissertations.Inordertoavoidsuchpitfalls,weshallonly
haverecoursetocommonsensehere.Whenphysicistscarryoutanexperiment,
theyarequestioninganexternalreality.Thephysicistdecideswhatquestionto
askandwhentoaskit.Andwhentheanswercomesback,forexample,inthe
form of a little red light that comes on, they do not ask themselves whether
xi