aspect - Bell’s inequality test: more ideal than ever...

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Unformatted text preview: Bell’s inequality test: more ideal than ever Alain napaet' | . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .' The experimental eiaiatieii iii aaiita inequalities eannrma that a pair of I entangled photons separated by hundreds of metres must he considered a single non-separable outset — it Is littlest-same to assign looal physleal reality to eaoh photon. ell's thooIItrnl. formulated in 1951, is B one ofthe profound scientific discover— ies of the century. Based on the Ein- stein. Podolsky anti Rosen {Britt} gedoiilrrri. or thoitght, experiment}, it shifted the arguv ments about the physical reality ofquantum systems from the realm of philosophy to the domain of experi mental physics. For almost three decades. experimental tests3 of Bell's inequalities have evolved closer and closer to the ideal EPR scheme. Ari experiment at the University of Innsbruck4 has. for the first time. fully enforced Bell’s requirement for strict relativistic Separation bet-ween measurements. it all started when Einstein er cl. pointed out that for certain quantum states (deseribed almosr simultaneously by Schrodinger. who coined the expression ‘ouantum entanglement'}, quantum mechanics predicts a strong correlation between distant measurements. Figure l shows a modern version of the EPR situa~ tion. where a pair ofentangled photons v. and I} are travelling in opposite directions away from a souroe. Results of polarization measurements with both polarlrers aligned are IDU% correlated. That is. each photon maybe found randomly either in channel + or — of the corresponding polarizer. but when photon v, is found positively polar— ised. then in; twin companion v2 is also found positively polarized. Because no signal can. connect the two measurements ifit travels at a wlocity less than or equal to the speed of light, c. and because the choice of the direc— tion of analysis can be made at the very last momentbefote measurementwlule the ph o- 1ons are in flight, how — argued Einstein — could one avoid the conclusion that each photon is carryinga property, determining The polarization outcome for any direction ofanalysisi' This seemingly logical conclusion pro- vides a simple image to understand the cor- relations between distant and simultaneous measurements. But it means specifying sup- plementary properties ('clements of i'ealjty' I tn. the Wfirtls of Einstein] beyond the quan- l'ln'l'lJRE I 'lr'ffil. 393_ Ill- MARCH IE'i'tl I www.na1 ii.ri.i.r.r.ir11 tum—mechanical description. To the ques— tion "Can a quantum-mechanical descrip- tion of physical reality be considered com- plate?"2 Einstein’s answer was clearly negaa titre, but this conclusion was incompatible with the ‘Copenhagen interpretation" defended by Bohr, for whom the quantum- mechanical description was the ultimate one’. This debate between Einstein and Bohr lasted until the end oflheir lives. its it was. it could hardly be settled. becausetherewasno apparent disagreement on the correlations predicted for an EPl't gcdenlreri experiment. The point undtn discussion was the world— view implied by the analysis ofthe situation. Bell’s theorem changed the nature of the debate. In a simple and illuminating paper‘. Bell proved that Einstein's point of view [local realism} leads to algebraic predictions [the celebrated Bell’s inequality} that are contradicted by the quantum-mechanical predictions for an EPR geriatrics experiment involving several polarizer orientations. The issue was nolonget a matter of taste. or epis- temological position: it was a quantitative question that could be answered experimen- tally.atleastin principle. Prompted -thc Clauscr—l-Iorne— news and views * Shimony—Holt paperIi that framed hell's 5 ineqtialities in a way better suited to real experiments, a first series of tests’. using photon pairs produced in atomic radiative cascades, was performed in the earlsr 19Tos at Berkeley. Harvard and 'liexas Alida-f. Most results agreed with quantum mechanics. but . the schemes used were far from ideal; in par— " ticular. the use of single—channel polarirers only gave access to the + outcome. Progress in laser physics and modern optics led in a new generation of experiments carried out by colleagues and myselfat Orsay in the early 1930s. They were based on a highly efficient source of pairs of correlated photons, pro 5 duoed by non—linear laser excitations of an I atomic radiative cascade. An experiment involving two—channel polarizers. as in die ideal EPR gea'rtnlren eirperiirieritJ gave an unambiguous violation ofBell's inequalities by tens of standard deviations. and an impressive with mechanics“. A third generation oftcsts, begun in the late l'ii'tlll's at Maryland and ltocliester9'l", .' used nonlinear splitting of ultraviolet pho- tons to produce pairs ofcorrelated EPR pho- tons. 1til-"ith such pairs. measurements can bear either on discrete variables such as polarisation or spin components. as consid- ered by Bell, or on continuous variables of the type originally considered by Einstein. Po-dolslry and Roach. and studied at lCal— tech“. a remarkable feature of such photon sources is the production of two narrow beams of correlated photons that can 11:: feet into two uptical fibres. allowing for tests with great distances between the source and the measuring apparatus. as demonstrated over four kilometresin Malvernuand overtens of icilomet resin Geneva”. The esperio‘ienters at Inrisbruclt' used this method to address a Fundamental point raised by Bell. to the experiment shown in , l‘ig. l..'a'l1ere the polanzers’ orientations are -' lcept Freed during a run, it is possible to rec— I agreement quantum I II Figure l Einstein—P‘odnlslty—Rosen gedcnlreri experts-ifit with photons. The tit-optimum. vl and tr}, are analysed by the linear polariters I and II. which malts: polarization measurements along}: and perpendicular to tl'iezaiu's. Each measurement hastwo possible outcomes, + or —, and one can. measure the probabilities ofsingle otjoint measurements at various orientations: and ii. For an entangled EPR state. violationofa Hell’s inequality indicates that the strong consolatiom between the measurements on the Iwn opposite sides cannot beeicptatned by an irntige‘d la Einstein" awaiting properties carried along by each photon. [n the Innsbruck experiment“, any p-flfijbflil‘f of communication between the pulat'izers. at a velocity less than or equal to that of light, is precliirkdbry random and uliralasr switching ofllte orientations oflhe polariaers. separated bye distance silent! in. On each sidc.a local computer registers the polarizer orientation and the result of earl-i ineasurentertlt with the timingmonitored by an atomic clock. Data are gathered and compared {or I correlation measureme nts after [lit-end ofa run. H In] 1959 lillaemlllan Magazines Ltd 135 news and storms oneile the quantum mechanical predictions and Einstein’s conceptions by invoking a possible exchange of signals between the polariaers. To avoid this loophole. llell stressed the importance of experiments “in which the settings are changed during the flight of the particles“. so that any direct signal exchange between polarizers would be impossible. provided that the choice of orientations is made randomly in a time shorter than the flight time oithc particle or ' photon. to ensure that relativistic separation is enforced. Prompted by hell's relnarlt. a first stc towards the realization ofthis ideal scheme * found a violation of Bell's inequality with rapidly switched polarizers.hut the polariaer separation (11 ml was too sinali to allow for a truly random resetting of the polarixers. 1rliith a separation of sill] In between their measuring stations, the physicists of Inns- bruclc‘ have 1.3 its to make random settings of the polarizer and to register the result of the measurement, as well as its exact timing monitored bye local rubidium atomic clock. It isonly at the end ofthe run that the experi- mentalisu gather the two series of data obtained on each side, and look for correla— tions. The results. in excellent agreement with the quantum mechanical predictions, show an unquestionable violation of Bell’s inequalities“. This experiment is remarkably close to die ideal gedonken experiment. used to dis- cuss the implications of Bell's theorem. Note that th ere remains sooth er loophole. due to the limited efficiency of the detectors. but this can he closed bya technological advance that seems plausible in the foreseeable future, and so does notcot respond to a radi- cal change in the scheme of the experiment. Although such an experiment is highlydesir- nhle,we can assume For the saltc ofargu ment that the present results will remain rlnch anged with high-efficiency detectors. The violatith of lie-Li's inequality, with strictrelativistic separation between the cho- sen measurements. means that it is impossir ble to maintain the image ‘ri in Einstein‘ where correlations are explained by com- mon properties determined at the common source and subsequently carried along by each photon. We must conclude that an entangled EPR photon pair is a non-separa— ble object; that is. it is impossible to assign individual local properties {local physical reality] to each photon. In some sc nse. both photons keep in contact through space and time. it is worth emphasizing that non-separa- bility, which is at the roots of quantum tele- portation”, does not imply the possibility of practical faster-than-light communication. An observer sitting behind a polarith only sees an apparently random series of — and + results. and single measurements on his side caimot make him aware that the distant operator has suddenly changed the orienta- tion of his polariaer. Should we then mn- riude thatthere is nothing remarkablein this experiment? 'I'o convince the reader of the contrary, I suggest we talte the point ofview of an external ohsen'er,whn collectsthe data From thetwo distant stations at the end oithe experiment. and compares the two series of results. This is what the lunsbruclt team has done. Looking at the data :I postedori. they found that the correlation immediately changed as soon as one ofthe poiarizers was sudtched. without any delay allowing for signal propagation: this reflects quantum non—separability. an tails? . 6*? a’f . 5 Einstein, Podolslcy, and Rosen did not doubt that quantum mechanics is correct, as far as it goes; they only claimed that it is an incomplete description of physical reality: The wave function is not the whole storyfiz-somo other quantity. it, is needed, in addition to lit. to charactdrize the state of a system fully. We call it the “hidden variable" because, at this stage. we have no idea how to calculate or measure it? 1iilver the years. a number of hidden variable theories have been proposed, to supplement quantum mechanics?" they tend to be cumbersome and implausible, but never mind—until 19164 the program seemed eminently worth pursuing. But in that year .1. 3. Bell proved that any local hidden variable tireon is incompatible with quantum mechanics. Bell suggested a generalization of the EPRiBohm experiment: Instead of ori- sso eating the electron and positron detocttns along the some direction, he allowed them to be rotated independently. The first measures the component of the electron spin in the direction of a unit vector a. and the second measures the spin of the positron along the direction b [Figure 12.2}. For simplicity, let‘s record the spins in units of M2; then each detector registers the value +1 (for spin up} or —l [spin down}. along the direction in question. a. table of results, for many rr“ decays, might look like this: fist} toss Macmillan Magazines Ltd Whether non-separa bilityofEPlt pairs is a realprublcm or not isadiffieultqttestion to settle. As Richard Feynman once said “3': “it has not yet become obvious to me that there is no real problem 1 have entertained tnyselfalv-‘ays by squeezing the difficulty of quantum mechanics into a smaller and smaller place, so as to get more and more worried about this particular item. It seems almost ridiculous th at you can squeeze it to a numerical question that one thing is bigger than another. But there you are — it is big- , gens". Yes. it is bigger by 3-D standard devia- r Lions. D Alain fispccris in the laborers-ire Chstrimt Hsbry, Uni-re Mixre rte Recherche alth on CNRS. instills? ri'Optique Iheorioue er Appliqrrdc. BF i4?—F9Hfi|5. firstly: Finn-tee. f-moil: dining.qspecrtE-liomJi—psun'j- _ |. Bell]. 5. in Sportinlllrwm' thgmdrmlt'eh Quorumnircfinmcc l-l-I'l lf-Il'llhrldur 'Lln'tIr. Flrss. lil‘li'u‘]. . Einstein,i‘t., llndoilslty, H. at Itosrn, N, ,l’fiyt, It”; 4?,711—130 [193%. I. Mpecl, A. In News. roller-mutton nnn' mutpriwt slimy-sic; le'. Prue. 't'iol. ll-l] ||L:l:|io Playtime, Bologna, 199.3]. 4-. “it-ills. Cm It‘l'lnt'utin. T.I Simon. ti, ‘rklnl'urler, N. & .Zeilinger. .l'l. Phys. REP. (NEIL 5039—5043 [IML s. titanic. H'Iys Err as, sat—1oz [Ii-I53]. o. Clauses. I. l'..llorn.e,. M}... El'LnTIDI'Ir. A. s: Holt, R. A. Pfip, Rev.ten.!!,flD—J-lu1tl'lld‘il‘l. i'. Clmstlfl. Ldt Shimmy,i'.. Rep. Frag. .Pllyi, 4|, IBBI—IBEE thHl. ll». .upocr,h.,fiirangier, P at Ronni; Plays Erl' [fit 49,5l—Slq ttifllil. "t Slfiht't'. H. d: nlleyfl; III.Phys.ll'ev.i.r:r.li1.1921—"92t|]9m:t to. EN. 7. 'f'.-5I Mandel. I. First Rn: Ler til. ill—SJ-tlgfll. llDu. Z.‘i'..l'uti|rera.:i. E. Ililnblt, H. Lti'IPeng, K. C, Plus. my, l to: transmit-s | lei-ti. ,' llTapsstr. P. IL Rarity._l.G.-§'On-ens,. P. L". M, Piqi fi'er first. 33, IFBR-IFJE [199“ l:|.'l'|rtel,"rl".lldrnilel,_|., thirds", H, *fiihirhr N, my; It“: [.211 ill. 3553-3555tl996l. l‘lu'tlpflfl. .l'l .IilalilmtvlJ. B: linger-J3. l’l'lys. Rev. LerL 45‘. lllU'l—ll‘vl'l? “Will. |5.B-enn.rl.tll-li.+rui. Plus. Hal: Latest], lays—mos “9‘95:- |ti.Ft'rt-1Inan. II. T“. int]: T-lIttIr. Phys Iltdo'lulllll truss]. Na't'Urslvot seal reunites-l Isvsi 'l'l'l-‘f'fi'.ntll.l,|fc otsm ...
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aspect - Bell’s inequality test: more ideal than ever...

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