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Hardy's paradox

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the 810 nm photons come back to the crystal, they are annihilated by interaction with the returning pump beam. In any case, the beam of photons that make it through the crystal and the beam of photons that pass through the beam splitter are both separated into "vertically polarized" and "horizontally polarized" beams, which correspond to the "electrons" and the "positrons" of Hardy's scheme. The two "electron" beams (the photons with one kind of polarization) are united at a beam splitter and go to one or two detectors, and the same for the "positrons" (the other photons). Classically, no photons should be detected at what the authors call the "dark ports" because if they take both directions from the first beam splitter, they will interfere with themselves, whereas if they take only one path, then one cannot detect them both at the dark ports because of the paradox. By introducing a 20° rotation in polarization and using
50:, and these have demonstrated that the phenomenon does occur. However, the consequence of these experiments is only that past events can be inferred after their occurrence as a probabilistic wave collapse. These weak measurements are considered to be an observation themselves, and therefore part of the causation of wave collapse, making the objective results only a probabilistic function rather than a fixed reality. However, a careful analysis of the experiment shows that Hardy's paradox only proves that a 1014: 762: 1352: 569: 1009:{\displaystyle \left|e^{+}e^{-}\right\rangle \to {\frac {1}{4}}\left(3\left|c^{+}\right\rangle \left|c^{-}\right\rangle +\left|c^{+}\right\rangle \left|d^{-}\right\rangle +\left|d^{+}\right\rangle \left|c^{-}\right\rangle -\left|d^{+}\right\rangle \left|d^{-}\right\rangle -2\left|\gamma \right\rangle \left|\gamma \right\rangle \right).} 1049: 389: 1539:
crystal to produce pairs of 810 nm photons with polarizations orthogonal to each other. These then hit a beam splitter, which sends photons back to the barium borate crystal with 50% probability. The 405 nm pumping beam also bounces from a mirror and comes back to the barium borate. If both
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that allowed them to calculate the "occupation" of different arms (paths) and combinations. As predicted by Aharonov and colleagues, they found a negative value for the combination in which both photons take the outer (no-annihilation) route. The results were not exactly as predicted, and they
250: 381: 1347:{\displaystyle {\frac {|v^{+}\rangle -i|w^{+}\rangle }{\sqrt {2}}}{\frac {|v^{-}\rangle -i|w^{-}\rangle }{\sqrt {2}}}={\frac {1}{2}}\left(|v^{+}\rangle |v^{-}\rangle -i|v^{+}\rangle |w^{-}\rangle -i|w^{+}\rangle |v^{-}\rangle -|w^{+}\rangle |w^{-}\rangle \right).} 54:
cannot exist, as there cannot be a theory that assumes that the system meets the states of reality regardless of the interaction with the measuring apparatus. This confirms that a quantum theory, to be consistent with the experiments, must be non-local (in the
564:{\displaystyle {\frac {1}{2}}\left(\left|v^{+}\right\rangle \left|v^{-}\right\rangle +i\left|v^{+}\right\rangle \left|w^{-}\right\rangle +i\left|w^{+}\right\rangle \left|v^{-}\right\rangle -\left|\gamma \right\rangle \left|\gamma \right\rangle \right).} 72: 137: 1479: 1357:
This is not orthogonal to the expression above for the state before the final beam splitters. The scalar product between them is 1/4, showing that there is a 1 in 16 chance of this happening, paradoxically.
754: 663: 308: 1849:; Botero, Alonso; Popescu, Sandu; Reznik, Benni; Tollaksen, Jeff (2002). "Revisiting Hardy's paradox: counterfactual statements, real measurements, entanglement and weak values". 83:
for quantum particles and antiparticles. We will describe the case using electrons and positrons. Each interferometer consists of bent paths and two beam splitters (labeled
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Aharonov, Yakir; Albert, David Z.; Vaidman, Lev (1988-04-04). "How the result of a measurement of a component of the spin of a spin-1/2particle can turn out to be 100".
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In the actual experiment the interferometers are arranged so that part of their paths overlap as shown in the diagram. If the amplitude for the particle in one arm, say
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Since the probabilities are the squares of the absolute values of these amplitudes, this means a 9 in 16 chance of each particle being detected in its respective
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The situation can be analyzed in terms of two simultaneous interaction-free measurements: from the point of view of the interferometer on the left, a click at
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Yokota, K.; Yamamoto, T.; Koashi, M.; Imoto, N. (2009). "Direct observation of Hardy's paradox by joint weak measurement with an entangled photon pair".
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paths in boxes and noting the effect of their mutual electrostatic attraction. They stated that one would actually find a repulsion between the boxes.
245:{\displaystyle \left|e^{-}\right\rangle \to {\frac {\left|v^{-}\right\rangle +i\left|w^{-}\right\rangle }{\sqrt {2}}}\to i\left|c^{-}\right\rangle .} 91:
in the accompanying diagram) and is tuned so that when operating individually, particles always exit to the same particle detector (the ones labeled
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detectors; and a 4 in 16 (1 in 4) chance that the electron and positron annihilate, so neither is detected. Notice that a detection in both
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is for "destructive interference"). For example, for the right-hand side interferometer, when operating alone, entering electrons (labeled
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and colleagues in 2001 pointed out that the number of electrons or positrons in each branch is theoretically observable and is 0 in the
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Lundeen, J. S.; Steinberg, A. M. (2009). "Experimental Joint Weak Measurement on a Photon Pair as a Probe of Hardy's Paradox".
376:{\displaystyle \left|w^{+}\right\rangle \left|w^{-}\right\rangle \to \left|\gamma \right\rangle \left|\gamma \right\rangle } 1520:! They proposed a way that this could be observed physically by temporarily trapping the electron and the positron in the 383:. There is a 1 in 4 chance of this happening. We can express the state of the system, before the final beam splitters, as 1936: 1550: 295: 51: 1535:
published work in which they set up a "Hardy's paradox" system using photons. A 405 nm laser goes through a
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term arises, in fact, from the nonmaximally entangled nature of the state just before the final beam splitters.
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Hardy, Lucien (1992). "Quantum mechanics, local realistic theories, and Lorentz-invariant realistic theories".
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Hardy, Lucien (1993). "Nonlocality for two particles without inequalities for almost all entangled states".
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observable and is not given by the product of the single-particle values. So we find that the number of
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on certain beams, and then measuring coincidence rates at the detectors, they were able to make
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paths in their respective interferometers, they will annihilate to produce two gamma rays:
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A paradox then arises because sometimes the particles do emerge simultaneously at
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amplitude would reach the second beam splitter and would split into arms
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respectively). If we assume the particles are independent (described by
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If (classically speaking) both the electron and the positron take the
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an annihilation taking place. For this reason, this scheme was named
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would thus indicate the presence of the obstructing particle, but
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The basic building block of Hardy’s thought experiment are two
1369:. Similarly, for the interferometer on the right, a click at 1397:), we conclude that they can never emerge simultaneously in 749:{\textstyle {\frac {1}{\sqrt {2}}}(|v\rangle -i|w\rangle )} 658:{\textstyle {\frac {1}{\sqrt {2}}}(|v\rangle +i|w\rangle )} 1413:, which cannot occur because of the annihilation process. 1549:
attribute this to imperfect switching (annihilation) and
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with equal amplitudes. The detection of a particle in
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implies the presence of the obstructing electron in
1496:branches. And yet, the number of electron–positron 1803:(14). American Physical Society (APS): 1351–1354. 1473: 1346: 1008: 748: 685: 657: 594: 563: 375: 244: 270:, were to be obstructed by a second particle in 1428: = 1/16). Quantum mechanically, the 1035:detector, or for both being detected in their 8: 1377:. Indeed, every time a click is recorded at 1333: 1315: 1294: 1276: 1252: 1234: 1210: 1192: 1148: 1124: 1095: 1071: 740: 723: 680: 649: 632: 589: 1862: 1755: 1689: 1671: 1669: 1461: 1443: 1433: 1327: 1318: 1309: 1300: 1288: 1279: 1270: 1261: 1246: 1237: 1228: 1219: 1204: 1195: 1186: 1177: 1162: 1142: 1133: 1118: 1109: 1106: 1089: 1080: 1065: 1056: 1053: 1051: 960: 942: 921: 903: 882: 864: 843: 825: 799: 785: 775: 764: 732: 715: 700: 698: 672: 670: 641: 624: 609: 607: 581: 579: 518: 500: 476: 458: 434: 416: 393: 391: 338: 320: 310: 229: 197: 173: 162: 149: 139: 31:in 1992–1993 in which a particle and its 1947:Thought experiments in quantum mechanics 1373:implies the presence of the positron in 119:(in the diagram, the latter part of the 1579: 1027:for one particle being detected in its 99:is for "constructive interference" and 1518:the number in the vv combination is −1 1405:. This would imply that they were in 7: 75:Setup for Hardy's thought experiment 42:Experiments using the technique of 1385:), the other particle is found in 14: 67:Setup description and the results 274:that collides with it, only the 1508:pairs (both particles in their 46:have studied an interaction of 1708:10.1103/PhysRevLett.102.020404 1319: 1301: 1280: 1262: 1238: 1220: 1196: 1178: 1134: 1110: 1081: 1057: 1031:detector and the other in its 796: 743: 733: 716: 712: 673: 652: 642: 625: 621: 582: 348: 255:Similarly, positrons (labeled 215: 159: 1: 1914:Lecture by Aephraim Steinberg 1881:10.1016/s0375-9601(02)00986-6 1857:(3–4). Elsevier BV: 130–138. 1774:10.1088/1367-2630/11/3/033011 1551:interaction-free measurements 111:of electrons taking the path 1043:detectors is represented by 296:interaction-free measurement 81:Mach–Zehnder interferometers 52:local hidden-variable theory 1817:10.1103/physrevlett.60.1351 1652:10.1103/PhysRevLett.71.1665 1609:10.1103/PhysRevLett.68.2981 1023:detector; a 1 in 16 chance 1963: 686:{\displaystyle |d\rangle } 595:{\displaystyle |c\rangle } 115:and electrons taking path 259:) are always detected at 1797:Physical Review Letters 1678:Physical Review Letters 1632:Physical Review Letters 1589:Physical Review Letters 1744:New Journal of Physics 1568:Wave function collapse 1500:in any combination is 1492:branches and 1 in the 1475: 1395:local hidden variables 1348: 1010: 750: 687: 659: 596: 565: 377: 246: 76: 1563:Uncertainty principle 1533:Aephraim M. Steinberg 1476: 1349: 1011: 751: 688: 660: 597: 566: 378: 247: 109:quantum superposition 74: 35:may interact without 1684:(2): 020404–000001. 1432: 1050: 763: 697: 669: 606: 602:detectors click for 578: 390: 309: 138: 1937:Quantum measurement 1873:2002PhLA..301..130A 1809:1988PhRvL..60.1351A 1766:2009NJPh...11c3011Y 1700:2009PhRvL.102b0404L 1644:1993PhRvL..71.1665H 1601:1992PhRvL..68.2981H 1919:2015-10-02 at the 1471: 1424:(with probability 1344: 1006: 746: 683: 655: 592: 561: 373: 242: 77: 21:thought experiment 1851:Physics Letters A 1728:. 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760: 756:, this becomes 695: 694: 667: 666: 604: 603: 576: 575: 542: 531: 514: 510: 496: 492: 472: 468: 454: 450: 430: 426: 412: 408: 407: 403: 388: 387: 362: 351: 334: 330: 316: 312: 307: 306: 225: 221: 193: 189: 169: 165: 164: 145: 141: 136: 135: 69: 17:Hardy's paradox 12: 11: 5: 1960: 1958: 1950: 1949: 1944: 1939: 1929: 1928: 1925: 1924: 1909: 1908:External links 1906: 1903: 1902: 1838: 1787: 1734: 1665: 1622: 1578: 1577: 1575: 1572: 1571: 1570: 1565: 1558: 1555: 1486:Yakir Aharonov 1484:An article by 1469: 1464: 1460: 1456: 1451: 1446: 1442: 1438: 1355: 1354: 1343: 1339: 1335: 1330: 1326: 1321: 1317: 1312: 1308: 1303: 1299: 1296: 1291: 1287: 1282: 1278: 1273: 1269: 1264: 1260: 1257: 1254: 1249: 1245: 1240: 1236: 1231: 1227: 1222: 1218: 1215: 1212: 1207: 1203: 1198: 1194: 1189: 1185: 1180: 1175: 1169: 1166: 1161: 1155: 1150: 1145: 1141: 1136: 1132: 1129: 1126: 1121: 1117: 1112: 1102: 1097: 1092: 1088: 1083: 1079: 1076: 1073: 1068: 1064: 1059: 1017: 1016: 1005: 1001: 996: 993: 990: 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Index

thought experiment
quantum mechanics
Lucien Hardy
antiparticle
annihilating
weak measurement
polarized photons
local hidden-variable theory
sense of Bell
contextual

Mach–Zehnder interferometers
quantum superposition
interaction-free measurement
local hidden variables
Yakir Aharonov
Jeff Lundeen
Aephraim M. Steinberg
barium borate
half-wave plates
weak measurements
interaction-free measurements
Uncertainty principle
Wave function collapse
Bibcode
1992PhRvL..68.2981H
doi
10.1103/PhysRevLett.68.2981
PMID
10045577

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