Spooky action … or Entanglement tales Gunnar Björk Department of Applied Phusics AlbaNova University center, Royal institute of Technology, Stockholm, Sweden Brief outline • There is something fishy about quantum mechanics • Classical and quantum correlations • Bell inequalities (for two particles) • FLASH — a proposal for superluminal communication • A funny (and highly original) review • Tying the knot ADOPT winter school, Romme, 2012 There is something fishy with quantum mecanics ̶ circa 1928-1935 Albert Einstein Boris Podolsky Nathan Rosen Erwin Schrödinger The EPR-“paradox” In their1935 paper Einstein, Podolsky, and Rosen showed that particles that intrinsically possess certain properties even before these properties are measured, is violated by quantum mechanics. They did not like what they found. In particular, for two entangled particles, one particle appears to acquire certain definite properties the very instant the other particle is measured, irrespective of their separation – Einstein, in a letter to max Born, called this “spooky action at a distance”. Polarization Measurement of State of other photon entangled photon one photon pair 1 2 Polarization correlations Entangled photons ”Classically” correlated photons ˆ 2 Polarizer Polarizer For a = b’= 0 a Atom a b’ P b’ 1 0 0,5 1 1 0 1 0 1 0,5 2 0 0 0 Photodetector Forbidden Photodetector transition Correlation coefficient 1 Polarization correlations, continued For a = b’= 45 degrees Entangled photons ”Classically” correlated photons a b’ P a b’ P 1 0 0,5 1 0 0,25 1 1 0 1 1 0,25 0 1 0,5 0 1 0,25 0 0 0 0 0 0,25 Correlation coefficient 1 Correlation coefficient 0 Many years pass ̶ come John Bell John Bell Bell claimed that quantum mechanics was at odds with locality ― and proposed an experiment to test locality v.s. quantum mechanics! A locally realistic model of anticorrelation and a Bell inequality Lets assume we measure each particle using one of three polarization settings a, b, or c. We are free to chose the setting at will. Left particle Right particle a b c a’ b’ c’ Pop. 1 1 1 0 0 0 N1 1 1 0 0 0 1 N2 P(a,b’) = (N3 + N4)/Ntot 1 0 1 0 1 0 N3 P(a,c’) = (N2 + N4)/Ntot 1 0 0 0 1 1 N4 P(c,b’) = (N3 + N7)/Ntot 0 1 1 1 0 0 N5 0 1 0 1 0 1 N6 A Bell inequality: 0 0 1 1 1 0 N7 P(a,b’) ≤ P(a,c’) + P(c,b’) 0 0 0 1 1 1 N8 J. Bell: "On the Einstein Podolsky Rosen paradox" Physics 1, 195 (1964). Quantum prediction for correlations of a singlet state a 2 P(a,b’) = Sin fa,b’, where fa,b’ is the angle between a and b’. c = (a + b)/2 Let’s make the following choice f a,c’ of measurement directions: fa,c’ b = fc,b’ = fa,b’ /2 = 30 degrees. P(a,b’) = Sin2(60) = 0.75 P(a,c’) = P(c,b’) = Sin2(30) = 0.25 0.75 ≤ 2 x 0,25 = 0.5 ? The quantum mechanical model does not obey locality Note that our model on the previous slide only assumed two things: 1) locality, and 2) anticorrelation between the two systems if measured in same direction, no correlation otherwise. => The model is the most general (but possibly unphysical) that meets these two conditions. What to use the ”spooky action” for? The story of FLASH Nick Herbert A funny review Early in 1981, the editor of Foundations of Physics asked me to be a referee for a manuscript by Nick Herbert, with title “FLASH—A superluminal communicator based upon a new kind of measurement.” It was obvious to me that the paper could not be correct, because it violated the special theory of relativity. However I was sure this was also obvious to the author. Anyway, nothing in the argument had any relation to relativity, so that the error had to be elsewhere. I recommended to the editor of Foundations of Physics that this paper be published [5]. I wrote that it was obviously wrong, but I expected that it Asher Peres would elicit considerable interest and that finding the error would lead to significant progress in our understanding of physics. Whats wrong with FLASH What is communicated/shared is not information – it is correlations. Useful for quantum key distribution, but not for communication! Herbert assumes noiseless phase-insensitive amplification or cloning can be done. It cannot ... whatever the means. Although ”erroneous”, Herbert’s paper was important (like the EPR paper) in that it spurred further development and understanding about the meaning/definition of communication (but Herbert obviously had not read Shannon’s papers) and about amplification and cloning. Here is an author who did not read about FLASH, but should have had ... but Peres also said something else Early in 1981, the editor of Foundations of Physics asked me to be a referee for a manuscript by Nick Herbert, with title “FLASH—A superluminal communicator based upon a new kind of measurement.” It was obvious to me that the paper could not be correct, because it violated the special theory of relativity. However I was sure this was also obvious to the author. Anyway, nothing in the argument had any relation to relativity, so that the error had to be elsewhere. I recommended to the editor of Foundations of Physics that this paper be published [5]. I wrote that it was obviously wrong, but I expected that it Asher Peres would elicit considerable interest and that finding the error would lead to significant progress in our understanding of physics. There also are many wrong papers that have been published in reputable journals, some of them by renowned scientists. Their bad influence may last for years. For these, I decline all responsibility. I was not the referee of these papers and I could not protect the good reputation of their authors. Tying the knot Ph.D. supervisor Ph.D. student Nathan Rosen Asher Peres Born Aristide Pressman The government agency … recommended to shorten Pressman into Peres (a mountain eagle, gypaetus barbatusYou should), or to work keep 24 all hours the consonants a day. And and if that's become not Afarsamonenough - you (a fruit,should diospiros work at virsinatanight too.). I chose Peres … With hindsight, I should have chosen Afarsamon, so as to be the first author of my future publications. For example the famous teleportation paper would have been Afarsamon et al.. .