Albert Einstein s Nightmare before Christmas. The spooky world of quantum light

Transcription

1 Albert Einstein s Nightmare before Christmas The spooky world of quantum light Peter J Mosley Christmas Lecture 2012 Department of Physics, University of Bath If all this is true then it means the end of physics Albert Einstein to Niels Bohr

2 Introduction 1905 Early-career revolutionary 1935 Mid-career conservative Einstein s transformation - revolutionary to conservative Two quantum paradoxes - thought experiments to refute quantum theory Einstein s legacy - current laboratory tests of quantum mechanics

3 Einstein s transformation - revolutionary to conservative A very brief history of quantum mechanics

4 Particles of light Einstein s revolutionary explanation of photoelectric effect in 1905 Light Electrons E k Metal [Light] consists of a finite number of energy quanta which... can be produced and absorbed only as discrete units Albert Einstein

5 Einstein s annus mirabilis Einstein as revolutionary......all while working as patent clerk!

6 Countdown to Copenhagen Planck E = n~! Einstein E k = ~! Bohr L = p l(l + 1)~ Einstein A 21,B 21,B 12 It is a weakness of the theory that it leaves the time and direction of the elementary process to chance I find the idea quite intolerable that an electron exposed to radiation should choose of its own free will not only its moment to jump off but also its direction. In that case I would rather be a cobbler, or even an employee of a gaming house, than a physicist. Albert Einstein

7 Countdown to Copenhagen Planck E = n~! Einstein E k = ~! Bohr L = p l(l + 1)~ Einstein A 21,B 21,B de Broglie = h p I had a sudden inspiration. Einstein's wave-particle dualism was an absolutely general phenomenon extending to all physical nature Louis de Broglie

8 Countdown to Copenhagen Planck E = n~! Einstein E k = ~! Bohr L = p l(l + 1)~ Einstein A 21,B 21,B de Broglie = h p Schrödinger ~ 2 2m r2 + V When one wishes to calculate `the future' from 'the present' one can only get statistical results since one can never discover every detail of the present. Werner Heisenberg Heisenberg x p ~ 2

9 Countdown to Copenhagen Planck E = n~! Einstein E k = ~! Bohr L = p l(l + 1)~ Einstein A 21,B 21,B de Broglie = h p Schrödinger ~ 2 2m r2 + V Heisenberg has laid a big quantum egg. In Göttingen they believe it. I don't. Einstein Heisenberg x p ~ 2

10 Countdown to Copenhagen Planck E = n~! Einstein E k = ~! Bohr L = p l(l + 1)~ Einstein A 21,B 21,B de Broglie = h p Schrödinger ~ 2 2m r2 + V Heisenberg x p ~ 2 Copenhagen interpretation

11 So what s so spooky? The philosophy bit... Copenhagen interpretation suggests... physical properties only have meaning within context of measurement uncertainty always exists in physical systems individual measurement outcomes are inherently unpredictable no underlying physical reality It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature Niels Bohr

12 So what s so spooky? The philosophy bit... Copenhagen interpretation suggests... physical properties only have meaning within context of measurement uncertainty always exists in physical systems individual measurement outcomes are inherently unpredictable no underlying physical reality Einstein believed... objective reality exists independent of measurement physics should be capable of predicting all outcomes existence of a more complete, realistic theory Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but it does not really bring us any closer to the secrets of the Old One. I at any rate am convinced that He does not play dice Albert Einstein

13 So what s so spooky? The philosophy bit... Copenhagen interpretation suggests... physical properties only have meaning within context of measurement uncertainty always exists in physical systems individual measurement outcomes are inherently unpredictable no underlying physical reality Einstein believed... objective reality exists independent of measurement physics should be capable of predicting all outcomes existence of a more complete, realistic theory Spent years in debate with Bohr, constructing thought experiments to refute QM

14 Two quantum paradoxes Einstein, Podolsky and Rosen and Schrödinger s cat

15 The EPR paradox Suggested by Einstein in 1933, published in 1935 Attempt to demonstrate that QM not complete description of reality

16 The EPR paradox 2 particles interact, then fly apart Individual variables unknown, but perfectly correlated to each other x 1 (x 1 + x 2 ) x 2 p 1 (p 1 p 2 ) p 2 Measure Measure x 1 know p 1 x 2 know p 2 Assume locality x 2, p 2 must be well-defined in advance! Hidden variables Suggests QM not complete! Physics should represent a reality in time and space, free from spooky action at a distance Albert Einstein

17 The EPR paradox 2 particles interact, then fly apart Individual variables unknown, but perfectly correlated to each other x 1 (x 1 + x 2 ) x 2 p 1 (p 1 p 2 ) p 2 Measure Measure x 1 know p 1 x 2 know p 2 Assume locality x 2, p 2 must be well-defined in advance! Hidden variables Suggests QM not complete! Bohr s response... Two particles behave as one system! Measurement of one particle instantaneously affects other! Particles are entangled

18 Entanglement Entanglement arises from the fact that the two bodies at some earlier time formed in a true sense one system, that is were interacting, and have left behind traces on one another Erwin Schrödinger I do not believe in it. This epistemology-soaked orgy ought to burn itself out. No doubt, however, you smile at me and think that, after all, many a young whore turns into an old praying sister, and many a young revolutionary becomes an old reactionary. Albert Einstein

19 Entanglement Example - polarisation of photons i = 1 p 2 ( H 1 V 2 i + V 1 H 2 i) Individually have 50% chance of being H or V But joint measurements correlated Nonlocal action at a distance (spooky!) Change measurement basis V i %i Hi &i Hi = 1 p 2 %i + &i) V i = 1 p 2 %i &i) i = 1 p 2 ( % 1 % 2 i & 1 & 2 i) Correlation preserved! Hallmark of entanglement

20 Schrödinger s cat Schrödinger s paradox was endorsed by Einstein Showed absurdity of quantum effects on everyday level

21 Schrödinger s cat Schrödinger s paradox was endorsed by Einstein Showed absurdity of quantum effects on everyday level

22 Schrödinger s cat Schrödinger s paradox was endorsed by Einstein Showed absurdity of quantum effects on everyday level Your cat shows we are in complete agreement concerning our assessment of the character of the current theory. A psi-function that contains the living as well as the dead cat just cannot be taken as a description of a real state of affairs.' Albert Einstein

23 Schrödinger s cat Schrödinger s paradox was endorsed by Einstein Showed absurdity of quantum effects on everyday level Debate continues over how to resolve this paradox Interaction of cat with environment is enough to collapse wavefunction! Measurement problem remains in Copenhagen interpretation

24 Einstein s legacy Laboratory tests of quantum theory

25 Young s double slit interference Experiment from nevertheless encapsulates quantum mechanics! Measure at slits - particle-like Measure at screen - wave-like OpenStax College, Measurement affects system - photons, electrons, or even molecules! Single photons Single electrons How does wavefunction collapse occur? Do particles have advance (hidden) knowledge of measurement?

26 Wheeler s delayed choice When does photon choose which behaviour to display? Does it have advance (hidden) information about measurement? Jacques et al, Science 315, 966 (2007) Photon makes choice after entering interferometer! Suggests QM is complete

27 How to test EPR - Bell s inequality x 1 (x 1 + x 2 ) x 2 p 1 (p 1 p 2 ) p 2 Set of measurements on individual particles Find correlation between them, S Hidden variables and locality QM complete, entanglement

28 We need some entanglement! Photon pair production in nonlinear crystal V i Select overlapped regions Hi i = 1 p 2 ( H 1 V 2 i + V 1 H 2 i)

29 Experimental Bell tests Ursin et al, Nature Physics 3, 481 (2007) S = 2.508(37) QM is nonlocal* *Subject to terms and conditions Detector loophole Fair sampling

30 Schrödinger cat states with light Mix quantum and classical light Interfere photon pairs with laser Produces optical Schrödinger cats! How big can they get? Decoherence limits size Ourjoumtsev et al, Nature 448, 784 (2007)

31 Optomechanical cats Quantum behaviour of macroscopic object? Requires object in ground state (very cold!) Cool micro-mirror coupled to EM field Like laser cooling atoms (but tougher!) Fiore et al, PRL 107, (2011) Aspelmeyer group University of Vienna Current state-of-the-art is n = (a few) Quantum effects should be observable soon! Wiederhecker et al, Nature 462, 633 (2009)

32 What would Einstein think? Huge progress in experimental tests of QM since Einstein s death in 1955 Hidden variables seem unlikely All tests have confirmed validity of QM Nevertheless, measurement problem still exists I hope Einstein would still be fighting!

33 Merry Christmas and a Happy New Year!