Strangeness of quantum world: Setting the speed limits for “spooky” communication

In a recent publication, physicists in Switzerland have shown that the “spooky action” (non-local correlations) observed in Bell experiments travels at least 10,000 times faster (>1.86 billion miles per second) than the speed of light (1). This throws off the physical reality envisioned by Einstein’s theory of relativity. The speed in the publication is defined in a specific frame of reference, which is not the same as the surface of earth. Measurements were performed at all possible orientations against this frame and based on a conservative assumption that earth speed in this frame is less than 1,000 times that of the speed of light. The distance over which entanglement has been shown to exist has increased over years since the early confirmation of Bell’s inequality theorem.

One of the biggest philosophical debates Einstein had with his peers was about the physical reality as perceived from the laws of quantum physics. In his letter of 1947 to Max Born Einstein ridiculed the idea of quantum entanglement as a “spooky action at a distance.” Einstein was irked by the suggestion that when a pair of particles is separated spatially, the physical property measured on one particle “instantly” determines the physical property of its twin pair no matter how far they are separated. This means that information between the two particles could travel faster than speed of light, which contradicted Special Theory of Relativity. Einstein did not like to accept physical realty of quantum mechanics, because it was different from the reality perceived from the laws of classical physics. He was not alone, Erwin Schrödinger, founder of wave mechanics was also uncomfortable with this idea. In fact he was first to use the term quantum entanglement to describe two quantum systems intimately correlated that could not be explained by classical laws of physics (2). Quantum entanglement is at the core oh his famous cat paradox. The famous EPR thought experiment published in 1935 by Einstein, Podolsky and Rosen under the title “Can quantum-mechanical description of physical reality be considered complete?” aimed to establish that quantum mechanics is not a complete theory because there are hidden variables (3). Einstein accepted that it is a correct theory since its predictions were confirmed by experiments, but maintained that theory has hidden variables that need to be understood and identified. Unfortunately Einstein did not live long enough to see that his position on quantum entanglement was not right. In 1964 John Bell derived an inequality theorem that showed that a testable difference between the predictions of quantum mechanics and local hidden variable theories (4). His theory postulated that EPR paradox had no hidden variables, and they would contradict predictions of quantum mechanics. Tests of Bell’s inequality are tests of Einstein’s EPR paradox, and Alain Aspect and his coworkers experimentally demonstrated that Bell’s inequality is violated, which means that the predictions of quantum mechanics is correct and there are no hidden variables as predicted by EPR experiment.

Experiments on quantum entanglement and EPR paradox involves a pair of particles, but entanglement involving more than two particles results in complex permutations and the nature of quantum reality becomes obscure than ever. Quantum entangled states have numerous applications in quantum information processing, which includes quantum teleportation, quantum computation, and quantum cryptography.

1. Salart, D., Baas, A., Gisin, N., and Zbinden, H. Testing the speed of “spooky action at a distance.” Nature, Volume 454 (August 14 issue), pages 861-864 (2008).
2. Schrödinger, E. Proc. Camb. Phil. Soc. 31, 555 - 565 (1935)
3. Einstein, A., Podolsky, A., and Rosen, N. Can quantum-mechanical description of physical reality be considered complete? Physical Review 41, 777 (1935)
4. Bell, J.S. On the Einstein, Podolsky Rosen Paradox. Physics 1, 195 (1964)
5. Aspect A. et al., Experimental tests of Bell’s inequalities using time-varying analyzers. Phys. Rev. Lett. 49, 1804 (1982)