Guest post by Federico Levi, Associate Editor Nature Communications
The experiments in this week ’s blog entry accompanying our poll of ‘ the most beautiful experiment with light’ were carried out in the second half of the twentieth century, in which physicists were still struggling to accept the counter-intuitive implications of quantum physics.
One of the most bewildering embodiments of quantum theory is quantum entanglement. When two particles are entangled, performing a measurement on one of them seems to instantaneously influence the other particle, even if it is light-years away. This paradox, famously termed ‘spooky action at a distance’ by Albert Einstein was formulated by Einstein, Podolsky and Rosen (EPR) in 1935. To restore reality, they argued that quantum physics could simply be our limited understanding of a deeper and less troubling theory, classically constructed over a set of ‘hidden variables’.
It took almost thirty years to devise a way to test their hypothesis. In 1964 John Stewart Bell demonstrated the famous theorem carrying his name, which showed how there would be an experimentally measurable difference between the prediction of quantum physics and that ‘less troubling theory’ imagined by EPR. Light provided the means to carry out this test. In 1972, by looking at the correlations in the linear polarization of photons emitted by an atomic cascade of calcium, Stuart J. Freedman and John F. Clauser tested Bell’s theorem at the Lawrence Berkeley Laboratory in California. The result was a landmark confirmation of quantum mechanics.
Jumping more than 10 years ahead, we find researchers dealing with the consequences of yet another quantum principle, namely the indistinguishability of fundamental particles. Two particles in exactly the same quantum state have to be considered essentially indistinguishable, and photons make no exception. In 1987 at Rochester University, New York, Chung Ki Hong, Zhe Yu Ou and Leonard Mandel showed what may be the most direct evidence of this principle. When a photon hits a beam splitter, it can continue in one of two ways with 50% probability. If two indistinguishable photons hit a beam splitter coincidentally, quantum interference forbids the outcome where they follow different output paths. The pronounced ‘dip’ in the measurement of simultaneous arrivals at the two outputs is a hallmark of the indistinguishability of a photon pair, a sought-after quality for quantum information applications.
In the same year, 1987, physicists were busy building on research initiated by Lord Rayleigh a century before, concerning the idea to influence the propagation of light in periodic structures. This led to the concept of photonic crystal: a structure with a well-defined band gap for electromagnetic waves, in analogy to the bandgap for electrons provided by conducting solids which have periodic atomic lattices. The first photonic crystal worked in the microwave regime, and was realized in 1991 by E. Yablonovich, T. J. Gmitter and K.M. Leung at the Bell Communications Research. It consisted of a block of plastic into which a series of holes were drilled in three different directions, an arrangement that gave rise to a true omni-directional photonic band gap. By now, photonic crystals provide a versatile platform for controlling the flow of light in a range of applications.
Experiments covered this week:
1972 Bell test
1987 HOM test
1990s Photonic crystals
Next week Congcong Huang and Nicky Dean will write about negative refraction with metamaterials and the first light from a hard X-ray free electron laser, among others.