Spotlight on quantum mechanics Some experiments on light particles uncover exceptional properties, such as the duality of the photon. Others point to the potential for miniaturization of photonics, making it a credible alternative to electronics. Wave-particle duality pushed to the limit ------------------------------------------------------------------------------------------------------------------------------------ In a naive view of quantum mechanics, a quantum object behaves either like a wave or a particle, depending on the experimental set-up used to observe it. However, researchers have now detected photons in a state where there is a superposition of their wavelike and particle-like properties. They did this by carrying out a highly sophisticated version of the experiment that is typically used to test wave-particle duality. In this experiment, a single photon is sent into an interferometer. If the interferometer is open, the particle takes either of the two possible paths between entering and exiting the device. If it is closed, the particle moves along both paths at the same time. In the first case, the photon shows particle-like behavior, whereas in the second, it behaves like a wave. By using entangled particles – in other words, particles whose quantum properties are inseparable – the researchers succeeded in creating situations where the photon takes either of the two paths, while at the same time following both! In this case, the photon shows both particle- and wave-like behavior, suggesting that it should not be seen as one or the other, but rather as an irreducible quantum object. Science November 2012 42 Towards photonic miniaturization In the future, photonics may well replace electronics, but only if the functions of electronics can be performed with light and in miniaturized devices. With this goal in mind, a group of physicists has succeeded in integrating especially dense light excitations known as surface plasmons into conventional photonic circuits via coupling with a silicon waveguide. These tiny elementary building blocks may make it possible to achieve optical functions that could be downsized by a factor of 50-100. Nano Letters January 2012 online Cold atoms do a U-turn Atoms moving in a disordered light field rapidly “forget” their initial direction of propagation. However, this no longer applies when the atoms are cooled to a temperature close to absolute zero, at which point they behave like matter waves. Physicists have now shown that interference between waves moving in opposite directions leads to greater scattering of the atoms in the opposite direction to that of initial propagation, rather as though quantum effects could to some extent offset those of disorder. Physical Review Letters November 2012 Artist’s impression of the behavior of single photons sent into an interferometer closed by a quantum beam splitter. Observation of the coherent backscattering of ultracold atoms launched in a disordered potential. 1 0,4 0,1 0,02 0,02 0 ms pz py 0 pi 0,5 ms 1 ms 1,5 ms 2 ms 2,5 ms 0,02 A year at CNRS 2012
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