Waving, one by one

  OpticsQuantum Mechanics

Bloch oscillations occur when electrons are driven by an external field through a periodic potential, such as that of a crystal. The phenomenon is usually interpreted in terms of the electron wave Bragg scattering off the periodic potential, causing the electron to oscillate rather than translate through the lattice. The optical analog of this effect is described in a classical picture as electromagnetic waves bouncing off a periodic structure. Writing in Physical Review Letters, Stefano Longhi of Politecnico di Milano in Italy has now formulated a fully quantum mechanical theory showing that nonclassical light consisting of only particle-like quanta can also produce optical Bloch oscillations.

Longhi considers photonic structures, where the spatial structure of the index of refraction of the material creates energy bands and band gaps analogous to those for electrons in solids, and applies a fully quantized photon field to them. He studies two situations. In the first case, photons interact with a simple singly periodic structure, and the author follows the photon number distribution as a function of distance, finding that the photons undergo Bloch oscillations just like classical particles. In a doubly periodic structure, two photon input states undergo correlated Bloch oscillations, meaning that the photons exhibit bunching and entanglement because the photons can tunnel between two energy bands. Longhi’s results may be applicable to fabricating and analyzing photonic structures useful for achieving more complex entanglements for optical quantum computing. – David Voss