Control of coherent backscattering of light by breaking optical reciprocity

Reciprocity is a universal principle that has a profound impact on
many areas of physics.  A fundamental phenomenon in condensed-matter
physics, optical physics and acoustics, arising from reciprocity, is
the constructive interference of waves which propagate along
time-reversed paths in disordered media, leading to, for example, weak
localization and the metal-insulator transition. Previous studies have
shown that such coherent effects are suppressed when reciprocity is
broken. In my talk I will present our recent experiment, in which we
have shown that by tuning a non-reciprocal phase we can coherently
control weak localization of light, also known as coherent
backscattering, rather than simply suppress it [1]. By utilizing a
magneto-optical effect, we controlled the interference between
time-reversed paths inside a multimode fiber with strong mode mixing,
observed for the first time the optical analogue of weak
anti-localization, and realized a continuous transition from weak
localization to anti-localization. In the last part of the talk I will
show how we can utilize the subtle interplay between reciprocity and
mode mixing in multimode fibers for secure optical communication [2].

[1] Y. Bromberg, B. Redding, S. M. Popoff, and H. Cao, Control of
coherent backscattering by breaking optical reciprocity,
arXiv:1505.01507.
[2] Y. Bromberg, B. Redding, S. M. Popoff, and H. Cao, Remote key
establishment by mode mixing in multimode fibers and optical
reciprocity, arXiv:1506.07892.