  Chirality is an important concept that describes the asymmetry property of a
system, which usually emerges spontaneously due to mirror symmetry breaking.
Such spontaneous chirality manifests predominantly as parity breaking in modern
physics, which has been studied extensively, for instance, in Higgs physics,
double-well Bose-Einstein condensates, topological insulators and
superconductors. In the optical domain, spontaneous chiral symmetry breaking
has been elusive experimentally, especially for micro- and nano-photonics which
demands multiple identical subsystems, such as photonic nanocavities,
meta-molecules and other dual-core settings. Here, for the first time, we
observe spontaneous emergence of a chiral field in a single ultrahigh-Q
whispering- gallery microresonator. This counter-intuitive effect arises due to
the inherent Kerr nonlinearity-modulated coupling between clockwise (CW) and
counterclockwise (CCW) propagating waves. At an ultra-weak input threshold of a
few hundred microwatts, the initial chiral symmetry is broken spontaneously,
and the CW-to-CCW output ratio reaches 20:1. The spontaneous chirality in
microresonator holds great potential in studies of fundamental physics and
applied photonic devices.
