  The Kerr nonlinearity can be a key enabler for many digital photonic circuits
as it allows access to bistable states needed for all-optical memories and
switches. A common technique is to use the Kerr shift to control the resonance
frequency of a resonator and use it as a bistable, optically-tunable filter.
However, this approach works only in a narrow power and frequency range or
requires the use of an auxiliary laser. An alternative approach is to use the
asymmetric bistability between counterpropagating light states resulting from
the interplay between self- and cross-phase modulation, which allows light to
enter a ring resonator in just one direction. Logical HIGH and LOW states can
be represented and stored as the direction of circulation of light, and
controlled by modulating the input power. Here we study the switching speed,
operating laser frequency and power range, and contrast ratio of such a device.
We reach a bitrate of 2 Mbps in our proof-of-principle device over an optical
frequency range of 1 GHz and an operating power range covering more than one
order of magnitude. We also calculate that integrated photonic circuits could
exhibit bitrates of the order of Gbps, paving the way for the realization of
robust and simple all-optical memories, switches, routers and logic gates that
can operate at a single laser frequency with no additional electrical power.
