  A scheme for heralded generation of frequency-bin photonic qubits via
spontaneous four-wave mixing in a system of coupled microring resonators
(photonic molecule) is developed so that the qubit state is fully controlled by
the frequency amplitude of the pump field. It is shown that coupling parameters
of the photonic molecule can be optimized to provide generation of nearly pure
heralded photons. The heralded qubit state does not depend on the microring
resonator mode frequency, thereby allowing to take advantage of frequency
multiplexing for improving efficiency of quantum information processing.
