  Motivated by developments in quantum information science, much recent effort
has been directed toward coupling individual quantum emitters to optical
microcavities. Such systems can be used to produce single photons on demand,
enable nonlinear optical switching at a single photon level, and implement
functional nodes of a quantum network, where the emitters serve as processing
nodes and photons are used for long-distance quantum communication. For many of
these practical applications, it is important to develop techniques that allow
one to generate outgoing single photons of desired frequency and bandwidth,
enabling hybrid networks connecting different types of emitters and
long-distance transmission over telecommunications wavelengths. Here, we
propose a novel approach that makes use of a nonlinear optical resonator, in
which the single photon originating from the atom-like emitter is directly
converted into a photon with desired frequency and bandwidth using the
intracavity nonlinearity. As specific examples, we discuss a high-finesse,
TE-TM double-mode photonic crystal cavity design that allows for direct
generation of single photons at telecom wavelengths starting from an InAs/GaAs
quantum dot with a 950 nm transition wavelength, and a scheme for direct
optical coupling of such a quantum dot with a diamond nitrogen-vacancy center
at 637 nm.
