  A deterministic "on demand" source of single photons is a basic building
block for linear quantum computation \cite{linear}, quantum cryptography
\cite{crypto}, quantum teleportation \cite{teleport}, and quantum networks
\cite{network}. In all these applications, quantum interference between two
single-photon pulses on a symmetric beam splitter has been exploited
\cite{review}, which imposes stringent requirement for the implemented single
photons to be indistinguishable in all degrees of freedom, including their
frequencies, spectral widths, pulse shapes, and polarizations. To generate
single photons one requires a pumping mechanism to excite a "two-level emitter"
and an efficient channeling of the subsequently emitted photons. The efficiency
of the source can be enhanced by coupling the emitter to the waveguide
\cite{manga} or a microcavity mode \cite{cklaw}. However, hitherto the
solid-state single photon sources realized by using a quantum dot coupled with
a microcavity rely on incoherent pumping of excitons, which leads to problems
with timing jitter \cite{kiraz}, leading to a trade off between efficiency and
indistinguishability. Here we introduce a means to realize a highly efficient
solid state source of indistinguishable single photons using cavity-assisted
adiabatic Raman passage in a single quantum dot - cavity system. We demonstrate
pulse-triggered single photons with 100% efficiency and $>90$%
indistinguishability using currently available experimental parameters.
