  Faithful storage and coherent manipulation of quantum optical pulses are key
for long distance quantum communications and quantum computing. Combining these
functions in a light-matter interface that can be integrated on-chip with other
photonic quantum technologies, e.g. sources of entangled photons, is an
important step towards these applications. To date there have only been a few
demonstrations of coherent pulse manipulation utilizing optical storage devices
compatible with quantum states, and that only in atomic gas media (making
integration difficult) and with limited capabilities. Here we describe how a
broadband waveguide quantum memory based on the Atomic Frequency Comb (AFC)
protocol can be used as a programmable processor for essentially arbitrary
spectral and temporal manipulations of individual quantum optical pulses. Using
weak coherent optical pulses at the few photon level, we experimentally
demonstrate sequencing, time-to-frequency multiplexing and demultiplexing,
splitting, interfering, temporal and spectral filtering, compressing and
stretching as well as selective delaying. Our integrated light-matter interface
offers high-rate, robust and easily configurable manipulation of quantum
optical pulses and brings fully practical optical quantum devices one step
closer to reality. Furthermore, as the AFC protocol is suitable for storage of
intense light pulses, our processor may also find applications in classical
communications.
