  Quantum technology is poised to enable a step change in human capability for
computing, communications and sensing. Photons are indispensable as carriers of
quantum information - they travel at the fastest possible speed and readily
protected from decoherence. However, the system requires thousands of
near-transparent components with ultra-low-latency control. For quantum
technology to be implemented, a new paradigm photonic system is required: one
with in-built coherence, stability, the ability to define arbitrary circuits,
and a path to manufacturability. Silicon photonics has unparalleled density and
component performance, which, with CMOS compatible fabrication, place it in a
strong position for a scalable quantum photonics platform. This paper is a
progress report on silicon quantum photonics, focused on developments in the
past five years. We provide an introduction on silicon quantum photonic
component and the challenges in the field, summarise the current
state-of-the-art and identify outstanding technical challenges, as well as
promising avenues of future research. We also resolve a conflict in the
definition of Hong-Ou-Mandel interference visibility in integrated quantum
photonic experiments, needed for fair comparison of photon quality across
different platforms. Our aim is the development of scalability on the platform,
to which end we point the way to ever-closer integration, toward silicon
quantum photonic systems-on-a-chip.
