  Astrophotonics is a burgeoning field that lies at the interface of photonics
and modern astronomical instrumentation. Here we provide a pedagogical review
of basic photonic functions that enable modern instruments, and give an
overview of recent and future applications. Traditionally, optical fibres have
been used in innovative ways to vastly increase the multiplex advantage of an
astronomical instrument, e.g. the ability to observe hundreds or thousands of
stars simultaneously. But modern instruments are using many new photonic
functions, some emerging from the telecom industry, and others specific to the
demands of adaptive optics systems on modern telescopes. As telescopes continue
to increase in size, we look to a future where instruments exploit the
properties of individual photons. In particular, we envisage telescopes and
interferometers that build on international developments in quantum networks,
the so-called quantum internet. With the aid of entangled photons and quantum
logic gates, the new infrastructures seek to preserve the photonic state and
timing of individual photons over a coherent network.
