  The ability to generate complex optical photon states involving entanglement
between multiple optical modes is not only critical to advancing our
understanding of quantum mechanics but will play a key role in generating many
applications in quantum technologies. These include quantum communications,
computation, imaging, microscopy and many other novel technologies that are
constantly being proposed. However, approaches to generating parallel multiple,
customisable bi- and multi-entangled quantum bits (qubits) on a chip are still
in the early stages of development. Here, we review recent developments in the
realisation of integrated sources of photonic quantum states, focusing on
approaches based on nonlinear optics that are compatible with contemporary
optical fibre telecommunications and quantum memory infrastructures as well as
with chip-scale semiconductor technology. These new and exciting platforms hold
the promise of compact, low-cost, scalable and practical implementations of
sources for the generation and manipulation of complex quantum optical states
on a chip, which will play a major role in bringing quantum technologies out of
the laboratory and into the real world.
