  The scaling up of quantum hardware is the fundamental challenge ahead in
order to realize the disruptive potential of quantum technology in information
science. Among the plethora of hardware platforms, photonics stands out by
offering a modular approach, where the main challenge is to construct
sufficiently high-quality building blocks and develop methods to efficiently
interface them. Importantly, the subsequent scaling-up will make full use of
the mature integrated photonic technology provided by photonic foundry
infrastructure to produce small foot-print quantum processors of immense
complexity. A fully coherent and deterministic photon-emitter interface is a
key enabler of quantum photonics, and can today be realized with solid-state
quantum emitters with specifications reaching the quantitative benchmark
referred to as Quantum Advantage. This light-matter interaction primer realizes
a range of quantum photonic resources and functionalities, including on-demand
single-photon and multi-photon entanglement sources, and photon-photon
nonlinear quantum gates. We will present the current state-of-the-art in
single-photon quantum hardware and the main photonic building blocks required
in order to scale up. Furthermore, we will point out specific promising
applications of the hardware building blocks within quantum communication and
photonic quantum computing, laying out the road ahead for quantum photonics
applications that could offer a genuine quantum advantage.
