Integrated quantum photonics

Integrated quantum photonics, uses photonic integrated circuits to control photonic quantum states for applications in quantum technologies. As such, integrated quantum photonics provides a promising approach to the miniaturisation and scaling up of optical quantum circuits. The major application of integrated quantum photonics is Quantum technology:, for example quantum computing, quantum communication, quantum simulation, quantum walks and quantum metrology. Linear optics was not seen as a potential technology platform for quantum computation until the seminal work of Knill, Laflamme, and Milburn, which demonstrated the feasibility of linear optical quantum computers using detection and feed-forward to produce deterministic two-qubit gates. Following this there were several experimental proof-of-principle demonstrations of two-qubit gates performed in bulk optics. It soon became clear that integrated optics could provide a powerful enabling technology for this emerging field. Early experiments in integrated optics demonstrated the feasibility of the field via demonstrations of high-visibility non-classical and classical interference. Typically, linear optical components such as directional couplers (which act as beamsplitters between waveguide modes), and phase shifters to form nested Mach–Zehnder interferometers are used to encode qubit in the spatial degree of freedom. That is, a single photon is in super position between two waveguides, where the zero and one state of the qubit correspond to the photon's presence in one or the other waveguide. These basic components are combined to produce more complex structures, such as entangling gates and reconfigurable quantum circuits. Reconfigurability is achieved by tuning the phase shifters, which leverage thermo- or electro-optical effects. Another area of research in which integrated optics will prove pivotal in its development is Quantum communication and has been marked by extensive experimental development demonstrating, for example, quantum key distribution (QKD), quantum relays based on entanglement swapping, and quantum repeaters.