Quantum information science aims to harness uniquely quantum mechanical properties to enhance measurement and information technologies, and to explore fundamental aspects of quantum physics. Of the various approaches to quantum computing , photons are particularly appealing for their low-noise properties and ease of manipulation at the single qubit level . Encoding quantum information in photons is also an appealing approach to quantum communication, metrology (eg. ), measurement (eg. ) and other quantum technologies . However, the implementation of optical quantum circuits with bulk optics has reached practical limits. We have developed an integrated waveguide approach to photonic quantum circuits for high performance, miniaturisation and scalability . Here we report high-fidelity silica-on-silicon integrated optical realisations of key quantum photonic circuits, including two-photon quantum interference and a controlled-NOT logic gate . We have demonstrated controlled manipulation of up to four photons on-chip, including high-fidelity single qubit operations, using a lithographically patterned resistive phase shifter . We have used this architecture to implement a small-scale compiled version of Shorʼs quantum factoring algorithm  and demonstrated heralded generation of tuneable four photon entangled states from a six photon input . We have combined waveguide photonic circuits with superconducting single photon detectors . We describe complex quantum interference behaviour in multi-mode interference devices with up to eight inputs and outputs , and quantum walks of correlated particles in arrays of coupled waveguides . Finally, we give an overview of our recent work on fundamental aspects of quantum measurement [14,15] and single photon sources [16,17].
© 2011 Optical Society of AmericaPDF Article