Integrated-optics heralded controlled-NOT gate for polarization-encoded qubits.

Recent progress in integrated-optics technology has made photonics a promising platform for quantum networks and quantum computation protocols. Integrated optical circuits are characterized by small device footprints and unrivalled intrinsic interferometric stability. Here, we take advantage of femtosecond-laser-written waveguides' ability to process polarization-encoded qubits and present an implementation of a heralded controlled-NOT gate on chip. We evaluate the gate performance in the computational basis and a superposition basis, showing that the gate can create polarization entanglement between two photons. Transmission through the integrated device is optimized using thermally expanded core fibers and adiabatically reduced mode-field diameters at the waveguide facets. This demonstration underlines the feasibility of integrated quantum gates for all-optical quantum networks and quantum repeaters. Quantum optics: On-chip quantum logic An on-chip quantum logic gate signals when it is successful so its output can be fed into further calculations. Photons are weakly interacting, making them good qubits but less able to perform operations where the state of one photon affects another. The properties of quantum measurements can be used to implement two-photon gates at the expense of making them random and sometimes causing failure. Jonas Zeuner at the University of Vienna and collaborators in Germany and Austria have implemented an integrated-optics device with a quantum two-photon gate that records the outcome in two 'herald' photons. These can be measured to determine the gate's outcome so later operations can be chosen appropriately. Combining this device with high performance single photon gates, sources and detectors may enable a modular, scalable approach to optical quantum computing. [ABSTRACT FROM AUTHOR]