Quantum information science has shown that quantum mechanical effects can dramatically improve performance for certain tasks in communication, computation and measurement. Of the various physical systems being pursued, single particles of light – photons – have been widely used in quantum communication, quantum metrology, and quantum lithography settings. Low noise (or decoherence) also makes photons attractive quantum bits (or qubits), and they have emerged as a leading approach to quantum information processing [1,2].
In addition to single photon sources and detectors, photonic quantum technologies require sophisticated optical circuits involving high-visibility classical and quantum interference with photons. While a number of photonic quantum circuits have been realized for quantum metrology , quantum lithography, and quantum logic gates . These demonstrations have relied on large-scale (bulk) optical elements bolted to large optical tables, thereby making them inherently unscalable.
Quantum technologies based on photons will likely require an integrated optics architecture for improved performance, miniaturization and scalability. We demonstrate high-fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference with a visibility of 94.8(5)%; a controlled-NOT gate with an average logical basis fidelity of 94.3(2)%; and a path entangled state of two photons, relevant to quantum metrology, with fidelity >92% .The monolithic nature of these devices means that the correct phase can be stably realized in what would otherwise be an unstable interferometer, greatly simplifying the task of implementing sophisticated photonic quantum circuits. We fabricated 100's of devices on a single wafer and find that performance across the devices is robust, repeatable and well understood. We have also demonstrated an all optical fibre CNOT gate .
These results show that it is possible to directly “write” sophisticated photonic quantum circuits onto a silicon chip, which will be of benefit to future quantum technologies based on photons, as well as the fundamental science of quantum optics.
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