Entanglement is the quintessential quantum mechanical phenomenon understood to lie at the heart of future quantum technologies and the subject of fundamental scientific investigations. Mixture, resulting from noise, is often an unwanted result of interaction with an environment, but is also of fundamental interest, and is proposed to play a role in some biological processes. Here we report an integrated waveguide device that can generate and completely characterize pure two-photon states with any amount of entanglement and arbitrary single-photon states with any amount of mixture. The device consists of a reconfigurable integrated quantum photonic circuit with eight voltage controlled phase shifters. We demonstrate that for thousands of randomly chosen configurations the device performs with high fidelity. We generate maximally and non-maximally entangled states, violate a Bell-type inequality with a continuum of partially entangled states, and demonstrate generation of arbitrary one-qubit mixed states.
Until recently, quantum photonic architecture comprised of large-scale (bulk) optical elements, leading to severe limitations in miniaturization, scalability and stability. The development of the first integrated quantum optical circuitry removes this bottleneck and allows realization of quantum optical schemes whose greatly increased capacity for circuit complexity is crucial to the progress of experimental quantum information science and the development of practical quantum technologies.Integrated quantum photonic circuits within Silica-on-Silicon waveguide chips were simulated, designed and tested. Hundreds of devices have been fabricated with the core components found to be robust and highly repeatable. Amongst these demonstrations, all the basic components required for quantum information applications are shown. The first integrated quantum metrology experiments are demonstrated by beating the standard quantum limit with twoand four-photon entangled states while providing the first re-configurable integrated quantum circuit capable of adaptively controlling levels of non-classical interference of photons. The tested integrated devices show no limitations to obtain high quality performances. It is reported near-unity visibility of two-photon non-classical interference and a Controlled-NOT gate that could in principle work in the fault tolerant regime.It is demonstrated the realization of a compiled version of Shors quantum factoring algorithm on an integrated waveguide chip. This demonstration serves as an illustration to the importance of using integrated optics for quantum optical experiments.
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We report a reconfigurable integrated photonic circuit with eight phase shifters. This device can generate and characterise entangled states, violate a Bell-type inequality with a continuum of partially entangled states, and generate one-qubit mixed states.The past forty years have seen very significant advances in the control of quantum systems. Much of this progress has been motivated by the remarkable discovery that quantum mechanics allows fundamentally new modes of computation 1,2 , simulation 3,4 and communication 5 , as well as enhanced methods of measurement and sensing 6,7 . Of the many prospective physical systems with which to encode quantum information, photons provide a particularly promising choice due to their properties of low noise, high speed transmission, and ease of single-qubit state manipulation.The majority of photonic quantum information experiments to date have been been implemented using bulk optical elements, with photons propagating in free space. Such experiments rapidly become impractically large as the complexity of the circuit is increased,and interferometric stability becomes increasingly difficult to maintain. Recently it has been shown that it is possible to miniaturize a subset of quantum optical circuits using optical fibre 8,9 and integrated waveguide chips 10-13 . Integrated waveguide devices are inherently stable and can be made orders of magnitude smaller than their bulk optical equivalents.Here we report a highly reconfigurable integrated quantum photonic circuit which can generate two-qubit entangled pure states with any amount of entanglement and single-qubit states with any amounts of mixture, perform two-qubit state tomography and process tomography, and implement the measurements necessary for a Bell violation experiment.The device described here is the silica-on-silicon waveguide circuit shown in Fig. 1. The circuit consists of a reconfigurable state preparation stage, which can prepare any separable two-qubit pure state (up to a global phase), a postselected two-qubit entangling gate, and a reconfigurable measurement stage, which together with measurement in the logical basis can perform arbitrary qubit measurement on each of the two outputs of the entangling gate.Two path-encoded qubits are prepared at the input of the device, using pairs of photons from a type-I down conversion source, in the |00 state -i.e. one photon in each upper waveguide. The preparation and measurement stages of the chip are each implemented via two Mach-Zehnder interferometers, each consisting of two directional couplers 11 and two voltage-controlled thermal phase shifters 11 . The central part of the circuit implements the maximally entangling cnot operation. The cnot gate is a postselected linear optical gate consisting of five directional couplers, whose operation depends FIG. 1: Waveguide architecture of the reconfigurable entangling circuit. The circuit is composed of Hadamardlike gates H , implemented using directional couplers, and Rz(φ) = e −iφσz /2 rotations, implemented using vol...
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