Measurement based nonlinear absorption for photonic QIP
Jeremy O'Brien
University of Bristol, United Kingdon
Abstract: In practice it is extremely difficult to make one photon coherently influence the state of another. The optical nonlinearities required are orders of magnitude beyond those commonly achieved in current photonic technology. Fortunately, strong effective nonlinearities can be induced in linear optical systems by combining quantum interference and projective measurement, opening the possibility of scalable linear-optical quantum computation. Such measurement-induced nonlinearities have had high impact in quantum information and optics, notably in optical quantum logic gate experiments and in exotic state production.
Most of these schemes achieve an effective nonlinearity via the lowest-order nonclassical interference, with one photon per mode input to a beamsplitter. Higher-order nonclassical interference, where more than one photon is allowed per mode, enables additional control over the quantum state. A single ancilla photon has been used to conditionally control the phase of a two-photon path entangled state, and to conditionally absorb either one-photon or two-photon input states. Applied to a superposition state, higher-order interference is predicted to act as a Fock-state filter, conditionally absorbing only terms with a specified number of photons. Here, we prove that this conditional absorption is coherent by applying it to a quantum superposition, and experimentally generating a path-entangled state [1]. We quantify the degree of entanglement by transforming the path information to polarisation information, and applying quantum state tomography.