Photonic two-qubit parity gate with tiny cross-Kerr nonlinearity

Details Photonic two-qubit parity gate with tiny cross-Kerr nonlinearity Photonic two-qubit parity gate with tiny cross-Kerr nonlinearity

2011-12-28

arxiv.org/abs/1112.6135v1

Physics

Quantum Physics

6 pages, 3 figures; comments are welcome

Scientific paper

The cross-Kerr nonlinear effect can induce effective photon interactions in principle, with which photonic multiqubit gates can be constructed using far fewer physical resources than linear optical schemes. Unfortunately, it is extremely challenging to generate the required giant cross-Kerr nonlinearities (XKNLs). In recent years, much effort has been made to create multiqubit gates via weak XKNLs. However, the required nonlinearity strengths are still difficult to achieve in experiment. We here propose a scheme for realizing a two-qubit parity gate, a universal two-qubit gate, using a several orders of magnitude weaker XKNL than previous schemes. The scheme employs a cross-Kerr medium to couple the signal mode (one of the two spatial modes of the two photons after passing through a polarizing beam splitter) with a ring cavity that is constituted by two mirrors and two beam splitters (BSs) and fed by a coherent-state beam, and to make the cavity field pick up a phase shift dependent on the number of photons in the signal mode. These potential phase shifts can then be measured using photon-number resolving (PNR) detectors at the output ports of the cavity, which reveals nondestructively the parity of the two qubits. The small value of the cross-phase modulation is compensated by both the amplitude of the coherent state and the reciprocal of the transmissivity of the BSs, which accounts for the fact that our scheme can work in the tiny XKNL regime. The scheme is also robust against imperfections of the PNR detectors.

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