Experimental realization of quantum logic gate with photons

Quantum computers promise great improvements in solving problems such as factoring large integers. Using a photon as quantum bit is one of the most promising ways because of the robustness against decoherence. Recently, a scheme for efficient quantum computation using only linear optical elements has been proposed [1]. Using the scheme, we are going to realize the first quantum logic gate operations with photons by the following procedure: (1) Development of a new photon-pair source for high order multi-photon experiment (2) Construction of interferometers for quantum logic gates (3) Demonstration of quantum logic gates with photons The optical quantum logic gates by the scheme require many additional photons (ancilla) for the construction. The actual source to prepare these photons is the spontaneous parametric down-conversion. However, with ordinary nonlinear crystals the source might not be sufficiently intense for higher order multi-photon experiments. To overcome this difficulty, we are planning to use waveguide-type nonlinear devices that have already been used to test some fundamental aspects of quantum theory with a greatly improved efficiency of producing photon pairs by the applicant [2]. The interferometers for the operation of quantum logic will be set up with linear optical elements and the optical modes of the interferometers must be matched in sub-wavelength level for many hours using a reference laser. The technology and the experience of Prof. Zeilinger`s laboratory combined with the applicant`s knowledge about waveguide parametric down-conversion will be a great advantage for the first realization of linear optic quantum logic. [1] K.Knill, R. Laflamme, G.J. Milburn; Nature 409, 46 (2001) [2] K. Sanaka, K. Kawahara, T. Kuga, Phys. Rev. Lett. 86, 5620 (2001)