  In this paper, we propose a method for building a two-qubit gate with the
Jaynes-Cummings model (JCM). In our scheme, we construct a qubit from a pair of
optical paths where a photon is running. Generating Knill, Laflamme and
Milburn's nonlinear sign-shift gate by the JCM, we construct the conditional
sign-flip gate, which works with small error probability in principle. We also
discuss two experimental setups for realizing our scheme. In the first
experimental setup, we make use of coherent lights to examine whether or not
our scheme works. In the second experimental setup, an optical loop circuit
made out of the polarizing beam splitter and the Pockels cell takes an
important role in the cavity.
