Quantum logic using photon-exchange interactions

Summary form only given. Nonlinear optical effects at the two-photon level are usually negligible because of the weak electric field associated with a single photon. In addition, conventional mechanisms for the production of nonlinear phase shifts require both photons to interact with the same atom, which is unlikely to occur in an ordinary medium containing a large number of atoms. Although nonlinear phase shifts of this kind can be enhanced using cavity QED or electromagnetically induced transparency (EIT), we are investigating a new approach that is based on the use of photon-exchange interactions. In this approach, a state containing two excited atoms in the medium could have been produced in two different ways: Photon 1 may have excited atom A while photon 2 excited atom B, or photon 1 may have excited atom B while photon 2 excited atom A. Under the appropriate conditions, constructive interference between these two processes can produce a factor of two enhancement in the probability of there being two excited atoms, which in turn can produce a nonlinear phase shift if a suitable sequence of laser pulses is applied to the medium while the photons are contained within it.