  Knill, Laflamme, and Milburn [Nature 409, 46 (2001)] showed that linear
optics techniques could be used to implement a nonlinear sign gate. They also
showed that two of their nonlinear sign gates could be combined to implement a
controlled-phase gate, which has a number of practical applications. Here we
describe an alternative implementation of a controlled-phase gate that only
requires the use of a single nonlinear sign gate. This gives a much higher
average probability of success when the required ancilla photons are generated
using heralding techniques. This implementation of a controlled-phase gate
destroys the control qubit, which is acceptable in a number of applications
where the control qubit would have been destroyed in any event, such as in a
postselection process.
