  We theoretically present a method to realize a deterministic photon-photon
(SWAP)^{1/2} gate using a three-level lambda system interacting with single
photons in reflection geometry. The lambda system is used completely passively
as a temporary memory for a photonic qubit; the initial state of the lambda
system may be arbitrary, and active control by auxiliary fields is unnecessary
throughout the gate operations. These distinct merits make this entangling gate
suitable for deterministic and scalable quantum computation.
