Generation of photonic superposition states by two-photon absorption

Pure superposition states obtained with two-level atoms, photons, nuclear spins, etc. are of great importance in quantum computing and quantum cryptography. The physical realization of photon qubits (quantum bits) is partially realized with the aid of two-photon absorption, which gives rise to strong coherence of the off-diagonal matrix element between the vacuum and the one-photon state. Although a true phase state is not obtained, nevertheless, the mixed state produced is rather close to being a pure state.