Generation and tomography of arbitrary optical qubits using transient collective atomic excitations.

We demonstrate the preparation of heralded Fock-basis qubits (a|0〉+b|1〉) from transient collective spin excitations in a hot atomic vapor. The preparation event is heralded by Raman-scattered photons in a four-wave mixing process seeded by a weak coherent optical excitation. The amplitude and phase of the seed field allow arbitrary control over the qubit coefficients. The qubit state is characterized using balanced homodyne tomography.