While the phase of a coherent light field can be precisely known, the phase of the individual photons that create this field, considered individually, cannot. Phase changes within single-photon wave packets, however, have observable effects. In fact,
actively controlling the phase of individual photons has been identified as a powerful resource for quantum communication protocols. Here we demonstrate the arbitrary phase control of a single photon. The phase modulation is applied without affecting the photons amplitude profile and is verified via a two-photon quantum interference measurement, which can result in the fermionic spatial behaviour of photon pairs. Combined with previously demonstrated control of a single photons amplitude, frequency, and polarisation, the fully deterministic phase shaping presented here allows for the complete control of single-photon wave packets.
An experiment is performed where a single rubidium atom trapped within a high-finesse optical cavity emits two independently triggered entangled photons. The entanglement is mediated by the atom and is characterized both by a Bell inequality violatio
n of S=2.5, as well as full quantum-state tomography, resulting in a fidelity exceeding F=90%. The combination of cavity-QED and trapped atom techniques makes our protocol inherently deterministic - an essential step for the generation of scalable entanglement between the nodes of a distributed quantum network.
