Interference of two single photons emitted by two atoms
Antoine Browaeys
Centre National de la Recherche Scientifique, Paris, France.
Abstract: Implementing a two-qubit quantum gate is a key step towards quantum computation. Such gates generally require a strong interaction between the particles that are used to carry the physical qubits. An alternative way to go is to bypass the requirement for a direct interaction between the qubits, and use instead an interference effect between single photons. A single photon source is thus the essential building block of such schemes. This talk will present our implementation of a single photon source consisting of a single rubidium atom trapped at the focal point of a tightly focused laser beam. The atom is excited by laser pulses short with respect to the lifetime of an optical transition. Some of the photons subsequently emitted are collected by a large numerical aperture lens. The intensity correlation function exhibit almost perfect antibunching, a signature of the single photon nature of the source. When two indistinguishable single photons are fed into the two input ports of a beam splitter, the photons will coalesce and leave together from the same output port. This is a quantum interference effect, which occurs because the two possible paths where the photons leave in different output ports interfere destructively. This effect was first observed in parametric down-conversion by Mandel in 1987. With the recent development of quantum information, a lot of attention has been devoted to this coalescence effect as a resource for entangling two atoms. We duplicated our single photon source by trapping a second atom in a second dipole trap and exciting the two atoms by the same laser beam. We have observed the coalescence of two single photons emitted by these sources on a beam splitter. Our data analysis shows that the coalescence observed is mostly limited by the wavefront matching of the light emitted by the two atoms, and to a lesser extent by the motion of each atom in its own trap.