  Zero-point electromagnetic fields were first introduced to explain the origin
of atomic spontaneous emission. Vacuum fluctuations associated with the
zero-point energy in cavities are now utilized in quantum devices such as
single-photon sources, quantum memories, switches and network nodes. Here we
present three-dimensional (3D) imaging of vacuum fluctuations in a high-Q
cavity based on the measurement of position-dependent emission of single atoms.
Atomic position localization is achieved by using a nanoscale atomic beam
aperture scannable in front of the cavity mode. The 3D structure of the cavity
vacuum is reconstructed from the cavity output. The root mean squared amplitude
of the vacuum field at the antinode is also measured to be 0.92+-0.07V/cm. The
present work utilizing a single atom as a probe for sub-wavelength imaging
demonstrates the utility of nanometre-scale technology in cavity quantum
electrodynamics.
