Imaging the vacuum field in a high-Q cavity with single atoms as a nanoscopic probe

When an excited atom is placed in a high-Q cavity, it is driven by the cavity vacuum field at its position and emits a photon into the cavity mode. Based on this principle, we have performed 3D imaging of the vacuum field in a Fabry-Perot cavity. Atomic position localization is achieved by using a nanohole-array atom-beam aperture scannable in front of the cavity mode. The transit-time broadening in the atom-cavity detuning curve provides the vacuum field profile along the direction of atomic beam. In this way the 3D profile of the vacuum-field intensity is imaged with a spatial resolution of about 170 nm, which is mostly limited by the nanohole diameter.