The idea of energy in the vacuum of quantum field theory is familiar, at least to physicists. Now it has been imaged in 3D for the first time. Moonjoo Lee of Seoul National University and colleagues used single barium atoms as a spontaneous-emission probe for the vacuum energy density inside high-Q microcavities that were built as a 2D array of 170 nm holes in a 75-nm-thick silicon-nitride membrane.

Excited atoms sent into a cavity emitted a photon with a probability that was proportional to the vacuum field-intensity at the cavity’s position. A measurement of transit-time broadening then gave the distribution of the vacuum energy in the transverse directions, while the spectrum of the emitted photons gave it along the atom’s flight path. This allowed for the construction of a full 3D image of the vacuum energy density in the cavity. The team could also measure the amplitude of the vacuum field in the cavity, which was as large as 1 V/cm, therefore putting a value on the emptiness of "empty space".