In one minute

Herter, Lindel, Gabriel, Buhmann and Faist tackle a long-standing ambiguity: whether the Lamb shift, Casimir forces and spontaneous emission come from vacuum fields or source radiation had only been decided by theoretical assumption. Ultrafast optics now allows an experimental analogue of Fermi's two-atom problem with two laser pulses in a nonlinear crystal. They detect correlations between the pulses caused by vacuum fluctuations and by source radiation, separated by their causal properties. The two correlate different quadratures of near-infrared pulses, so phase-sensitive detection probes each separately. The results verify the time-domain fluctuation-dissipation theorem at the quantum level and open the way to entanglement harvesting from the vacuum and field detection in curved-space analogues.

Experimentally separating vacuum fluctuations from source radiation