Subcycle quantum electrodynamics

Squeezed states(1-4) of electromagnetic radiation have quantum fluctuations below those of the vacuum field. They offer a unique resource for quantum information systems(5) and precision metrology(6), including gravitational wave detectors, which require unprecedented sensitivity(7). Since the first experiments on this non-classical form of light(8,9), quantum analysis has been based on homodyning techniques and photon correlation measurements(10,11). These methods currently function in the visible to near-infrared and microwave(12) spectral ranges. They require a well-defined carrier frequency, and photons contained in a quantum state need to be absorbed or amplified. Quantum non-demolition experiments(13,14) may be performed to avoid the influence of a measurement in one quadrature, but this procedure comes at the expense of increased uncertainty in another quadrature. Here we generate mid-infrared time-locked patterns of squeezed vacuum noise. After propagation through free space, the quantum fluctuations of the electric field are studied in the time domain using electro-optic sampling with few-femtosecond laser pulses(15,16). We directly compare the local noise amplitude to that of bare (that is, unperturbed) vacuum. Our nonlinear approach operates off resonance and, unlike homodyning or photon correlation techniques, without absorption or amplification of the field that is investigated. We find subcycle intervals with noise levels that are substantially less than the amplitude of the vacuum field. As a consequence, there are enhanced fluctuations in adjacent time intervals, owing to Heisenberg's uncertainty principle, which indicate generation of highly correlated quantum radiation. Together with efforts in the far infrared(17,18), this work enables the study of elementary quantum dynamics of light and matter in an energy range at the boundary between vacuum and thermal background conditions.