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This work asks what quantum light looks like at its most fundamental timescale, a single oscillation cycle of a mode. The authors show theoretically that local quantum measurements can reconstruct and visualise a quantum field below one cycle, even when its temporal mode structure is unknown in advance. They describe generating and performing tomography on ultrabroadband squeezed states and on photon-subtracted states derived from them, including single-photon states. Building phase-space distributions from subcycle-resolved tomography, they track how these states evolve in phase space and push temporal resolution to its physical limit. The results lay a cornerstone for time-domain quantum optics.

Subcycle tomography of quantum light