Attosecond Access to the Quantum Noise of Light

Characterizing the quantum state of intense light fields on sub-cycle timescales remains beyond the reach of existing methods. Here, we show that attosecond streaking provides direct, phase-sensitive access to the quantum properties of the driving field through delay-resolved photoelectron spectra. Using a Feynman--Vernon treatment, we decompose the influence of the quantized driving field on the photoelectron into coherent and fluctuation contributions. This yields a simple, moment-based characterization of the light state: the first moment of the photoelectron momentum distribution reveals the coherent displacement, while the second central moment captures the fluctuation contribution and, for squeezed states, exhibits a clear modulation at twice the driving frequency, directly signaling phase-sensitive quantum noise. Time-dependent Schr\"odinger equation simulations confirm these relations and enable retrieval of the coherent phase, the squeezing phase, and the relative strengths of the coherent and fluctuation contributions from delay-resolved spectra. Taken together, these results establish attosecond streaking as a route to sub-cycle quantum-optical metrology in the strong-field regime.