Single-cycle THz-field electro-optical sampling with single-photon detectors

Terahertz time-domain spectroscopy (THz-TDS) using electro-optic sampling and ultrashort pulsed probes is a well-established technique for directly measuring the electric field of THz radiation. Traditionally, a balanced detection scheme relies on measuring an optical phase shift brought by THz-induced birefringence radiation using photodiodes, where sensitivity is limited by the shot-noise of the optical-sampling probe. Improvement to the sensitivity of such an approach could be achieved by applying quantum metrology, such as using NOON states for Heisenberg-limited phase estimation. We report on the first step in that direction, demonstrating that THz electric fields can be measured with single-photon detectors using a squeezed vacuum as the optical probe. Our approach achieves THz electro-optical sampling using phase-locked single-photon detectors at the shot-noise limit and thus paves the way toward quantum-enhanced THz sensing.