  We propose and analyze a new time-domain method for subcycle metrology of
quantum electric fields using a combination of a 3rd order nonlinear optical
process and homodyne detection with a local oscillator (LO) field. The new
method enables isolation of intrinsically weak quantum noise contribution by
subtraction of the shot noise of the LO on a pulse-by-pulse basis. Together
with the centro-symmetric character of the nonlinearity, our method unlocks
novel opportunities toward terahertz and mid-infrared quantum field
metrologies.
