Definition
Plain language
A clean, signed signal pulled out of light measurements that captures how two beams interact.
As stated in the literature
The complex-valued bilinear cross-term isolated via four-phase demodulation in coherent optical setups, separating it from the magnitude-squared terms of each input.
Also called: Complex-B field
Why it matters: It lets analog optical hardware perform signed multiplications cleanly, despite detectors that fundamentally only measure positive intensities.
For example, by stepping the laser's phase through four settings and combining the readings, an optical computer extracts just the part of the signal that depends on both input beams together.