Success of precision measurements often depends on the use of amplifiers. The sensitivity of these measurements is, therefore, limited by the noise that the amplifier adds to the signal. For electronic signals, the noise floor is set by thermal fluctuations. At optical frequencies, however, the thermal noise gets negligibly small. In this case, the noise floor of a phase-insensitive amplifier (PIA)—a linear amplifier whose gain does not depend on the signal phase—is determined by a fundamental quantum limit, which arises ultimately from zero-point field fluctuation.