A phase sensitive SIS mixer to circumvent the quantum limit
A conventional superconductor-insulator-superconductor (SIS) mixer, that is pumped by a single-frequency local oscillator (LO) functions as a phase-insensitive linear amplifier and may achieve a noise level near the quantum limit of one-half photon added noise per unit bandwidth. A phase-sensitive linear amplifier may have less noise than this quantum limit. To overcome the quantum noise limit with an SIS mixer, a two-LO technique which makes the mixer's gain dependent upon the phase of the incoming signal was used. The phase sensitive gain of this two-LO mixer is experimentally demonstrated. The gain variation from maximum to minimum is more than 20 dB. Theoretical predictions of the noise of the two-LO mixer are presented.