Figure 2: The noise levels for the NCI.

(a,b) Graphic representation for (a) correlated amplification (PA) with quantum interference or (b) uncorrelated amplification (UA) with no interference. Solid circles represent the noise with colours corresponding to the traces of the same colours in (c). The sizes of the circles correspond to the noise levels in (c) (not to scale). In (a), a coherent state at vacuum noise level (red) is input to the NCI together with the vacuum state at the unused input port (dashed line) and then amplified by PA1 (green and dark brown) but quantum destructive interference at PA2 brings the noise level back close to shot noise level (light blue). However in (b), without the interference, the noise level is further amplified by PA2 (yellow). (c) Measured noise levels at zero frequency span at the output: the dark blue trace is the noise level when the phase φ of the NCI is scanned; the yellow trace is the noise level when one arm of the NCI is blocked; the green and dark brown traces are the noise levels (noise gain) for individual amplifiers with vacuum input; the light blue trace is the noise level at dark fringe (φ=π) of the NCI; the red trace is the vacuum noise level, which is also the noise level for a CI at dark fringe. The centre frequency=1 MHz, frequency span=0, resolution bandwidth=100 kHz, video bandwidth=100 Hz. The scanning time is the time for phase scan via PZT voltage ramp and the scope running time.