1. A quantum coherent system comprising: a first controlled phase shifters having a first phase constant; a second controlled phase shifter having a second phase constant that is the negative of the first phase constant; a probe electromagnetic mode passing through the first controlled phase shifter and the second controlled phase shifter; a detector positioned to measure a phase shift in the probe electromagnetic mode; a first state separator having first and second output modes, wherein the first output mode of the first state separator controls the first controlled phase shifter; and a second state separator having first and second output modes, wherein the first output mode of the second state separator controls the second controlled phase shifter.