Continuous Feedback on Quantum Superconducting Circuits

Quantum measurement theory describes the dynamics of a quantum system when it transfers information to an observer. Within the framework of continuous measurement, this outflow of information and resultant back-action occur at a finite rate. Continuously measured systems do not instantaneously collapse to eigenstates, but rather undergo stochastic evolution highly correlated with the observer’s received information. When the measurement rate is on the same timescale as the processing speed of classical control electronics, we can perform feedback on a quantum system during its collapse process. We experimentally realize such a protocol by using an adaptively controlled quantum amplifier to implement a canonical phase measurement on a flying photon. Continuous measurements can also be used as a tool in quantum error correction. Error correction typically requires discrete rounds of measurement, often using entangling gates and ancilla qubits. In contrast, continuous error correction is implemented via always-on measurements of parity syndromes. Using direct parity measurements of pairs of qubits in a three qubit system along with a custom classical controller, we implement the first continuous quantum error correction code