Quantum Characterization and Control Using Continuous Weak Measurements

Continuous weak measurement provides a unique resource for probing the time evolution of quantum systems. This functionality has been used to faithfully reconstruct individual quantum trajectories in isolated qubits and in entangled pairs coupled to a Markovian bath. We now extend these techniques to execute more complex quantum information processing tasks including tracking non-Markovian dynamics, continuous quantum error correction, and Hamiltonian reconstruction in superconducting circuits. In particular, we can reconstruct an a priori unknown time-dependent process with an algorithm to recover the density matrix from an incomplete set of continuous measurements. We show that it reliably extracts amplitudes of a variety of single-qubit and entangling two-qubit Hamiltonia. We further demonstrate how this technique reveals deviations from a theoretical control Hamiltonian that would have otherwise been missed by conventional techniques, thereby suggesting methods for identifying non-idealities in gates, certifying analog quantum simulators, and performing quantum metrology.