Abstract: A closed-loop feedback system and method of active noise cancellation to maintain a desired operating frequency of a qubit during a quantum computation, even when that frequency is relatively sensitive to flux noise. A series of Ramsey experiments is performed on the qubit to estimate an offset between its actual and desired operating frequencies, and the error is accumulated. After the probing is complete, the accumulated error is supplied to an arbitrary waveform generator that produces a magnetic flux that is coupled to the qubit, thereby tuning the qubit and actively controlling its operating frequency. Having corrected the operating frequency of the qubit and extended its coherence time, the quantum state of the qubit is allowed to evolve according to the computation.

Abstract: According to some embodiments, a method can identify and discriminate contributions from one or more noise sources using the multi-level structure of a quantum system with three or more levels. The method can include: preparing the quantum system in a predetermined state; applying one or more control signals to the quantum system; measuring values of one or more observables of the quantum system that quantify the quantum system's response to the noise sources and the one or more applied control signals; extracting noise spectra information associated with the noise sources from the measured values; and identifying contributions from the one or more noise sources based on the noise spectra information.

Abstract: According to some embodiments, a method can identify and discriminate contributions from one or more noise sources using the multi-level structure of a quantum system with three or more levels. The method can include: preparing the quantum system in a predetermined state; applying one or more control signals to the quantum system; measuring values of one or more observables of the quantum system that quantify the quantum system's response to the noise sources and the one or more applied control signals; extracting noise spectra information associated with the noise sources from the measured values; and identifying contributions from the one or more noise sources based on the noise spectra information.

Abstract: Techniques for machine learning assisted qubit state readout are disclosed. A system a set of training data that describes states of multiple qubits, and trains a neural network to determine qubit states based on the set of training data. The system obtains one or more unlabeled qubit signals, and determines one or more states corresponding to the unlabeled qubit signal(s), using the neural network. The unlabeled qubit signal(s) may include one or more multiplexed qubit signals, and the state(s) corresponding to the unlabeled qubit signal(s) may include one or more multi-qubit states based on the multiplexed qubit signal(s).