Noise spectroscopy through dynamical decoupling with a superconducting flux qubit

Abstract

The characterization and mitigation of decoherence in natural and artificial two-level systems (qubits) is fundamental to quantum information science and its applications. Decoherence of a quantum superposition state arises from the interaction between the constituent system and the uncontrolled degrees of freedom in its environment. Within the standard Bloch-Redfield picture of two-level sys tem dynamics, qubit decoherence is characterized by two rates: a longitudinal relaxation rate 1 due to the exchange of energy with the environment, and a transverse relaxation rate 2 = 1=2 + ' which contains the pure dephasing rate '. Irreversible energy relaxation can only be mitigated by reducing the amount of environmental noise, reducing the qubit’s internal sensitivity to that noise, or through multi-qubit encoding and error correction protocols (which already presume ultra-low error rates). In contrast, dephasing is in principle reversible a nd can be refocused dynamically through the application of coherent control pulse methods [1‐3]. In this work we demonstrate how dynamical-decoupling techniques can moderate the dephasing effects of low-frequency noise on a superconducting qubit [4‐6] with energy-relaxation time T1 = 1= 1 = 12� s. Using the CPMG sequence [7, 8] with up to 200 � -pulses, we demonstrate a 50-fold improvement in the transverse relaxation time T2 over its baseline value. We observe relaxation-limited times T CPMG 2 = 23� s � 2T1 resulting from CPMG-mediated Gaussian pure-dephasing times in apparent excess of 100� s. We leverage the filtering property of this sequence in conj unction with Rabi and energy relaxation measurements to facilitate the spectroscopy and reconstruction of the environmental noise power spectral density (PSD) [9, 10].