Enhancing qubit noise spectroscopy using a quantum quench

Morgan W. Mitchell, Vito Giovanni Lucivero, Aleksandra Dimić +2 more

Anonymous, Christopher A. Watson, Andrew Murphy +2 more

Yuxin Wang, Aashish A. Clerk

Quantum sensing protocols that exploit the dephasing of a probe qubit are powerful and ubiquitous methods for interrogating an unknown environment. They have a variety of applications, ranging from noise mitigation in quantum processors, to the study of correlated electron states. Here, we discuss a simple strategy for enhancing these methods, based on the fact that they often give rise to an inadvertent quench of the probed system: there is an effective sudden change in the environmental Hamiltonian at the start of the sensing protocol. These quenches are extremely sensitive to the initial environmental state, and lead to observable changes in the sensor qubit evolution. We show how these new features give access to environmental response properties. This enables methods for direct measurement of bath temperature, and for detecting non-thermal equilibrium states. We also discuss how to deliberately control and modulate this quench physics, which enables reconstruction of the bath spectral function. Extensions to non-Gaussian quantum baths are also discussed, as is the application of our ideas to a range of sensing platforms (e.g., nitrogen-vacancy (NV) centers in diamond, semiconductor quantum dots, and superconducting circuits).

R. J. Schoelkopf, Aashish A. Clerk, S. M. Girvin +2 more

Yue Fu, Yang Wu, Yingqiu Dai +3 more

With the power of chemistry for inexpensive synthesis of tailored systems, molecular qubits are attractive candidates for quantum computing. Here decoherence persists as a major obstacle; for its effective suppression, the properties of environmental noise must be elucidated. Traditional noise spectroscopy with spurious harmonics leads to false identification of nuclear spins. The authors demonstrate a protocol for noise-spectrum analysis without spurious harmonics, through dynamical decoupling. This result enables enhanced coherence for molecular qubits, and could be quite significant for quantum information processing based on such systems.

Uwe von Lüpke, Félix Beaudoin, Leigh Norris +9 more

Noise that exhibits significant temporal and spatial correlations across multiple qubits can be especially harmful to both fault-tolerant quantum computation and quantum-enhanced metrology.However, a complete spectral characterization of the noise environment of even a two-qubit system has not been reported thus far.We propose and experimentally demonstrate a protocol for two-qubit dephasing noise spectroscopy based on continuous-control modulation.By combining ideas from spin-locking relaxometry with a statistically motivated robust estimation approach, our protocol allows for the simultaneous reconstruction of all the single-qubit and two-qubit cross-correlation spectra, including access to their distinctive nonclassical features.Only single-qubit control manipulations and state-tomography measurements are employed, with no need for entangled-state preparation or readout of two-qubit observables.While our experimental demonstration uses two superconducting qubits coupled to a shared, colored engineered noise source, our methodology is portable to a variety of dephasing-dominated qubit architectures.By pushing quantum noise spectroscopy beyond the single-qubit setting, our work heralds the characterization of spatiotemporal correlations in both engineered and naturally occurring noise environments.