A Two-Level System as a Nanoscale Quantum Sensor in a Transmon Qubit

Transmon qubits are a promising candidate for building scalable quantum processors, offering fast, high-fidelity gate operations, ease of manufacture and scalability using standard fabrication techniques, and compatibility with existing control electronics. However, their performance is limited by environmental noise, with two-level systems (TLSs) defects in amorphous oxide layers emerging as a significant source of decoherence.

A key challenge in studying TLSs is their short relaxation times. This is due to TLSs having an elastic dipole moment, which can couple to phonon modes in their hosting material. To mitigate this phonon-mediated relaxation and better study TLS dynamics, we employ a phononic crystal to suppress the resonant decay of microwave-frequency TLSs into their phonon bath, significantly extending the energy relaxation time (T₁) of the TLSs. By directly driving the TLS and measuring its coherence time (T₂), we perform noise spectroscopy using dynamical decoupling protocols. Given their atomic scale, these TLSs act as highly sensitive nanoscale quantum sensors, enabling us to probe their microscopic noise environment and resolve interactions with individual noise sources like two-level fluctuators (TLFs). This ability to observe and potentially control these TLFs provides further insight into noise sources that degrade qubit performance and hinder the scalability of quantum processors.