Correlated Coherence Loss from Controlled Quasiparticles Injection in Superconducting Circuits

Understanding the dynamics of quasiparticles is critical for improving qubit coherence and achieving fault-tolerant quantum computing. Non-equilibrium quasiparticles, and in some devices vortex-related excitations, are a significant source of decoherence in superconducting qubits and can limit gate fidelities. These excitations can propagate across the chip via pair-breaking phonons, leading to spatially correlated noise that reduces the effectiveness of quantum error correction.

In this work, we study quasiparticle-induced decoherence by increasing the trapping rate, using induced vortices in the device electrodes to raise the quasiparticle density. Coherence times are measured across multiple qubits on the chip, and both their values and correlations are examined under increased quasiparticle density. Correlated T₁–T₂ fluctuations then serve as a probe of the spread and influence of non-equilibrium quasiparticles. In addition to characterizing the noise spectrum from quasiparticle injection, we use T₂ Ramsey spectroscopy and tools from quantum noise spectroscopy. This approach provides frequency-dependent information about the noise environment and enables a deeper understanding of quasiparticle dynamics in large-scale quantum circuits.

Acknowledgement: Devices were fabricated and provided by the Superconducting Qubits at Lincoln Laboratory (SQUILL) Foundry at MIT Lincoln Laboratory, with funding from the Laboratory for Physical Sciences (LPS) Qubit Collaboratory.