Quantifying Trapped Magnetic Vortex Losses in Niobium Resonators at mK Temperatures

Abstract

Trapped magnetic vortices in niobium introduce microwave losses that degrade the performance of superconducting resonators. While such losses have been extensively studied above 1~K, we report here their direct quantification in the millikelvin and low-photon regime relevant to quantum devices. Using a high-quality factor 3-D niobium cavity cooled through its superconducting transition in controlled magnetic fields, we isolate vortex-induced losses and find the resistive component of the sensitivity to trapped flux $S$ to be approximately 2~n$\Omega$/mG at 10~mK and 6~GHz. The decay rate is initially dominated by two-level system (TLS) losses from the native niobium pentoxide, with vortex-induced degradation of $T_1$ occurring above $B_{\text{trap}}\sim$~50~mG. In the absence of the oxide, even 10~mG of trapped flux limits performance $Q_0\sim$~10${}^{10}$, or $T_1\sim$~350~ms, underscoring the need for stringent magnetic shielding. The resistive sensitivity $S$ decreases with temperature and remains largely field-independent, whereas the reactive component, $S^{\prime }$, exhibits a maximum near 0.8~K. These behaviors are well modeled within the Coffey-Clem framework in the zero-creep limit, under the assumption that vortex pinning is enhanced by thermally activated processes. Our results suggest that niobium-based transmon qubits can tolerate vortex-induced dissipation at trapped field levels up to several hundred mG, but achieving long coherence times still requires careful magnetic shielding to suppress lower-field losses from other mechanisms.

References in corpus (18)

• Towards understanding two-level-systems in amorphous solids -- Insights from quantum circuits
• The Superconducting TESLA Cavities
• Measurement and Control of Quasiparticle Dynamics in a Superconducting Qubit
• Three-dimensional superconducting resonators at $T<20$ mK with the photon lifetime up to $\tau =2$ seconds
• Microwave response of vortices in superconducting thin films of Re and Al
• Evidence of a nonequilibrium distribution of quasiparticles in the microwave response of a superconducting aluminium resonator
• Systematic Improvements in Transmon Qubit Coherence Enabled by Niobium Surface Encapsulation
• Ultra-high quality factors in superconducting niobium cavities in ambient magnetic fields up to 190 mG
• Two-level systems in superconducting quantum devices due to trapped quasiparticles
• Dependence of the residual surface resistance of superconducting RF cavities on the cooling dynamics around $T_{\mathrm{c}}$
• Superconducting cavity qubit with tens of milliseconds single-photon coherence time
• Tunneling of a Quantized Vortex: Roles of Pinning and Dissipation
• Efficient expulsion of magnetic flux in superconducting RF cavities for high $Q_0$ applications
• Proximity breakdown of hydrides in superconducting niobium cavities
• Effect of interstitial impurities on the field dependent microwave surface resistance of niobium
• Microwave measurements of the high magnetic field vortex motion pinning parameters in Nb${}_3$Sn
• Frequency dependence of trapped flux sensitivity in SRF cavities
• Oxygen Vacancies in Niobium Pentoxide as a Source of Two-Level System Losses in Superconducting Niobium