Assessing the Sensitivity of Niobium- and Tantalum-Based Superconducting Qubits to Infrared Radiation

TL;DR

This work addresses infrared radiation as a source of non-thermal quasiparticles that decohere Nb- and Ta-based superconducting qubits. It compares Nb and Ta offset-charge-sensitive transmons using a Ramsey-based charge-parity readout to measure quasiparticle tunneling rates under controlled infrared exposure, and then evaluates mitigation strategies with in-line infrared filters and ambient foam absorbers. The authors report that Ta exhibits higher background tunneling and stronger sensitivity to radiative power, but that combining filters and foam dramatically reduces rates, achieving approximately $100\ \mathrm{Hz}$ for Nb and $300\ \mathrm{Hz}$ for Ta under filtration, with rates continuing to decline over days–weeks after cooldown. The findings underscore radiative backgrounds as a critical consideration when deploying new material platforms and call for optimized experimental designs and deeper studies of quasiparticle diffusion and trapping to maximize qubit coherence.

Abstract

The use of tantalum films for superconducting qubits has recently extended qubit coherence times significantly, primarily due to reduced dielectric losses at the metal-air interface. However, the choice of base material also influences the sensitivity to quasiparticle-induced decoherence. In this study, we investigate quasiparticle tunneling rates in niobium and tantalum-based offset-charge-sensitive qubits. Using a source of thermal radiation, we characterize the sensitivity of either material to infrared radiation and explore the impact of the infrared background through the targeted use of in-line filters in the wiring and ambient infrared absorbers. We identify both radiation channels as significant contributions to decoherence for tantalum but not for niobium qubits and achieve tunneling rates of 100 Hz and 300 Hz for niobium and tantalum respectively upon installation of infrared filters. Additionally, we find a time-dependence in the observed tunneling rates on the scale of days, which we interpret as evidence of slowly cooling, thermally radiating components in the experimental setup. Our findings indicate that continued improvements in coherence times may require renewed attention to radiative backgrounds and experimental setup design, especially when introducing new material platforms.

Figures (7)

• Figure 1: Experimental protocol for quasiparticle tunneling measurement and design of the quantum device. (a) Numerically obtained energy levels of an offset-charge sensitive transmon for typically observed device parameters. The eigenenergies of the transmon Hamiltonian depend on the offset charge $n_g$ on the qubit capacitor and show a periodicity of $2e$. (b) Experimental sequence for mapping the charge parity state of the qubit into the $\ket{0}/\ket{1}$ subspace. (c, d) Evolution of the qubit state on the Bloch sphere in case of (c) even and (d) odd charge parity when starting in the state $\ket{0}$. (e) False colored micrograph of one of the investigated quantum devices and (f) a zoom-in on the SQUID of one of the qubits, see main text for details.
• Figure 2: Investigation of the sensitivity of niobium and tantalum devices to infrared radiation. (a) Exemplary excerpt of a time-trace of the tunneling rate extraction experiment for a niobium qubit, with regions corresponding to even (odd) charge parity shaded in blue (red). The time trace shows the absolute value of the discrete derivative of the assigned qubit states (mapped to zero for $\ket{0}$ and one for $\ket{1}$), smoothed -- for visualization purposes only -- with a Gaussian filter of width $\sigma = 10$. (b) Example of a power spectral density dataset with applied moving average filter taken for both materials (gray dots), together with a Lorentzian model fit (solid lines, fitted to unfiltered data). (c) Quasiparticle tunneling rate $\Gamma_0$ (taken for qubit one of device A (niobium) and device B (tantalum)) versus electrical power $P_W$ applied to the Manganin wire (dots), with power law fits (dashed lines) with fitted exponents $n$. (d) Schematic of the experimental setup. (e) Correlation between tunneling rate and qubit relaxation time for data in (c). The region for which the qubit's $T_1$-time is limited by quasiparticle tunneling is indicated in red.
• Figure 3: Suppressing infrared radiative background through in-line filters and infrared-absorbing foam. (a) Schematic of the experimental setup. Eccosorb filters are placed at three positions along the readout output and signal lines, before and after the TWPA. All filters are thermally anchored to the base plate using copper braids (not shown in the schematic). Eccosorb foam is placed inside the shield assembly. (b) Tunneling rates from multiple measurements and devices are compared for six filter and foam configurations, for niobium- and tantalum-based devices.
• Figure 4: Reduction in background quasiparticle tunneling rates over the course of a cooldown. The tunneling times for one niobium and one tantalum device are compared across two thermal cycles: one with Eccosorb foam installed (filled markers) and one without (empty markers). In all cases, we fit a material-characteristic power law to the data, see main text for details.
• Figure 5: Experimental setup. (a) Diagram of the cryogenic wiring for the setup. We show the line configuration for flux lines in green, for charge lines in orange, for readout input in red, for readout output in purple and for TWPA pump lines in teal. Components are thermalized at the indicated temperature stage. (b) Photograph of an Eccosorb filter for drive lines. (c) Photograph of the sample package consisting of a copper base and aluminum lid enclosing the quantum device. (d) Photograph of the sample mount with Eccosorb HR foam installed and removed shield assembly. The copper mezzanine together with the shield assembly (not shown in the image) provide a light-tight enclosure for the sample package.