Tantalum Quantum Bits Hampered by Infrared, Niobium Proves More Resilient

Researchers have long sought to minimise decoherence in superconducting qubits, a critical challenge for advancing quantum computation. Michael Kerschbaum, Felix Wagner, and Uroš Ognjanović, from the Department of Physics at ETH Zurich, alongside Giovanni Vio, Kuno Knapp, Dante Colao Zanuz et al., now present a detailed assessment of how infrared radiation impacts the performance of niobium and tantalum-based superconducting qubits. Their work is significant because it identifies previously underestimated radiation channels contributing to decoherence in tantalum qubits, while demonstrating niobium’s relative resilience. By characterising quasiparticle tunneling rates with and without infrared filtering, the team reveals a clear pathway for improving coherence times and highlights the importance of careful experimental design as new qubit materials are explored.

While tantalum offers reduced dielectric losses at metal-air interfaces, the underlying base material profoundly influences susceptibility to quasiparticle-induced decoherence.

This work investigates quasiparticle tunneling rates in both niobium and tantalum-based offset-charge-sensitive qubits, meticulously characterizing their sensitivity to infrared radiation. Researchers employed a source of thermal radiation alongside in-line filters and ambient infrared absorbers to explore the impact of the infrared background on qubit performance.
The study identifies radiation channels as substantial contributors to decoherence in tantalum qubits, a phenomenon not observed in niobium. Upon implementing infrared filters, tunneling rates were reduced to 100Hz for niobium and 300Hz for tantalum, representing a measurable improvement in qubit stability.

Furthermore, a time-dependent variation in tunneling rates was observed over several days, suggesting the presence of slowly cooling, thermally radiating components within the experimental apparatus. These findings underscore the importance of addressing radiative backgrounds and refining experimental setup design to further enhance coherence times, particularly when integrating novel material platforms into quantum computing architectures.

This research centers on the precise measurement of quasiparticle tunneling, a critical decoherence mechanism in superconducting qubits. By utilizing offset-charge-sensitive transmons with a Josephson to charging energy ratio of approximately 20, the team was able to resolve individual tunneling events through frequency shifts dependent on charge parity.

A Ramsey-type sequence was implemented to map the charge parity state, enabling accurate extraction of tunneling rates. The experimental setup, adhering to established cryogenic engineering practices, allowed for controlled exposure to out-of-equilibrium infrared radiation generated by a current-biased resistive source.

Detailed analysis revealed that tantalum qubits exhibit a higher susceptibility to quasiparticle tunneling compared to their niobium counterparts, leading to diminished coherence. However, the integration of both in-line filters within the coaxial wiring and ambient infrared absorbers effectively mitigated this effect, bringing the performance of tantalum qubits to a level comparable with niobium.

Moreover, tracking tunneling rates over extended cooldown periods revealed a gradual reduction, attributed to the slow thermal stabilization of components within the cryostat. These results highlight the necessity of careful consideration of radiative environments and meticulous experimental design for achieving optimal qubit performance and scalability.

Quasiparticle Tunneling Rates and Infrared Radiation Sensitivity in Niobium and Tantalum Devices

Tantalum films have recently demonstrated extended coherence times, largely due to reductions in dielectric losses at metal-air interfaces. This work investigates quasiparticle tunneling rates in both niobium and tantalum-based offset-charge-sensitive devices to determine the influence of base material on sensitivity to quasiparticle-induced decoherence.

A source of thermal radiation was employed to characterise the sensitivity of each material to infrared radiation, with in-line filters and ambient infrared absorbers strategically incorporated into the wiring and surrounding the experimental setup. Researchers identified radiation channels as significant contributors to decoherence in tantalum, but not in niobium, achieving tunneling rates of 100Hz for niobium and 300Hz for tantalum prior to filter installation.

Subsequent installation of infrared filters reduced these rates to 100Hz and 300Hz respectively. Daily measurements spanning several weeks revealed a time-dependent reduction in observed tunneling rates, occurring over a period of days, which the study attributes to slowly cooling, thermally radiating components within the experimental apparatus.

To quantify the impact of infrared radiation, the team implemented two mitigation strategies: in-line filters and Eccosorb foam absorbers. In-line filters reduced tunneling rates by 14.4kHz for tantalum and 0.28kHz for niobium, while foam absorbers reduced them by 14.6kHz and 0.56kHz respectively. The combined effect of both methods yielded a total reduction of 15.7kHz for tantalum and 0.52kHz for niobium, suggesting that infrared radiation propagates both through free space and within the connecting cables.

Analysis of long-term data, collected over three weeks for the first thermal cycle and two weeks for the second, demonstrated a power-law reduction in quasiparticle tunneling rates for both materials. The addition of foam absorbers reduced the rate at one day from 93 ±4Hz to 48 ±3Hz for niobium, and from 1.97 ±0.07kHz to 0.96 ±0.02kHz for tantalum, representing a factor of two decrease. However, the rate of decrease over time remained unaffected by the foam absorbers, indicating that the observed reduction in tunneling rates is likely due to thermal radiation from poorly anchored components such as aluminum shielding and polymer-based dielectrics.

Mitigation of quasiparticle tunneling via infrared radiation suppression in superconducting qubits

Quasiparticle tunneling rates of 100Hz were measured in niobium-based offset-charge-sensitive qubits, while tantalum-based qubits exhibited rates of 300Hz under identical conditions. These rates represent the frequency of charge parity flips induced by quasiparticle tunneling across the Josephson junction.

The study characterized the sensitivity of niobium and tantalum materials to infrared radiation and its impact on decoherence processes within superconducting qubits. Researchers utilized a Ramsey-type sequence to map the charge parity state of the qubit, enabling the extraction of tunneling rates and assessment of coherence times.

Implementation of in-line filters within the coaxial control wiring and ambient infrared filtering using foam absorbers reduced the tunneling rate in tantalum qubits to match that of niobium, achieving rates of 100Hz. This demonstrates a significant mitigation of quasiparticle-induced decoherence through targeted infrared radiation suppression.

The experimental setup followed established cryogenic engineering practices for superconducting quantum devices, allowing for precise control and measurement of qubit parameters. Analysis revealed that radiation channels contribute significantly to decoherence in tantalum but not in niobium, highlighting material-specific sensitivities.

Furthermore, a time-dependent behavior in the observed tunneling rates was identified, with rates decreasing over a period of days. This temporal evolution is attributed to the gradual cooling of thermally radiating components within the experimental cryostat. The observed reduction in tunneling rates suggests that maintaining a stable and cold thermal environment is crucial for optimizing qubit coherence. These findings emphasize the importance of addressing radiative backgrounds and refining experimental setup design to further enhance coherence times in superconducting qubit systems.

Radiative backgrounds limit tantalum qubit coherence via quasiparticle tunnelling

Scientists have demonstrated that infrared radiation significantly limits the coherence of tantalum qubits in standard superconducting qubit setups. Investigations into quasiparticle tunneling rates in niobium and tantalum revealed that tantalum is particularly susceptible to decoherence induced by infrared photons.

The implementation of in-line filters and ambient infrared absorbers reduced tunneling rates from approximately 1.97kHz to 0.96kHz for tantalum and from 100Hz to 300Hz for niobium. These findings establish a clear link between radiative backgrounds and qubit performance, suggesting that improvements in coherence times require careful attention to experimental setup design and material choices.

Observed time-dependent changes in tunneling rates, lasting days, indicate that slowly cooling components within the experimental apparatus contribute to the infrared background. Although foam absorbers effectively reduced ambient infrared radiation, the rate of decrease in tunneling over time remained unaffected, implying that other thermally radiating elements are also present.

The authors acknowledge that the observed spread in tunneling rates is partially attributable to differing measurement times relative to the start of the cooldown process. Future research should focus on systematically investigating quasiparticle diffusion and its impact on junction performance. Revisiting experimental configurations may become necessary as qubit coherence times improve or new materials are introduced, ensuring optimal performance and minimizing the effects of low-energy radiative backgrounds. Understanding these phenomena is crucial for advancing the field of superconducting quantum computing.