Rhenium as a material platform for long-lived transmon qubits

Abstract

Dielectric loss at the interfaces of superconducting films has long been recognized as limiting the performance of state-of-the-art superconducting circuits. Notably, the presence of a native oxide layer on the film is hypothesized to contribute to dielectric loss at the metal-air interface. Here, we explore rhenium as a candidate for the film, motivated by its remarkable property to suppress native oxide formation. We demonstrate rhenium on sapphire as a promising material platform for superconducting circuits through the realization of transmons with mean relaxation times $T_1$ up to 407 microseconds at 5 GHz. Our transmons are supplemented with a loss characterization study, in which we separate the dominant loss mechanisms and construct a loss budget that agrees with our $T_1$ measurements. Further characterization may establish rhenium as a leading candidate for maximizing decoherence time.

Figures (8)

• Figure 1: Measurement of rhenium-based transmons.(a) Schematic of our transmon chips, measured in hanger configuration in a superconducting tunnel package. Each chip consists of a transmon dispersively coupled to a stripline-based readout resonator and Purcell filter. (b) Relaxation time $T_1$ of our best-performing transmon ("Transmon 5") over the duration of 24 hours. Each data point (purple) is an averaged $T_1$ trace, along with the temporal mean and standard deviation underlaid in gray. (c) Averaged $T_1$ trace (orange) for the highlighted data point in (b), fit by an exponential decay (blue).
• Figure 2: Loss analysis of the tripole stripline resonator.(a) Schematic of our tripole stripline resonators, measured in hanger configuration in a superconducting tunnel package. The 10µm-wide strip is not visible at this scale. (b)--(d) Cross-sectional illustrations of the electric field profile of the first differential (D1), second differential (D2) and common (C) modes of the tripole stripline resonator respectively. The strip widths and separations are not to scale. The D1 mode has the highest surface participation $p_\mathrm{surf}$, rendering it more sensitive to dielectric loss at the interfaces. (e) Dependence of the internal quality factor $Q_\mathrm{int}$ on the circulating photon number $\bar{n}$ for each of the three fundamental modes of a representative tripole stripline resonator ("Tripole 1"). The data points are fit by the two-level system (TLS) model, allowing extraction of $Q_\mathrm{int}$ at single-photon power, $\bar{n}=1$. (f) Power dependence of the loss factors $\Gamma_i$ for Tripole 1, extracted using $Q_\mathrm{int}$ data from (e) and inverting the participation matrix. Unlike surface and bulk, the package seam loss factor is not dimensionless and has been plotted separately. The shaded area represents the error propagated from $Q_\mathrm{int}$ to $\Gamma_i$, and is too small to be visible for surface and bulk. (g) Relative loss budget at $\bar{n}=1$ for the three fundamental modes of Tripole 1, constructed using loss factors from (f) and participations from simulation. The contribution by package seam is too small to be visible in the budgets for the D1 and D2 modes; similarly for package metal-air and package conductor in the D1 mode.
• Figure 3: Constructing the loss budget for the transmon.(a) Schematic of our segmented stripline resonators, measured in hanger configuration in a superconducting tunnel package. (b) Top-down illustration of the alternating rhenium and aluminum segments in the strip. The notch geometry replicates the rhenium-aluminum interface in the transmon. (c) Dependence of the internal quality factor $Q_\mathrm{int}$ on the circulating photon number $\bar{n}$ for the fundamental mode of a representative segmented stripline resonator ("Segmented 1"). The data points are fit by the two-level system (TLS) model, allowing extraction of $Q_\mathrm{int}$ at single-photon power, $\bar{n}=1$. (d) Relative loss budget at $\bar{n}=1$ for the fundamental mode of Segmented 1, constructed using $Q_\mathrm{int}$ data from (c) and participations from simulation. (e) Absolute loss budget of the transmon in comparison to the $Q_\mathrm{int}$ of measured transmons in Table \ref{['tab:T1']}. The error in the loss budget is propagated from the sample standard deviation of $\Gamma_i$. The contributions by the rhenium-aluminum interface and the package are too small to be visible in the budget.
• Figure 4: Materials characterization of the rhenium film. (a) Temperature dependence of electrical resistance, indicating the onset of the superconducting transition at 1.9K and a residual-resistivity ratio (RRR) of 19. The temperature floor of the measurement system is 1.8K. (b) Transmission electron micrograph (TEM) of the metal-air (MA) interface with rhenium on the left and carbon on the right, demonstrating the absence of a native oxide layer on the nanometer scale. The carbon is introduced as part of TEM sample preparation. (c) TEM of the metal-substrate (MS) interface with sapphire on the left and rhenium on the right. (d), (e) Distribution of rhenium and oxygen counts respectively at the MA interface, obtained by energy dispersive X-ray spectroscopy (EDS). The orientation and magnification of these data compared to (b) are different. Colored pixels, which are exaggerated for clarity, indicate the location of at least one count. (f) Depth profile of rhenium and oxygen counts obtained by EDS. The number of counts along the horizontal axis of (d), (e) is summed to generate this plot. The elements are plotted on different axes to exaggerate the oxygen count, where no elevated oxygen count beyond the background is visible at the MA interface. The transition from metal to air in the rhenium count is not sharp due to the imprecise alignment of the interface along the horizontal axis.
• Figure 5: Designs of devices used in this work.(a)--(c) Top-down illustration of the important design parameters in the tripole stripline resonator, segmented stripline resonator and transmon respectively, not to scale. The lengths of the tripole stripline and segmented stripline resonators are marginally varied to separate the resonant frequencies and allow multiplexed measurement.