Rhenium Transmon Relaxation Times Up To 5 GHz

Rhenium‑Based Transmons Reach 5 GHz Relaxation Times

Superconducting qubits continue to push the limits of coherence, and a recent study demonstrates that rhenium films deposited on sapphire can support transmons with mean energy‑relaxation times (T₁) approaching 5 GHz. By targeting the suppression of native oxide formation—a known source of dielectric loss—the researchers provide a detailed loss‑budget analysis that validates the material’s promise for next‑generation quantum processors.

Why Rhenium?

Rhenium’s key advantage lies in its resistance to forming a native oxide layer at the metal‑air interface. In superconducting circuits, such oxides host two‑level systems (TLS) that absorb microwave energy, shortening qubit lifetimes. By minimizing this interfacial oxide, rhenium reduces TLS participation and therefore dielectric loss. The material also exhibits good lattice matching with sapphire, creating a low‑loss metal‑substrate interface that further curtails energy dissipation.

Fabrication and Characterization

The team sputtered 150 nm rhenium films onto high‑purity sapphire substrates that had been annealed to reduce bulk loss. Reactive‑ion etching defined the transmon pads, after which aluminum Josephson junctions were deposited. Because rhenium does not survive aggressive piranha cleaning, the researchers used concentrated sulfuric acid for post‑patterning cleaning—a step that may still leave room for surface‑loss improvement.

A companion set of rhenium stripline resonators was measured to extract loss factors for individual components. These resonators, operating in λ/2 modes, provided a reliable bound on the internal quality factor (Q_int) and helped separate contributions from:

• Rhenium‑aluminum interface
• Rhenium and aluminum surfaces
• Sapphire bulk
• Package seams and metal‑air interfaces

Loss Budget Highlights

The constructed loss budget aligns closely with the observed T₁ values, confirming the accuracy of the model. Key contributions are:

• Surface loss (overall) – ~36 % of total loss, with aluminum surfaces accounting for 38 % of that share.
• Bulk loss – ~23 %, comparable between rhenium and previously studied tantalum devices.
• Package‑related losses – each below 1 %, indicating that the packaging design is not a limiting factor.

Notably, the rhenium‑aluminum interface exhibited a loss factor of roughly 3 × 10⁻¹², on par with the low‑loss tantalum‑aluminum interface achieved using similar ion‑milling processes. This suggests that rhenium can be integrated into existing aluminum‑based fabrication flows without introducing additional interfacial loss.

Near‑Junction TLS Hotspot

Despite the low overall loss, the aluminum region surrounding the Josephson junction contributed disproportionately to decoherence. The high electric‑field concentration in this area amplifies the impact of any TLS, making even a small amount of surface contamination significant. This observation underscores the importance of refining cleaning protocols and possibly exploring alternative junction materials to further suppress TLS density.

Outlook

Rhenium’s performance rivals that of tantalum while offering distinct processing benefits, such as reduced oxide formation. Future work could focus on:

1. Optimizing surface cleaning – identifying chemical treatments that remove organic residues without damaging rhenium.
2. Engineering the junction vicinity – reducing TLS participation through improved junction design or passivation.
3. Scaling to larger arrays – assessing whether the low‑loss characteristics persist in multi‑qubit chips.

If these avenues prove successful, rhenium‑based transmons may become a mainstream platform for high‑coherence superconducting quantum processors, helping to extend gate depths and algorithmic complexity.

--- This article synthesizes the findings of the recent rhenium transmon study without introducing new data or quotations.