Transmon Qubits Sustain Coherence for Ten Times Longer Than Gatemons

Zhenhai Sun and colleagues at the Department of Electrical and Computer Engineering, University of Colorado Boulder, have identified a significant performance difference between gatemon qubits and conventional transmons fabricated using identical materials and designs. The research reveals that gatemon qubits exhibit markedly shorter relaxation times than transmons, with the discrepancy originating from an additional, temperature-independent source of energy loss within the gatemon’s Josephson junction. A thorough loss budget and direct comparison of gatemon and transmon coherence identifies junction-intrinsic dissipation as a key obstacle to improving gatemon qubit performance and offers vital insight for future qubit designs.

Gatemon decoherence originates from interface dissipation not conventional loss mechanisms

Superconducting transmon qubits represent a leading platform for realising quantum computation. These qubits, characterised by a non-linear inductor-capacitor circuit, routinely achieve coherence times, a measure of how long a qubit maintains its quantum state, in the tens of microseconds. However, alternative qubit designs, such as gatemons, which utilise hybrid superconductor-semiconductor Josephson junctions, have lagged behind in performance. Specifically, gatemons, while offering the attractive feature of gate tunability, have historically demonstrated relaxation times significantly shorter than their transmon counterparts. Across multiple fabricated devices, transmons routinely achieve relaxation times exceeding 50 microseconds, whereas co-fabricated gatemons saturate at a few microseconds, representing a performance difference of over one order of magnitude. This disparity is important because it previously prevented gatemons from reaching the coherence levels necessary for implementing complex quantum algorithms requiring a substantial number of gate operations. The co-fabrication technique, building both qubit types on the same chips, was crucial. It enabled a direct, side-by-side comparison, effectively eliminating fabrication inconsistencies and material variations as potential causes of the observed performance gap. This approach ensured that any differences in coherence were attributable to the fundamental properties of the Josephson junction itself, rather than extrinsic factors.

Detailed analysis of potential decoherence pathways reveals that standard loss mechanisms, including Purcell decay (caused by interaction with the electromagnetic environment), spontaneous emission, and dielectric loss within the supporting materials, cannot adequately explain the reduced coherence of gatemons. These mechanisms contribute to decoherence in all superconducting qubits, but their impact appears insufficient to account for the substantial difference observed between gatemons and transmons. Instead, the research points to junction-intrinsic dissipation occurring within the gatemon’s semiconductor-superconductor interface as the dominant source of energy loss. The best-performing transmon reached a relaxation time of 71.6 microseconds, while gatemons, utilising a Josephson junction fabricated from a hybrid semiconductor-superconductor interface, peaked at 9.1 microseconds. Temperature-dependent measurements were conducted to investigate the origin of this dissipation. These measurements confirmed similar superconducting gaps, a key parameter characterising the superconducting state, for both junction types, indicating that the observed reduction in gatemon coherence is not related to differences in the fundamental superconducting properties of the materials. However, these numbers do not yet clarify the precise microscopic origin of this dissipation, such as the presence of interface states or non-equilibrium quasiparticles, nor do they demonstrate scalability to larger, more complex quantum circuits with a higher density of qubits.

Josephson junction characteristics limit gatemons’ quantum coherence times

Building more complex and scalable quantum processors is a key goal for scientists in the field of quantum information science, and hybrid superconducting circuits offer a promising route towards increased functionality and qubit connectivity. Gatemons offer a key advantage over traditional transmons: the ability to precisely tune their properties via external signals, such as microwave drives or magnetic fields. This tunability allows for more flexible control and manipulation of the qubit state, potentially enabling the implementation of more sophisticated quantum algorithms. Despite careful fabrication and design, however, this work highlights a frustrating roadblock, as gatemons consistently exhibit sharply reduced coherence compared to standard transmon qubits. The Josephson junction, acting as the non-linear element in the qubit circuit, is therefore a critical component influencing overall performance.

Understanding these fundamental limitations in gatemons is vital, directing future material science and device engineering efforts towards improving hybrid qubit designs, even if immediate performance gains remain elusive. The identification of junction-intrinsic dissipation as the primary source of decoherence is a significant step forward, as it moves the field beyond previously considered loss mechanisms. Energy dissipation originates not from expected sources, but from within the qubit’s core junction itself, a finding important for advancing hybrid quantum circuit designs. Gatemons, incorporating both a superconductor (typically aluminium or niobium) and a semiconductor (such as indium arsenide), exhibit a relaxation time of approximately 30 microseconds, significantly lower than state-of-the-art transmons. This discovery moves the field beyond previously considered loss mechanisms, opening questions regarding the microscopic origins of this dissipation, potentially related to defects at the semiconductor-superconductor interface or the presence of two-level systems, and directing future material science towards mitigating it. Further investigation is needed to fully characterise the dissipation, including its dependence on junction parameters and material properties, and to explore its impact on scalability to larger, more complex quantum circuits. Addressing this dissipation is crucial for realising the full potential of gatemon qubits and enabling the development of practical quantum computers.

The research demonstrated that gatemon qubits, utilising hybrid superconductor-semiconductor junctions, exhibited significantly reduced coherence times, reaching a maximum of a few microseconds, compared to standard transmons achieving tens of microseconds. This difference in performance is not attributable to external loss factors, but instead originates from dissipation within the gatemon’s Josephson junction itself. Identifying this junction-intrinsic dissipation is important as it directs future material science and device engineering towards improving hybrid qubit designs. The authors suggest further work is needed to fully characterise this dissipation and its impact on scalability.