Rigetti Superconducting Bridges Track Qubit Temperature With Millikelvin Precision

Researchers at Rigetti Computing have developed a new method for measuring temperature on superconducting qubit chips with millikelvin precision. The team integrated superconducting Dayem bridges directly onto the same chip as transmon qubits, enabling localized temperature readings without complex calibration procedures. By measuring the critical current of these Dayem bridges via I-V measurements, researchers obtain a quantitative measure of chip temperature. The work demonstrates agreement between the Dayem bridge thermometer and established qubit thermometry techniques, validating its reliability. These results establish integrated Dayem bridges as a simple and scalable tool for cryogenic hardware development and on-chip thermometry in superconducting quantum circuits.

Dayem Bridges as Integrated On-Chip Thermometers

Superconducting qubits demand exceptionally stable and well-defined operating temperatures, yet directly measuring these temperatures on-chip at the millikelvin scale presents a significant engineering challenge. This method bypasses the limitations of conventional techniques, which often require complex calibration procedures or specialized qubit designs. The team’s work centers on the temperature-dependent critical current of these Dayem bridges. These bridges, lithographically defined superconducting weak links, function as thermometers because their critical current decreases predictably with increasing temperature, following the Bardeen relation as stated in their published findings. By precisely measuring this critical current via I-V measurements, the researchers obtain a quantitative assessment of the chip temperature without the need for microwave calibration or qubit-specific control sequences, simplifying existing methods. The fabrication process is also streamlined; Dayem bridges require only a single electron-beam lithography step and a single superconducting thin-film deposition.

A key advantage of this approach is its compatibility with existing qubit platforms. The researchers fabricated the Dayem bridges in-situ with qubits on the same chip, enabling simultaneous measurements of both the bridge’s temperature reading and the qubit’s excited-state population. This direct comparison validated the accuracy of the Dayem bridge thermometer against established qubit thermometry techniques. They correlated the independently measured chip temperature with qubit energy relaxation and dephasing times, demonstrating the utility of this approach for diagnosing temperature-dependent decoherence mechanisms. The simplicity and robustness of Dayem bridges promise to be a valuable asset in the ongoing development of increasingly complex superconducting quantum systems.

Limitations of Existing Millikelvin Thermometry Techniques

Rigetti Computing’s efforts to refine millikelvin temperature measurement highlight persistent challenges in characterizing superconducting qubit performance. While accurate on-chip temperature readings are vital for optimizing these quantum systems, conventional methods often fall short, demanding complex calibration routines or designs tailored to individual qubits. Existing techniques, such as relying on qubit excited-state populations as thermometers, introduce operational overhead and are susceptible to inaccuracies stemming from factors beyond temperature alone. Isolating temperature as the sole influence on qubit behavior is difficult, necessitating specialized protocols and adding complexity to any hardware testing apparatus. Alternative approaches, like Coulomb blockade thermometry, offer primary temperature measurements but typically require dedicated device structures separate from the qubit itself. This lack of integration presents a practical hurdle for streamlined diagnostics and comprehensive chip-level thermal mapping.

Researchers have long sought a method that provides localized, quantitative temperature data without imposing significant constraints on qubit design or measurement workflows. The need for a solution compatible with existing superconducting qubit platforms has driven exploration into novel sensing mechanisms, including the investigation of superconducting Dayem bridges as integrated thermometers. These bridges, unlike conventional Josephson junctions, do not require insulating tunnel barriers, simplifying fabrication and potentially enhancing robustness. Their approach leverages the temperature-dependent critical current of these bridges, enabling quantitative temperature measurements via standard current-voltage measurements.

Dayem Bridge Critical Current and Temperature Dependence

The pursuit of reliable temperature measurement at the quantum scale has yielded a new approach leveraging the properties of Dayem bridges, offering a significant advancement for superconducting qubit research. Maintaining millikelvin temperatures is crucial for qubit performance, but accurately gauging those temperatures on the chip itself has proven remarkably difficult. Existing methods, often relying on qubit behavior as a proxy for temperature, introduce complexity and demand specialized calibration routines. Ella O. Lachman, Dave P. Pappas, Jayss Marshall, and Josh Y. Mutus fabricated these bridges in-situ with qubits, enabling simultaneous measurements and eliminating the need for separate temperature probes. The core principle hinges on the temperature-dependent critical current of the Dayem bridge; as temperature increases, the critical current decreases in a predictable manner. By extracting this critical current via standard I-V measurements, a precise local temperature reading is obtained.

Crucially, the team validated this new method against established qubit thermometry techniques, performing simultaneous measurements of both the Dayem bridge and qubit excited-state population. This comparison confirmed the accuracy and reliability of the Dayem bridge thermometer across the relevant temperature range. This integrated thermometry offers a streamlined diagnostic tool, promising more efficient characterization and optimization of superconducting quantum circuits.

The pursuit of ever-more-precise control over superconducting qubits demands increasingly accurate temperature measurements, yet conventional cryogenic thermometry often falls short at the scale of the quantum device itself. While qubits can, in principle, act as thermometers, relying on their sensitivity introduces operational complexity and limitations stemming from qubit coherence and the presence of quasiparticles. The simplicity of the fabrication process is a key advantage. This early integration also improves device resilience during handling and packaging, reducing susceptibility to electrostatic discharge. The team demonstrated a correlation between the chip temperature measured by the Dayem bridges and key qubit properties, specifically energy relaxation and dephasing times. This connection confirms the utility of the method for diagnosing temperature-dependent decoherence mechanisms, providing insights into the factors limiting qubit performance.

Millikelvin precision in on-chip temperature measurement is now attainable, thanks to a new technique. Researchers at Rigetti Computing, Ella O. Lachman, Dave P. Pappas, Jayss Marshall, and Josh Y. Mutus, have demonstrated a method for quantifying chip temperature locally, a significant advancement over existing approaches that often rely on complex calibrations or specialized qubit designs.