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arXiv:2607.06670

Localized Thermometry via Dayem Bridges Integrated on Superconducting Qubit Chips

Ella O. Lachman, Dave P. Pappas, Jayss Marshall, Josh Y. Mutus

quant-phcond-mat.mes-hallcond-mat.supr-conSubmitted Jul 7, 2026• Announced Jul 9, 2026• 0 citations
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Abstract

Accurate knowledge of the on-chip temperature is essential for understanding and optimizing the performance of superconducting qubits, yet direct thermometry at millikelvin temperatures remains challenging. While qubits themselves are sensitive to the temperature of their environment, other factors may affect the qubits` effective temperature, and using them as thermometers with any accuracy requires specialized measurement protocols and qubit designs, limiting their practicality for routine diagnostics and adding complex infrastructure to any hardware testing apparatus. Here we demonstrate a complementary on-chip thermometry method based on superconducting Dayem bridges that are integrated on the same chip as transmon qubits. By extracting the critical current of the Dayem bridge from I-V measurements, we obtain a local, quantitative measure of the chip temperature without the need for microwave calibration or qubit-specific control sequences. To demonstrate the utility of the Dayem bridges as thermometers, we fabricate them in-situ with qubits on the same chip, calibrate the Dayem bridge critical current as a function of temperature, and characterize its resolution and stability at cryogenic temperatures. We additionally perform simultaneous measurements of the Dayem bridge thermometer and qubit excited-state population, and show agreement over the relevant temperature range, validating the method against established qubit thermometry. Furthermore, we correlate the independently measured chip temperature with qubit energy relaxation and dephasing times, demonstrating the utility of this approach for diagnosing temperature-dependent decoherence mechanisms. These results establish integrated Dayem bridges as a simple, non-invasive, and scalable tool for cryogenic hardware development, and on chip thermometry in superconducting quantum circuits.

Comments
18 pages, 5 figures

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Accurate knowledge of the on-chip temperature is essential for understanding and optimizing the performance of superconducting qubits, yet direct thermometry at millikelvin temperatures remains challenging. While qubits themselves are sensitive to the temperature of their environment, other factors may affect the qubits` effective temperature, and using them as thermometers with any accuracy requires specialized measurement protocols and qubit designs, limiting their practicality for routine diagnostics and adding complex infrastructure to any hardware testing apparatus. Here we demonstrate a complementary on-chip thermometry method based on superconducting Dayem bridges that are integrated on the same chip as transmon qubits. By extracting the critical current of the Dayem bridge from I-V measurements, we obtain a local, quantitative measure of the chip temperature without the need for microwave calibration or qubit-specific control sequences. To demonstrate the utility of the Dayem bridges as thermometers, we fabricate them in-situ with qubits on the same chip, calibrate the Dayem bridge critical current as a function of temperature, and characterize its resolution and stability at cryogenic temperatures. We additionally perform simultaneous measurements of the Dayem bridge thermometer and qubit excited-state population, and show agreement over the relevant temperature range, validating the method against established qubit thermometry. Furthermore, we correlate the independently measured chip temperature with qubit energy relaxation and dephasing times, demonstrating the utility of this approach for diagnosing temperature-dependent decoherence mechanisms. These results establish integrated Dayem bridges as a simple, non-invasive, and scalable tool for cryogenic hardware development, and on chip thermometry in superconducting quantum circuits.
We report temperature measurements using a transmon qubit by detecting the population of its first three energy levels after applying a sequence of π pulses and performing projective dispersive readout. We measure the effective temperature of the qubit and characterize its relaxation and coherence times τ 1 , τ 2 for three devices in the temperature range from 20 to 300 mK. We analyze the process of qubit thermalization to its effective environment consisting of multiple heat baths and support this with experimental data. The signal-to-noise ratio of the temperature measurement depends strongly on τ 1 , which drops at higher temperatures due to quasiparticle excitations, adversely affecting the measurements and setting an upper bound of the dynamic temperature range of the thermometer. The measurement relies on coherent dynamics of the qubit during the π pulses. The effective qubit temperature follows closely that of the cryostat in the range from 100 to 200 mK. We present a numerical model of the qubit population distribution and find hat it compares favorably with the experimental results. Finally, we compare our technique with previous work on qubit thermometry and discuss its application prospects.
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Electron-spin qubits hosted in silicon are a promising platform for quantum computing, since in recent years operation and readout fidelities have exceeded the fault tolerant threshold. Now the manufacturing techniques used to fabricate these devices have to be scaled up. To enable scaling the number of qubits beyond 1000, the quantum processor needs to be integrated with local control electronics. However, the heat dissipated by operation of these electronic control circuits can negatively affect qubit performance. In this work, I develop a primary temperature sensor based on a quantum dot, to monitor the qubit temperature under varying conditions. Real-time monitoring of the local temperature can be used to study the impact of on-chip dissipated heat on the qubit. These sensors are used to define a power budget for integrated electronics. Furthermore, a simple thermal model is developed to predict thermal transport in a quantum chip. This methodology is universal and can be applied to any qubit platform with a local temperature sensor. Finally, I compare the performance of this quantum dot thermometer against other cryogenic thermometers and find this primary thermometer is more sensitive than other common thermometry techniques. To reduce the number of control lines required to control individual qubits, I investigate the use of a 3D microwave resonator to deliver a global oscillating magnetic field to control electron spin qubits. When using the resonator to rotate electron spins, no spin flips are detected. This is attributed to a significant decrease in resonator quality factor which originates from materials with a different thermal contraction. This leads to a misalignment of the two sapphire ring oscillators making up the resonator. Despite this, I’ve used these experiments to characterize various types of charge sensors, and I have identified the RF-SET and RF-SEB as the best sensors for near term applications.
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