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arXiv:2403.17720

Low-temperature benchmarking of qubit control wires by primary electron thermometry

Elias Roos Hansen, Ferdinand Kuemmeth, Joost van der Heijden

Published Jan 1, 2024• 0 citations
arXivHTMLPDF

Abstract

Low-frequency qubit control wires require non-trivial thermal anchoring and low-pass filtering. The resulting electron temperature serves as a quality benchmark for these signal lines. In this technical note, we make use of a primary electron thermometry technique, using a Coulomb blockade thermometer, to establish the electron temperature in the millikelvin regime. The experimental four-probe measurement setup, the data analysis, and the measurement limitations are discussed in detail. We verify the results by also using another electron thermometry technique, based on a superconductor-insulator-normal metal junction. Our comparison of signal lines with QDevil's QFilter to unfiltered signal lines demonstrates that the filter significantly reduces both the rms noise and electron temperature, which is measured to be 22 $\pm$ 1 mK.

DOI
10.48550/arxiv.2403.17720

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Low-frequency qubit control wires require non-trivial thermal anchoring and low-pass filtering. The resulting electron temperature serves as a quality benchmark for these signal lines. In this technical note, we make use of a primary electron thermometry technique, using a Coulomb blockade thermometer, to establish the electron temperature in the millikelvin regime. The experimental four-probe measurement setup, the data analysis, and the measurement limitations are discussed in detail. We verify the results by also using another electron thermometry technique, based on a superconductor-insulator-normal metal junction. Our comparison of signal lines with QDevil's QFilter to unfiltered signal lines demonstrates that the filter significantly reduces both the rms noise and electron temperature, which is measured to be 22 ±\pm± 1 mK.
The performance and accuracy of quantum electronics is substantially degraded when the temperature of the electrons in the devices is too high. The electron temperature can be reduced with appropriate thermal anchoring and by filtering both the low frequency and radio frequency noise. Ultimately, for high performance filters the electron temperature can approach the phonon temperature (as measured by resistive thermometers) in a dilution refrigerator. In this application note, the method for measuring the electron temperature in a typical quantum electronics device using Coulomb blockade thermometry is described. This technique is applied to find the readily achievable electron temperature in the device when using the QFilter provided by QDevil. With our thermometry measurements, using a single GaAs/AlGaAs quantum dot in an optimized experimental setup, we determined an electron temperature of 28 ±\pm± 2 mK for a dilution refrigerator base temperature of 18 mK.
Coulomb blockade thermometers (CBTs) have attracted more interest in recent years, as the demand for sub-one Kelvin thermometry has increased, especially due to the prevalence of dilution refrigerators in research and applications in quantum technology. CBTs can be operated both as a primary thermometer, requiring no prior calibration, or as a simple resistance thermometer in the secondary mode after calibration. As new scalable fabrication processes for quantum devices and cryogenic electronics are being developed, cryogenic wafer characterization methods must also scale up to provide statistical data on device parameters. Currently, characterization throughput of cryogenic devices is limited by the turnover time and sample capacity of traditional cryostats, where a measurement cycle for only a few devices can take several days. In this work, we demonstrate wafer-scale cryogenic characterization of a recently developed TiW/Al-AlOx/TiW normal-metal tunnel junction technology using CBTs. Measurements performed in a 300 mm cryogenic wafer prober (CWP) show on-chip electron temperatures below 700 mK across a full 150 mm wafer, as determined by primary thermometry. These results establish wafer-level testing below 1 K as a viable approach for large-scale cryogenic characterization of electrical devices, opening a pathway toward high-throughput screening of quantum devices and direct wafer-scale characterization of aluminum-based superconducting circuits.
Careful filtering is necessary for observations of quantum phenomena in superconducting circuits at low temperatures. Measurements of coherence between quantum states require extensive filtering to protect against noise coupled from room temperature electronics. We demonstrate distributed transmission line filters which cut off exponentially at GHz frequencies and can be anchored at the base temperature of a dilution refrigerator. The compact design makes them suitable to filter many different bias lines in the same setup, necessary for the control and measurement of superconducting qubits.
Superconducting nanowires are widely used in quantum computing, superconducting electronics, and low-temperature physics due to their high sensitivity, compact footprint, and low noise. Here, we present a gate-tunable superconducting nanowire thermometer that enables high-precision temperature measurements in the liquid-helium regime by applying a gate voltage. Experimental results demonstrate stable operation in the 2.5-6.2 K range, with a maximum sensitivity of 4.51×105 cps/K, indicating the potential for sub-millikelvin resolution. The device further exhibits a minimum noise-equivalent temperature difference (NETD) of 5.5 mK, corresponding to millikelvin-level measurement accuracy. Its on-chip integrability, fast response, and tunable performance make it a promising solution for low-temperature sensing and on-chip thermal monitoring in quantum chips. This work provides an experimental platform and theoretical basis for electric-field-controlled superconducting electronics and highlights the potential of gate-controlled nanowires for integrated cryogenic sensing technologies.
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.
We developed a quantum noise measurement system in a dilution refrigerator by using three kinds of cryogenic low pass filters. One of them is a commercial low pass filter inserted into the noise measurement lines instead of the conventional powder filter, which assures well-defined circuit parameters necessary for the noise measurement at a finite frequency. We checked that this filter gives sufficiently large attenuation up to 20 GHz at room temperature, 77 and 4.2 K. The electron temperature of the mesoscopic device placed in the present system was confirmed to be down to around 20 mK by measuring the thermal noise of the device.