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Physical Review Research

Microkelvin electronics on a pulse-tube cryostat with a gate Coulomb-blockade thermometer

Mohammad Samani, C. P. Scheller, Omid Sharifi Sedeh, Dominik M. Zumbühl, Nikolai Yurttagül, Kestutis Grigoras, David Gunnarsson, Mika Prunnila, Alexander Thomas Jones, Jonathan R. Prance, R. P. Haley

Published Sep 19, 2022• 11 citations
Publisher

Abstract

Temperatures below 1 mK on-chip hold great potential for quantum physics but present a great challenge due to the lack of suitable thermometry and the detrimental pulse-tube vibrations of cryogen-free refrigerators. Here, we solve the pulse-tube problem using a rigidly wired metallic sample holder, which provides a microkelvin environment with low heat leaks despite the vibrations. Further, we demonstrate an improved type of temperature sensor, the gate Coulomb blockade thermometer (gCBT), employing a gate metallization covering the entire device. This immunizes against nanofabrication imperfections and uncontrollable offset charges, and extends the range to lower temperatures compared to a junction CBT with the same island capacitance, here down to $\ensuremath{\approx}$160 $\ensuremath{\mu}$K for a 10% accuracy. Using on- and off-chip cooling, we demonstrate electronic temperatures as low as 224 $\ifmmode\pm\else\textpm\fi{}$ 7 $\ensuremath{\mu}$K, remaining below 300 $\ensuremath{\mu}$K for 27 hours, thus providing time for experiments. Finally, we give an outlook for cooling below 50 $\ensuremath{\mu}$K for a future generation of microkelvin transport experiments.

DOI
10.1103/physrevresearch.4.033225

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Temperatures below 1 mK on-chip hold great potential for quantum physics but present a great challenge due to the lack of suitable thermometry and the detrimental pulse-tube vibrations of cryogen-free refrigerators. Here, we solve the pulse-tube problem using a rigidly wired metallic sample holder, which provides a microkelvin environment with low heat leaks despite the vibrations. Further, we demonstrate an improved type of temperature sensor, the gate Coulomb blockade thermometer (gCBT), employing a gate metallization covering the entire device. This immunizes against nanofabrication imperfections and uncontrollable offset charges, and extends the range to lower temperatures compared to a junction CBT with the same island capacitance, here down to \ensuremath≈\ensuremath{\approx}\ensuremath≈160 \ensuremathμ\ensuremath{\mu}\ensuremathμK for a 10% accuracy. Using on- and off-chip cooling, we demonstrate electronic temperatures as low as 224 \ifmmode±\else\textpm\fi\ifmmode\pm\else\textpm\fi{}\ifmmode±\else\textpm\fi 7 \ensuremathμ\ensuremath{\mu}\ensuremathμK, remaining below 300 \ensuremathμ\ensuremath{\mu}\ensuremathμK for 27 hours, thus providing time for experiments. Finally, we give an outlook for cooling below 50 \ensuremathμ\ensuremath{\mu}\ensuremathμK for a future generation of microkelvin transport experiments.
Access to lower temperatures has consistently enabled scientific breakthroughs. Pushing the limits of \emph{on-chip} temperatures deep into the microkelvin regime would open the door to unprecedented quantum coherence, novel quantum states of matter, and also the discovery of unexpected phenomena. Adiabatic demagnetization is the workhorse of microkelvin cooling, requiring a dilution refrigerator precooling stage. Pulse-tube dilution refrigerators have grown enormously in popularity due to their vast experimental space and independence of helium, but their unavoidable vibrations are making microkelvin cooling very difficult. On-chip thermometry in this unexplored territory is also not a trivial task due to extreme sensitivity to noise. Here, we present a pulse-tube compatible microkelvin sample holder with on-board cooling and microwave filtering and introduce a new type of temperature sensor, the gate Coulomb blockade thermometer (gCBT), working deep into the microkelvin regime. Using on- and off-chip cooling, we demonstrate electronic temperatures as low as 224$\pm777μK,remainingbelow300K, remaining below 300K,remainingbelow300μKfor27hours,thusprovidingsufficienttimeformeasurements.Finally,wegiveanoutlookforcoolingbelow50K for 27 hours, thus providing sufficient time for measurements. Finally, we give an outlook for cooling below 50Kfor27hours,thusprovidingsufficienttimeformeasurements.Finally,wegiveanoutlookforcoolingbelow50μ$K for a new generation of microkelvin transport experiments.
Abstract Fragile quantum effects such as single electron charging in quantum dots or macroscopic coherent tunneling in superconducting junctions are the basis of modern quantum technologies. These phenomena can only be observed in devices where the characteristic spacing between energy levels exceeds the thermal energy, kBT, demanding effective refrigeration techniques for nanoscale electronic devices. Commercially available dilution refrigerators have enabled typical electron temperatures in the 10 to 100 mK regime, however indirect cooling of nanodevices becomes inefficient due to stray radiofrequency heating and weak thermal coupling of electrons to the device substrate. Here, we report on passing the millikelvin barrier for a nanoelectronic device. Using a combination of on-chip and off-chip nuclear refrigeration, we reach an ultimate electron temperature of Te = 421 ± 35 μK and a hold time exceeding 85 h below 700 μK measured by a self-calibrated Coulomb-blockade thermometer.
A Coulomb-blockade thermometer (CBT) provides calibration-free measurement of low temperatures, even in strong magnetic fields---but slowly. A standard conductance measurement with a CBT takes typically minutes, which is useless in applications where relatively fast changes in temperature are monitored. Besides, a slow measurement is prone to drifts, and low-frequency noise. The authors solve this issue by hooking a CBT sensor into a radio-frequency tank circuit, making the measurement about 1000 times faster. Their approach is expected to impact low-temperature thermometry, for e.g.e.g.e.g. applications in quantum technology.
Accessibility to the microkelvin temperature regime is important for quantum technology research. Cryogen-free dilution refrigerators opened up the millikelvin temperature range, but the perception is that ultralow temperatures are difficult to reach and require specialist infrastructure. The authors report the design of a microkelvin platform based on a nuclear-demagnetization stage, engineered and optimized for operation on a cryogen-free dilution refrigerator. The study demonstrates temperatures as low as 395 $\ensuremath{\mu}$K, and a protocol that enables experiments operated below 1 mK for 95% of the time, providing an efficient cryogen-free microkelvin environment for a wide range of applications.
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.
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.