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Europhysics news

Breaking the millikelvin barrier in nanoelectronics

R. P. Haley, Jonathan R. Prance, Dominik M. Zumbühl

Published Jan 1, 2021• 0 citations
Publisher

Abstract

In labs across Europe physicists are pushing the boundaries of how far we can cool the electrons in nano-fabricated circuits and quantum-enhanced devices. The cryogen-free revolution in dilution refrigeration has liberated researchers from a reliance on helium, a costly and non-renewable resource, and hugely expanded the numbers of cooling machines available for new science and quantum technology applications which exploit the properties of materials at kelvin and millikelvin temperatures.

DOI
10.1051/epn/2021406

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In labs across Europe physicists are pushing the boundaries of how far we can cool the electrons in nano-fabricated circuits and quantum-enhanced devices. The cryogen-free revolution in dilution refrigeration has liberated researchers from a reliance on helium, a costly and non-renewable resource, and hugely expanded the numbers of cooling machines available for new science and quantum technology applications which exploit the properties of materials at kelvin and millikelvin temperatures.
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.
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.
Cryogen-free dilution refrigerators (dry fridges) have become an increasingly popular solution to researchers’ cryogenic measurement needs due to their user friendliness and long term use cost savings (4He does not need to be replenished). However, commercially available dry fridges frequently have an effective electron temperatures much warmer than the base temperature of the mixing chamber of the dry fridge. This thesis documents efforts taken to reduce the electron temperature in a dry fridge and the direct probing of the electron temperature via Coulomb blockade thermometry in a 500 nm wide quantum dot.
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.
Standard quantum architectures rely on dilution refrigeration to suppress thermal noise, tethering the industry to a non-scalable 20 mK thermal floor. The cooling power at millikelvin temperatures scales poorly, creating a “mainframe constraint” that limits widespread deployment. Here, a 4-Kelvin topological processor is proposed, utilizing twistronic symmetry breaking in Bi-2212 to engineer a 25 meV spectral gap. By integrating this intrinsic protection with RSFQ control logic and pulse tube cryogenics via a flip-chip architecture, a stability margin of Γ≈20\Gamma \approx 20Γ≈20 is achieved even with interface losses. This architecture eliminates the need for dilution refrigeration, enabling scalable, rack-mounted quantum processing.