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arXiv:1909.10906 · Review of Scientific Instruments

Quantum dot thermometry at ultra-low temperature in a dilution refrigerator with a 4He immersion cell

Giorgio Nicolí, Peter Märki, Beat A. Braem, Marc P. Röösli, Szymon Hennel, Andrea Hofmann, Christian Reichl, W. Wegscheider, Thomas Ihn, K. Ensslin

Published Nov 1, 2019• 12 citations
arXivHTMLPDF

Abstract

Experiments performed at a temperature of a few millikelvins require effective thermalization schemes, low-pass filtering of the measurement lines, and low-noise electronics. Here, we report on the modifications to a commercial dilution refrigerator with a base temperature of 3.5 mK that enable us to lower the electron temperature to 6.7 mK measured from the Coulomb peak width of a quantum dot gate-defined in an [Al]GaAs heteostructure. We present the design and implementation of a liquid 4He immersion cell tight against superleaks, implement an innovative wiring technology, and develop optimized transport measurement procedures.

DOI
10.1063/1.5127830

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Experiments performed at a temperature of a few millikelvins require effective thermalization schemes, low-pass filtering of the measurement lines, and low-noise electronics. Here, we report on the modifications to a commercial dilution refrigerator with a base temperature of 3.5 mK that enable us to lower the electron temperature to 6.7 mK measured from the Coulomb peak width of a quantum dot gate-defined in an [Al]GaAs heteostructure. We present the design and implementation of a liquid 4He immersion cell tight against superleaks, implement an innovative wiring technology, and develop optimized transport measurement procedures.
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.
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.
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.
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.
Rapid thermal cycling is a central bottleneck in the development of superconducting quantum devices: conventional dilution refrigerators require cooldowns of a day or more and substantial cryogenic infrastructure, which throttles the fabricate-measure-redesign loop. We present an ultracompact dilution refrigerator (3 kg in mass and 100 mm in diameter) that completes a full cooldown-warm-up cycle to a base temperature of 70 mK in 1.2 hours when unloaded, and in 2.1 hours when fully equipped with the microwave wiring required for qubit measurements, while delivering 20 microW of cooling power at 100 mK. We validate the platform through a complete characterization of a two-fluxonium device: we extract the full circuit Hamiltonian by two-tone spectroscopy, measure energy-relaxation and coherence times, and benchmark single-qubit control. Although the relaxation time is limited by the base temperature of the system, we reach a single-qubit gate fidelity of up to 99%, at the coherence limit set by our operating temperature. These results demonstrate that compact, fast-cycling dilution refrigeration can support state-of-the-art quantum-device characterization without sacrificing measurement quality, offering a practical route to high-throughput quantum-hardware development.
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.