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The potential of Si and SiGe-based devices for the scaling of quantum circuits is tainted by device variability. Each device needs to be tuned to operation conditions and each device realisation requires a different tuning protocol. We demonstrate that it is possible to automate the tuning of a 4-gate Si FinFET, a 5-gate GeSi nanowire and a 7-gate Ge/SiGe heterostructure double quantum dot device from scratch with the same algorithm. We achieve tuning times of 30, 10, and 92 min, respectively. The algorithm also provides insight into the parameter space landscape for each of these devices, allowing for the characterization of the regions where double quantum dot regimes are found. These results show that overarching solutions for the tuning of quantum devices are enabled by machine learning. [ABSTRACT FROM AUTHOR]

Metal contacts in semiconductor quantum electronic devices can offer advantages over doped contacts, primarily due to their reduced fabrication complexity and lower temperature requirements during processing. Some metals can also facilitate ambipolar device operation or form superconducting contacts. Furthermore, a sharp metal–semiconductor interface allows for contact placement in close proximity to the active device area avoiding damage caused by dopant implantation. However, in the case of gate-defined quantum dots in intrinsic silicon, the formation of a Schottky barrier at the silicon–metal interface can lead to large, nonlinear contact resistances at cryogenic temperatures. We investigate this issue by examining hole transport through metal oxide-semiconductor transistors with platinum silicide contacts on intrinsic silicon substrates. We extract the contact and channel resistances as a function of temperature and improve the cryogenic conductance of the device by more than an order of magnitude by implementing meander-shaped contacts. In addition, we observe signatures of enhanced transport through localized defect states, which we attribute to platinum clusters in the depletion region of the Schottky contacts that form during the silicidation process. These results showcase the prospects of silicide contacts in the context of cryogenic quantum devices and address associated challenges. [ABSTRACT FROM AUTHOR]

Fault-tolerant spin-based quantum computers will require fast and accurate qubit read out. This can be achieved using radiofrequency reflectometry given sufficient sensitivity to the change in quantum capacitance associated with the qubit states. Here, we demonstrate a 23-fold improvement in capacitance sensitivity by supplementing a cryogenic semiconductor amplifier with a SQUID preamplifier. The SQUID amplifier operates at a frequency near 200 MHz and achieves a noise temperature below 600 mK when integrated into a reflectometry circuit, which is within a factor 120 of the quantum limit. It enables a record sensitivity to capacitance of 0.07 aF / Hz . The setup is used to acquire charge stability diagrams of a gate-defined double quantum dot in a short time with a signal-to-noise ration of about 38 in 1 μ s of integration time. [ABSTRACT FROM AUTHOR]

This article was originally published online on 2 July 2024 with errors to the affiliation links for the eighth, tenth, and eleventh authors. All affiliation links are correct as they appear above.All online versions of this article were corrected on 10 July 2024.By K. Tsoukalas; F. Schupp; L. Sommer; I. Bouquet; M. Mergenthaler; S. Paredes; N. Vico Triviño; M. Luisier; G. Salis; P. Harvey-Collard; D. Zumbühl and A. FuhrerReported by Author; Author; Author; Author; Author; Author; Author; Author; Author; Author; Author; Author [Extracted from the article]

Quantum devices with a large number of gate electrodes allow for precise control of device parameters. This capability is hard to fully exploit due to the complex dependence of these parameters on applied gate voltages. We experimentally demonstrate an algorithm capable of fine-tuning several device parameters at once. The algorithm acquires a measurement and assigns it a score using a variational auto-encoder. Gate voltage settings are set to optimize this score in real-time in an unsupervised fashion. We report fine-tuning times of a double quantum dot device within approximately 40 min. [ABSTRACT FROM AUTHOR]

We report highly tunable control of holes in Ge/Si core/shell nanowires. We demonstrate the ability to create single quantum dots of various sizes, with low hole occupation numbers and clearly observable excited states. For the smallest dot size, we observe indications of single-hole occupation. Moreover, we create double and triple tunnel-coupled quantum dot arrays. In the double quantum dot configuration, we observe Pauli spin blockade. These results open the way to perform hole spin qubit experiments in these devices. [ABSTRACT FROM AUTHOR]

Cooling nanoelectronic devices below 10mK is a great challenge since thermal conductivities become very small, thus creating a pronounced sensitivity to heat leaks. Here, we overcome these difficulties by using adiabatic demagnetization of both the electronic leads and the large metallic islands of a Coulomb blockade thermometer. This reduces the external heat leak through the leads and also provides on-chip refrigeration, together cooling the thermometer down to 2.8±0.1 mK. We present a thermal model which gives a good qualitative account and suggests that the main limitation is heating due to pulse tube vibrations. With better decoupling, temperatures below 1mK should be within reach, thus opening the door for μK nanoelectronics. [ABSTRACT FROM AUTHOR]

We present a parallel network of 16 demagnetization refrigerators mounted on a cryofree dilution refrigerator aimed to cool nanoelectronic devices to sub-millikelvin temperatures. To measure the refrigerator temperature, the thermal motion of electrons in a Ag wire--thermalized by a spot-weld to one of the Cu nuclear refrigerators--is inductively picked-up by a superconducting gradiometer and amplified by a SQUID mounted at 4 K. The noise thermometer as well as other thermometers are used to characterize the performance of the system, finding magnetic field independent heat-leaks of a few nW/mol, cold times of several days below 1 mK, and a lowest temperature of 150 μK of one of the nuclear stages in a final field of 80 mT, close to the intrinsic SQUID noise of about 100 μK. A simple thermal model of the system capturing the nuclear refrigerator, heat leaks, and thermal and Korringa links describes the main features very well, including rather high refrigerator efficiencies typically above 80%. [ABSTRACT FROM AUTHOR]

We propose a new scheme aimed at cooling nanostructures to microkelvin temperature based on the well established technique of adiabatic nuclear demagnetization: we attach each device measurement lead to an individual nuclear refrigerator, allowing efficient thermal contact to a microkelvin bath. On a prototype consisting of a parallel network of nuclear refrigerators, temperatures of ∼1 mK simultaneously on ten measurement leads have been reached upon demagnetization, thus completing the first steps toward ultracold nanostructures. [ABSTRACT FROM AUTHOR]

We present transport measurements of cleaved edge overgrowth GaAs quantum wires. The conductance of the first mode reaches 2e2/h at high temperatures T≳10 K, as expected. As T is lowered, the conductance is gradually reduced to 1e2/h, becoming T independent at T≲0.1 K, while the device cools far below 0.1 K. This behavior is seen in several wires, is independent of density, and not altered by moderate magnetic fields B. The conductance reduction by a factor of 2 suggests lifting of the electron spin degeneracy in the absence of B. Our results are consistent with theoretical predictions for helical nuclear magnetism in the Luttinger liquid regime., Physics

We present an improved nuclear refrigerator reaching 0.3 mK, aimed at microkelvin nanoelectronic experiments, and use it to investigate metallic Coulomb blockade thermometers (CBTs) with various resistances R. The high-R devices cool to slightly lower T, consistent with better isolation from the noise environment, and exhibit electron-phonon cooling ∝ T 5 and a residual heat-leak of 40 aW. In contrast, the low-R CBTs display cooling with a clearly weaker T-dependence, deviating from the electron-phonon mechanism. The CBTs agree excellently with the refrigerator temperature above 20 mK and reach a minimum-T of 7.5 ± 0.2 mK. [ABSTRACT FROM AUTHOR]

We demonstrate low leakage surface gating of an indium arsenide heterostructure with the two-dimensional electron gas close to the surface. Gating is made possible by growing an aluminum oxide layer on top of the device. We find that the depletion point can be changed by applying a positive gate voltage and we see hysteresis when the voltage is swept below depletion. [ABSTRACT FROM AUTHOR]

We report a thermally activated metastability in a GaAs double quantum dot exhibiting real-time charge switching in diamond shaped regions of the charge stability diagram. Accidental charge traps and sensor backaction are excluded as the origin of the switching. We present an extension of the canonical double dot theory based on an intrinsic, thermal electron exchange process through the reservoirs, giving excellent agreement with the experiment. The electron spin is randomized by the exchange process, thus facilitating fast, gate-controlled spin initialization. At the same time, this process sets an intrinsic upper limit to the spin relaxation time. [ABSTRACT FROM AUTHOR]

We perform momentum-conserving tunneling spectroscopy using a GaAs cleaved-edge overgrowth quantum wire to investigate adjacent quantum Hall edge states. We use the lowest five wire modes with their distinct wave functions to probe each edge state and apply magnetic fields to modify the wave functions and their overlap. This reveals an intricate and rich tunneling conductance fan structure which is succinctly different for each of the wire modes. We self-consistently solve the Poisson-Schrödinger equations to simulate the spectroscopy, reproducing the striking fans in great detail, thus, confirming the calculations. Further, the model predicts hybridization between wire states and Landau levels, which is also confirmed experimentally. This establishes momentum-conserving tunneling spectroscopy as a powerful technique to probe edge state wave functions. [ABSTRACT FROM AUTHOR]

Introduction: The most common cause of acute renal colic is a ureteral obstruction caused by ureterolithiasis. Urgent intervention is often necessary due to intractable pain. Early extracorporeal shockwave lithotripsy (SWL) as an alternative treatment option to ureteral stenting becomes forgotten in times of rising ureterorenoscopy. However, definitive guidelines are lacking, in which urgent treatment should be preferred in the absence of signs of infection. Therefore, we assessed efficacy and safety of early SWL (eSWL) to secondary SWL (sSWL) after urgent ureteral stenting. Patients and Methods: One hundred four patients treated between January 2015 and November 2017 for obstructive ureterolithiasis were matched regarding stone size, stone localization, and assigned to group eSWL ( n  = 52) or group sSWL ( n  = 52). The eSWL group received shock waves (without prior ureteral stenting) and sSWL group ureteral stenting within 48 hours from diagnosis. Thereafter, patients in group sSWL were treated with shock waves for a median of 23 ± 14.6 days after ureteral stenting. Stone-free rates, complication rates, and reintervention rates were assessed. Univariable and multivariable logistic regression was applied to find predictors of outcomes in the two treatment groups. Results: Overall, there was no statistically significant difference between both groups regarding stone-free rate and complication rate. Reinterventions were more often addressed for patients in group sSWL ( p  = 0.05). eSWL was significantly superior to sSWL regarding stone-free rates for stones between 6 and 9 mm ( p  = 0.04). At the multivariable multinomial logistic regression none of the two treatment modalities was associated with better outcomes. A body mass index ≥30 was associated with a reduced 6-week stone-free status ( p  = 0.04), whereas stones ≥8 mm were associated with an increased need of reintervention ( p  = 0.04). Conclusion: eSWL seems to be an effective and safe emergency procedure compared with sSWL after urgent stenting within 6 weeks and should be considered as a treatment option in patients without absolute indications for immediate ureteral drainage. Clinical trial registration number: 2019-00155.

We demonstrate electrical control of the spin relaxation time T1 between Zeeman-split spin states of a single electron in a lateral quantum dot. We find that relaxation is mediated by the spin-orbit interaction, and by manipulating the orbital states of the dot using gate voltages we vary the relaxation rate W identical withT1(-1) by over an order of magnitude. The dependence of W on orbital confinement agrees with theoretical predictions, and from these data we extract the spin-orbit length. We also measure the dependence of W on the magnetic field and demonstrate that spin-orbit mediated coupling to phonons is the dominant relaxation mechanism down to 1 T, where T1 exceeds 1 s.

We present measurements of the rates for an electron to tunnel on and off a quantum dot, obtained using a quantum point contact charge sensor. The tunnel rates show exponential dependence on drain-source bias and plunger gate voltages. The tunneling process is shown to be elastic, and a model describing tunneling in terms of the dot energy relative to the height of the tunnel barrier quantitatively describes the measurements.

The symmetry properties of transport beyond the linear regime in chaotic quantum dots are investigated experimentally. A component of differential conductance that is antisymmetric in both applied source-drain bias V and magnetic field B, absent in linear transport, is found to exhibit mesoscopic fluctuations around a zero average. Typical values of this component allow a measurement of the electron interaction strength.

Few-electron quantum dots are investigated in the regime of strong tunneling to the leads. Inelastic cotunneling is used to measure the two-electron singlet-triplet splitting above and below a magnetic field driven singlet-triplet transition. Evidence for a nonequilibrium two-electron singlet-triplet Kondo effect is presented. Cotunneling allows orbital correlations and parameters characterizing entanglement of the two-electron singlet ground state to be extracted from dc transport.

In situ control of spin-orbit coupling in coherent transport using a clean GaAs/AlGaAs two-dimensional electron gas is realized, leading to a gate-tunable crossover from weak localization to antilocalization. The necessary theory of 2D magnetotransport in the presence of spin-orbit coupling beyond the diffusive approximation is developed and used to analyze experimental data. With this theory the Rashba contribution and linear and cubic Dresselhaus contributions to spin-orbit coupling are separately estimated, allowing the angular dependence of spin-orbit precession to be extracted at various gate voltages.

We investigate antilocalization due to spin-orbit coupling in ballistic GaAs quantum dots. Antilocalization that is prominent in large dots is suppressed in small dots, as anticipated theoretically. Parallel magnetic fields suppress both antilocalization and also, at larger fields, weak localization, consistent with random matrix theory results once orbital coupling of the parallel field is included. In situ control of spin-orbit coupling in dots is demonstrated as a gate-controlled crossover from weak localization to antilocalization.