Electric-field dependent g-factor anisotropy in Ge-Si core-shell nanowire quantum dots

We present angle-dependent measurements of the effective g-factor g* in a Ge-Si core-shell nanowire quantum dot. g* is found to be maximum when the magnetic field is pointing perpendicular to both the nanowire and the electric field induced by local gates. Alignment of the magnetic field with the electric field reduces g* significantly. g* is almost completely quenched when the magnetic field is aligned with the nanowire axis. These findings confirm recent calculations, where the obtained anisotropy is attributed to a Rashba-type spin-orbit interaction induced by heavy-hole light-hole mixing. In principle, this facilitates manipulation of spin-orbit qubits by means of a continuous high-frequency electric field

Highly tuneable hole quantum dots in Ge-Si core-shell nanowires

We define single quantum dots of lengths varying from 60 nm up to nearly half a micron in Ge-Si core-shell nanowires. The charging energies scale inversely with the quantum dot length between 18 and 4 meV. Subsequently, we split up a long dot into a double quantum dot with a separate control over the tunnel couplings and the electrochemical potential of each dot. Both single and double quantum dot configurations prove to be very stable and show excellent control over the electrostatic environment of the dots, making this system a highly versatile platform for spin-based quantum computing

Single-hole tunneling through a two-dimensional hole gas in intrinsic silicon

In this letter we report single-hole tunneling through a quantum dot in a two-dimensional hole gas, situated in a narrow-channel field-effect transistor in intrinsic silicon. Two layers of aluminum gate electrodes are defined on Si/SiO$_2$ using electron-beam lithography. Fabrication and subsequent electrical characterization of different devices yield reproducible results, such as typical MOSFET turn-on and pinch-off characteristics. Additionally, linear transport measurements at 4 K result in regularly spaced Coulomb oscillations, corresponding to single-hole tunneling through individual Coulomb islands. These Coulomb peaks are visible over a broad range in gate voltage, indicating very stable device operation. Energy spectroscopy measurements show closed Coulomb diamonds with single-hole charging energies of 5--10 meV, and lines of increased conductance as a result of resonant tunneling through additional available hole states.Comment: 4 pages, 4 figures. This article has been submitted to Applied Physics Letter

AC Josephson effect in a gate-tunable Cd3_3As2_2 nanowire superconducting weak link

Three-dimensional topological Dirac semimetals have recently gained significant attention, since they possess exotic quantum states. When constructing Josephson junctions utilizing these materials as the weak link, the fractional ac Josephson effect emerges in the presence of a topological supercurrent contribution. We investigate the ac Josephson effect in a Dirac semimetal Cd$_3$As$_2$ nanowire using two complementary methods: by probing the radiation spectrum and by measuring Shapiro patterns. With both techniques, we find that conventional supercurrent dominates at all investigated doping levels and that any potentially present topological contribution falls below our detection threshold. The inclusion of thermal noise in a resistively and capacitively shunted junction (RCSJ) model allows us to reproduce the microwave characteristics of the junction. With this refinement, we explain how weak superconducting features can be masked and provide a framework to account for elevated electronic temperatures present in realistic experimental scenarios

Anisotropic Pauli spin blockade in hole quantum dots

We present measurements on gate-defined double quantum dots in Ge-Si core-shell nanowires, which we tune to a regime with visible shell filling in both dots. We observe a Pauli spin blockade and can assign the measured leakage current at low magnetic fields to spin-flip cotunneling, for which we measure a strong anisotropy related to an anisotropic g-factor. At higher magnetic fields we see signatures for leakage current caused by spin-orbit coupling between (1,1)-singlet and (2,0)-triplet states. Taking into account these anisotropic spin-flip mechanisms, we can choose the magnetic field direction with the longest spin lifetime for improved spin-orbit qubits

Phase-dependent microwave response of a graphene Josephson junction

Gate-tunable Josephson junctions embedded in a microwave environment provide a promising platform to in situ engineer and optimize novel superconducting quantum circuits. The key quantity for the circuit design is the phase-dependent complex admittance of the junction, which can be probed by sensing a radio frequency SQUID with a tank circuit. Here, we investigate a graphene-based Josephson junction as a prototype gate-tunable element enclosed in a SQUID loop that is inductively coupled to a superconducting resonator operating at 3 GHz. With a concise circuit model that describes the dispersive and dissipative response of the coupled system, we extract the phase-dependent junction admittance corrected for self-screening of the SQUID loop. We decompose the admittance into the current-phase relation and the phase-dependent loss, and as these quantities are dictated by the spectrum and population dynamics of the supercurrent-carrying Andreev bound states, we gain insight to the underlying microscopic transport mechanisms in the junction. We theoretically reproduce the experimental results by considering a short, diffusive junction model that takes into account the interaction between the Andreev spectrum and the electromagnetic environment, from which we estimate lifetimes on the order of ∼10 ps for nonequilibrium populations

Performance of high impedance resonators in dirty dielectric environments

High-impedance resonators are a promising contender for realizing long-distance entangling gates between spin qubits. Often, the fabrication of spin qubits relies on the use of gate dielectrics which are detrimental to the quality of the resonator. Here, we investigate loss mechanisms of high-impedance NbTiN resonators in the vicinity of thermally grown SiO\textsubscript{2} and Al\textsubscript{2}O\textsubscript{3} fabricated by atomic layer deposition. We benchmark the resonator performance in elevated magnetic fields and at elevated temperatures and find that the internal quality factors are limited by the coupling between the resonator and two-level systems of the employed oxides. Nonetheless, the internal quality factors of high-impedance resonators exceed $10^3$ in all investigated oxide configurations which implies that the dielectric configuration would not limit the performance of resonators integrated in a spin-qubit device. Because these oxides are commonly used for spin qubit device fabrication, our results allow for straightforward integration of high-impedance resonators into spin-based quantum processors. Hence, these experiments pave the way for large-scale, spin-based quantum computers.Comment: 10 pages, 6 figure

Zeeman effect induced 0-π\pi transitions in ballistic Dirac semimetal Josephson junctions

One of the consequences of Cooper pairs having a finite momentum in the interlayer of a Josephson junction, is $\pi$-junction behavior. The finite momentum can either be due to an exchange field in ferromagnetic Josephson junctions, or due to the Zeeman effect. Here, we report the observation of Zeeman effect induced 0-$\pi$ transitions in Bi$_{1-x}$Sb$_x$, 3D Dirac semimetal-based Josephson junctions. The large g-factor of the Zeeman effect from a magnetic field applied in the plane of the junction allows tuning of the Josephson junctions from 0- to $\pi$- regimes. This is revealed by sign changes in the modulation of the critical current by applied magnetic field of an asymmetric superconducting quantum interference device (SQUID). Additionally, we directly measure a non-sinusoidal current-phase relation in the asymmetric SQUID, consistent with models for ballistic Josephson transport

Serial cardiovascular magnetic resonance feature tracking indicates early worsening of cardiac function in Fontan patients

Background: In Fontan patients, attrition of ventricular function is well recognized, but early detection of ventricular dysfunction is difficult. The aim of this study is to longitudinally assess ventricular strain in Fontan patients using a new method for cardiac magnetic resonance (CMR) feature tracking, and to investigate the relationship between ventricular strain and cardiac systolic function. Methods and results: In this prospective, standardized follow-up study in 51 Fontan patients, age ≥ 10 years, CMR and concomitant clinical assessment was done at the start of the study and after 2 years. CMR feature tracking was done combining the dominant and hypoplastic ventricles. Global longitudinal strain (GLS) (−17.3% versus −15.9%, P = 0.041) and global circumferential strain (GCS) (−17.7 versus −16.1, P = 0.047) decreased over 2 years' time. Ejection fraction (EF) (57%), cardiac index (CI) (2.7 l/min/m2) and NYHA functional class (97% in class I/II) were preserved. The strain values of the combined dominant and hypoplastic ventricles were significantly worse compared to those of the dominant ventricle only (GLS −16.8 (−19.5 to −14.0) versus −18.8 (−21.3 to −15.3) respectively, P = 0.001, GCS −18.3 (−22.1 to −14.8) versus −22.5 (−26.3 to −19.4) respectively, P < 0.001). Conclusions: This study showed a decrease in cardiac strain over 2 years in Fontan patients without clinical signs of Fontan failure, where EF, CI and clinical status were still preserved. Cardiac strain might be a sensitive early indicator of systolic ventricular decline. Furthermore, combined strain of the hypoplastic and dominant ventricles seems a more accurate representation of cardiac strain in functionally univentricular hearts