Superconductors and semiconductors are crucial platforms in the field of quantum computing. They can be combined to hybrids, bringing together physical properties that enable the discovery of new emergent phenomena and provide novel strategies for quantum control. The involved semiconductor materials, however, suffer from disorder, hyperfine interactions or lack of planar technology. Here we realise an approach that overcomes these issues altogether and integrate gate-defined quantum dots and superconductivity into germanium heterostructures. In our system, heavy holes with mobilities exceeding 500,000 cm2 (Vs)−1 are confined in shallow quantum wells that are directly contacted by annealed aluminium leads. We observe proximity-induced superconductivity in the quantum well and demonstrate electric gate-control of the supercurrent. Germanium therefore has great promise for fast and coherent quantum hardware and, being compatible with standard manufacturing, could become a leading material for quantum information processing.
We investigate the magnetic field and temperature dependence of the single-electron spin lifetime in silicon quantum dots and find a lifetime of 2.8 ms at a temperature of 1.1 K. We develop a model based on spin-valley mixing and find that Johnson noise and two-phonon processes limit relaxation at low and high temperature respectively. We also investigate the effect of temperature on charge noise and find a linear dependence up to 4 K. These results contribute to the understanding of relaxation in silicon quantum dots and are promising for qubit operation at elevated temperatures.
We fabricate Josephson field-effect-transistors in germanium quantum wells contacted by superconducting aluminum and demonstrate supercurrents carried by holes that extend over junction lengths of several micrometers. In superconducting quantum point contacts we observe discretization of supercurrent, as well as Fabry-Perot resonances, demonstrating ballistic transport. The magnetic field dependence of the supercurrent follows a clear Fraunhofer-like pattern and Shapiro steps appear upon microwave irradiation. Multiple Andreev reflections give rise to conductance enhancement and evidence a transparent interface, confirmed by analyzing the excess current. These demonstrations of ballistic superconducting transport are promising for hybrid quantum technology in germanium.Quantum information processing in the solid-state is being pursued using superconducting and semiconducting platforms [1,2]. In both platforms, rudimentary quantum algorithms have already been demonstrated [3,4]. While decoherence is a central topic, advanced superconducting systems are now capable of entangling 10 qubits [5]. Spin qubits based on silicon (Si) and germanium (Ge), on the other hand, can be isotopically enriched to remove magnetic decoherence [6,7], resulting in extremely long coherence times [8,9]. Crucially, these qubits can be defined using conventional semiconductor technology. A hybrid approach may build upon the strengths of each platform motivating extensive research. Superconducting qubits with semiconductor elements have led to electric gate-tuneable superconducting qubits [10,11], or gatemons, while spin qubits interfaced with superconducting resonators have reached the regime of strong spin-photon coupling [12][13][14], an important step toward long-range entanglement.Hybrid technology in condensed matter physics has even more surprises and can host exotic excitations. In particular, a topological phase transition may occur in superconductor-semiconductor systems in the presence of spin-orbit coupling and magnetism [15,16]. At the topological transition, excitations emerge that represent Majorana fermion states that can exhibit non-Abelian exchange statistics. Next to their fundamental interest, these states are argued to be excellent building blocks for quantum computation as they bear some topological protection against decoherence. Despite protection limited only to operations inside the Clifford group, coupling topological qubits to spin qubits may offer an effective pathway toward universal quantum computation [17]. In addition, integrating topological systems to the spin qubit platform may enable the coupling of spatially separated spin qubits via topologically protected braiding operations [18,19].Germanium has the potential to become an excellent material platform for the construction of these hybrid systems. It can be isotopically purified, thereby removing decoherence by nuclear spins [6], and can host strong-spin orbit coupling [20], in particular when the charge carriers are holes. In addition, mobilities reach...
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