Germanium Quantum-Well Josephson Field-Effect Transistors and Interferometers

Abstract

Hybrid superconductor-semiconductor structures attract increasing attention owing to a variety of potential applications in quantum computing devices. They can serve to the realization of topological superconducting systems, as well as gate-tunable superconducting quantum bits. Here we combine a SiGe/Ge/SiGe quantum-well heterostructure hosting high-mobility two-dimensional holes and aluminum superconducting leads to realize prototypical hybrid devices, such as Josephson field-effect transistors (JoFETs) and superconducting quantum interference devices (SQUIDs). We observe gate-controlled supercurrent transport with Ge channels as long as one micrometer and 1 arXiv:1810.05012v2 [cond-mat.mes-hall] 23 Oct 2018 estimate the induced superconducting gap from tunnel spectroscopy measurements in superconducting point-contact devices. Transmission electron microscopy reveals the diffusion of Ge into the aluminum contacts, whereas no aluminum is detected in the Ge channel.Modern quantum nanoelectronics takes increasing advantage of newly synthesized hybrid superconductor-semiconductor (S-Sm) interfaces. 1 One of the main motivations is the search for Majorana zero modes that are predicted to appear in a topological superconductor. 2-4 A Josephson field effect transistor (JoFET) is one of the basic devices. It consists of a gatetunable semiconductor channel allowing Cooper-pair exchange between two superconducting contacts mediated by the superconducting proximity effect. 5 Gate control on the Josephson coupling has eventually led to the realization of electrically tunable transmon quantum bits, now often referred to as gatemons. 6-8 Many of the reported experimental realizations of hybrid S-Sm devices rely on bottomup fabrication starting from semiconductor nanowires or carbon nanotubes. 9-16 Recently, new hybrid S-Sm devices were demonstrated using top-down fabrication processes based on two-dimensional systems made of graphene, 17 InAs, 18,19 GaAs, 20 InGaAs 21 or Ge/SiGe. 22,23Top-down nanoscale devices offer significant advantages in terms of complexity and scalability. Those based on p-type SiGe heterostructures are readily compatible with silicon technology, 24 and, thanks to their intrinsically strong spin-orbit coupling, they are an attractive candidate for the development of topological superconducting systems. 22,[25][26][27][28][29][30][31][32] In this work, we present proof-of-concept S-Sm devices in which the semiconducting element consists of an undoped SiGe heterostucture embedding a strained Ge quantum-well (QW). A high-mobility two-dimensional hole gas (2DHG) is electrostatically accumulated in the QW by means of a surface gate electrode. (Hole mobilities as high as 5×10 5 cm 2 /Vs were reported for similar heterostructures. 12,22,33,34 ) The superconducting proximity effect induces gate-tunable superconductivity in the 2DHG enabling JoFET operation. This functionality is exploited for the realization of gate-controlled superconducting quantum interference

Fast two-qubit logic with holes in germanium

“…The full π phase shift on Q2 for a conditional 2π rotation on Q1, as a result of the θ/2 phase that is accumulated by the control qubit, demonstrates the application of a coherent CX gate.The demonstration of a universal gate set with all electrical control and without the need of any microscopic structures provide great prospects to scale up spin qubits using holes in strained germanium. These quantum dots are furthermore contacted by superconductors [22,36,37] that may be shaped into microwave resonators for spin-photon coupling, providing opportunities for a platform that can combine semiconducting, superconducting, and topological systems for hybrid technology with fast and coherent control over individual hole spins. Moreover, the demonstrated quantum coherence and level of control 4 0 20 40 m 0 50 100 I SD (fA) F = 99.3 % X Y -X -Y X/2 Y/2 -X/2 -Y/2 Interleaved gate 99 100 Gate fidelity 100 200 300 t (ns) -20 -15 -10 P (arb.…”

A single-hole spin qubit

“…More recently, hole spins in group-IV materials have gained attention as a platform for quantum information processing [19][20][21][22] . In particular, hole states in germanium can provide a high degree of quantum dot tunability [23][24][25] , fast and all-electrical driving 20,21 and Ohmic contacts to superconductors for hybrids 26,27 . These experiments culminated in the recent demonstration of full two-qubit logic 21 .…”

Highly Transparent Contacts to the 1D Hole Gas in Ultrascaled Ge/Si Core/Shell Nanowires

“…To overcome these limitations, material combinations with no intermetallic phase formation, such as the Al-Ge system, enabling true metal-semiconductor heterostructures with abrupt metal-semiconductor interfaces received a considerable amount of attention. 12,13,14,15,16,17 Within this paper, we apply quasi 1D…”

“…19 Further, this architecture due to the strong spin-orbit coupling of holes in Ge could be an attractive candidate for the study of Majorana zero modes and development of topological superconducting qubits. 16,20 Our approach for the synthesis of a monolithic Al-Ge/Si-Al NW heterostructure featuring highly transparent contacts to a 1D hole gas is based on vapor-liquid-solid 21 to the converted c-Al region the intensity changes over about 1 nm. However, if we measure the local lattice spacing, as shown in the inset, we find the last plane of higher HAADF STEM intensity has the lattice spacing close to the value of a Ge (111) plane (about 0.32 nm).…”