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Science

Building a nanowire superconducting qubit

Jelena Stajic

Published Oct 1, 2015• 0 citations
Publisher

Abstract

Superconduction A major ingredient of a superconducting qubit is a Josephson junction, typically containing an insulator that acts as a bridge (“weak link”) between two superconductors. Larsen et al. and de Lange et al. fabricated superconducting qubits that, instead of an insulator, used a semiconducting nanowire as the weak link. Both groups used InAs nanowires, in one case enclosed in a shell of superconducting Al and in the other contacted by superconducting NbTiN. The researchers controlled the frequency of the qubit—a complex operation in the conventional setup—by simply modulating the carrier concentration of the nanowire with gate voltage. The new architecture may lead to more-scalable quantum computers and address fundamental issues such as the quantum statistics of Majorana fermions. Phys. Rev. Lett. 115 , 127001; 127002 (2015).

DOI
10.1126/science.350.6256.53-b

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Superconduction
A major ingredient of a superconducting qubit is a Josephson junction, typically containing an insulator that acts as a bridge (“weak link”) between two superconductors. Larsen et al. and de Lange et al. fabricated superconducting qubits that, instead of an insulator, used a semiconducting nanowire as the weak link. Both groups used InAs nanowires, in one case enclosed in a shell of superconducting Al and in the other contacted by superconducting NbTiN. The researchers controlled the frequency of the qubit—a complex operation in the conventional setup—by simply modulating the carrier concentration of the nanowire with gate voltage. The new architecture may lead to more-scalable quantum computers and address fundamental issues such as the quantum statistics of Majorana fermions.
Phys. Rev. Lett. 115 , 127001; 127002 (2015).
We introduce a hybrid qubit based on a semiconductor nanowire with an epitaxially grown superconductor layer. Josephson energy of the transmonlike device ("gatemon") is controlled by an electrostatic gate that depletes carriers in a semiconducting weak link region. Strong coupling to an on-chip microwave cavity and coherent qubit control via gate voltage pulses is demonstrated, yielding reasonably long relaxation times (~0.8 μs) and dephasing times (~1 μs), exceeding gate operation times by 2 orders of magnitude, in these first-generation devices. Because qubit control relies on voltages rather than fluxes, dissipation in resistive control lines is reduced, screening reduces cross talk, and the absence of flux control allows operation in a magnetic field, relevant for topological quantum information.
A practical quantum computer demands a physical quantum bit (qubit) that is scalable, has a well-defined initial state, and brings together coherence with reliable control and read-out. A compelling idea is to put together a locally protected Majorana qubit with a superconducting qubit. The superconducting qubit is used to read out the Majorana states while the Majorana qubit may act as a quantum memory. This thesis presents a series of studies on quantum transport in semiconducting nanowires coupled to superconductors to investigate various platforms for realizing Majorana. These studies are followed by microwave measurements on superconducting circuits compatible with semiconducting-superconducting heterostructures. We first evaluate the potential of Ge/Si core/shell nanowires by achieving induced superconductivity as well as estimating spin-orbit coupling. Next we explore the transport mediated by Andreev bound states formed in InSb nanowire quantum. A subgap negative differential conductance is investigated together with the coalescing Andreev resonances at zero bias relevant for the correct interpretation of Majorana experiments done on the same structures. We conclude our studies of semiconducting nanowires by exploring tunnel junctions in Sn-InSb nanowires that are prepared by in-situ shadowing using nearby nanowires and flakes. Tin shells are found to induce a hard superconducting gap persisting up to high magnetic fields. We observe the two-electron charging effect from a small superconducting island of Sn-InSb. This effect is attributed to charge parity stability, which makes this nanowire system an intriguing candidate for superconducting and topological quantum circuits. These Sn-InSb junctions are then used as the nonlinear element in a transmon qubit design where we observe a dispersive coupling between this nanowire Josephson junction and a superconducting resonator. We also present our progress towards building a magnetic field resilient superconducting circuit that allows integration with semiconducting and topological structures. In the last chapter, InSb semiconducting nanowires are utilized as shadow masks prior to superconductor deposition on an InAs quantum well. We study Josephson current properties in Josephson junctions made from these nanowire shadows. Our results point to highly transparent junctions that can be developed further for hybrid superconductor-semiconductor qubit systems.
Superconductor qubits typically use aluminum–aluminum oxide tunnel junctions to provide the nonlinear inductance. Junctions with semiconductor barriers make it possible to vary the superconductor material and explore beyond aluminum. We use InAs semiconductor nanowires coated with thin superconducting shells of β-Sn to realize transmon qubits. By tuning the Josephson energy with a gate voltage, we adjust the qubit frequency over a range of 3 GHz. The longest energy relaxation time, T 1 = 27 μs, is obtained at the lowest qubit frequencies, while the longest echo dephasing time, T 2 = 1.8 μs, is achieved at higher frequencies. We assess the possible factors limiting coherence times in these devices and discuss steps to enhance performance through improvements in materials fabrication and circuit design.
Superconducting qubits are among the most promising platforms for realizing practical quantum computers. One requirement to create a quantum processor is nonlinearity, which in superconducting circuits is typically achieved by sandwiching a layer of aluminum oxide between two aluminum electrodes to form a Josephson junction. These junctions, however, face several limitations that hinder their scalability: the small superconducting gap of aluminum necessitates millikelvin operating temperatures, the material interfaces lead to dissipation, and the sandwich geometry adds unwelcome capacitance for high-frequency applications. In this work, we address all three limitations using a novel superconducting weak link based on the superconductor-insulator transition. By locally thinning a single film of niobium nitride, we exploit its thickness-driven superconductor-insulator transition to form a weak link employing only atomic layer deposition and atomic layer etching. We utilize our weak links to produce a transmon qubit, 'planaronplanaronplanaron', with a measured anharmonicity of α/2π=235α/2π= 235α/2π=235 MHz; at present, the linewidth is κ/2π=15 MHzκ/2π= 15 \mathrm{\: MHz}κ/2π=15MHz. The high superconducting gap of niobium nitride can enable operation at elevated temperatures in future devices, and the fully planar geometry of the weak link eliminates superfluous material interfaces and capacitances. The investigation of small patches of material near the SIT can shed new light on the nature of the transition, including the role of dissipation and finite-size effects.
Abstract In this perspective article, we review the current state of research on integrating quantum materials (QMs) into superconducting quantum devices. We begin with the role of QMs as weak links in Josephson junctions, enabling gate- and flux-tunable transmons. We then explore their application in more complex superconducting circuits, such as gate-tunable fluxonium qubits, or gatemonium, which provide additional control over qubit parameters. We also discuss QM-based vertical junctions and their potential for creating merged-element transmons. Further, we highlight QMs’ role in topological superconducting circuits, where they facilitate the study of Majorana zero modes through signatures such as 4π-periodic supercurrents. Additionally, we review the integration of QMs into 3D cavity architectures and discuss how they differ from their 2D counterparts. Beyond weak links, we examine the use of 2D superconducting and insulating materials, such as NbSe2 and hBN, in parallel-plate capacitors, offering a compact alternative to conventional large-footprint transmon capacitors. Finally, we outline the current challenges and future directions for exploration.
Abstract We demonstrate that superconducting gatemon qubits based on superconductor-semiconductor-superconductor Josephson junctions can be constructed on hole-type Ge/Si core/shell nanowires. The frequency of the qubit can be set firstly by controlling the diffusion of Al in the nanowire via thermal annealing, which yields a roughly suitable critical supercurrent for the junction, and then by fine tuning of a gate voltage, by which an accurate adjustment of the frequency can be realized. On the resulted qubit, Rabi oscillation with an energy relaxation time T1~180 ns was observed in the time domain, and the gate voltage dependence of both T1 and the dephasing time T2 was investigated. Such a hole-type superconducting gatemon qubit, based on materials with strong spin-orbit coupling and the absence of hyperfine interaction, could be used for exploring the quantum coherence phenomena of hole-gas and potentially also Majorana physics in Ge-based quantum devices.