[Paper Review] Superconducting Gatemon Qubit based on a Proximitized Two-Dimensional Electron Gas

Lucas Casparis, M. R. Connolly+12 more|arXiv (Cornell University)|Nov 21, 2017

Quantum and electron transport phenomena4 citations

TL;DR

This paper demonstrates a scalable superconducting gatemon qubit platform based on a proximitized two-dimensional electron gas (2DEG), enabling all-electric qubit control via local gates. The authors achieve coherent single-qubit rotations and two-qubit swap operations with qubit coherence times up to ~2 μs, limited by dielectric loss in the 2DEG substrate, establishing a viable route toward scalable, flux-insensitive quantum processors.

ABSTRACT

The coherent tunnelling of Cooper pairs across Josephson junctions (JJs) generates a nonlinear inductance that is used extensively in quantum information processors based on superconducting circuits, from setting qubit transition frequencies and interqubit coupling strengths, to the gain of parametric amplifiers for quantum-limited readout. The inductance is either set by tailoring the metal-oxide dimensions of single JJs, or magnetically tuned by parallelizing multiple JJs in superconducting quantum interference devices (SQUIDs) with local current-biased flux lines. JJs based on superconductor-semiconductor hybrids represent a tantalizing all-electric alternative. The gatemon is a recently developed transmon variant which employs locally gated nanowire (NW) superconductor-semiconductor JJs for qubit control. Here, we go beyond proof-of-concept and demonstrate that semiconducting channels etched from a wafer-scale two-dimensional electron gas (2DEG) are a suitable platform for building a scalable gatemon-based quantum computer. We show 2DEG gatemons meet the requirements by performing voltage-controlled single qubit rotations and two-qubit swap operations. We measure qubit coherence times up to ~2 us, limited by dielectric loss in the 2DEG host substrate.

Motivation & Objective

• To develop a scalable, all-electric superconducting qubit platform free from flux noise and crosstalk.
• To demonstrate coherent control of qubits using local gates on a 2DEG-based Josephson junction.
• To achieve high-fidelity single- and two-qubit operations in a wafer-scale 2DEG heterostructure.
• To evaluate qubit coherence times and identify dielectric loss as the primary limitation.
• To enable frequency-tunable qubits without magnetic flux control, reducing complexity in 3D integration.

Proposed method

• Fabricated gatemon qubits using a wafer-scale Al/InGaAs/InP 2DEG heterostructure with a proximitized superconducting Al layer.
• Defined Josephson junctions via wet etching of a 100 nm segment of Al, forming a 2DEG JJ with tunable width w.
• Employed a 20 nm AlOx gate dielectric and a top Al gate to capacitively tune the Josephson energy EJ.
• Integrated T-shaped Al islands capacitively coupled to coplanar waveguide cavities for readout and control.
• Used IQ-modulated microwave pulses and dispersive heterodyne detection for single-qubit operations and state readout.
• Performed Rabi and Ramsey spectroscopy to characterize qubit coherence and gate fidelity.

Experimental results

Research questions

• RQ1 Can a 2DEG-based gatemon qubit achieve coherent single-qubit rotations with all-electric control?
• RQ2 What is the coherence time of a 2DEG gatemon, and what limits it?
• RQ3 Can two-qubit swap operations be coherently implemented on a 2DEG gatemon platform?
• RQ4 How does the Josephson energy EJ scale with the junction width w in a 2DEG JJ?
• RQ5 Can the 2DEG gatemon platform support scalable, flux-insensitive quantum computation?

Key findings

• Qubit coherence times up to ~2 μs were measured, limited by dielectric loss in the 2DEG substrate.
• Coherent Rabi oscillations were observed with a Gaussian-damped sinusoidal fit, confirming high-fidelity single-qubit control.
• Ramsey interference fringes demonstrated coherent Z-axis rotations with a visibility consistent with high-fidelity gate operations.
• Two-qubit swap operations were coherently implemented, confirming entangling gate capability.
• The Josephson energy EJ varied with junction width w, enabling tunable qubit frequency via gate voltage.
• The 2DEG gatemon platform supports six independently addressable qubits on a single chip, demonstrating scalability.