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  • Open Access

Fast Quantum Gates for Exchange-Only Qubits Using Simultaneous Exchange Pulses

Irina Heinz1,*, Felix Borjans2, Matthew J. Curry2, Roza Kotlyar2, Florian Luthi2, Mateusz T. Mądzik2, Fahd A. Mohiyaddin2, Nathaniel Bishop2, and Guido Burkard1

  • *Contact author: i.heinz@fz-juelich.de
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PRX Quantum 6, 030353 – Published 17 September, 2025

DOI: https://doi.org/10.1103/njq3-fcdd

Abstract

The benefit of exchange-only qubits compared to other spin-qubit types is the universal control using only voltage-controlled exchange interactions between neighboring spins. As a compromise, qubit operations have to be constructed from nonorthogonal rotation axes of the Bloch sphere and result in rather long pulsing sequences. This paper aims to develop a faster implementation of single-qubit gates using simultaneous exchange pulses and manifests their potential for the construction of two-qubit gates. We introduce pulse sequences in which single-qubit gates could be executed faster and show that subsequences on three spins in two-qubit gates could be implemented in fewer steps. Our findings can particularly speed up gate sequences for realistic idle times between sequential pulses and we show that this advantage increases with more interconnectivity of the quantum dots. We further demonstrate how a phase operation can introduce a relative phase between the computational and some of the leakage states, which can be advantageous for the construction of two-qubit gates. In addition to our theoretical analysis, we experimentally demonstrate and characterize a simultaneous exchange implementation of X rotations in a SiGe quantum dot device and compare to the state of the art with sequential exchange pulses.

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Popular Summary

The growing interest in quantum computers has triggered various approaches for the realization of large-scale quantum processors that can eventually outperform classical high-performance computers. One promising platform for the implementation of a quantum processor consists of the spins of charge carriers in semiconductor quantum dots. The use of purified silicon as a host material reduces nuclear noise significantly, thus improving the performance of quantum operations. Single-spin qubits are usually controlled electrically by microwave pulses, which in larger arrays gives rise to crosstalk and heating. A promising alternative is the exchange-only (EO) spin qubit consisting of three electron spins and only requiring baseband control of the exchange interaction between spins to achieve universal quantum computation. As a consequence, the quantum gate construction usually requires rather complicated sequences of exchange pulses. We present fast quantum operations for EO qubits using simultaneous exchange pulses and demonstrate a novel single-pulse X gate experimentally.

Simultaneous pulsing of exchange interactions reduces the number of pulses for single-qubit gates, as it enables flexible rotations of the spin, but it can also facilitate the construction of two-qubit gates, as shown in this work. The EO qubit is encoded in a subspace of a larger Hilbert space; thus, charge noise during two-qubit gates and local magnetic fluctuations can cause leakage out of the qubit space. We discover a quantum operation induced by simultaneous exchange pulses that introduces relative phases between computational and leakage states, and we discuss their application for new quantum gate constructions.

Since spin-qubit-based quantum chips are moving toward two-dimensional arrays, we extend our description from a linear to an all-to-all connectivity and exhibit an even more flexible quantum gate construction with simultaneous exchange.

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Outline

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PRX Quantum 6, 030353– Published 17 September, 2025
  • Received 17 October 2024
  • Revised 14 May 2025
  • Accepted 21 July 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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