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  • Open Access

Singlet-Triplet and Exchange-Only Flopping-Mode Spin Qubits

Simon Stastny and Guido Burkard*

  • *Contact author: guido.burkard@uni-konstanz.de
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PRX Quantum 6, 030360 – Published 23 September, 2025

DOI: https://doi.org/10.1103/v1b6-r6jm

Abstract

Semiconductor-based spin qubits embedded into a superconducting microwave cavity constitute a fast-progressing and promising platform for realizing fast and fault-tolerant qubit control with long-range two-qubit coupling. The flopping-mode spin qubit consists of a single electron in a double quantum dot; it combines a charge qubit with a spin qubit. With its strong and tunable cavity coupling, the flopping-mode qubit is proven to be well suited for low-power qubit control and cavity-mediated long-range quantum gates. The singlet-triplet (ST) and exchange-only (EO) qubits are multielectron realizations that go without broadband control and are protected from some types of noise, but are challenging to couple to each other and to microwave cavities. We combine the flopping-mode concept with the ST and EO qubits and propose two new flopping-mode qubits that consist of three (four) quantum dots, occupied by two (three) electrons near the (1,0,1)↔(0,1,1)[(1,0,1,1)↔(0,1,1,1)] charge transition. The two-electron system augments the ST0 spin qubit with a charge qubit that interacts transversally and longitudinally with a cavity. Both couplings are highly tunable, and the longitudinal coupling distinguishes the flopping-mode ST qubit from the regular flopping-mode qubit. The longitudinal coupling allows for nondissipative universal control similar to superconducting transmon qubits. The EO flopping-mode qubit comprises four dots occupied by three electrons and opens a new possibility to perform two-qubit gates for EO qubits that are challenging to perform directly with the exchange coupling. We use input-output theory to provide means of extracting the coupling strengths from cavity transmission data.

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Popular Summary

Spins in semiconductor quantum dots are used to construct qubits, which allow for long coherence and high-fidelity quantum gates driven by oscillating magnetic fields and the exchange coupling between nearby spins. Microwave control to rotate spins is challenging when scaling up a quantum processor, as it entails many parallel control cables and potentially heats the device. Singlet-triplet and exchange-only spin qubits are multielectron systems that operate in a decoherence-free subspace, allowing for low-frequency control and benefitting from additional protection against noise. However, two-qubit operations between these multispin qubits can be elaborate, involving many sequential applications of pairwise exchange coupling, while making sure to return to the computational subspace after finishing the gate. Also, the exchange coupling, while extremely versatile and strong, is short ranged and can only be utilized to couple nearest-neighbor qubits.

This work introduces new flavors of multispin qubits, which can be coupled to microwave photons in a superconducting on-chip resonator, thus allowing for short-range and long-range two-qubit interactions that address both of the above problems. The coupling works by adding an empty quantum dot to the two quantum dots forming the singlet-triplet qubit, or to the three dots forming an exchange-only qubit. The resulting charge degree of freedom gives rise to an electric dipole moment that interacts with the quantized electric field of the cavity. The coupling mechanism between the spin qubit and the charge involves either magnetic-field gradients or the exchange interaction and can be switched on and off by applying voltage signals to the gate electrodes of the additional quantum dot.

Article Text

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PRX Quantum 6, 030360– Published 23 September, 2025
  • Received 6 March 2025
  • Revised 15 June 2025
  • Accepted 18 August 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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