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arXiv:2312.09471 · Physical Review A

Topological nonlocal operations on toroidal flux qubits

Adel Ali, Alexey A. Belyanin

Published Aug 6, 2024• 4 citations
arXivHTMLPDF

Abstract

We propose a method to achieve coherent, field-free coupling between the toroidal flux qubit (tfluxon), which is isolated from the environment, and a charged particle on a quantum ring. The resulting nonlocal coupling is mediated only by the vector potential and can be used to control the tfluxon by acting on the quantum ring's degrees of freedom. Furthermore, the emergent coupling can facilitate coherent interaction between states of distant tfluxons mediated by an electron on a quantum ring. We demonstrate that the topological and nonlocal aspects of this system are of fundamental interest and can have important applications in quantum information, benefiting from the decoupling from the environment and the topological protection.

DOI
10.1103/physreva.110.022604

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We propose a method to achieve coherent, field-free coupling between the toroidal flux qubit (tfluxon), which is isolated from the environment, and a charged particle on a quantum ring. The resulting nonlocal coupling is mediated only by the vector potential and can be used to control the tfluxon by acting on the quantum ring's degrees of freedom. Furthermore, the emergent coupling can facilitate coherent interaction between states of distant tfluxons mediated by an electron on a quantum ring. We demonstrate that the topological and nonlocal aspects of this system are of fundamental interest and can have important applications in quantum information, benefiting from the decoupling from the environment and the topological protection.
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The realization of a quantum computer represents a tremendous scientific and technological challenge due to the extreme fragility of quantum information. The physical support of information, namely the quantum bit or qubit, must at the same time be strongly coupled to other qubits by gates to compute information, and well decoupled from its environment to keep its quantum behavior. An interesting physical system for realizing such qubits are magnetic impurities in semiconductors, such as bismuth donors in silicon. Indeed, spins associated to bismuth donors can reach an extremely long coherence time - of the order of seconds. Yet it is extremely difficult to establish and control efficient gates between these spins. Here we demonstrate a protocol where single bismuth donors can coherently transfer their quantum information to a superconducting flux qubit, which acts as a mediator or quantum bus. This superconducting device allows to connect distant spins on-demand with little impact on their coherent behavior. Donor spin impurities in silicon are promising qubit candidates, but efficient control and coupling of distant spins remains a key challenge. In this work, the authors experimentally demonstrate coherent coupling between a superconducting flux qubit and individual bismuth donor spins in silicon.
Hybrid semiconductor-superconductor qubits have recently emerged as a promising alternative to traditional platforms, combining material advantages with device-level tunability. A defining feature is their gate-tunable Josephson coupling, enabling superconducting qubit architectures with full electric-field control and offering a path toward scalable, low-crosstalk quantum processors. This approach seeks to merge benefits of superconducting and semiconductor qubits, for instance by encoding quantum information in the spin of a quasiparticle occupying an Andreev bound state, thus combining long coherence times with fast, flexible control. Progress has accelerated through bottom-up engineering of Andreev states in coupled quantum dot arrays, leading to architectures such as minimal Kitaev chains hosting Majorana zero modes. In parallel, Hamiltonian-protected designs aim to enhance resilience against local noise and decoherence by exploiting superconducting phase dynamics and discrete charge or flux degrees of freedom. This article reviews recent theoretical and experimental advances in hybrid qubits, providing an overview of physical mechanisms, device implementations, and emerging architectures, with emphasis on their potential for (topologically) protected quantum information processing. While many designs remain at proof-of-concept stage, rapid progress suggests practical demonstrations may soon be achievable.