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arXiv:2212.12019 · Physical Review Research

Quantum computing on magnetic racetracks with flying domain wall qubits

Ji Zou, Stefano Bosco, Banabir Pal, S. Parkin, Jelena Klinovaja, Daniel Loss

Published Sep 7, 2023• 29 citations
arXivHTMLPDF

Abstract

Domain walls (DWs) on magnetic racetracks are at the core of the field of spintronics, providing a basic element for classical information processing. Here, we show that mobile DWs also provide a blueprint for large-scale quantum computers. Remarkably, these DW qubits showcase exceptional versatility, serving not only as stationary qubits, but also performing the role of solid-state flying qubits that can be shuttled in an ultrafast way. We estimate that the DW qubits are long-lived because they can be operated at sweet spots to reduce potential noise sources. Single-qubit gates are implemented by moving the DW, and two-qubit entangling gates exploit naturally emerging interactions between different DWs. These gates, sufficient for universal quantum computing, are fully compatible with current state-of-the-art experiments on racetrack memories. Further, we discuss possible strategies for qubit readout and initialization, paving the way toward future quantum computers based on mobile topological textures on magnetic racetracks.

DOI
10.1103/physrevresearch.5.033166

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Physical Review B•Sep 21, 2007•30 citations
Domain walls (DWs) on magnetic racetracks are at the core of the field of spintronics, providing a basic element for classical information processing. Here, we show that mobile DWs also provide a blueprint for large-scale quantum computers. Remarkably, these DW qubits showcase exceptional versatility, serving not only as stationary qubits, but also performing the role of solid-state flying qubits that can be shuttled in an ultrafast way. We estimate that the DW qubits are long-lived because they can be operated at sweet spots to reduce potential noise sources. Single-qubit gates are implemented by moving the DW, and two-qubit entangling gates exploit naturally emerging interactions between different DWs. These gates, sufficient for universal quantum computing, are fully compatible with current state-of-the-art experiments on racetrack memories. Further, we discuss possible strategies for qubit readout and initialization, paving the way toward future quantum computers based on mobile topological textures on magnetic racetracks.
Magnetic domain walls have long been pursued as carriers of classical information for storage and processing. With the ability to create, control, and probe domain walls at the nanoscale, they are recently recognized as an ideal platform for studying macroscopic quantum effects and provide a natural blueprint for building scalable quantum computing architectures. In particular, the experimentally demonstrated high mobility of domain walls makes them not only suitable as stationary qubits but also as flying qubits, which may offer advantages over currently explored quantum computing platforms. In this Perspective, we outline our current understanding of the essential ingredients and key requirements for realizing universal quantum computation based on magnetic domain walls. We highlight promising concrete material platforms and identify the experiments that are still needed to advance this concept. We also discuss the potential challenges and point to new opportunities in this emerging research direction at the interface between magnetism and quantum information science.
Nanoscale topological spin textures in magnetic systems are emerging as promising candidates for scalable quantum architectures. Despite their potential as qubits, previous studies have been limited to semiclassical approaches, leaving a critical gap: the lack of a fully quantum demonstration. Here, we address this challenge by employing the density matrix renormalization group (DMRG) method to establish domain wall (DW) qubits in coupled quantum spin-1/2 chains. We calculate the ground-state energies and excitation gaps of the system and find that DWs with opposite chiralities form a well-defined low-energy sector, distinctly isolated from higher excited states in the presence of anisotropies. This renders the chirality states suitable for encoding quantum information, serving as robust qubits. Interestingly, when a magnetic field is applied, we observe tunneling between quantum DW states with opposite chiralities. Through quantum simulations, we construct an effective qubit Hamiltonian that exhibits strongly anisotropic ggg factors, offering a way to implement single-qubit gates. Furthermore, we obtain an effective interacting Hamiltonian for two mobile DWs in coupled quantum spin chains from DMRG simulations, enabling the implementation of two-qubit gates. Single-qubit and two-qubit gates are also demonstrated in real-time simulations using the time-dependent variational principle. Our work represents a critical step from semiclassical constructions to a fully quantum demonstration of the potential of DW textures for scalable quantum computing, establishing a solid foundation for future quantum architectures based on topological magnetic textures.
Quantum computers allow to solve efficiently certain problems that are intractable for classical computers. For the realization of a quantum computer, a qubit design as the basic building block is a nontrivial starting point. Within a nanoscale ferromagnetic domain wall stabilized by achiral energy, two degenerate chirality forms exist which can be regarded as the two qubit states. Our numerical demonstration shows that the manipulation of spin configurations of the ferromagnetic domain walls is governed by magnetic and electric fields for single-qubit quantum gates, while the Ising exchange coupling facilitates the two-qubit gates. The incorporation of these quantum gates permits universal quantum computation. Furthermore, we discuss the estimation of the coherence time, as well as the initialization and readout of the qubits. Our findings show a practical implementation of quantum computing architectures based on the domain-wall qubits in ferromagnetic materials.
Domain walls (DWs), as interfaces separating different magnetic domains, are widely regarded as classical objects, even though the underlying constituents, namely, electron spins and spin-spin exchange interaction, are intrinsically of quantum nature. One intriguing question is, how do DWs behave when the size of the magnetic domains is shrunk to the atomistic level? Would quantum fluctuations destroy the stability of the DWs? These questions are partly addressed for ferromagnets, but seldom studied for antiferromagnets. Here we present a microscopic quantum spin model of antiferromagnets where the ground states not only share several common properties with the classical domain walls, but also exhibit nontrivial quantum properties. Specifically, the magnetization profile highly resembles its classical counterparts, even though the spins inside the domain walls are highly entangled. Furthermore, a nonzero spin angular momentum of a quantum DW emerges and it does not depend on the system size. Finally, we show that quantum DWs can be generated through a quantum phase transition from a quantum domain and the critical point can be characterized by the global entanglement. These results provide additional physical insights on the quantum effects in spintronics, and can be experimentally verified within the current technologies in superconducting circuits and trapped ions.
Abstract The scalability and power of quantum computing architectures depend critically on high-fidelity operations and robust and flexible qubit connectivity 1–3 . In this respect, mobile qubits are particularly attractive as they enable dynamic and reconfigurable qubit arrays. This approach allows quantum processors to adapt their connectivity patterns during operation, implement different quantum error correction codes on the same hardware and optimize resource use through dedicated functional zones for specific operations such as measurement or entanglement generation 4–7 . Such flexibility also relieves architectural constraints, as recently demonstrated in atomic systems based on trapped ions 4,5 and neutral atoms manipulated with optical tweezers 6,7 . In solid-state platforms, highly coherent shuttling of electron spins was recently reported 8,9 . A key outstanding question is whether it may be possible to perform quantum gates directly on the mobile spins. Here we demonstrate two-qubit operations between two electron spins carried towards each other in separate travelling potential minima in a semiconductor device. We find that the interaction strength is highly tunable by their spatial separation. When we shuttle the two spins towards the centre by 120 nm each for a total displacement of 240 nm, we achieve an average two-qubit gate fidelity of about 99%. Furthermore, we implement conditional post-selected quantum state teleportation between qubits separated by 320 nm with an average gate fidelity of 87%, showcasing the potential of mobile spin qubits for non-local quantum information processing. We expect that operations on mobile qubits will become a universal feature of future large-scale semiconductor quantum processors.
We present a method for encoding and transporting qubits within a dimerized Heisenberg spin-12\frac{1}{2}21​ chain. Logical qubits are localized at the domain walls that separate the two possible dimerized states. The domain walls can be moved to produce ``flying spin qubits.'' The topological nature of these states makes them stable against local perturbations of the exchange profile. Pairs of domain walls can be used to generate Einstein-Podolsky-Rosen pairs of entangled qubits. We discuss speed limitations within an exactly solvable three-spin model and describe a possible physical realization using quantum dot arrays.