Session N44: Nanomagnets for Quantum Information

Topological Magnons for Quantum Information

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 [1]. 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.

We uncover that antiskyrmion crystals provide an experimentally accessible platform to realize a magnonic quadrupole topological insulator [2-6], whose hallmark signatures are robust magnonic corner states. Furthermore, we show that tuning an applied magnetic field can trigger the self-assembly of antiskyrmions carrying a fractional topological charge along the sample edges. Crucially, these fractional antiskyrmions restore the symmetries needed to enforce the emergence of the magnonic corner states. Using the machinery of nested Wilson loops, adapted to magnonic systems supported by noncollinear magnetic textures, we demonstrate the quantization of the bulk quadrupole moment, edge dipole moments, and corner charges.   

Quantum sensing of magnon excitations with a superconducting qubit

Magnons are quanta of collective spin excitations, such as in magnetostatic modes and spin waves, ubiquitously found in magnetic materials. Conventional experimental techniques like magnetic resonance and spintronics can only access large-amplitude "classical" signals in those modes. However, the recent progress in quantum magnonics has enabled us to investigate the properties of magnons at the single-magnon limit [1]. In this presentation, we discuss how we can control and detect single magnons in a macroscopic-scale ferromagnetic crystal. We strongly couple a superconducting qubit, an artificial two-level system realized in a superconducting circuit, to a magnon via a virtual photon excitation in a microwave cavity. The interaction allows us to generate entanglement between the qubit and the magnon excitation, and the qubit readout results in detecting a magnon [2,3].   

Magnon engineering in YIG-based heterostructures - from strong coupling to short wavelength modes

Magnetic insulators such as yttrium iron garnets (YIGs) are of paramount importance for spin-wave or magnonic devices due to their ultralow power dissipation, exotic magnon state, and coherent coupling to other wave excitations. Magnetic insulator heterostructures bestow superior structural and magnetic properties and house many novel properties due to their strong and engineerable exchange interactions between individual layers. In this talk, I will discuss several YIG-based heterostructures in the context of strong magnon-magnon coupling, which enable a variety of coherent coupling phenomena, such as the magnonic-induced transparency, short wavelength excitation and selectivity, and nonlinear magnonic couplings. Finally, I will move from the conventional metal-insulator systems to a recent, all-insulating system exhibiting strong magnon-magnon hybridization. The versatility of magnon-magnon coupling in such heterostructures can prepare tailored magnon modes for their further hybridization with other solid-state excitations, including but not limited to optical photon, phonon, and qubits.   

Quantum Magnonics with Synthetic Antiferromagnets

Synthetic antiferromagnets are magnetic multilayers consisting of two or more ferromagnetic layers that are coupled antiferromagnetically. They play an important role in spintronic devices, e.g., as field sensors, and as synthetic materials for fundamental explorations. In this talk, I will highlight the use of synthetic antiferromagnets for quantum information science with spin waves, i.e., for quantum magnonics. Examples that are discussed are unidirectionally-coupled magnetic layers that give rise to magnon quantum amplification, and new ways to entangle magnons between two ferromagnetic layers. Both these examples rely on the possibility to engineer both the interactions between the layers, and the interactions of the magnetic layers with the environment. This tunability highlights the potential of synthetic antiferromagnets for quantum magnonics.   

Cavity quantum electrodynamics with magnons

Hybridizing collective spin excitations and a cavity with high cooperativity provides a new research subject in the field of cavity quantum electrodynamics and can have potential applications in quantum information. Here we report the quantum control of a single magnon in a macroscopic spin system (i.e., 1 mm-diameter yttrium-iron-garnet sphere) embedded in a microwave cavity. In this hybrid quantum system, an auxiliary superconducting qubit is also embedded in the cavity. Via this microwave cavity, we can implement strong coupling between the magnon and the superconducting qubit. By tuning the qubit frequency via the Autler-Townes effect, we manipulate a single magnon to generate its nonclassical quantum states, including the single-magnon state and its coherent superposition with a vacuum (zero magnon state). We also confirm the deterministic generation of these nonclassical states by Wigner tomography. This experiment offers the first reported deterministic generation of the nonclassical quantum states in a macroscopic spin system. Moreover, we demonstrate the deterministic generation of the macroscopically entangled Bell state between this millimeter-sized spin system and the micrometer-sized superconducting qubit. We develop a joint tomography approach to confirming this deterministic generation of the Bell state, which gives a generation fidelity of about 0.90. This work makes the macroscopic spin system the largest system capable of generating the maximally entangled quantum state and paves a way to explore its promising applications in quantum information.