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arXiv:2406.14967 · Physical review. B./Physical review. B

Magnon-mediated quantum gates for superconducting qubits

Martijn Dols, Sanchar Sharma, Lenos Bechara, Yaroslav M. Blanter, Marios Kounalakis, Silvia Viola Kusminskiy

Published Sep 9, 2024• 7 citations
arXivHTMLPDF

Abstract

We propose a hybrid quantum system consisting of a magnetic particle inductively coupled to two superconducting transmon qubits, where qubit-qubit interactions are mediated via magnons. We show that the system can be tuned into three different regimes of effective qubit-qubit interactions, namely, a transverse ($XX+YY$), a longitudinal ($ZZ$), and a nontrivial $ZX$ interaction. In addition, we show that an enhanced coupling can be achieved by employing an ellipsoidal magnet, carrying anisotropic magnetic fluctuations. We propose a scheme for realizing two-qubit gates, and simulate their performance under realistic experimental conditions. We find that iswap and cz gates can be performed in this setup with an average fidelity $\ensuremath{\gtrsim}99%$, while an icnot gate can be applied with an average fidelity $\ensuremath{\gtrsim}88%$. Our proposed hybrid circuit architecture offers an alternative platform for realizing two-qubit gates between superconducting qubits and could be employed for constructing qubit networks using magnons as mediators.

DOI
10.1103/physrevb.110.104416

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We propose a hybrid quantum system consisting of a magnetic particle inductively coupled to two superconducting transmon qubits, where qubit-qubit interactions are mediated via magnons. We show that the system can be tuned into three different regimes of effective qubit-qubit interactions, namely, a transverse (XX+YYXX+YYXX+YY), a longitudinal (ZZZZZZ), and a nontrivial ZXZXZX interaction. In addition, we show that an enhanced coupling can be achieved by employing an ellipsoidal magnet, carrying anisotropic magnetic fluctuations. We propose a scheme for realizing two-qubit gates, and simulate their performance under realistic experimental conditions. We find that iswap and cz gates can be performed in this setup with an average fidelity \ensuremath≳99\ensuremath{\gtrsim}99%\ensuremath≳99, while an icnot gate can be applied with an average fidelity \ensuremath≳88\ensuremath{\gtrsim}88%\ensuremath≳88. Our proposed hybrid circuit architecture offers an alternative platform for realizing two-qubit gates between superconducting qubits and could be employed for constructing qubit networks using magnons as mediators.
Coherent coupling between a ferromagnetic magnon and a superconducting qubit demonstrates a special fascination in the quantum information field. Here, we construct a hybrid system to realize tunable strong coupling between a transmon and a magnon. In a hybrid system, the transmon couples to a cavity while the magnon couples to the other cavity. Meanwhile, the two cavities are coupled via a hopping interaction. In addition, we demonstrate some applications of this system to quantum information. Specifically, a high-fidelity iiiswap${}^{\ifmmode\dagger\else\textdagger\fi{}}$ gate and a maximally entangled state between the transmon and the magnon with this coupling system can be realized even if the dissipation is considered. Moreover, by employing more cavities and magnons, the WWW-type entangled state can be created with multiple transmons. This hybrid system provides an alternative platform for scalable quantum information processing.
We propose an efficient two-stage protocol for generating distant entanglement in a magnon-mediated hybrid quantum system, where magnons serve dual roles as both interaction mediators and qubits. This integrated design reduces the physical component count while leveraging the inherent advantages of magnons, such as their strong coupling via magnetic dipole interactions, low dissipation, and high integrability. In our setup, a superconducting resonator interfaces between a local superconducting qubit (SQ) and a local magnonic system (QM1), which is waveguide-coupled to a remote magnonic system (QM2). The protocol comprises two stages: (i) deterministic Bell-state generation between the SQ and QM1 using shortcuts to adiabaticity, and (ii) coherent state transfer to QM2 via engineered Hamiltonian dynamics. This adiabatic characteristic enhances robustness against environmental dissipation. Numerical simulations under realistic noise conditions confirm strong resilience to decoherence, achieving fidelity F>0.90F > 0.90F>0.90 and negativity N2>0.40\mathcal{N}_2 > 0.40N2​>0.40. These results establish the protocol as a scalable and practical building block for distributed quantum networks.
To scale superconducting quantum processors, it is essential to achieve long coherence times while engineering interactions that do not introduce additional decoherence channels. In superconducting qubit systems, this can be realized using multimode circuits that feature a protected qubit mode alongside a distinct mediator mode. Building on this concept, our recently developed P-mon qubit provides intrinsic protection against decoherence from the readout environment. We extend this approach to controlled two-qubit interactions, by exploiting the mediator modes of P-mons for on-demand coupling. Because direct interactions between the qubit modes are strongly suppressed, unwanted ZZZZZZ-type interactions are significantly reduced to below 3.6(5) kHz3.6(5)~\text{kHz}3.6(5) kHz in the idle state. When tuning the coupled mediator modes on resonance, the cross-Kerr interaction between the qubit and the hybridized mediator modes leads to a qubit-state dependent frequency shift. By selectively addressing these transitions, we implement a 180 ns180~\text{ns}180 ns long CZ gate and determine a fidelity of $99.62(4)~\text{%}$. These results represent a significant step toward a scalable superconducting architecture that maintains high performance at scale.
Hybrid superconducting architectures combining different types of qubits offer a promising platform for exploiting their complementary advantages, yet high-fidelity entangling gates remain challenging because of strong nonlinearities and residual qubit-qubit interactions. Here, we propose a high-fidelity controlled-Z (CZ) gate for a hybrid circuit comprising a fluxonium qubit, a fixed-frequency transmon qubit, and a flux-tunable transmon coupler. By modulating only the external magnetic flux applied to the coupler, the qubit-qubit interaction is dynamically engineered for conditional-phase accumulation while the residual interaction is suppressed at idle, mitigating spectator-induced errors. We employ a low-dimensional Fourier-cosine pulse parameterization and a physically motivated cost function to independently suppress conditional-phase errors and leakage from the computational subspace. Numerical simulations demonstrate that a microwave-free CZ gate can be realized within 25ns25 \mathrm{ns}25ns, with an average gate fidelity exceeding 99.99%99.99\%99.99% and leakage below 10−510^{-5}10−5. Using experimentally relevant superconducting-qubit parameters and accounting for decoherence, the proposed scheme maintains a CZ-gate fidelity of approximately 99.9%99.9\%99.9%. We further extend the analysis to larger coupled architectures and find that the CZ-gate infidelity remains below 10−410^{-4}10−4 in the presence of spectator qubits. These results establish single-parameter flux control as a simple and robust approach for realizing high-fidelity entangling gates in heterogeneous superconducting quantum architectures.
Abstract We put forward a proposal to construct a quantum network using a hybrid cavity–magnonics system as with two nodes. At each node, a cascade of the quantum system consists of cavity–magnonics and magnonic–qubit interactions, and the quantum interface between the flying qubit and superconducting qubit is mediated by a magnon. Considering the phase resulting from the distance between the two nodes, we derive a master equation for two superconducting qubits and show that, by adiabatically controlling the cavity–magnon coupling, perfect quantum state transfer between two qubits can be realized. We also consider the influence of intrinsic dissipation of the magnetic mode and the cavity mode. In an unideal case, the design time-dependent cavity–magnonics couplings obtained in the ideal case are still employed. Our results show that low intrinsic loss in the magnetic mode and the cavity mode is still welcome for the high fidelity of state transfer.
Spin qubits and superconducting qubits are promising candidates for realizing solid-state quantum information processors. Designing a hybrid architecture that combines the advantages of different qubits on the same chip is a highly desirable but challenging goal. Here we propose a hybrid architecture that utilizes a high-impedance SQUID array resonator as a quantum bus, thereby coherently coupling different solid-state qubits. We employ a resonant exchange spin qubit hosted in a triple quantum dot and a superconducting transmon qubit. Since this hybrid system is highly tunable, it can operate in a dispersive regime, where the interaction between the different qubits is mediated by virtual photons. By utilizing such interactions, entangling gate operations between different qubits can be realized in a short time of 30 ns with a fidelity of up to 96.5% under realistic parameter conditions. Further utilizing this interaction, remote entangled state between different qubits can be prepared and is robust to perturbations of various parameters. These results pave the way for exploring efficient fault-tolerant quantum computation on hybrid quantum architecture platforms.