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arXiv:2309.16514 · Physical review. B./Physical review. B

Engineering entangled coherent states of magnons and phonons via a transmon qubit

Marios Kounalakis, Silvia Viola Kusminskiy, Yaroslav M. Blanter

Published Dec 11, 2023• 11 citations
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Abstract

Entangled coherent states exhibit a special form of entanglement between two or more bosonic modes, which is useful for several tasks in quantum information and fundamental studies of quantum mechanics. Here, the authors propose a scheme for generating and controlling such states between distant magnets or mechanical resonators, using a superconducting qubit that couples via its inductance to the quanta of the collective magnetic excitations (magnons) or the vibrational excitations in mechanical beams (phonons). The results establish a promising platform for quantum information using magnonic and mechanical hybrid quantum devices.

DOI
10.1103/physrevb.108.224416

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Entangled coherent states exhibit a special form of entanglement between two or more bosonic modes, which is useful for several tasks in quantum information and fundamental studies of quantum mechanics. Here, the authors propose a scheme for generating and controlling such states between distant magnets or mechanical resonators, using a superconducting qubit that couples via its inductance to the quanta of the collective magnetic excitations (magnons) or the vibrational excitations in mechanical beams (phonons). The results establish a promising platform for quantum information using magnonic and mechanical hybrid quantum devices.
We propose to directly and quantum-coherently couple a superconducting transmon qubit to magnons-the quanta of the collective spin excitations, in a nearby magnetic particle. The magnet's stray field couples to the qubit via a superconducting quantum interference device. We predict a resonant magnon-qubit exchange and a nonlinear radiation-pressure interaction that are both stronger than dissipation rates and tunable by an external flux bias. We additionally demonstrate a quantum control scheme that generates magnon-qubit entanglement and magnonic Schrödinger cat states with high fidelity.
Abstract Optimized generation of entangled states between solid-state qubits is of fundamental significance to quantum information science and technology. Here, we develop an effective scheme to fast and robustly create the entangled states in a hybrid system containing a superconducting transmon qubit and a spin magnon of yttrium iron garnet. The transmon qubit and the magnon system are coupled via a microwave cavity field. Within the framework of invariant-based shortcuts to adiabaticity, the transmon-magnon entangled states can be rapidly induced by engineering the Rabi couplings between the cavity mode and transmon (magnon) with constant amplitudes. Compared with a non-shortcut counterpart, the shortcut-based operations of interest not only considerably shorten the generation time, but also greatly mitigate the decoherence effects. Furthermore, the fast dynamics is well insensitive to the control deviation of Rabi couplings. Thus our approach could lead to advances in information processing and state engineering with composite systems.
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.
Generation of entanglement between spatially separated macroscopic systems enables quantum networks and fundamental tests of quantum theory. Here, we propose a scheme to generate a long-range entangled state between two macroscopic spheres. The hybrid system under consideration consists of two massive ferromagnetic spheres and a superconducting transmon qubit, which are both coupled to a microwave cavity array. When the qubit or magnon frequency lies outside of the frequency band of the cavity array, the qubit or magnon mode can seed a tunable, localized cavity mode with an exponentially decaying envelope around its position. This effective cavity mediates the long-range qubit-magnon and magnon-magnon interactions in a coherent way, enabling the long-distance magnon-magnon entanglement via the superconducting qubit.
Abstract Massive mechanical resonators operating at the quantum scale can enable a large variety of applications in quantum technologies as well as fundamental tests of quantum theory. Of crucial importance in that direction is both their integrability into state-of-the-art quantum platforms as well as the ability to prepare them in generic quantum states using well-controlled high-fidelity operations. Here, we propose a scheme for controlling a radio-frequency mechanical resonator at the quantum scale using two superconducting transmon qubits that can be integrated on the same chip. Specifically, we consider two qubits coupled via a capacitor in parallel to a superconducting quantum interference device (SQUID), which has a suspended mechanical beam embedded in one of its arms. Following a theoretical analysis of the quantum system, we find that this configuration, in combination with an in-plane magnetic field, can give rise to a tuneable three-body interaction in the single-photon strong-coupling regime, while enabling suppression of the stray qubit-qubit coupling. Using state-of-the-art parameters and qubit operations at single-excitation levels, we numerically demonstrate the possibility of ground-state cooling as well as high-fidelity preparation of mechanical quantum states and qubit-phonon entanglement, i.e. states having negative Wigner functions and obeying non-classical correlations. Our work significantly extends the quantum control toolbox of radio-frequency mechanical resonators and may serve as a promising architecture for integrating such mechanical elements with transmon-based quantum processors.
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.