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Physical Review Applied

Floquet Engineering of Two Weakly Coupled Superconducting Flux Qubits

Wan-Lu Song, Jia-Bin You, J.K. Xu, W.L. Yang, Jun‐Hong An

Published Nov 19, 2020• 6 citations
Publisher

Abstract

We explore the periodically driven dynamics of two weakly coupled flux qubits and propose a scheme to generate a maximally entangled state based on Floquet engineering. The periodic driving modifies the quasienergy spectrum of the system and induces crossing of quasienergies, and the weak coupling between them lifts the degeneracy of quasienergies. It is found that the phenomena of entanglement resonance of Floquet state appear at certain values of the driving parameters, which could be demonstrated by virtue of the Rabi-frequency spectrum and average concurrence during one driving period. Our scheme relaxes the experimental difficulty in adjusting the static parameters of the system to establish entanglement, and may open interesting perspectives for devising active decoherence-immune quantum-information devices.

DOI
10.1103/physrevapplied.14.054049

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It remains challenging to preserve entanglement between distant solid-state qubits with high-fidelity, such as nitrogen vacancy centers (NVCs). We propose a Floquet engineering strategy to protect the maximal entanglement between two weakly interacting NVCs separated in long spatial distance by locally applying periodic strong driving on the NVCs. It is found that entanglement of the Floquet states of the NVCs resonantly reaches its maximum during the whole driving period at certain values of the driving parameters. Our analysis reveals that it is the occurrence of the avoided level crossing in the Floquet quasienergy spectrum which results in such entanglement resonance. The dissipation effect on the generated entanglement has also been analyzed. Our results may be of both theoretical and experimental interests in exploring the long-lasting entanglement between weakly interacting spins to long distances in realistic environments.
We study the manipulation of quantum entanglement by periodic external fields. As an entanglement measure we compute numerically the concurrence of two flux qubits coupled inductively and/or capacitively, both driven by a dc+ac magnetic flux. Also we find an analytical lower bound for the concurrence, where the dominant terms correspond to the concurrence in the Floquet states. We show that it is possible to create or destroy entanglement in a controlled way by tuning the system at or near multiphoton resonances. We find that when the driving term of the Hamiltonian does not commute with the qubit-qubit interaction term, the control of the entanglement induced by the driving field is more robust in parameter space. This implies that capacitively coupled two flux qubits are more convenient for controlling entanglement through ac driving fluxes.
The advancement of magnonics has facilitated the utilization of hybrid magnetic systems in quantum technologies. A hybrid magnetic lattice formed by an array of superconducting loops and magnetic particles has been devised as a quantum bus to disseminate quantum resources among magnetic quantum entities serving as nodes of a quantum network. However, the lattice also exerts a decoherence effect on the quantum entities, which impairs its practical performance. By studying the non-Markovian dynamics of nitrogen-vacancy centers and magnon modes coupled to two independent hybrid magnetic lattices, we propose a Floquet-engineering scheme via periodic driving on the quantum entities to suppress decoherence. We find that significant steady-state entanglement is preserved when a Floquet bound state exists in the quasienergy spectrum of the system consisting of each driven quantum entity and its lattice. This result enables a precise manipulation of hybrid magnetic systems and benefits their applications in quantum networks. The hybrid magnetic lattice has been proposed as a quantum bus for integrating the quantum network, but the decoherence depletes the quantum resources carried by it. By studying the non-Markovian dynamics of nitrogen-vacancy centers and magnon modes coupled to two independent hybrid magnetic lattices, the authors demonstrate a Floquet-engineering scheme using periodic driving to suppress decoherence.
A one-dimensional waveguide is a crucial medium for transferring propagating quantum information between remote qubits, yet the photon-mediated interaction is typically fixed by the static device geometry, qubit parameters, and their couplings to the transmission line. Such an interaction can be engineered to be coherent or dissipative via virtual- or real-photon exchange, respectively, yet its effective strength remains fixed in either case. Here we demonstrate dynamical control over this interaction by applying Floquet engineering to two superconducting qubits coupled to a waveguide terminated by a capacitive mirror. By positioning one qubit at an antinode and the other qubit at a node or an antinode of the resonant standing wave, we realize a Hermitian or non-Hermitian coupling between the two qubits. A periodic flux drive of the qubit frequencies then enables on-demand control of the coherent energy splitting and the collective decay. Our results establish Floquet engineering as a route to dynamically reshape both coherent and dissipative photon-mediated interactions between remote quantum emitters.
Quantum technology resorts to efficient utilization of quantum resources to realize technique innovation. The systems are controlled such that their states follow the desired manners to realize different quantum protocols. However, the decoherence caused by the system-environment interactions causes the states deviating from the desired manners. How to protect quantum resources under the coexistence of active control and passive decoherence is of significance. Recent studies have revealed that the decoherence is determined by the feature of the system-environment energy spectrum: Accompanying the formation of bound states in the energy spectrum, the decoherence can be suppressed. It supplies a guideline to control decoherence. Such idea can be generalized to systems under periodic driving. By virtue of manipulating Floquet bound states in the quasienergy spectrum, coherent control via periodic driving dubbed as Floquet engineering has become a versatile tool not only in controlling decoherence, but also in artificially synthesizing exotic topological phases. We will review the progress on quantum control in open and periodically driven systems. Special attention will be paid to the distinguished role played by the bound states and their controllability via periodic driving in suppressing decoherence and generating novel topological phases.