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arXiv:2106.12960 · Physical Review A

Efficient steady-state-entanglement generation in strongly driven coupled qubits

Ana Laura Gramajo, Daniel Domı́nguez, María José Sánchez

Published Sep 10, 2021• 7 citations

Abstract

We report on a mechanism to optimize the generation of steady-state entanglement in a system of coupled qubits driven by microwave fields. Due to the interplay between Landau-Zener-St\"uckelberg-Majorana pumping involving three levels and a subsequent fast relaxation channel, which is activated by tuning the qubits-reservoir couplings, a maximally entangled state can be populated. This mechanism does not require the fine tuning of multiphoton resonances but depends on the sign of the qubit-qubit coupling. In particular, we find that by a proper design of the system parameters and the driving protocol, the two-qubit steady-state concurrence can attain values close to 1 in a wide range of driving amplitudes. Our results may be useful to gain further insight into entanglement control and manipulation in dissipative quantum systems exposed to strong driving.

DOI: 10.1103/physreva.104.032410

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An entangled-state generation protocol for a system of two qubits driven with an ac signal and coupled through a resonator is introduced. We explain the mechanism of entanglement generation in terms of an interplay between unitary Landau-Zener-St\"uckelberg (LZS) transitions, induced for appropriate frequencies and strong amplitudes of the applied ac signal and dissipative processes dominated by photon loss. In this way, we find that the steady state of the system can be tuned to be arbitrarily close to a Bell state, which is stable against photon loss. Effective two-qubit Hamiltonians that reproduce the resonance patterns associated with LZS transitions are derived.

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A system, comprised of a qubit pair coupled to a common cavity, is studied with the aim of establishing qubit entanglement. This study is the sequel of the paper Phys. Rev. A 111, 043705 (2025), where similar model was investigated for an initially vacuum cavity. In the present manuscript the cavity with finite initial occupancy is considered and the effect of asymmetric qubit cavity couplings is investigated. For a closed system scenario, the ratio of the qubit-cavity couplings shows a threshold beyond which no maximally-entangled qubit state is available. The threshold value is shown to depend on the excitation level of the cavity. For a driven-dissipative case steady state entanglement is shown to depend non-monotonically on the qubit drive. Intricate interplay of drive, dissipation, and coupling asymmetry is shown to be pivotal for steady-state entanglement generation.

An analytical theory to calculate the dissipatively stable concurrence in the system of two coupled flux superconducting qubits in the strong driving field is developed. The conditions for the entanglement state generation and destruction during the formation of the multiphoton transitions regions due to the interference of Landau--Zener--Stückelberg--Majorana are found. Based on the solution of the Floquet--Markov equation, the technique is proposed to adjust the amplitudes of dc- and ac-fields for effective control of the entanglement between qubit states while taking into account the effects of the decoherence.

We propose a scheme for driving a dimer of spatially separated qubits into a maximally entangled nonequilibrium steady state. A photon-mediated retarded interaction between the qubits is realized by coupling them to two tunnel-coupled leaky cavities where each cavity is driven by a coherent microwave tone. The proposed cooling mechanism relies on striking the right balance between the unitary and driven-dissipative dynamics of the qubit subsystem. We map the dimer to an effective transverse-field isotropic

XY

model coupled to a nonequilibrium bath that can be suitably engineered through the choice of drive frequencies and amplitudes. We show that both singlet and triplet states can be obtained with remarkable fidelities. The proposed protocol can be implemented with a superconducting circuit architecture that was recently experimentally realized and paves the way to achieving large-scale entangled systems that are arbitrarily long lived.