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Scientific Reports

Dynamic generation of multi-qubit entanglement in the ultrastrong-coupling regime

Xin Liu, Qinghong Liao, Guangyu Fang, Shutian Liu

Published Feb 27, 2019• 8 citations
Publisher

Abstract

Abstract We propose a dynamic evolution protocol for generating multi-qubit GHZ states in the ultrastrong-coupling regime of circuit QED. By varying the time length of sequences, the protocol works for any coupling strength g/ωr ≥ 0.25. The time for generating the GHZ states in our protocol can be in the subnanoseconds. By taking into account realistic parameters of circuit QED, the degeneracy of fidelity due to decoherence can be as low as 0.02%.

DOI
10.1038/s41598-019-39265-4

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Abstract We propose a dynamic evolution protocol for generating multi-qubit GHZ states in the ultrastrong-coupling regime of circuit QED. By varying the time length of sequences, the protocol works for any coupling strength g/ωr ≥ 0.25. The time for generating the GHZ states in our protocol can be in the subnanoseconds. By taking into account realistic parameters of circuit QED, the degeneracy of fidelity due to decoherence can be as low as 0.02%.
We analyze a multi-qubit circuit QED system in the regime where the qubit-photon coupling dominates over the system's bare energy scales. Under such conditions a manifold of low-energy states with a high degree of entanglement emerges. Here we describe a time-dependent protocol for extracting these quantum correlations and converting them into well-defined multi-partite entangled states of non-interacting qubits. Based on a combination of various ultrastrong-coupling effects the protocol can be operated in a fast and robust manner, while still being consistent with experimental constraints on switching times and typical energy scales encountered in superconducting circuits. Therefore, our scheme can serve as a probe for otherwise inaccessible correlations in strongly-coupled circuit QED systems. It also shows how such correlations can potentially be exploited as a resource for entanglement-based applications.
Ultrastrong coupling in circuit quantum electrodynamics makes enhanced fast quantum operations possible. We present a scheme to realize controllable qubit-resonator interactions in ultrastrong circuit quantum electrodynamics with the aid of a unitary transformation. The controllable qubit-resonator interaction is just one type of the so-called selective resonant interactions. From the effective dynamics, it is possible to create the multipartite Dicke states at nanoseconds with high fidelity if the large detuning constraint of the selective resonant interaction is fulfilled. We also investigate the performance of our scheme in the presence of decoherence. Our results present a promising way toward achieving enhanced fast generation of the Dicke states.
We analyze a multiqubit circuit QED system in the regime where the qubit-photon coupling dominates over the system's bare energy scales. Under such conditions a manifold of low-energy states with a high degree of entanglement emerges. Here we describe a time-dependent protocol for extracting these quantum correlations and converting them into well-defined multipartite entangled states of noninteracting qubits. Based on a combination of various ultrastrong-coupling effects, the protocol can be operated in a fast and robust manner, while still being consistent with experimental constraints on switching times and typical energy scales encountered in superconducting circuits. Therefore, our scheme can serve as a probe for otherwise inaccessible correlations in strongly coupled circuit QED systems. It also shows how such correlations can potentially be exploited as a resource for entanglement-based applications.
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We propose methods for the preparation and entanglement detection\nof multi-qubit Greenberger–Horne–Zeilinger (GHZ) states in circuit quantum\nelectrodynamics. Using quantum trajectory simulations appropriate for the\nsituation of a weak continuous measurement, we show that the joint dispersive\nreadout of several qubits can be utilized for the probabilistic production of\nhigh-fidelity GHZ states. When employing a nonlinear filter on the recorded\nhomodyne signal, the selected states are found to exhibit values of the\nBell–Mermin operator exceeding 2 under realistic conditions. We discuss the\npotential of the dispersive readout to demonstrate a violation of the Mermin\nbound, and present a measurement scheme avoiding the necessity for full\ndetector tomography.
We give an analytical description of the dynamics of the three-qubit Dicke model using the adiabatic approximation in the parameter regime where the qubit transition frequencies are far off-resonance with the field frequency and the interaction strengths reach the ultrastrong-coupling regimes. Qualitative differences arise upon comparison to single- and two-qubit systems. Simple analytic formulas show that three revival sequences produce a three-frequency beat note in the time evolution of the population. We find an explicit way to estimate the dynamics for qubit-field and qubit-qubit entanglement inside the three-qubit system in the ultrastrong-coupling regime, and the resistance to sudden death proves that the entanglement in the Greenberger-Horne-Zeilinger state is more robust than that in the WWW state.