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Applied Physics Letters

Strong coupling in the entanglement dynamics of two qubits interacting with a graphene nanodisk

Nikos Iliopoulos, Ioannis Thanopulos, Vasilios D. Karanikolas, Emmanuel Paspalakis

Published Oct 24, 2022• 8 citations
Publisher

Abstract

We investigate the entanglement dynamics of two qubits interacting with a graphene nanodisk using the macroscopic quantum electrodynamics method. By modifying the free-space decay rate of each qubit, we study the coupling strength between the nanoparticle and the qubits. We find that as the free-space decay rate increases, the decaying Rabi oscillations featured in the qubit population dynamics change to complex non-Markovian dynamical population evolution. This is also reflected on the concurrence, which at weak or moderate light–matter coupling conditions, attains values up to 0.5, while as the coupling conditions become stronger, larger values are also transiently observed. Our findings indicate that graphene nanostructures can provide a platform for the realization of high degree of entanglement in the strong coupling regime at the nanoscale, essential for quantum technology applications.

DOI
10.1063/5.0119264

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We investigate the entanglement dynamics of two qubits interacting with a graphene nanodisk using the macroscopic quantum electrodynamics method. By modifying the free-space decay rate of each qubit, we study the coupling strength between the nanoparticle and the qubits. We find that as the free-space decay rate increases, the decaying Rabi oscillations featured in the qubit population dynamics change to complex non-Markovian dynamical population evolution. This is also reflected on the concurrence, which at weak or moderate light–matter coupling conditions, attains values up to 0.5, while as the coupling conditions become stronger, larger values are also transiently observed. Our findings indicate that graphene nanostructures can provide a platform for the realization of high degree of entanglement in the strong coupling regime at the nanoscale, essential for quantum technology applications.
In this work, we theoretically analyze the population and entanglement dynamics for two identical qubits interacting with a graphene nanodisk. We consider qubits with various free-space decay rates in order to study how the coupling strength influences the dynamics of our composite system, as well as their entanglement evolution as quantified by concurrence. We find that for relatively large free-space decay rates, the population dynamics of each qubit features Rabi oscillations, while for smaller decay rates it exhibits a complex behavior, which nevertheless can be understood by semianalytical expressions using the sub- and superradiant states of the system. Additionally, we observe phenomena, such as population and entanglement trapping, induced by the strong interaction of the qubits with the graphene nanodisk, with trapping values depending solely on the initial conditions of the two qubits. Lastly, we compare our results with those obtained in the case of the two qubits interacting independently with a graphene nanodisk.
We investigate the dynamical evolution of the spontaneous emission of a two-level quantum emitter near a graphene nanodisk. We employ the macroscopic quantum electrodynamics methodology to study the reversible population dynamics of the excited state of the quantum emitter in the non-Markovian limit. Our results indicate that the quantum-emitter--nanodisk interaction enters the strong light-matter coupling regime, as manifested in the excited-state probability where Rabi oscillations and population trapping effects are observed. The level of non-Markovianity is estimated by computing three different well-established measures and relate them to the quantum speedup due to the strong-coupling interaction of the emitter with the graphene nanodisk. Importantly, high values of the non-Markovianity and quantum speedup measures are achieved when the emitter-nanodisk interaction is strong, whereas decreasing coupling strength leads to smaller values.
Abstract In this work, we analyze the spontaneous emission dynamics of a V‐type quantum emitter near a graphene nanodisk based on the combination of electromagnetic and quantum dynamical calculations. The presence of the graphene nanodisk gives strong anisotropy to the Purcell factors of the quantum emitter, leading to interference effects in spontaneous emission appearing as coupling between the emitter’s upper levels. This effect is further enhanced by the strong light–matter interaction of the quantum emitter with the modified electromagnetic mode continuum, which induces non‐Markovian spontaneous emission dynamics. We have studied the population dynamics of the quantum emitter at a specific distance from the center of the graphene nanodisk for various free‐space decay widths and different quantum emitter’s initial conditions and have shown weak coupling results appearing with Markovian decay dynamics, obtained for quantum emitters with small free‐space decay widths, and population dynamics that exhibits distinctly non‐Markovian features, such as prominent decaying Rabi oscillations in the population evolution of the quantum emitter’s excited states and energy exchange between them during the overall population decay into the photonic mode continuum for largest free‐space decay widths. Also, for the largest value of the free‐space decay width, we obtain significant population trapping effects in the excited states of the quantum emitter. Furthermore, we find that the population dynamics for specific light–matter interaction strength conditions between the quantum emitter and the graphene nanodisk depend distinctively on the initial state of the quantum emitter, whether it is a single state or a superposition state.
We study the spontaneous emission dynamics of a quantum emitter near a graphene nanodisk. We analyze the population dynamics of the excited state of the quantum emitter and also explore its dynamics as a non-Markovian open system. Specifically, we quantify the non-Markovian spontaneous emission dynamics using different non-Markovianity measures and calculate the quantum speed limit under non-Markovian evolution. We find strong light-matter coupling conditions for the quantum emitter near the graphene nanodisk, which are manifested in either distinct decaying Rabi oscillations or population trapping effects in the excited state population dynamics of the quantum emitter, depending on the parameters of the system. We also show that the values of the non-Markovianity measures and of the potential quantum speed up are large under strong light–matter coupling conditions.
This work studies the absorption spectrum of a two-level quantum emitter coupled to a photonic environment modified by the plasmon modes of a graphene nanodisk. Using macroscopic quantum electrodynamics and a Lorentzian mapping, the system is described within an effective cavity quantum electrodynamics framework. Analytical expressions are derived for the linear susceptibility, and three distinct spectral regimes are identified. In vacuum induced transparency, destructive quantum interference opens a narrow transparency window in the absorption spectrum. In vacuum Rabi splitting, coherent energy exchange between the emitter and the plasmonically modified photonic mode produces a well-resolved doublet. When plasmon losses are large, the required interference is suppressed, leading to the absence of induced optical transparency. It is shown that the transitions between these regimes are strictly determined by the exceptional points of the system's effective non-Hermitian Hamiltonian, and a complete phase diagram is provided as a function of the emitter-nanodisk distance and the free-space decay rate. The results highlight graphene nanodisks as a highly tunable platform for the dynamic control of quantum interference, offering potential applications in reconfigurable nanophotonic spectral switching and transparency-window engineering.
We analyze the dynamics of quantum correlations between two qubits coupled to a linear chain of oscillators. The chain mediates interactions between the qubits and acts as a non-Markovian reservoir. The model is amenable to an analytical solution when the initial state of the chain is Gaussian. We study the dynamics of the qubits' concurrence starting from a separable state and assuming that the chain spectrum is gapped and the chain is initially in a thermal state. We identify three relevant regimes that depend on the strength of the qubit-chain coupling in relation to the spectral gap. These are (i) the weak-coupling regime, where the qubits are entangled at the asymptotics, (ii) the strong-coupling regime, where the concurrence can exhibit collapses followed by revivals with exponentially attenuated amplitude, and (iii) the thermal damping regime, where the concurrence rapidly vanishes due to the chain's thermal excitations. In all cases, if entanglement is generated, this occurs after a finite time has elapsed. This time scale depends exponentially on the qubits' distance and is determined by the spectral properties of the chain. Entanglement irreversible decay, on the other hand, is due to the dissipative effect induced by the coupling with the chain and is controlled by the coupling strength between the chain and qubits. This study unravels the basic mechanisms leading to entanglement in a non-Markovian bath and allows one to identify the key resources for realizing quantum coherent dynamics of open systems.