Abstract We demonstrate the impact of detunings on quantum discord between two qubits in a hybrid system composed of two quantum emitters (QEs) and a resonant Ag nanoparticle (AgNP). The dynamics and steady-state quantum discord in weak- and strong-driving regimes are both investigated. In these regimes, the existence of detunings between QEs can enhance quantum discord because of the superposition of entangled states. By the aid of the strong localized field of the AgNP, quantum discord of two QEs is effectively improved. Our protocol aims to not only complete the dynamics model of cavity-based quantum discord, but also pave a way to on-chip quantum information devices.
Abstract We demonstrate the impact of detunings on quantum discord between two qubits in a hybrid system composed of two quantum emitters (QEs) and a resonant Ag nanoparticle (AgNP). The dynamics and steady-state quantum discord in weak- and strong-driving regimes are both investigated. In these regimes, the existence of detunings between QEs can enhance quantum discord because of the superposition of entangled states. By the aid of the strong localized field of the AgNP, quantum discord of two QEs is effectively improved. Our protocol aims to not only complete the dynamics model of cavity-based quantum discord, but also pave a way to on-chip quantum information devices.
We theoretically investigate a general scheme for determining quantum entanglement of two detuned quantum dots (QDs) mediated by plasmon nanoparticle for weakly driven dissipative systems. By adiabatically eliminating the field operators, an effective model is proposed to find the underlying mechanism for this determination. Two specific hybrid systems composed of two QDs and a metallic nanoparticle (MNP) are considered. In the QD-MNP-QD system, the concurrence maxima generated by the detunings are several times larger than those in the resonance condition because of the robust superposition between the two separated entangled states. However, in the MNP-QD-QD system, the detunings generally reduce entanglement because the dipole-dipole interaction strongly suppresses the superposition of the entangled states. Moreover, the plateau of large concurrence exists for a broad range of parameters in both cases. The scheme is robust to dissipative systems and paves the way to improve entanglement in quantum information devices at the nanoscale.
This theoretical study presents a hybrid quantum system that combines the geometry of a T-shaped plasmonic waveguide with an embedded common cavity and applies quantum feedback control to enhance and preserve quantum discord between two quantum dots (QDs). Building upon previous work on entanglement generation in T-shaped waveguides and feedback-enhanced discord in V-shaped waveguides, we demonstrate that the T-shape geometry with its specific boundary conditions, coupled with active feedback, offers superior control over quantum correlations. The system consists of two QDs placed at strategic positions within the waveguide: one at the node and the other at the end of the finite arm, both embedded in a common cavity. We derive the system's dynamics using a real-space Hamiltonian approach and solve the time-independent Schrödinger equation to obtain scattering amplitudes. Quantum discord is then calculated for the two-QD subsystem. Our results show that by applying symmetric quantum feedback based on Wiseman-Milburn formalism, we can significantly enhance the steady-state quantum discord, achieving values up to [Formula: see text] for Werner states under optimal parameter conditions. We identify three distinct decay regimes for discord and demonstrate that parameters such as the phase accumulation ([Formula: see text], where [Formula: see text] is the plasmon wavevector) along the finite arm, providing cavity-QD couplings ([Formula: see text], [Formula: see text]), detunings ([Formula: see text], [Formula: see text]), and dipole-dipole interaction strength (f) provide multiple control knobs for tuning quantum correlations. This work provides a comprehensive framework for designing actively controlled nanophotonic quantum devices that leverage both plasmonic field enhancement and quantum feedback for robust quantum information processing at room temperature.
In the field of quantum information, which is subdivided into quantum computing and quantum cryptography, quantum correlations are essential for a performance or security boost not achievable with classical means. Various quantum correlation measures exist for evaluating a state’s potential to be a qubit (quantum bit). Entanglement, or nonseparability of quantum states, is the older, better known class of measures. However, for a mixed state, quantum entanglement is an incomplete measure of quantumness. Quantum discord, and its multibody extension global discord, encompass all quantum correlations. We study systems of coupled quantum dots using these measures. We study the discord of two quantum dots in the steady state with dissipation and detuning. The entanglement of the system was previously studied by Mitra and Vyas. We compare quantum discord to entanglement, finding high discord in an unentangled region, namely the upper branch of the bistability curve, where the driving field is high. By adjusting the detuning between the dots and the driving field, we can optimize the quantum discord and entanglement. We present an efficient numerical method for calculating global discord and analyze its speed and scaling. We verify that the method works for two, three, and four qubits and run speed tests. We present further simplifications that greatly enhance the scaling of the method with system size provided that the bodies are identical, but which also improve the speed otherwise. We compare our system of two quantum dots to an otherwise identical system with three quantum dots. We observe tristability in the cavity field as a function of driving field. The high field limit of global discord is a larger portion of the maximum than the discord in the same limit for the system of two quantum dots. The improvement in correlations due to detuning is still present but diminished compared to that in our two quantum dot system. The decreased effectiveness of detuning is, however, offset by the increased effect of high dot coupling, whereby the peak found in said limit with no detuning may exceed the asymptotic value without the aid of detuning.
A dissipative-driven system can generate steady-state entanglement robust to control parameters, while a quantum plasmonic system has the advantages of being strongly coupled to qubits and manipulation of electromagnetic waves in the subwavelength regime. We propose an effective multiqubit dissipative-driven entanglement model for a plasmonic system which takes into account coupling of the plasmonic cavity and qubits as well as the direct coupling between any two qubits and develop a numerical solution of the effective model on a scalable composite plasmonic structure. In this structure, the multiqubit entanglement under asymmetric detuning, especially for those combinations containing pairs of antisymmetric detuning qubits, achieves stronger steady-state entanglement measured by negativity and pairwise concurrence. The robustness of multiqubit entanglement against variations of the control parameters is also presented. In addition, the phenomenon of entanglement sudden death caused by the driving field in entanglement dynamics is investigated. Our model shows evidence that quantum plasmonics has great potential in multiqubit entanglement.
We investigate relaxation dynamics in the system of two identical quantum dipole emitters (QDEs) located near a metal nanoparticle (MNP) exhibiting a dipolar localized surface plasmon (LSP) resonance at the frequency of the QDE radiative transition. Considering one QDE to be brought into an optically active excited state and weakly coupled to the resonant LSP, we show that a stable superposition state of two QDEs is formed during the transition time, which is much shorter than the QDE spontaneous decay time and determined by the efficiency of resonant interaction between the QDEs and induced LSP. It is elucidated that the superposition state is established as a result of redistribution of the energy of the initially excited QDE so that the corresponding steady-state QDE fields induced at the MNP site cancel each other. The degree of steady-state entanglement characterized by the concurrence is found dependent only on the ratio of distances between the QDEs and the MNP, reaching its maximum value of $\ensuremath{\sim}0.65$, when the separation between the MNP and the initially excited QDE is larger by $\ensuremath{\sim}20%$ than the distance from the other QDE to the MNP.
We study the quantum discord for a system of two identical coupled quantum dots interacting with quantized cavity field in the presence of cavity as well as dot decay and detuning. The cavity is externally driven by a coherent light. These results are compared with the entanglement of the quantum dots in various parameter regimes in which system may or may not show bistability. We show that the discord in the steady state is nonzero for any nonzero cavity field amplitude. The system has higher discord in the upper branch of the bistability curve where the entanglement is zero. We also find many other interesting results including high discord and entanglement in the presence of detuning, a phenomenon which we further examine by approximating the density matrix in the appropriate limit.