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Optics Express

Optical-Coupling of distant spins via collective enhancement in multi-mode whispering gallery resonators

Daniel Lombardo, Jason Twamley

Published Feb 2, 2015• 0 citations
Publisher

Abstract

The quantum coupling of spatially distant spins via optical photons using cavity quantum electrodynamic (cQED) methods has proved experimentally challenging due to the large spin-photon coupling strengths required. To achieve such coupling strengths using traditional cQED methods requires either individual spins and ultra-small cavities or an ensemble of identical spins coupled to larger cavities. In this work we describe a method to couple distant spins via the collective enhanced coupling to a large ensemble ∼ N, of degenerate optical Whispering Gallery Modes (WGM) in a spherical resonator where the spins are spatially located at the antipodes. The setup can be scaled-up to build 1D, 2D and 3D cQED lattices to enable quantum simulation or computing.

DOI
10.1364/oe.23.002945

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The quantum coupling of spatially distant spins via optical photons using cavity quantum electrodynamic (cQED) methods has proved experimentally challenging due to the large spin-photon coupling strengths required. To achieve such coupling strengths using traditional cQED methods requires either individual spins and ultra-small cavities or an ensemble of identical spins coupled to larger cavities. In this work we describe a method to couple distant spins via the collective enhanced coupling to a large ensemble ∼ N, of degenerate optical Whispering Gallery Modes (WGM) in a spherical resonator where the spins are spatially located at the antipodes. The setup can be scaled-up to build 1D, 2D and 3D cQED lattices to enable quantum simulation or computing.
Cavity quantum electrodynamics (CQED) investigates the interaction between light confined in a resonator and particles, such as atoms. In recent years, CQED experiments have reached the optical domain resulting in many interesting applications in the realm of quantum information processing. For many of these application it is necessary to overcome limitations imposed by photon loss. In this context whispering-gallery mode (WGM) resonators have obtained significant interest. Besides their small mode volume and their ultra high quality, they also exhibit favorable polarization properties that give rise to chiral light--matter interaction. In this chapter, we will discuss the origin and the consequences of these chiral features and we review recent achievements in this area.
We present a flexible design to realize the entanglement between two distant semiconductor quantum dots (QDs) embedded in separated photonic crystal nanobeam cavities. When bridged by a largely detuned microring cavity, photonic supermodes between two distant nanobeam cavities are formed via whispering gallery modes (WGMs). Due to the large detuning, WGMs in the microring exhibit almost no photonic excitation, showing the "dark WGMs." With the dyadic Green's functions of the nano-structure and the resolvent operators of the Hamiltonian, we numerically investigate the entanglement dynamics of two distant QDs. Furthermore, we prove that the entanglement can be tuned by adjusting the distances between the cavities. Such a scheme paves an efficient way for realizing a scalable quantum network in a solid-state system.
In the two-atom or multiatom system, the atoms can interact with each other through exchange of virtual photon. This kind of energy exchange is often referred as the dipole-dipole interaction (DDI). Here we consider this DDI system consisting of a pair of two-level atoms strongly coupled with a bimodal whispering-gallery-mode (WGM) microresonator which is driven by an external laser field. Our aim is to explore the photon correlation characteristics of the proposed architecture using realistic experimental parameter values. We compare in detail the quality of photon antibunching (i.e., the smallness of the second-order correlation function) from three involved configurations in cavity quantum electrodynamics (QED): (i) only one two-level atom, (ii) two far apart two-level atoms without DDI, and (iii) two DDI (dipole-coupled) two-level atoms are respectively coupled to the driven WGM microresonator through the evanescent field. We clearly show that the DDI between both atoms can distinctly enhance the photon antibunching even in the weak-coupling regime in configuration (iii) with feature-rich line shapes. We also find that the photon antibunching can be modulated by properly adjusting the atom-cavity coupling strength. In addition, we display that this strong photon antibunching is robust against the cooperative atomic decay. Our DDI-based cavity QED scheme may provide an alternative way to the construction of integrated on-chip single-photon sources.
We showcase an erbium-doped whispering-gallery-mode resonator with optical modes that display intrinsic quality factors better than 10810^8108 (linewidths less than 2 MHz), and coupling strengths to collective erbium transitions of up to 2$π\times$1.2 GHz - enough to reach the ensemble strong coupling regime. Our optical cavity sits inside a microwave resonator, allowing us to probe the spin transition which is tuned by an external magnetic field. We show a modified optically detected magnetic resonance measurement that measures population transfer by a change in coupling strength rather than absorption coefficient. This modification was enabled by the strong coupling to our modes, and allows us to optically probe the spin transition detuned by more than the inhomogeneous linewidth. We contrast this measurement with electron paramagnetic resonance to experimentally show that our optical modes are confined in a region of large microwave magnetic field and we explore how such a geometry could be used for coherent microwave-optical transduction.
Strong coupling between an emitter and a cavity is fundamental for quantum networks, enabling deterministic quantum state transfer, distributed quantum computing, and on-demand entanglement generation. Rare-earth ions (REIs) are compelling candidates for such networks due to their long optical and spin coherence times. However, their inherently weak intra-4f optical transitions typically result in low coupling strengths, restricting most current achievements to the weak coupling regime. Our work proposes a realistic approach for achieving strong coupling between rare-earth ions and high-Q whispering-gallery-mode (WGM) microcavities. Our simulations further demonstrate that electro-optic tuning in this microdisk resonator system enables precise frequency alignment between rare-earth ions and the optical cavity mode across arbitrary target frequencies. Additionally, we provide numerical validation for a parametric amplification technique to enhance the emitter-cavity coupling strength. As an extension of this approach, the coupled system efficiently achieves the quantum entanglement of local and flying qubits. This study deepens the understanding of emitter-cavity interactions and contributes to realizing REIs-based photonic platforms, which are crucial to distributed quantum computing and developing robust quantum networks.
Atom coherence in the cooperative interaction between the atoms and a single photon creates ultrafast spontaneous emission and ultrastrong intensity peaks, namely single-photon superradiance, which has attracted considerable research interest in the recent years. As the investigation for such a single-photon superradiance usually involves only quantum emitters, in this paper we present the cooperative atom-photon interaction in a system including whispering gallery mode resonators, which are one of the most essential components in nanophotonic devices. Here we both analytically and computationally show that the separated atoms on a resonator can obtain cooperative interaction, manifesting superradiant coupling strength, which results in the ultrafast spontaneous emission and ultrastrong intensity peaks. The conditions of such single-photon superradiance are called whispering gallery mode superradiance conditions. Furthermore, the atoms are proved to have cooperative interaction on a cascade of resonators by a renormalization approach.