[Paper Review] Collective super- and subradiant dynamics between distant optical quantum emitters

Alexey Tiranov, Vasiliki Angelopoulou+10 more|arXiv (Cornell University)|Oct 5, 2022

Photonic and Optical Devices4 citations

TL;DR

This study demonstrates coherent, long-range radiative dipole-dipole coupling between distant optical quantum emitters in a photonic crystal waveguide, enabling dynamic control of collective super- and subradiant emission. By tuning magnetic fields and excitation conditions, the authors observe both enhanced (superradiant) and suppressed (subradiant) decay dynamics, establishing a foundational platform for scalable quantum information processing with on-chip photonic networks.

ABSTRACT

Photon emission is the hallmark of light-matter interaction and the foundation of photonic quantum science, enabling advanced sources for quantum communication and computing. Although single-emitter radiation can be tailored by the photonic environment, the introduction of multiple emitters extends this picture. A fundamental challenge, however, is that the radiative dipole-dipole coupling rapidly decays with spatial separation, typically within a fraction of the optical wavelength. We realize distant dipole-dipole radiative coupling with pairs of solid-state optical quantum emitters embedded in a nanophotonic waveguide. We dynamically probe the collective response and identify both super- and subradiant emission as well as means to control the dynamics by proper excitation techniques. Our work constitutes a foundational step toward multiemitter applications for scalable quantum-information processing.

Motivation & Objective

• To realize long-range radiative dipole-dipole coupling between distant optical quantum emitters beyond the sub-wavelength limit.
• To dynamically probe and control collective emission behavior—specifically super- and subradiance—in a solid-state nanophotonic platform.
• To overcome the challenge of rapidly decaying dipole-dipole coupling in free space by leveraging photonic crystal waveguides to extend interaction range.
• To demonstrate coherent control of single-excitation collective states using spectrally tuned excitation and magnetic field tuning.
• To provide experimental evidence of coherent coupling through time-resolved emission dynamics, advancing multi-emitter quantum networks.

Proposed method

• Utilized a photonic crystal waveguide (PCW) to mediate long-range radiative coupling between two self-assembled InGaAs quantum dots (QDs), extending interaction beyond the sub-wavelength limit.
• Employed magnetic field tuning (Zeeman effect) to spectrally align the QD transition frequencies and achieve resonance between emitters.
• Performed time-resolved single-photon emission measurements using a Hanbury Brown and Twiss setup to detect dynamics from both waveguide output ports.
• Applied spectrally shaped laser pulses for resonant excitation, with polarization control to independently address each QD and tune the relative phase of excitation.
• Used a theoretical model based on coupled-dipole master equations including radiative decay, spectral diffusion, dephasing, and coherent coupling to fit experimental data.
• Calibrated excitation pulse areas via Rabi oscillations and used frequency-filtered lasers with a 3 GHz linewidth to suppress background excitation.

Experimental results

Research questions

• RQ1 Can radiative dipole-dipole coupling between distant optical emitters be extended beyond the sub-wavelength range using a photonic waveguide?
• RQ2 Can both super- and subradiant collective emission dynamics be experimentally observed and controlled in a solid-state system?
• RQ3 How does the relative phase and amplitude of excitation fields affect the coherent dynamics of the two-emitter system?
• RQ4 What role do dephasing and spectral diffusion play in the visibility and lifetime of subradiant states?
• RQ5 Can the coherent coupling strength and decay rates be quantitatively extracted from time-resolved emission traces?

Key findings

• The authors observed both super- and subradiant decay dynamics in a pair of quantum dots separated by 1.25 µm, with decay rates of 0.61 GHz (Γ_sup) and 0.12 GHz (Γ_sub), respectively.
• Subradiant states exhibited a lifetime of approximately 10 ns, with a dephasing rate of 0.03 GHz (γ_d), indicating long-lived coherence despite experimental imperfections.
• The radiative coupling rate Γ_23 was measured at 0.61 GHz, and the dispersive coupling J_23 at 0.03 GHz, confirming strong photon-mediated interaction.
• By tuning the excitation phase θ to -0.48π, the system was driven into a superposition state that enhanced the visibility of coherent oscillations in emission.
• Theoretical fits to experimental data using a master equation model with parameters from Table S3 showed excellent agreement, validating the model's predictive power.
• Resonant excitation of both QDs with orthogonal polarization components enabled coherent control of the collective state, with excitation pulse areas Ω₂ = 0.87(6) and Ω₃ = 1.33(5) in units of Rabi frequency.