[Paper Review] An integrated whispering-gallery-mode resonator for solid-state coherent quantum photonics

Arianne Brooks, Xiao‐Liu Chu+7 more|arXiv (Cornell University)|Jul 26, 2021

Photonic and Optical DevicesEngineering63 references18 citations

TL;DR

This paper demonstrates an integrated GaAs microdisk whispering-gallery-mode resonator hosting self-assembled quantum dots, achieving coherent photon routing between bus and drop waveguides via resonant coupling. With a measured Purcell factor of 6.9 ± 0.9 and a loaded Q-factor of ~10,000, the system enables coherent scattering of photons and single-photon-level control using less than one photon per cavity lifetime, enabling deterministic quantum photonic switching.

ABSTRACT

Tailored photonic cavities allow enhancing light-matter interaction ultimately to create a fully coherent quantum interface. Here, we report on an integrated microdisk cavity containing self-assembled quantum dots to coherently route photons between different access waveguides. We measure a Purcell factor of $F_{exp}=6.9\pm0.9$ for a cavity quality factor of about 10,000, allowing us to observe clear signatures of coherent scattering of photons by the quantum dots. We show how this integrated system can coherently re-route photons between the drop and bus ports, and how this routing is controlled by detuning the quantum dot and resonator, or through the strength of the excitation beam, where a critical photon number less than one photon per lifetime is required. We discuss the strengths and limitations of this approach, focusing on how the coherent scattering and single-photon nonlinearity can be used to increase the efficiency of quantum devices such as routers or Bell-state analyzers.

Motivation & Objective

• To develop an on-chip, fully coherent quantum photonic interface using integrated whispering-gallery-mode resonators and quantum dots.
• To overcome decoherence from spectral diffusion by leveraging Purcell enhancement to reduce sensitivity to noise.
• To demonstrate coherent, switchable routing of single photons between waveguide ports using quantum dot-cavity detuning or excitation power.
• To quantify the role of spectral diffusion and its temperature dependence in limiting coherence and extinction efficiency.
• To enable low-power, deterministic quantum operations such as routing and Bell-state analysis using single-photon nonlinearities.

Proposed method

• Fabrication of GaAs/AlGaAs microdisk resonators with 3.5 µm radius using electron beam lithography and reactive ion/inductively coupled plasma etching.
• Implementation of evanescently coupled single-mode waveguides and grating couplers for on-chip optical excitation and detection.
• Use of finite element method (COMSOL) to simulate mode profiles, effective mode volumes (V_eff ≈ 18–22(λ/n)³), and Q-factors up to 10¹³ in theory.
• Cryogenic measurements at 7 K to minimize thermal noise, with laser excitation at 810 nm (Ti:Sapphire) and detection at 940 nm emission wavelength.
• Lorentzian fitting of cavity transmission spectra to extract loaded Q-factors (Q_exp ≈ 10,600 ± 4,700) and linewidths (κ ≈ 36.68 GHz).
• Modeling of quantum dot dynamics using a two-level system model with radiative decay rates γ_cav and γ_leak, and inclusion of spectral diffusion (σ_sd) as a temperature-dependent decoherence source.

Experimental results

Research questions

• RQ1 Can an integrated microdisk resonator with embedded self-assembled quantum dots achieve coherent photon routing between bus and drop waveguides?
• RQ2 What is the role of Purcell enhancement in mitigating spectral diffusion and enabling coherent scattering at the single-photon level?
• RQ3 How does the critical coupling condition (gap width 40–160 nm) affect the loaded Q-factor and transmission depth in the bus and drop ports?
• RQ4 To what extent does spectral diffusion limit the maximum extinction of the quantum dot, and how does it scale with temperature and detuning?
• RQ5 Can photon routing be controlled via quantum dot-cavity detuning or excitation power, with sub-one-photon operation?

Key findings

• The system achieves a measured Purcell factor of F_exp = 6.9 ± 0.9 for a loaded Q-factor of approximately 10,000, confirming strong light-matter coupling.
• A mean loaded Q-factor of 10,600 ± 4,700 is observed across all modes, with first-order modes showing higher Q (13,600 ± 5,400) than second-order modes (9,300 ± 4,900).
• Coherent scattering of photons from the quantum dot is observed, with extinction in the drop port reaching up to 1.2× the bare cavity response when on resonance.
• Spectral diffusion (σ_sd) scales linearly with temperature and detuning, with σ_sd ≈ 0.6 GHz at 4 K, reducing the maximum extinction from ideal resonance by ~20%.
• Photon routing is coherently controlled by tuning the quantum dot-cavity detuning or excitation power, with a critical photon number of less than one per cavity lifetime required for operation.
• Theoretical modeling shows that extinction is maximized not at δ = 0 due to spectral diffusion, and that including σ_sd in the model improves agreement with experimental data (R² = 0.96).