[Paper Review] GaAs quantum dots grown by droplet etching epitaxy as quantum light sources

Saimon Filipe Covre da Silva, Gabriel Undeutsch+7 more|arXiv (Cornell University)|Sep 3, 2021

Semiconductor Quantum Structures and Devices110 references67 citations

TL;DR

This paper demonstrates that GaAs quantum dots grown via local droplet etching epitaxy (LDE) on GaAs(001) substrates serve as high-performance sources of single and polarization-entangled photons, achieving record-breaking photon indistinguishability (93.0(8)%) and entanglement fidelity (0.978(5)), with ultra-low multi-photon emission probability (g(2)(0) = 0.00075(16)) under two-photon excitation, making them ideal for scalable quantum communication and repeater technologies.

ABSTRACT

This paper presents an overview and perspectives on the epitaxial growth and optical properties of GaAs quantum dots obtained with the droplet etching method as high-quality sources of quantum light. We illustrate the recent achievements regarding the generation of single photons and polarization entangled photon pairs and the use of these sources in applications of central importance in quantum communication, such as entanglement swapping and quantum key distribution.

Motivation & Objective

• To develop a scalable, high-quality solid-state source of single and entangled photons for quantum technologies.
• To overcome limitations of conventional SPDC and InGaAs QD sources, such as multi-photon emission and low entanglement fidelity.
• To achieve near-unity photon indistinguishability and entanglement fidelity through material and device engineering.
• To enable practical applications in quantum key distribution, entanglement swapping, and quantum repeaters using GaAs QDs.
• To establish GaAs QDs via LDE as a leading platform for future photonic quantum networks by optimizing material quality and photonic integration.

Proposed method

• Employing molecular beam epitaxy (MBE) to grow GaAs/AlGaAs quantum dots via the local droplet etching (LDE) method on GaAs(001) substrates.
• Using Al droplets to induce nanohole formation in AlGaAs layers through As flux-assisted etching, followed by GaAs overgrowth to form QDs.
• Applying strain-tuning and charge-tunable devices to suppress blinking and reduce excitonic fine structure splitting (FSS) to near-zero values.
• Integrating QDs into circular Bragg grating cavities to enhance photon collection efficiency and Purcell effect for improved indistinguishability.
• Using resonant optical excitation and two-photon excitation (TPE) to achieve sub-Poissonian emission and minimize multi-photon events.
• Implementing Hong-Ou-Mandel (HOM) interferometry to measure photon indistinguishability between photons from two remote QDs.

Experimental results

Research questions

• RQ1 Can GaAs QDs grown via LDE achieve near-unity entanglement fidelity and ultra-low multi-photon emission probability?
• RQ2 To what extent can photon indistinguishability be enhanced in remote GaAs QDs through material quality and device engineering?
• RQ3 Can the combination of low FSS, short excitonic lifetimes, and charge-tunable devices suppress blinking and enable high-fidelity quantum operations?
• RQ4 How does the integration of GaAs QDs into photonic cavities affect photon collection efficiency and indistinguishability?
• RQ5 Can GaAs QDs serve as a scalable platform for quantum repeater nodes via entanglement swapping and spin-photon interfaces?

Key findings

• GaAs QDs via LDE achieved the highest reported two-photon interference visibility of 93.0(8)% between photons from two remote QDs, setting a new state-of-the-art for solid-state emitters.
• The entanglement fidelity reached 0.978(5) with a concurrence of 0.97(1), the highest to date for any quantum dot type, due to ultralow ensemble-averaged FSS and short excitonic lifetimes.
• The multi-photon emission probability was measured at g(2)(0) = 0.00075(16) under two-photon excitation without background subtraction or temporal filtering, indicating near-ideal single-photon emission.
• Photon pair collection efficiency reached 0.65(4) in broadband cavities, and single-photon collection efficiency reached 0.85(3), demonstrating high efficiency for practical integration.
• Blinking was completely suppressed under both resonant single-photon and two-photon excitation in p-i-n diode-integrated QDs, enabling stable operation.
• Theoretical and experimental results confirm that GaAs QDs are promising candidates for quantum repeater nodes due to low strain, large size, and potential for long spin coherence times.