A solid-state entangled photon pair source with high brightness and indistinguishability (1903.01339v1)

Abstract: The generation of high-quality entangled photon pairs has been being a long-sought goal in modern quantum communication and computation. To date, the most widely-used entangled photon pairs are generated from spontaneous parametric downconversion, a process that is intrinsically probabilistic and thus relegated to a regime of low pair-generation rates. In contrast, semiconductor quantum dots can generate triggered entangled photon pairs via a cascaded radiative decay process, and do not suffer from any fundamental trade-off between source brightness and multi-pair generation. However, a source featuring simultaneously high photon-extraction efficiency, high-degree of entanglement fidelity and photon indistinguishability has not yet been reported. Here, we present an entangled photon pair source with high brightness and indistinguishability by deterministically embedding GaAs quantum dots in broadband photonic nanostructures that enable Purcell-enhanced emission. Our source produces entangled photon pairs with a record pair collection probability of up to 0.65(4) (single-photon extraction efficiency of 0.85(3)), entanglement fidelity of 0.88(2), and indistinguishabilities of 0.901(3) and 0.903(3), which immediately creates opportunities for advancing quantum photonic technologies.

• The paper reports a GaAs quantum dot design using circular Bragg resonators that achieves a record photon pair collection probability of 0.65.
• It employs pulsed two-photon resonant excitation to reach an entanglement fidelity of 0.88 while significantly reducing photon leakage.
• The advanced nanostructure design yields photon indistinguishability indices above 0.90, paving the way for scalable quantum networks.

Overview of a Solid-State Entangled Photon Pair Source with High Brightness and Indistinguishability

The paper presents a significant advancement in the generation of entangled photon pairs using semiconductor quantum dots (QDs) embedded in photonic nanostructures. The authors propose a solid-state source that achieves high brightness, entanglement fidelity, and photon indistinguishability, utilizing GaAs QDs within a novel broadband structure termed circular Bragg resonators on highly-efficient broadband reflectors (CBR-HBR). This paper makes notable strides in overcoming traditional challenges faced by quantum photonic technologies that rely on spontaneous parametric downconversion (SPDC), an inherently probabilistic photon pair generation process with limited brightness.

Research Design and Methodology

The paper details the engineering of nanostructures that enhance the photon-emission characteristics of quantum dots. The CBR-HBR employs a combination of circular Bragg gratings and broadband reflectors, which serve to boost both the collection efficiency and the Purcell factor over a significant bandwidth. This novel design surpasses existing methods by incorporating a high-efficiency reflector that curtails downward photon leakage, thus amplifying photon collection efficiency.

Fabricated via deterministic processes, these structures precisely position single GaAs QDs at the resonant cavity centers, ensuring optimal entangled photon pair generation. The experimental setup includes pulsed two-photon resonant excitation for the quantum dots, leveraging weak coherent control protocols to maximize photon emissions.

Key Findings and Results

Several key performance metrics illustrate the advantages of this approach:

• Pair Collection Probability: The authors report a photon pair collection probability per pulse of approximately 0.65, a record-high figure that significantly outperforms traditional SPDC and other quantum dot-based systems.
• Entanglement Fidelity: An entanglement fidelity of 0.88 is achieved, indicating robust quantum state integrity without requiring stringent noise filtering methods.
• Photon Indistinguishability: The system achieves indistinguishability indices of 0.901 and 0.903 for X and XX photons, respectively, which is critical for applications requiring quantum interference.

Implications and Future Directions

The innovations described pave the way for practical deployment in quantum communication systems and on-chip quantum processing applications. Given the demonstrated scalability of the fabrication techniques, this source could integrate well into photonic circuits, bridging the gap between experimental quantum mechanics and applied technologies.

The high collection efficiency, achieved through enhanced photonic structure design, suggests a strong potential for this method to enable quantum networks using entangled photons over extended distances. Future research could focus on extending these techniques into telecom wavelengths for integration with existing fiber-optic networks. Moreover, improving strain-tuning methodologies may further optimize entanglement fidelity across a broader range of quantum dots.

In conclusion, this paper provides a comprehensive exploration of how semiconductor quantum dots embedded in sophisticated photonic structures can advance the capabilities of quantum light sources. The reported breakthroughs have tangible implications for the evolution of quantum photonic technologies, with the potential to facilitate significant advancements in quantum information science.