The realization of a functional quantum repeater is one of the major research goals in long-distance quantum communication. Among the different approaches that are being followed, the one relying on quantum memories interfaced with deterministic quantum emitters is considered as among one of the most promising solutions. In this work, we focus on memory-based quantum-repeater schemes that rely on semiconductor quantum dots for the generation of polarization entangled photons. Going through the most relevant figures of merit related to efficiency of the photon source, we select significant developments in fabrication, processing and tuning techniques aimed at combining high degree of entanglement with on-demand pair generation, with a special focus on the progress achieved in the representative case of the GaAs system. We proceed to offer a perspective on integration with quantum memories, both highlighting preliminary works on natural-artificial atomic interfaces and commenting a wide choice of currently available and potentially viable memory solutions in terms of wavelength, bandwidth and noise-requirements. To complete the overview, we also present recent implementations of entanglement-based quantum communication protocols with quantum dots and highlight the next challenges ahead for the implementation of practical quantum networks.
The realization of a functional quantum repeater is one of the major research goals in long-distance quantum communication. Among the different approaches that are being followed, the one relying on quantum memories interfaced with deterministic quantum emitters is considered as among one of the most promising solutions. In this work, we focus on memory-based quantum-repeater schemes that rely on semiconductor quantum dots for the generation of polarization entangled photons. Going through the most relevant figures of merit related to efficiency of the photon source, we select significant developments in fabrication, processing and tuning techniques aimed at combining high degree of entanglement with on-demand pair generation, with a special focus on the progress achieved in the representative case of the GaAs system. We proceed to offer a perspective on integration with quantum memories, both highlighting preliminary works on natural-artificial atomic interfaces and commenting a wide choice of currently available and potentially viable memory solutions in terms of wavelength, bandwidth and noise-requirements. To complete the overview, we also present recent implementations of entanglement-based quantum communication protocols with quantum dots and highlight the next challenges ahead for the implementation of practical quantum networks.
Abstract The realization of a functional quantum repeater is one of the major research goals in long-distance quantum communication. Among the different approaches that are being followed, the one relying on quantum memories interfaced with deterministic quantum emitters is considered as one of the most promising solutions. In this work, we focus on the hardware to implement memory-based quantum-repeater schemes that rely on semiconductor quantum dots (QDs) for the generation of polarization entangled photons. Going through the most relevant figures of merit related to efficiency of the photon source, we select significant developments in fabrication, processing and tuning techniques aimed at combining high degree of entanglement with on-demand pair generation, with a special focus on the progress achieved in the representative case of the GaAs system. We proceed to offer a perspective on integration with quantum memories, both highlighting preliminary works on natural–artificial atomic interfaces and commenting a wide choice of currently available and potentially viable memory solutions in terms of wavelength, bandwidth and noise-requirements. To complete the overview, we also present recent implementations of entanglement-based quantum communication protocols with QDs and highlight the next challenges ahead for the implementation of practical quantum networks.
The generation and long-haul transmission of highly entangled photon pairs is a cornerstone of emerging photonic quantum technologies with key applications such as quantum key distribution and distributed quantum computing. However, a natural limit for the maximum transmission distance is inevitably set by attenuation in the medium. A network of quantum repeaters containing multiple sources of entangled photons would allow overcoming this limit. For this purpose, the requirements on the source's brightness and the photon pairs' degree of entanglement and indistinguishability are stringent. Despite the impressive progress made so far, a definitive scalable photon source fulfilling such requirements is still being sought after. Semiconductor quantum dots excel in this context as sub-Poissonian sources of polarization entangled photon pairs. In this work, we present the state-of-the-art set by GaAs based quantum dots and use them as a benchmark to discuss the challenges toward the realization of practical quantum networks.
The generation and long-haul transmission of highly entangled photon pairs is a cornerstone of emerging photonic quantum technologies, with key applications such as quantum key distribution and distributed quantum computing. However, a natural limit for the maximum transmission distance is inevitably set by attenuation in the medium. A network of quantum repeaters containing multiple sources of entangled photons would allow to overcome this limit. For this purpose, the requirements on the source's brightness and the photon pairs' degree of entanglement and indistinguishability are stringent. Despite the impressive progress made so far, a definitive scalable photon source fulfilling such requirements is still being sought for. Semiconductor quantum dots excel in this context as sub-poissonian sources of polarization entangled photon pairs. In this work we present the state-of-the-art set by GaAs based quantum dots and use them as a benchmark to discuss the challenges to overcome towards the realization of practical quantum networks.
Entanglement-based quantum key distribution (e-QKD) is a cornerstone of quantum-secure communication, offering advantages in device-independent security, compatibility with quantum networks, and resilience to specific attacks. While the probabilistic nature of entangled photon generation has slowed down experimental adoption compared to prepare-and-measure approaches, deterministic solid-state emitters—particularly epitaxial quantum dots (QDs)—have recently emerged as promising candidates for scalable, high-rate implementations. In this review, we survey the main e-QKD protocols along with their practical requirements and assess current technologies for entangled photon pair generation. We place particular emphasis on the state of the art in QD-based entangled photon sources, analyzing design strategies and performance metrics that directly impact secure key generation. Recent experimental advances demonstrating entanglement distribution and e-QKD with QDs over both fiber and free-space channels are discussed in detail. We conclude with an outlook on remaining challenges in source engineering, transmission capacity, and system integration, highlighting the potential of QDs to enable next-generation quantum communication technologies.
Quantum technology promises to revolutionize various aspects of information processing, communication, and data security, by surpassing the capabilities of classical systems. Central to these advancements are photons, due to their resilience to external influence and ease of transport in air and glass fibers. However, attenuation in glass fiber cores hinders the realization of a worldwide quantum network. To circumvent this limit, the use of quantum repeaters for long-distance communication is required. Pivotal to the realization of quantum repeaters are entangled photons, enabling the distribution of entanglement across intermediate nodes, eventually extending the range of the envisioned quantum network. Despite the availability of numerous photon sources, stringent requirements–such as deterministic photon generation with high purity, high indistinguishability, high efficiency and a high degree of entanglement–hinder the realization of quantum repeaters, as they need to be fulfilled within the same device. Epitaxial quantum dots have emerged as promising candidates for generating pairs of entangled photons with a high purity and a high degree of entanglement. These semiconductor nanostructures, often referred to as “artificial atoms,” confine charge carriers and discretize energy levels like atoms but in a manipulable solid state environment. They exhibit exceptional emission characteristics owing to their high quantum efficiency and tunability via external fields such as electric, magnetic, and strain fields. Nevertheless, the fact that they are embedded in a semiconductor matrix with high refractive index limits their brightness due to challenging light extraction. This PhD thesis investigates the potential of strain-tuned GaAs droplet-etched quantum dots–a system known to generate highly entangled photon pairs–embedded within a photonic cavity, aiming to enhance light extraction efficiency, for an ideal entangled photon source. Device fabrication involves bonding the semiconductor with a micromachined piezoelectric substrate for strain-tuning quantum dots and their deterministic integration with a circular Bragg grating resonator, known as a “bullseye cavity.” Key advancements include cryogenic photoluminescence imaging for precise quantum dot localization, achieving high spatial overlap of the confined excitons with the resonator’s mode volume. Furthermore, postfabrication tuning of the resonator mode via wet chemical etching is employed to optimize spectral overlap. The achievements enable significant Purcell enhancement and high extraction efficiency. Simultaneously, multi-axial strain tuning the quantum dot emission eliminates entanglement degradation caused by the anisotropic exchange interaction of charge carriers. Such an entangled photon source additionally allows to experimentally reveal the limits of entanglement set by the excitation type. Finally, with optimized excitation conditions, a near-ideal entangled photon pair source is realized, which may surpass any other quantum emitter in terms of simultaneous brightness and degree of entanglement. The advancements presented within this PhD thesis represent a significant step towards the integration of quantum dot-based entangled photon sources into real-world quantum technology applications, facilitating the realization of a quantum internet.
We propose and analyze a hybrid quantum repeater architecture that combines two leading hardware platforms: quantum dots (QDs) as bright, deterministic sources of entangled photon pairs and group-IV-vacancy centers in diamond as efficient, heralded quantum memories. This combination leverages high-rate entanglement generation together with long-lived storage, enabling scalable entanglement distribution over long distances. A key challenge is the large bandwidth mismatch between QD photons and the narrow optical transitions of the memories. We combine a comprehensive model of the spin-photon interface, including full spin-photon coupling dynamics, and explore mitigation strategies such as frequency filtering and optimized magnetic-field orientation. Our results show that with optimized designs, photon-to-memory transfer can be achieved with high efficiency and fidelity, supporting the feasibility of such hybrid systems. Finally, we analyze a full repeater chain using experimentally achievable parameters and find that a network with thousands of memories across several repeater nodes could achieve a secret-key rate of 500 bit/s over 1,000 km, demonstrating the strong potential of this approach for next-generation quantum networks.