Limited spatial overlap of self-assembled semiconductor quantum dots with photonic structures hinders the scalability of entangled photon sources. Here we present a marker-based cryogenic imaging system enabling strain-tunable and cavity-enhanced quantum dots emitting highly-entangled photons.
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.
Many of the quantum information applications rely on indistinguishable sources of polarization-entangled photons. Semiconductor quantum dots are among the leading candidates for a deterministic entangled photon source; however, due to their random growth nature, it is impossible to find different quantum dots emitting entangled photons with identical wavelengths. The wavelength tunability has therefore become a fundamental requirement for a number of envisioned applications, for example, nesting different dots via the entanglement swapping and interfacing dots with cavities/atoms. Here we report the generation of wavelength-tunable entangled photons from on-chip integrated InAs/GaAs quantum dots. With a novel anisotropic strain engineering technique based on PMN-PT/silicon micro-electromechanical system, we can recover the quantum dot electronic symmetry at different exciton emission wavelengths. Together with a footprint of several hundred microns, our device facilitates the scalable integration of indistinguishable entangled photon sources on-chip, and therefore removes a major stumbling block to the quantum-dot-based solid-state quantum information platforms.
The prospect of using the quantum nature of light for secure communication keeps spurring the search and investigation of suitable sources of entangled-photons. Semiconductor quantum dots are arguably the most attractive. They can generate indistinguishable entangled-photons deterministically, and are compatible with current photonic-integration technologies, a set of properties not shared by any other entanglement resource. However, as no two quantum dots are identical, they emit entangled-photons with random energies. This hinders their exploitation in communication protocols requiring entangled-states with well-defined energies. Here, we introduce scalable quantum-dot-based sources of polarization-entangled-photons whose energy can be controlled via dynamic strain-engineering without degrading the degree of entanglement of the source. As a test-bench, we interface quantum dots with clouds of atomic vapours, and we demonstrate slow-entangled-photons from a single quantum emitter. These results pave the way towards the implementation of hybrid quantum networks where entanglement is distributed among distant parties using scalable optoelectronic devices.
Abstract Semiconductor quantum dots are leading candidates for the on-demand generation of single photons and entangled photon pairs. High photon quality and indistinguishability of photons from different sources are critical for quantum information applications. The inability to grow perfectly identical quantum dots with ideal optical properties necessitates the application of post-growth tuning techniques via e.g. temperature, electric, magnetic or strain fields. In this review, we summarize the state-of-the-art and highlight the advantages of strain tunable non-classical photon sources based on epitaxial quantum dots. Using piezoelectric crystals like PMN-PT, the wavelength of single photons and entangled photon pairs emitted by InGaAs/GaAs quantum dots can be tuned reversibly. Combining with quantum light-emitting diodes simultaneously allows for electrical triggering and the tuning of wavelength or exciton fine structure. Emission from light hole exciton can be tuned, and quantum dot containing nanostructure such as nanowires have been piezo-integrated. To ensure the indistinguishability of photons from distant emitters, the wavelength drift caused by piezo creep can be compensated by frequency feedback, which is verified by two-photon interference with photons from two stabilized sources. Therefore, strain tuning proves to be a flexible and reliable tool for the development of scalable quantum dots-based non-classical photon sources.
Single self-assembled quantum dots are established emitters of single photons and entangled photon pairs. The entanglement of photons generated by quantum dots has been shown in polarization [1] , in time bin [2] , and also as hyperentanglement [3] . However, the achievable degree of entanglement and readiness of the source for use in quantum communication protocols, depend on several additional functionalities such as high collection efficiency, coherence of the emitted photon pairs, and fabrication scalability.
Photonic quantum technologies require the integration of high-quality quantum light sources with programmable photonic circuitry to enable on-chip quantum information processing. Semiconductor quantum dots have emerged as a leading platform for quantum light generation, yet their monolithic integration with cryogenic-compatible, programmable photonic circuitry remains challenging. Here we demonstrate a programmable quantum photonic platform that enables simultaneous control of highly coherent quantum dot-based single photon emitters and reconfigurable optical elements at cryogenic temperatures. System-level integration enables individual electrical and optical addressing of active components on a single chip, allowing their concurrent operation while preserving coherent non-classical light emission. We demonstrate multiple quantum-photonic functionalities, including pure single-photon generation, two-photon interference, and resonant transmission and reflection measurements of integrated emitters. These results mark a step from isolated quantum-optical devices towards programmable quantum-photonic hardware platforms capable of combining coherent quantum emitters with large-scale photonic functionality.