A source of entangled photons based on a cavity-enhanced and strain-tuned GaAs quantum dot
Michele B. Rota, Tobias M. Krieger, Quirin Buchinger, Mattia Beccaceci, Julia Neuwirth, Hêlio Huet, Nikola Horová, Gabriele Lovicu, Giuseppe Ronco, Saimon Filipe Covre da Silva, Giorgio Pettinari, Magdalena Moczała-Dusanowska, Christoph Kohlberger, Santanu Manna, Sandra Stroj, Julia Freund, Xueyong Yuan, Christian Schneider, Miroslav Ježek, Sven Höfling, Francesco Basso Basset, Tobias Huber, Armando Rastelli, Rinaldo Trotta
A quantum-light source that delivers photons with a high brightness and a high degree of entanglement is fundamental for the development of efficient entanglement-based quantum-key distribution systems. Among all possible candidates, epitaxial quantum dots are currently emerging as one of the brightest sources of highly entangled photons. However, the optimization of both brightness and entanglement currently requires different technologies that are difficult to combine in a scalable manner. In this work, we overcome this challenge by developing a novel device consisting of a quantum dot embedded in a circular Bragg resonator, in turn, integrated onto a micromachined piezoelectric actuator. The resonator engineers the light-matter interaction to empower extraction efficiencies up to 0.69(4). Simultaneously, the actuator manipulates strain fields that tune the quantum dot for the generation of entangled photons with corrected fidelities to a maximally entangled state up to 0.96(1). This hybrid technology has the potential to overcome the limitations of the key rates that plague QD-based entangled sources for entanglement-based quantum key distribution and entanglement-based quantum networks. Supplementary Information: The online version contains supplementary material available at 10.1186/s43593-024-00072-8.
A quantum-light source that delivers photons with a high brightness and a high degree of entanglement is fundamental for the development of efficient entanglement-based quantum-key distribution systems. Among all possible candidates, epitaxial quantum dots are currently emerging as one of the brightest sources of highly entangled photons. However, the optimization of both brightness and entanglement currently requires different technologies that are difficult to combine in a scalable manner. In this work, we overcome this challenge by developing a novel device consisting of a quantum dot embedded in a circular Bragg resonator, in turn, integrated onto a micromachined piezoelectric actuator. The resonator engineers the light-matter interaction to empower extraction efficiencies up to 0.69(4). Simultaneously, the actuator manipulates strain fields that tune the quantum dot for the generation of entangled photons with corrected fidelities to a maximally entangled state up to 0.96(1). This hybrid technology has the potential to overcome the limitations of the key rates that plague QD-based entangled sources for entanglement-based quantum key distribution and entanglement-based quantum networks. Supplementary Information: The online version contains supplementary material available at 10.1186/s43593-024-00072-8.
On demand single-photon and entangled-photon sources are key building-blocks for many proposed photonic quantum technologies. For practical device applications, epitaxially grown quantum dots (QDs) are of increasing importance due to their bright photon emission with sharp line width. Particularly, they are solid-state systems and can be easily embedded within a light-emitting diode (LED) to achieve electrically driven sources. Therefore, one would expect a full-fledged optoelectronic quantum network that is running on macroscopically separated, QD-based single- and entangled-photon devices. An all-electrically operated wavelength-tunable on demand single-photon source (SPS) is demonstrated first. The device consists of a LED in the form of self-assembled InGaAs QDs containing nanomembrane integrated onto a piezoelectric crystal. Triggered single photons are generated via injection of ultra-short electrical pulses into the diode, while their energy can be precisely tuned over a broad range of about 4.8 meV by varying the voltage applied to the piezoelectric crystal. High speed operation of this single-photon emitting diode up to 0.8 GHz is demonstrated. In the second part of this thesis, a fast strain-tunable entangled-light-emitting diode (ELED) is demonstrated. It has been shown that the fine structure splitting of the exciton can be effectively overcome by employing a specific anisotropic strain field. By injecting ultra-fast electrical pulses to the diode, electrically triggered entangled-photon emission with high degree of entanglement is successfully realized. A statistical investigation reveals that more than 30% of the QDs in the strain-tunable quantum LED emit polarization-entangled photon-pairs with entanglement-fidelities up to f+ = 0.83(5). Driven at the highest operation speed ever reported so far (400 MHz), the strain-tunable quantum LED emerges as unique devices for high-data rate entangled-photon applications. In the end of this thesis, on demand and wavelength-tunable LH single-photon emission from strain engineered GaAs QDs is demonstrated. Fourier-transform spectroscopy is performed, from which the coherence time of the LH single-photon emission is studied. It is envisioned that this new type of LH exciton-based SPS can be applied to realize an all-semiconductor based quantum interface in the foreseeable distributed quantum networks.
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.
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.
Entanglement resources are key ingredients of future quantum technologies. If they could be efficiently integrated into a semiconductor platform, a new generation of devices could be envisioned, whose quantum-mechanical functionalities are controlled via the mature semiconductor technology. Epitaxial quantum dots (QDs) embedded in diodes would embody such ideal quantum devices, but a fine-structure splitting (FSS) between the bright exciton states lowers dramatically the degree of entanglement of the sources and hampers severely their real exploitation in the foreseen applications. In this work, we overcome this hurdle using strain-tunable optoelectronic devices, where any QD can be tuned for the emission of photon pairs featuring the highest degree of entanglement ever reported for QDs, with concurrence as high as 0.75 ± 0.02. Furthermore, we study the evolution of Bell's parameters as a function of FSS and demonstrate for the first time that filtering-free violation of Bell's inequalities requires the FSS to be smaller than 1 μeV. This upper limit for the FSS also sets the tuning range of exciton energies (∼1 meV) over which our device operates as an energy-tunable source of highly entangled photons. A moderate temporal filtering further increases the concurrence and the tunability of exciton energies up to 0.82 and 2 meV, respectively, though at the expense of 60% reduction of count rate.
We report on the observation of nearly maximally entangled photon pairs from semiconductor quantum dots, without resorting to postselection techniques. We use GaAs quantum dots integrated on a patterned piezoelectric actuator capable of suppressing the exciton fine structure splitting. By using a resonant two-photon excitation, we coherently drive the biexciton state and demonstrate experimentally that our device generates polarization-entangled photons with a fidelity of 0.978(5) and a concurrence of 0.97(1) taking into account the nonidealities stemming from the experimental setup. By combining fine-structure-dependent fidelity measurements and a theoretical model, we identify an exciton spin-scattering process as a possible residual decoherence mechanism. We suggest that this imperfection may be overcome using a modest Purcell enhancement so as to achieve fidelities >0.99, thus making quantum dots evenly matched with the best probabilistic entangled photon sources.