Periodic Reporting for period 2 - Qurope (Quantum Repeaters using On-demand Photonic Entanglement)
Reporting period: 2021-09-01 to 2024-02-29
Today’s society is based on the fast access to information. Getting a head start on information is key in business, finance, politics and security. Most of our information exchange is done via the internet. However, not only has the current structure of our internet limits in capacity but also data transfer is not secure. Therefore, we are in need to invest in a future network, capable of handling the massive data flow and allowing for secure data communication. Physics offers a solution to this difficult task in the so-called quantum internet. By using quantum mechanics, it is possible to encode information on the smallest quantum of energy, a single light particle called photon. Information encoded on single photons cannot be eavesdropped without the sender and original receiver noticing it. The basic concept relies on network nodes and special links, which are the quantum mechanical analogue to classical fiber amplifiers currently used to overcome transmission losses in standard network, to connect physically separated nodes with each other. However, the same quantum mechanical principle (non-cloning theorem) which makes the network totally secure also renders classical signal amplification impossible. Qurope develops quantum communication links taking advantage of another quantum mechanical effect to overcome transmission losses: Entanglement swapping using quantum repeaters. This allows transferring quantum information without physically sending a single information carrier the full distance to the receiver. Realizing such quantum repeaters requires quantum memories and entangled photon pair sources. The goal of Qurope is to develop a hybrid quantum repeater architecture based on dissimilar quantum systems and to test its performances in real-word applications. The envisioned implementation is based on two disruptive technologies that will be pioneered during the project: (i) Near-ideal quantum-dot-based sources of entangled photon pairs that will simultaneously feature high brightness, near-unity degree of entanglement and indistinguishability, wavelength-tuneability, and on-demand operation. (ii) Efficient and broad-band quantum memories that will be specifically designed and engineered to store and retrieve polarization-entangled photons from quantum dots. Different quantum dot-quantum memory systems will be combined to develop near-infrared and telecom-based quantum repeaters, which will then be tested using both free-space and fiber-based quantum key distribution protocols based on entanglement. This will be performed in the elementary quantum-network infrastructure available in the consortium – a major breakthrough that will open the path towards future large-scale implementation of secure quantum communication. The project combines semiconductor physics, nanofabrication technology, atomic physics, and quantum optics, and strongly benefits from the emerging synergy effects. This gives Qurope all required tools at hand to finally realize a functional hybrid quantum repeater between quantum network nodes, bringing us one big step closer to the quantum internet.
We have realized a near-ideal on-demand entangled photon pair source to which we have dedicated our work package 1. To reach this goal we worked on two approaches: i) aluminum droplet etched GaAs quantum dots emitting at near infrared wavelengths for free-space quantum communications. ii) InAs quantum dots on GaAs wafers using metamorphic buffer layers emitting at telecom frequencies. Both systems need to be integrated in circular Bragg resonators for efficient photon outcouping and increased photon indistinguishability. We have successfully fabricated such devices for the NIR sources by spatial localization of quantum dots using in-situ imaging. The localization accuracy is below 15nm and the overlap nanofabrication precision is around 35nm. Our fabrication specifications are excellent to achieve maximal coupling to the circular Bragg resonators. Furthermore, Qurope has achieved for the first time circular Bragg resonators for telecom O-band emitting quantum dots with a record high extraction efficiency of 23%. For experiments with several emitters, a tuning mechanism is needed. We have integrated circular Bragg grating cavity quantum dot samples on piezo-electric substrates.
In work package 2, we develop and optimize our second disruptive technology: quantum memories. We have agreed to commit to operate a telecom wavelength ORCA and have stored and retrieved single-photons from a InAs quantum dot. In addition, we have worked on our solid-state approach, performing spectroscopic characterizations of the telecom wavelength transition in praseodymium (Pr) doped yttrium orthosilicate. We found a transition at 1550nm that partners with one at 995nm to form an ORCA ladder to a long-lived excited state.
Finally, we have achieved our goals in work package 3 and showed quantum key distribution using entangled photons emitted from quantum dots in a fiber network as well as free-space (back-to-back publications in science advances). We have used a commercial deployed fiber network to distribute entanglement and we have managed to do quantum key distribution after entanglement swapping, showcasing that quantum dots are capable of realizing quantum relays. With these groundbreaking experiments, we also tested our sources in other quantum communication test-beds and successfully performed quantum key distribution over a 80km long fiber link between two cities using our telecom quantum dot sources.
We have disseminated our results in 43 journal publications, more than 50 invited talks at conferences and workshops and organized 3 scientific workshops as well as one summer school. In work package 4 we evaluated our results in internal screenings, made market analysis for our main innovations, and developed a technology exploitation strategy. We found 4 key innovations which will be further commercially exploited.
In work package 2, we develop and optimize our second disruptive technology: quantum memories. We have agreed to commit to operate a telecom wavelength ORCA and have stored and retrieved single-photons from a InAs quantum dot. In addition, we have worked on our solid-state approach, performing spectroscopic characterizations of the telecom wavelength transition in praseodymium (Pr) doped yttrium orthosilicate. We found a transition at 1550nm that partners with one at 995nm to form an ORCA ladder to a long-lived excited state.
Finally, we have achieved our goals in work package 3 and showed quantum key distribution using entangled photons emitted from quantum dots in a fiber network as well as free-space (back-to-back publications in science advances). We have used a commercial deployed fiber network to distribute entanglement and we have managed to do quantum key distribution after entanglement swapping, showcasing that quantum dots are capable of realizing quantum relays. With these groundbreaking experiments, we also tested our sources in other quantum communication test-beds and successfully performed quantum key distribution over a 80km long fiber link between two cities using our telecom quantum dot sources.
We have disseminated our results in 43 journal publications, more than 50 invited talks at conferences and workshops and organized 3 scientific workshops as well as one summer school. In work package 4 we evaluated our results in internal screenings, made market analysis for our main innovations, and developed a technology exploitation strategy. We found 4 key innovations which will be further commercially exploited.
Our research efforts in quantum technologies have had a significant impact on various fields and have brought tangible progress. In particular, our work with GaAs quantum dots has led to unprecedented performance, outperforming competitors in crucial areas such as indistinguishable photon emission, entangled photon pair emission, multiple photon emission and spin coherence times. This will enable efficient spin-photon entanglement and a new source for either cluster state generation or quantum network nodes. In addition, our pioneering work with InAs telecom dots has enabled breakthrough applications in quantum communication and provided the first proof-of-concept for use in fiber optic networks. While our focus on fundamental research and the development of a quantum repeater may not have an immediate economic impact, the innovations resulting from our work will have a significant impact on the economy in the years to come. In addition, our partnerships have provided valuable opportunities for young researchers to secure prestigious fellowships, tenure-track positions and grants, thereby strengthening their contribution to quantum research. The expansion of our SME partners' portfolio and increased public engagement underlines the societal impact of our work and paves the way for the widespread adoption of quantum technologies in the near future.