Periodic Reporting for period 1 - Qurope (Quantum Repeaters using On-demand Photonic Entanglement)
Reporting period: 2020-09-01 to 2021-08-31
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.
In the first year we have extensively worked on our first objective: near-ideal on-demand entangled photon pair source to which we have dedicated our work package 1. To reach this goal we envisioned two routes to success: 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 already 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 work both on electric field and piezo-electric tuning. In the first reporting period, we have fabricated quantum dot diode structures to match the rubidium resonances in the near infrared region. Furthermore, we already realized circular Bragg grating cavities 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 protocol on a magneto-optical trap of rubidium. This decision is informed by our working memory prototypes based on warm alkali vapours, together with the simulations performed in this work package which predict high efficiency operation. 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. Based on these promising results we have started to work on the integrated quantum memories in fs-written waveguides and stoichiometric crystals to enhance the light-matter interaction.
Finally, we have started working on 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). With these groundbreaking experiments, we also have established two more test-beds for quantum communication within our consortium, enabling fast feedback to our work package 1 and 2 technologies.
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 protocol on a magneto-optical trap of rubidium. This decision is informed by our working memory prototypes based on warm alkali vapours, together with the simulations performed in this work package which predict high efficiency operation. 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. Based on these promising results we have started to work on the integrated quantum memories in fs-written waveguides and stoichiometric crystals to enhance the light-matter interaction.
Finally, we have started working on 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). With these groundbreaking experiments, we also have established two more test-beds for quantum communication within our consortium, enabling fast feedback to our work package 1 and 2 technologies.
In addition to our results described above, we have investigated the origin of the anti-bunching in resonance fluorescence (Physical Review Letters 125 (17), 170402 2020). Given the high quality of the quantum dots developed in Qurope, we were able to perform crucial experiments to finally understand this counterintuitive quantum effect. Furthermore, these experiments helped us to better understand the fundamental properties of our quantum dots developed in work package 1 and will enable faster optimization of relevant specifications, such as blinking, linewidth, and auger recombination. We would like to also highlight our results on quantum key distribution that we have achieved faster than expected, which clearly go beyond the state-of-the-art. We expect to keep up the momentum and further push the boundaries of quantum key distribution rates with on-demand generated entangled photon pairs. Within Qurope we have a dedicated dissemination strategy including market research and screening for our disruptive technologies. We have identified three key innovations, which we will carefully monitor to maximize future socio-economic impact. Secure communication and cyber security is a pressing sustainable development goal and we expect Qurope to make a substantial contribution to it in the coming two years.