Periodic Reporting for period 4 - QuPoPCoRN (Quantum Particles on Programmable Complex Reconfigurable Networks) Reporting period: 2022-01-01 to 2023-06-30 Summary of the context and overall objectives of the project We are currently witnessing a radical change in the way we perceive information. The unintuitive concepts of quantum mechanics gave birth to a novel breed of information technologies—quantum information technologies. In these technologies, information is carried by quantum particles such as photons and encoded on qubits, which are quantum superposition states. Processing them in controlled networks gives rise to the field of quantum computers, which can be more powerful than their classical counterparts; they can solve tasks that intractable for today’s machines. The implementation of quantum computers, however, will not happen soon. Too many fundamental questions remain unanswered. How large can a quantum system be? How do we build scalable quantum systems? How can we mitigate the effects of noise?The project QuPoPCoRN has brought us closer to answering these questions. During the project’s lifetime, we have realized quantum networks—tool kits for quantum information technologies,—that are, in principle, resource efficient and scalable. We have firmly established time-multiplexed networks as a route towards scalability. This approach has been pioneered in my group and has now internationally been accepted as promising path towards photonic quantum computing with global companies adopting time-multiplexing for their realizations of quantum computers. QuPoPCoRN has also significantly influenced the European research in the field. We have studied the intricate interplay between fragile quantum properties such as entanglement and unavoidable noise from the environment. We fed our networks with quantum particles and controlled their evolution, which yields unique insights into both fundamental and applied concepts of quantum mechanics. All this has been possible because we leveraged the supreme capabilities of integrated quantum photonics to build light powered optical networks. As a conclusion of QuPoPCorn, we can say that a scalable quantum system must be based on different information encoding strategies and that time- and frequency-multiplexing are both resource efficient and compatible with existing information infrastructure based on single mode fibre. We have learned how to feed many quantum particles into these systems and how to control them with an unprecedented degree of precision. Ultimately, the results of QuPoPCoRN highlight important strategies for scaling quantum photonics and pave the way to quantum computing and information technologies. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far The work in QuPoPCoRN was structured into three columns – WP1: Noise in quantum networks, WP2: Simulating complex topological structures, and WP3: Enhancing entanglement “on the fly” – whose foundations were laid by the development of an accompanying hardware toolbox.We built photonic quantum networks in a time-multiplexing architecture. In this type of architecture, photons travel through a simple interferometer-like setup many times. Each roundtrip constitutes one step, and state of the art technologies allow us to control the state of each photon at each step. This enables, for instance, the controlled introduction of different types of noise into the network. At the end of the network, we monitor the photon properties with single photon detectors. The main workhorse of our work has been the so-called quantum walk, the quantum analogue of a classical random walk. In a random walk, a walker tosses a coin and, depending on the outcome of the toss, takes a step to either the left or right. In a quantum walk, thanks to quantum mechanical superpositions, the walker can take a step to the left and the right at the same time. This enables novel applications, for instance quantum simulation, where a well-controllable quantum system is used to simulate and hence study an inaccessible quantum system. Using our time-multiplexing architecture, we developed novel methods for creating and measuring entanglement and non-classicality in large networks with many photons. We studied the role of measurements in the evolution of quantum walks, which we extended to studies of open quantum systems. Furthermore, we investigated the behaviour of entanglement during a quantum walk. On a different note, we made use of the capability for quantum simulation to study topology in quantum networks. By appropriate structuring of the network, it is possible to create states that are robust against noise, so-called topological states. Finally, we implemented both quantum feed forward and feedback and demonstrated the generation of multi-particle entanglement with exponentially increased rates. All this has been carried by the development of tailored quantum sources, innovative photon counting detectors and fast modulators for manipulating the state of the photons.Our results have been published in prominent scientific journals and have been presented in numerous invited and plenary talks at renowned conferences all over the world. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) We can identify a few areas where the results of QuPoPCoRN have progressed beyond the state of the art. One such area is the size of the quantum networks. Prior to this project, quantum networks were limited to only a few photons (typically one or two) in small networks (up to ten modes). Now, we can precisely control the quantum interference of two photons inside a network. We can observe the interference of up to eight photons on a beam splitter and, from these measurements, draw conclusions about the quantum state of the photons. We also implemented a network in which we distributed ten photons coherently over 64 modes thus significantly increasing the size of experimentally accessible quantum networks. Another area is feed-forward in quantum networks. It is widely accepted that quantum networks require fast feed-forward operations to become scalable. Until QuPoPCoRN, however, no practical demonstration of feed-forward in quantum networks existed. The setting we investigated was simple: given a probabilistic quantum light source, how can we reliably generate large, entangled states? The answer to this question is time-multiplexing. We used the probabilistic source many times over and stored generated photons in a quantum memory until the next photon was generated. Then, we interfered and thus entangled the photons and repeated the process. We could demonstrate the generation of four- and six-photon entangled states with exponentially increased rates compared to non-multiplexed scenarios. This significantly advanced the state of the art in generating multi-photon entangled states.The final area is hardware. Future quantum technologies will require supreme hardware components. During QuPoPCoRN, we developed novel integrated quantum light sources that create tailored quantum states with high brightness, which are well suited for use in quantum networks. We further built novel photon-counting detectors that interface with our networks and our sources. Finally, we laid the groundworks for a novel thin-film lithium niobate on insulator technology, which will enable industrial-style miniaturization of photonic quantum networks in the future. Lastly it is worthwhile mentioning that the results of QuPoPCoRN were the starting point for building the Institute of Photonic Quantum Systems (PhoQS) at Paderborn University. The PhoQS is on its way to becoming a national centre for photonic quantum technologies with high international visibility. Development of innovative devices for quantum simulation setups
