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.
