Building blocks for a hybrid quantum repeater

Erika K. Carlson

Peter van Loock, Wolfgang Alt, Christoph Becher +9 more

Abstract Elementary building blocks for quantum repeaters based on fiber channels and memory stations are analyzed. Implementations are considered for three different physical platforms, for which suitable components are available: quantum dots, trapped atoms and ions, and color centers in diamond. The performances of basic quantum repeater links for these platforms are evaluated and compared, both for present‐day, state‐of‐the‐art experimental parameters as well as for parameters that can in principle be reached in the future. The ultimate goal is to experimentally explore regimes at intermediate distances—up to a few 100 km—in which the repeater‐assisted secret key transmission rates exceed the maximal rate achievable via direct transmission. Two different protocols are considered, one of which is better adapted to the higher source clock rate and lower memory coherence time of the quantum dot platform, while the other circumvents the need of writing photonic quantum states into the memories in a heralded, nondestructive fashion. The elementary building blocks and protocols can be connected in a modular form to construct a quantum repeater system that is potentially scalable to large distances.

Shuo Sun

I will discuss our recent proposal on deterministic generation of photonic repeater graph states using only a single quantum emitter, our plans for its experimental implementation, and its applications in quantum repeaters and networks.

A. Kuzmich

Kristiaan De Greve

Filip Rozpędek

Future quantum networks will enable the realisation of a large family of quantum protocols, hence allowing for implementation of various tasks ranging from quantum cryptography through quantum computing to quantum metrology.In this thesis we analyse and develop the building blocks of quantum repeater networks.These networks consist of quantum repeater nodes, which are equipped with quantum memories enabling storage of quantum information.The repeater nodes are connected using optical fibres that enable for the transmission of photonic quantum signals between the repeater nodes.We specifically focus on the two building blocks: remote entanglement generation and entanglement distillation.We then investigate various proof of principle quantum repeater schemes used for generating shared secret key between distant parties.The repeater schemes that we consider utilise one to three memory nodes capable of generating memory-photon entanglement.Finally, we introduce a novel framework for investigating a foundational aspect in the use of quantum communication related to the uncertainty principle.Efficient remote entanglement generation is an indispensable building block of most near-term quantum repeater networks.In this thesis we consider various remote entanglement generation schemes that permit multiplexing, which aims at increasing the throughput of this procedure.We find that for the specific schemes that are suitable for platforms with a single communication qubit and multiple memory qubits, the performance of the multiplexing approach is strongly dependent on the time it takes to perform local operations within the memory nodes.Entanglement distillation enables for filtering out the noise from imperfect entangled states using only local operations and classical communication.Using a general framework for optimising such schemes, which we have developed, we have proven optimality of specific generic and experimentally relevant distillation schemes.We have developed a general framework for optimising such schemes, which we have used to prove optimality of various generic and experimentally relevant distillation schemes.In designing proof of principle quantum repeater schemes we assume an informationtheoretic approach, which enables us to assess their performance in a hardware-agnostic way.We find that all the schemes that we consider can prove useful in specific parameter regimes for the nitrogen-vacancy centre platform.Moreover, we find that one scheme which has already been realised experimentally in the context of remote entanglement generation, is expected to significantly outperform every possible system based on direct communication without repeaters.However, our analysis shows that encapsulating the nitrogen-vacancy centres in optical cavities for the enhancement of the emission rates is a necessary requirement for such a successful demonstration of the first proofof-principle quantum repeater.xi xii SUMMARY Finally, we investigate the quantum uncertainty principle which is a fundamental feature of quantum mechanics, allowing for security in many quantum cryptographic protocols.We find that in a particular scenario, relevant from the perspective of quantum key distribution, a significant part of the observed uncertainty is in fact due to lack of information rather than intrinsic.If the eavesdropper could access that information, she would be able to much better, or in some cases even with certainty, guess the value of the raw bit of the generated key.