Networked Quantum Systems
The MQV Lighthouse Project Networked Quantum Systems (NeQuS) brings together ten groups from the Technical University of Munich (TUM), the Ludwig-Maximilians-Universität (LMU), the Walther-Meißner-Institute, and the Max Planck Institute for Quantum Optics (MPQ) that aim to develop novel quantum interfaces and transducers, operating in both the microwave and optical regimes, needed to connect different quantum systems – atoms in a vacuum, quantum dots, superconducting quantum bits, and dopant atoms in silicon – and take the first steps toward the quantum internet. At WMI, we are focusing on two directions: (1) the development of remote entanglement of superconducting qubits by exploiting propagating squeezed microwaves and (2) the transduction of quantum microwave excitations to optical signals by using magneto-electro-mechanical systems and quantum dot molecules in collaboration with the Walter-Schottky-Institute.
Our first direction is based on superconducting cavity-qubits systems. We made progress with modelling and fabricating superconducting aluminum cavities coupled to niobium-aluminum transmon qubits, which are known for their long coherence times and reproducibility. In this context, we also focus on preliminary theory simulations of relevant remote entanglement protocols, which are based on the illumination of remote qubits with propagating two-mode squeezed microwave signals.
Our second direction is focused on transduction between microwave and optical frequency domains while trying to preserve fragile quantum correlations. Here, we would like to exploit magneto-electro-mechanical nanodevices. These devices rely on a hybrid frequency conversion by combining opto-mechanics and opto-magnonics. This conversion concept uses a combined three-step conversion process, involving the coupling of microwave photons with magnons, the coupling of the magnonic and tailored phononic degrees of freedom harnessing magneto-elasticity, and the opto-mechanical interaction linking the photonic and phononic degree of freedom. In particular, the three-step conversion should allow us to overcome the cooperativity matching criterion of a one-step conversion process and, hereby, will allow for much larger bandwidths. Along these lines we are also collaborating with the group of Prof. Jonathan Finley towards microwave-to-optical transduction schemes based on quantum dot molecules in semiconductor heterostructures. Coupling quantum dot molecules to microwave photons in superconducting waveguide resonators via the ac-Stark effect leads to sidebands in the optical spectrum, which can be used for coherent transduction.
In summary, we are currently in the process of developing two complementary approaches towards distributing quantum correlations over long distances with a final aim to form hybrid, microwave-microwave and microwave-optical, quantum networks.