In the first reporting period (RP1) of the SuperQuLAN project, important steps toward realizing an operational prototype of a QuLAN setup have already been achieved. In particular, the experimental group at ETH Zurich and our industry partner Zurich Instruments have realized a cryogenic transmission line connecting two qubits that are located in separated dilution refrigerators 30m apart. This transmission line is essentially a long cable that is cooled to a temperature of less than 50mK above absolute zero. This achievement enabled the team at ETH Zurich to transmit quantum states through this cryogenic cable and to demonstrate for the first time entanglement between two spatially separated superconducting chips. This is the essential first step for implementing now also more sophisticated quantum networking protocols.
In parallel, the experimental group at IST Austria has developed a so-called electro-optical quantum transducer. This device can convert quantum signals at microwave frequencies into corresponding quantum signals in the optical regime and vice versa. Optical signals can be transmitted conveniently over long distances via commercial glass fibers. Therefore, the development of such opto-electrical transducers offers an alternative indirect way to couple two superconducting quantum processors via an optical channel. Previous transducer designs faced the problem that during the conversion process most of the signal was lost and a lot of noise was added, which smeared out all quantum features. With their new design and using a pulsed conversion scheme, the team at IST Austria has now managed to overcome both problems simultaneously and to reach a transducer performance that will allow the transmission of true quantum signals.
Apart from work on the experimental components, the theory groups at CSIC in Madrid, the TU Wien, the Max Planck Institute for Quantum Optics and our associate partner aQa Leiden have proposed and analyzed various novel schemes, for example, for implementing gate operations between distant qubits in a network, for distributing quantum entanglement via correlated photon sources, for generating photonic tensor network states, and also described a quantum algorithm that can be improved by running them on two coherently linked quantum processors. Many of these schemes will form the basis of even more complex protocols and algorithms that may in the long run be used to operated large scale QuLANs and quantum computing clusters much more efficiently.
In RP2 continued progress on the development of quantum networks and quantum transducers has been made. A specific focus in this period was placed on the implementation of quantum operations and quantum communication protocols in the 30-m long QuLAN setup at ETH Zurich. A highlight in this respect was the demonstration of non-local quantum correlations between qubits located at two ends of the network. This enabled ETH Zürich to perform fundamental tests of quantum mechanical principles with macroscopic superconducting circuits and use these principle to demonstrate device-independent quantum security tests. The researcher at ISTA in turn realized for the first time and entangled state between microwave and optical photons. This paves the way for coherent quantum interfaces between superconducting qubits and flying optical qubits, which, on the long, run can be used to connect superconducting quantum processors to the quantum internet.
In parallel to those experimental advances, the theory groups continued their efforts in developing and analyzing new quantum networking protocols. This includes optimal control pulses for quantum state transfer operations in realistic QuLAN settings, new robust ways to distribute entanglement in multi-node networks, as well as cross-verification strategies and distributed quantum optimization algorithms. Further, based on their work on QuLAN control electronics,our industry partner Zurich Instruments developed the Quantum System Hub "QHub" a device for controlling networks with many tens to hundreds of qubits.
