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Quantum Local Area Networks with Superconducting Qubits

Periodic Reporting for period 1 - SuperQuLAN (Quantum Local Area Networks with Superconducting Qubits)

Reporting period: 2020-09-01 to 2021-08-31

With the transition into the digital age, processing and distribution of information has become the backbone of our society and an abiding driving force for science, innovation and economic growth. At the same time, the continuing digitalization of our lives is generating higher and higher demands for computational power beyond current technological capabilities. Quantum technologies promise a radically new way to meet these demands by exploiting the quantum mechanical principles of superposition and entanglement, which do not have a counterpart in classical information processing.

Superconducting quantum circuits are currently one of the most promising platforms for realizing large-scale quantum computing devices, where in the near future a coherent integration of 100-1000 quantum bits (qubits) is feasible. However, the required temperatures of only a few mK currently restrict quantum operations to qubits that are located within a single, heavily shielded dilution refrigerator. This imposes a serious constraint on the realization of even larger quantum processors or the implementation of local- and wide-area quantum networks based on this technology.

The project SuperQuLAN is set out to address this important open problem and to demonstrate a first operational prototype quantum local area network (QuLAN) of separated superconducting quantum processors. This work is carried out by a multi-national team of scientists and industry partners who will develop key network components and quantum communication protocols that will facilitate in the long term the realization of large quantum computing clusters or even city-wide quantum networks using superconducting circuits.
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.
The progress that has been achieved in SuperQuLAN in the first year already goes considerably beyond the state of the art in this field. The cryogenic quantum link at ETH Zurich is a world-wide unique device and can be considered the first QuLAN prototype. The electro-optical transducer developed at IST Austria uses a unique design, which pushed the combined performance in terms of efficiency and noise into a regime, where true quantum transduction becomes possible. Based on these very promising results, it is very likely that the overall goal of the project, namely to demonstrate a fully operational QuLAN device will be reached. This will provide a completely new technology for scaling-up quantum computers and will eventually make this transformative new way of processing information available for enduser in academia and industry.
Vision of a Quantum Local Area Network (QuLAN).