Scaling-up quantum computers - the challenge
The technological vision behind the SuperQuLAN project
The targeted breakthrough that our team has set out to for this project is to show that QuLANs are not just a visionary idea by demonstrating for the first time their basic operation principles prototype device.
To achieve this goal, the SuperQuLAN consortium is formed by an interdisciplinary team of experts in the fields of superconducting circuits, nanophotonics and quantum information theory. The complementary expertise of this consortium and a close collaboration with our industry partner Zurich Instruments makes this ambitious undertaking possible.
A cryogenic quantum link
Superconducting qubits must be operated at temperatures that are only a few hundreds of a degree above absolute zero, otherwise they lose their quantum properties. Such temperature can only be reached inside a dilution refrigerators where the quantum processor is highly isolated from the outside world. This means that, unlike their classical counterparts, quantum computers cannot be simply be connected through conventional cables.
One approach explored within SuperQuLAN is thus to connect separated quantum processors through cryogenic quantum links. This means that the connecting cable itself is cooled down to millikelvin temperatures such that even during transfer quantum states are protected from the "hot" environment and retain their coherence properties.
First steps towards this technology have been already demostrated by the group at ETH Zürich, in the longest-to-date link (5m) between separate cryostats. The group not only showed that the linked could be cooled sufficiently down, but they also confirmed that it acts as a viable quantum link between separate chips, as explained in this paper and in this talk of the Virtual March Meeting 2020.
Optical quantum links for superconducting circuits
In contrast to microwave signals, quantum information that is transmitted via optical photons is not affected by temperature since at those high frequencies thermal fluctuations, even without any cooling, are negligible. At the same time, photons in the telecom band can be transmitted through standard optical fibers over kilometer distances with only very little loss. This makes the use of optical photons for quantum communication applications much more practical. Yet, the quantum processors still operate in the microwave regime and there is no way to make them talk to the optical photons directly.
A second line of research in SuperQuLAN is thus focused on the development of so-called quantum transducers, which allow us to convert microwave into optical quantum signals and vice versa. Such a device has been developed over the past years by the group of Johannes Fink at IST Austria. This device relies on the electro-optical Pockels effect, which changes the resonance frequency of a high-quality optical resonator proportional to an applied microwave signal. The challenge that we address in this project is to reduce the residual noise in this device to much less than a single photon and to show that quantum properties such as superpositions and entanglement are preserved during the conversion process.