On the hardware side, multiple circuit components have been established as part of the final multi-qubit prototype. Among the completed components, a wideband nearly quantum-limited amplifier was produced with a bandwidth spanning several gigaherz and a center frequency which can be tuned over several gigaherz. At the same time, a flexible, scalable cabling technology has been adapted for quantum annealing needs and is ready to be tested with the multi-qubit devices. This cabling contains a high density of lines while exhibiting very low thermal conductivity, making it an ideal component for experiments involving large numbers of qubits and control channels, as is the case of annealers.
Individual coherent flux qubits of low and high impedance were built and characterized. Both types of qubits exhibit long coherence times in the microsecond range, making them suitable to build coherent quantum annealers at the few-qubit scale. As part of the production of low-impedance qubits, a new fabrication method employing niobium has been demonstrated, which may lead to flux qubits made with a larger-gap superconductor leading to lower losses.
A chain of three coupled high-impedance qubits was fabricated, using contactless flip-chip technology that enables replacing single qubit chips to select optimal devices. With such a system a tunably coupled qubit pair was demonstrated, using capacitive interactions which represent a model that goes beyond the standard Ising Hamiltonians and is necessary to overcome classical computational methods.
On the theory side, a circuit design generating a nonclassical interaction between flux-like qubits was produced using capacitive interactions. On the software side, a classical simulator was developed which allows the simulation of noisy quantum systems with reasonable accuracy for qubit numbers well above one hundred. In parallel, a variety of algorithms and simulation protocols have been developed, including a sorting algorithm, a method to engineer thermal states with a quantum annealer, and the simulation of interacting fermions.
The results of the project have been disseminated throughout the scientific community in important events worldwide, as well as in publications of high-impact journals such as Nature Physics and Physical Review Letters. All information regarding project progress and production has been updated on the project website avaqus.eu. Some of the results have been protected in the form of patents, with a total of 5 patents produced within the reporting period, 4 of which have already been published. It is expected that at least one more patent will be produced coming out of the results of the project.
