Recent progress in quantum technologies is underpinning significant advances across many sectors including defence, healthcare, and communication. At the same time, several challenges emerge as scientists strive to manipulate quantum states for signal enhancement, noise reduction, and ultimately quantum computing. A paradigmatic example is the recent demonstration of noise mitigation on a 127-qubit chip, but this has highlighted the limitations of coherence time, gate fidelity, and error suppression, as well as the challenge of connecting large numbers of physically separate qubits. Therefore, whilst improvements on established technologies remain crucial for scaling them up, the search for alternative routes towards quantum computing remains a most promising pathway towards useful quantum supremacy. Quantum Information with Mechanical Systems (QuIMS) explores the potential of mechanical resonators as a novel computing platform, both in support of existing qubit technologies (e.g., for quantum memories) and as a stand-alone qubit technology. To this end, we will build mechanical resonators with ultra-high coherence times that are manipulated with extreme precision by means of light fields. In this opto-mechanical system, we will attempt for the first time to embed several qubits in a single mechanical resonator, removing the need for cumbersome connecting wires that impedes, for instance, spin qubit devices. These mechanical qubits are expected to offer exciting opportunities to implement multi-qubit gates directly on a single resonator, which can greatly suppress the main sources of errors encountered in current platforms. The core novelty on which QuIMS leverages is the quadratic opto-mechanical coupling, which is needed for mechanical quantum computing but has so far been out of reach. We will design new devices that exploit symmetry and phononic crystals to suppress detrimental contributions such as heating of the mechanical resonators. We will work with graphene and carbon nanotubes that are uniquely suited to achieve the quadratic regime owing to their extremely low mass and strong interaction with radio-frequency light. The synergy of our complementary state-of-the-art facilities and of experimental and theoretical expertise at the Universities of Exeter and Lancaster are ideally suited to nurture the ambitious aims of this proposal. Upon demonstrating the quadratic opto-mechanical interaction, in collaboration with project partners including the National Quantum Computing Centre, we will explore the potential of our devices for applications such as signal enhancement, noise reduction, mechanical signal processing, filtering, and transduction. Hence, we will benchmark the performance of our opto-mechanical quantum systems against that of other known platforms such as superconducting, photonic, Rydberg and ion devices. Finally, we will investigate how our platforms can be combined with existing technologies, to be employed, e.g., as highly coherent memories (due to the extreme quality factors attainable by mechanical resonators).

Superconducting quantum circuits are one of the most promising platforms for realizing large-scale quantum computing devices, where in the near future a coherent integration of 100-1000 qubits is feasible. However, the required temperatures of only a few mK currently restrict quantum operations to superconducting qubits that are located within the same 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 superconducting technology. The targeted breakthrough of this project is to overcome this limitation by demonstrating for the first time the operation of a quantum local area network (QuLAN), where superconducting qubits housed in spatially separated refrigerators are connected via a cryogenic transmission line. Using this setup, we will implement state transfer protocols and distributed quantum algorithms between superconducting qubits that are tens of meters apart. In parallel, we will develop and demonstrate new electro-optical quantum transducer designs for fast microwave-to-optics conversion and many other essential components and protocols for efficiently integrating multiple superconducting quantum computing units into a single coherent network. The outcomes of this project will enable the non-incremental step from intra- to inter-fridge quantum communication and will facilitate the implementation of first quantum computing clusters. In the long run, this technology provides the basis for the realization of metropolitan-area scale quantum networks using superconducting circuits. The project will be carried out by an interdisciplinary team of experts in the fields of superconducting circuits, nanophotonics and quantum information theory, and in close collaboration with industry partners. The complementary expertise of this consortium will ensure the scientific and economic success of this project.