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Details of Grant 

EPSRC Reference: EP/Y003152/1
Title: Josephson Parametric Amplifiers using CVD graphene junctions
Principal Investigator: Thompson, Dr MD
Other Investigators:
Researcher Co-Investigators:
Project Partners:
Tel Aviv University
Department: Physics
Organisation: Lancaster University
Scheme: Standard Research - NR1
Starts: 01 January 2024 Ends: 31 December 2025 Value (£): 99,772
EPSRC Research Topic Classifications:
EPSRC Industrial Sector Classifications:
No relevance to Underpinning Sectors
Related Grants:
Panel History:
Panel DatePanel NameOutcome
24 May 2023 ECR International Collaboration Grants Panel 3 Announced
Summary on Grant Application Form
This collaborative project brings together the capabilities of international collaborators who are experts in fabricating superconducting junctions devices using graphene, with UK based expertise in performing low-noise electronic measurements at ultra-low temperatures to develop superconducting amplifiers that will improve the performance of superconducting quantum circuits.

Quantum systems are generally very sensitive to noise. In quantum computing, for example, qubits are characterised by coherence, a property which describes how long a qubit can remain in a given quantum state. Noise causes decoherence which limits the lifetime of these delicate systems. One source of noise is the amplifiers that we use to amplify the very low power signals used to control these quantum systems.

In superconducting circuits we can use parametric amplifiers that employ Josephson junctions to create amplifiers that can operate at the very lowest possible noise levels, limited only by quantum mechanics. Most commonly, these amplifiers use Josephson junctions that are formed by connecting two superconductors with an insulator, a so-called SIS junction. The operating frequency of parametric amplifiers employing SIS junctions can be tuned using magnetic flux, however due to the long range effects of magnetic fields, this magnetic flux can interfere with our delicate quantum devices, or create cross-talk between multiple amplifiers. Screening this flux therefore creates additional challenges.

Superconducting junctions can also be formed by connecting two superconductors with graphene, an SgS junctions. Unlike parametric amplifiers using SIS junctions, parametric amplifiers using SgS junctions can instead be tuned electrostatically. This is achieved by applying a voltage to a gate electrode near the graphene. In this way, we can avoid the perils of the interference caused by stray magnetic flux. However, the development of SgS junctions is overwhelmingly focused on using graphene that is exfoliated from high quality graphite crystals. Although this produces the highest quality graphene, the flakes are only a few micrometers in size, which limits the number of junctions that can be fabricated from a single flake. This makes a practical realisation of devices using these junctions very challenging.

Graphene can also be produced in large areas, enough to cover a 6-inch diameter silicon wafer and can be readily purchased. This form of graphene is generally of a lower quality, however Junctions using large-area graphene have been demonstrated. Unfortunately, there has been almost no development of devices which exploit these junctions. This project seeks to develop a robust method for fabricating superconducting junctions and then to develop parametric amplifiers using the junctions. Through this proof-of-concept work we aim to demonstrate that scalable, electrostatically tuned parametric amplifiers is a possibility.

These amplifiers could be used for the readout of superconducting qubits and by operating with very low noise, could help in reducing decoherence of these systems. As they are less sensitive to magnetic fields, these amplifiers would also be more robust against such interference. More widely, superconducting microwave amplifiers are used in experiments searching for Axions, a dark matter candidate and can also be used for radio astronomy. The low heat capacity of graphene will also allow the fabrication of very sensitive bolometers with low noise parametric amplifiers.

Success in this project would bring the expertise of a world leading research group to the UK and deliver a key enabling technology that would strengthen the UK's position as a world leader in quantum technologies.
Key Findings
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Organisation Website: http://www.lancs.ac.uk