Photometric and Spectroscopic Superconducting Imaging Technology for Astrophysics

The microwave (3 cm-3 mm), submillimetre-wave (3 mm-300 um) and far-infrared (300 um-20 um) regions of the electromagnetic spectrum contain a wealth of information about the cool, dark Universe. For example, the Cosmic Microwave Background radiation can be found at the longest wavelengths, and thermal radiation from distant, highly redshifted galaxies can be found at the shortest wavelengths. This part of the spectrum also contains thousands of spectral lines from numerous molecular and atomic species, which are important for studying the chemistry and physics of regions where stars and planets are formed. It is exceptionally difficult to carry out astronomy at submillimetre wavelengths because observations must be made from high dry sites or from space. The detection of signals requires large, precision telescopes, and complex instruments must be cooled to temperatures of between 4 K and 100 mK. It is simply not possible to buy suitable cameras and spectrometers, and so astronomers must develop their own imaging technology. The proposed programme aims to develop a new generation of extremely sensitive detectors and receivers for microwave, submillimetre-wave and far-infrared wavelengths by fabricating microcircuits out of materials called superconductors. The superconducting state is a distinct state of matter, which has many curious properties. By fabricating microcircuits from Nb, Ta, Al, Mo, NbN, TiN and NbTiN and by using modern Si and SiN micromachining techniques, it is possible to make complex electronic devices having extraordinary characteristics. For example, some of our infrared detectors are capable of sensing a light bulb being turned on and off for just 1 second at a distance of 10 million miles, whilst others operate in a truly quantum mechanical way, displaying non-classical conversion gain and sensitivities limited by the Heisenberg uncertainty principle. The work described in this proposal concentrates on four specific devices: (i) Transition Edge Sensors (TESs), which operate by using the sharp transition of a superconductor, to its normal state, to measure the minute change in temperature that occurs when infrared power is absorbed by a tiny free-standing micro-machined SiN membrane; (ii) Kinetic Inductance Detectors (KIDs), which essentially measure a small change that occurs in the amount by which magnetic field penetrates into the surface of a superconductor when photons are absorbed; (iii) parametric amplifiers, which use the non-linear characteristics of certain superconducting materials to achieve ultra-low-noise amplification; (iv) Superconductor Insulator Superconductor (SIS) mixers, which use extremely thin layers of superconducting and insulating material to create diodes in which quantum mechanical tunnelling occurs, creating highly sensitive radio receivers. Each of these devices can be used singly or packed into arrays of multiple pixels to form cameras. Superconducting mixers require reference sources called local oscillators, which are extremely difficult to realise at THz frequencies. The development of suitable coherent source technology is therefore an essential part of our programme. Another innovative part of our proposed work is to develop microscopically patterned phononic filters that control the flow of heat onto devices, and reduce thermal fluctuation noise, by forming filters that attenuate elastic waves in support structures. The core themes of our proposed research into quantum sensor physics are intrinsically intellectually fruitful, and are of central importance to enabling major areas of astronomy. At the end of the work, we will have demonstrated various new imaging technologies based on advanced superconducting devices, and this technology will then be available to construct the highly sensitive instruments needed for the next generation of ground-based and space-based astronomy.

We see our role as exploring and understanding the physics of quantum sensors, innovating and developing photometric and spectroscopic imaging technology, and providing the community with ultra-low-noise components in the form of well-characterised subsystems for major projects. In this way we help lever opportunity for the UK in high-profile areas of fundamental science. Our combined track record of project advancement and delivery is excellent. We have contributed significantly to the success of major astronomy infrastructure (JCMT, ALMA, Herschel HIFI, etc.), participated extensively in technical networks (e.g. RadioNet, ESA CTP), and are well connected to many high-profile organisations (SRON, NRAO, ESA, ESO, PTB, VTT, IRAM, NASA-JPL, GSFC, CalTech, CfA, SWRI, PMO). We have instrumentation projects with organisations such as ESO, ESA and the Greenland Telescope. Yassin and Ellison are members of the H2020 RadioNet Board, and Yassin has lead a consortium of European research institutions to develop Supra-THz mixers in task 4 of AETHER project in the FP7 frame. They both participated in the UK-led M5 mission proposal 'FIRSPEX'. Oxford work on the development of THz feed-horns is being exploited worldwide in astronomical receivers and features future generation CMB experiments such as BLASTPol, the spectrograph of SuperSpec and the spectrometer of CCAT X-Spec. Withington worked with SRON, Univ. Cardiff and Airbus (and a consortium of UK universities and international partners) to submit an ESA/UKSA M5 bid for the far-infrared space telescope SPICA. Cambridge's selection by the SPICA project to supply half of the superconducting focal plane technology, with JPL providing the other half, is a measure of our reputation for developing ultra-low-noise imaging technology. Cambridge is also working with SRON and GSFC on understanding the physics of the sensors being developed for the approved X-ray mission ATHENA. In the context of CMB astrophysics, we are supporting Cardiff and Manchester in engaging with the US initiative CMB-S4 and the recently funded Simons Observatory. We also contributed to the recent ESA M5 proposal for a CMB polarization mission CORE. Not only is our work of pivotal importance for astronomy, it is highly intellectually rich in its own right. Since 2010, the Consortium has published well over 200 unique journal and conference papers, many in top-quality journals such as Phys. Rev. A & B, Phys. Rev. Lett., Sup. Sci. Tech., Teraherz Sci. Tech., JOSA: available on www.mrao.cam.ac.uk/projects/cg2017, username `reviewer', password `heaviside'. Additionally, we submitted over 200 technical reports to various astronomy projects. The partner groups provide exceptional training for graduate students in areas such as device and optical physics, materials science and fabrication, cryogenic, microwave and electronic test. Our research not only enables internationally competitive astronomy, but has wider applications in areas such as weather monitoring and forecasting, communications, security surveillance, biological sensing, medical imaging and plasma diagnostics. For example, we worked for a year with British Antarctic Survey to assess the use of our technology in solving key problems in polar atmospheric science. Our work responds to a growing commercial interest in far-infrared laboratory based spectroscopy for industrial process control, non-destructive materials examination and pollution monitoring. As evidenced by Cambridge's success in raising funding for device-processing equipment through an EPSRC Quantum Technologies Call, there are close synergies with the major field of quantum computing and communications. All of these topics have excellent growth potential and, through industrial exploitation, represent opportunities for delivering intellectual status and financial return to the UK. Our many spin-out activities with large and small companies are listed in the Impact document.