Photometric and Spectroscopic Superconducting Imaging Technology for Astrophysics

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The submillimetre-wave (3mm-300um) and far-infrared (300um-20um) regions of the electromagnetic spectrum are of considerable importance for astronomy because they contain a wealth of information about the cool, optically dark Universe. For example, the Cosmic Microwave Background radiation, which is a relic of the Big Bang, can be found at the longest wavelengths, and thermal radiation from distant, highly redshifted galaxies can be found at the shortest wavelengths. These regions also contain 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 being formed. It is exceptionally difficult to carry out astronomy at submillimetre wavelengths because observations must be made from high dry sites in remote places or from space. The detection of signals requires large and expensive telescopes, and complex instruments must be cooled to temperatures of between 4K and 100 mK. It is simply not possible to buy suitable cameras and spectrometers, and instead astronomers must develop their own imaging technology. The proposed programme aims to develop a new generation of extremely sensitive detectors and receivers by fabricating microcircuits out of materials called superconductors. Superconductors have the property that their electrical resistance falls to zero below a critical temperature, and magnetic flux is expelled. Indeed, the superconducting state is a distinct state of matter, which has many curious properties. By fabricating microcircuits from Nb, Ta, Al, Mo, NbN and NbTiN and by using modern Si and SiN micromachining techniques, it is possible to make detectors 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 programme described in this application concentrates on three 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 island; (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) 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 device types can be used singly or packed into arrays of multiple pixels to form cameras. Superconducting mixers require coherent, phased locked reference sources called local oscillators, which are extremely difficult to realise at supra-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 a new generation of imaging technology based on advanced superconducting devices that will be available to construct the highly sensitive submillimetre-wave and far-infrared instruments needed for the next generation of ground-based and space-borne astronomy.