CAMELS - The Cambridge Emission Line Surveyor for the Greenland Telescope

It is widely recognised that the development of a mature chip spectrometer technology, where each pixel in a focal-plane array is intrinsically capable of yielding detailed spectroscopic information, would revolutionise far-infrared and submm-wave(4 mm to 300 um) astronomy. Low spectral-resolution channels (R = 5-20) could be used for CMB and SZ astronomy, and for determining dust temperatures through simultaneous multicolour observations of continuum sources; medium spectral-resolution channels (R = 500-1000) could be used for wide-field blind surveys of high-redshift spectral-lines; and high spectral-resolution channels (R = 2000-4000) could be used for multiline mapping of molecular gas in star-forming regions and extended nearby galaxies. Once the core technology is available, a large number of pixels could be packed into arrays for mapping and surveys, or a small number of pixels could be positioned sparsely over a wide field of view to enable multi-object spectroscopy.

Although a number of organisations are working on chip spectrometers, the international community is falling short of demonstrating science-grade observations. To some extent this situation has occurred because a significant amount time is needed on a submm-wave telescope to understand behaviour and refine designs. To address this situation, we propose to demonstrate a high-resolution (R = 3000) superconducting filter-bank spectrometer for the 70-115 GHz (4.3-2.6 mm) atmospheric window. The Cambridge Emission Line Surveyor (CAMELS) is a collaborative project between the Cavendish Laboratory and the Harvard Smithsonian Center for Astrophysics (CFA). CAMELS will be installed on the Greenland Telescope (GLT) and used to map isotopic abundances in low-z galaxies by measuring 12CO (vr = 115.271 GHz) and 13CO (vr =110.201 GHz) line strengths simultaneously. We will assess two slightly different designs: One will map bright lines from extended galaxies (z = 0.005 - 0.05) against high backgrounds (NEP = 2 x 10-17 WHz-1/2), and the other will detect faint lines from point sources (z = 0.05 - 0.12) against low backgrounds (NEP = 4 x 10-18 WHz-1/2). As well as being scientifically important, operation in the 70-115 GHz window will allow us to explore performance, without worrying about scheduling limitations imposed by the atmosphere.

The elements of a chip spectrometer are easy to understand in principle, but the realisation of a complete instrument that is capable of making science-grade observations requires detailed knowledge. Our pixels will comprise a single-mode antenna, a bank of superconducting RF filters, coupling terminations to an array of Kinetic Inductance Detectors (KIDs), and a single superconducting readout line. All of these will be realised on a single wafer using multi-layer superconducting microcircuit technology. The chip will be read out using fast digital electronics and Software Defined Radio (SDR) techniques. The Cambridge Group runs a state of the art facility for manufacturing superconducting quantum sensors, and has considerable expertise in fabricating multi-layer microcircuits using bcc-Ta, beta-Ta, NbN, Nb, Al, Mo, Hf, Ir, Au, Cu, SiO, SiO2 films on Si substrates and SiN membranes. This facility will be used to realize the spectrometer modules.

The outlook for the technology is considerable, and our programme contributes strongly to STFC's vision. All existing and planned ground-based and space-borne far-infrared observatories are completely reliant on superconducting imaging arrays and receivers. Superconducting device processing technology is now well established, and the next step is to produce microcircuits having complex on-chip functionality. For example, the ability to realise hyperspectral imaging where each pixel is capable of measuring the temperature of the continuum background and the strengths of certain widely space lines simultaneously would have a major impact on the design of future space telescopes.

Astrophysics is an area of science that is still easily capable of making major discoveries. It continues to thrill the public, it is one of the primary reasons why many young people choose to study Physics at University, and it contributes to the general cultural and intellectual health of the nation. To make discoveries and to allow the testing of ideas requires the development of new technology that pushes forward the limits of what is possible. Our programme will use superconducting microcircuits to create low-noise imaging arrays where each pixel is intrinsically capable of providing detailed spectroscopic information. The work will bring together a team of scientists working on quantum sensor physics, all-wafer superconducting microcircuit and materials technology, fast digital electronics, observational astronomy, and theoretical astrophysics to yield a technology base that is well able to open up new horizons. In addition to having astronomical significance, the work on quantum sensors is intellectually rich and will find applications elsewhere. For example, our recent modelling of the non-equilibrium quasiparticle and phonon energy spectra in thin-films, and our work on tunnel-junction and resonator fabrication, are closely related to studies being undertaken to realise qubits for quantum computing.

We passionately believe that the next generation of astronomical facilities can only come from building and testing prototype instruments on telescopes. During the course of the work we will build a prototype spectrometer, and study its behaviour on the Greenland Telescope (GLT) in Thule. Eventually the telescope will be moved to the centre of the Greenland ice sheet, and then the UK be well placed to help build a major spectrometer for this observatory. The development of technology for an Arctic project will create many opportunities for exciting Outreach. The Cavendish Laboratory has a fully staffed office aimed solely at Outreach. A wide range of activities are undertaken. This office will use the CAMELS-GLT project to help promote exciting science to those people who come onto its radar. The Detector Physics Group (DPG), regularly engages in Outreach activities. For example, in the summer of 2012 we have a work experience student joining the Group during her holiday, we have an undergraduate student carrying out a project during the summer, and we a have a visiting graduate student joining from Berkeley for a year. Members of the project team often give public talks, and CAMELS on the GLT will be an excellent topic around which prominent lectures can be given.

The DPG has strong connections with a number of international organisations. Formal research agreements are currently held with the Space Research Organisation of the Netherlands (SRON), the European Space Agency (ESA), the European Southern Observatory (ESO), the Goddard Space Flight Center Washington (GSFC), the national standards agency of Germany, Physikalisch-Technische Bundesanstalt (PTB), the National Physical Laboratory (NPL), the South West Research Institute Texas (SWRI), and the Dublin Institute for Advanced Studies (DIAS) . Some of these relate to astronomy, but others are aimed at developing advanced technology for utilitarian applications. The DPG continues to operate its 20-year licensing agreement with Oxford Instruments. The Detector Physics and Astrophysics Groups have numerous other active collaborations, and are well connected to the international community. The DPG is highly active writing conference and journal papers, and all of the work will be disseminated widely. A number of post doctoral researchers and PhD students will work on the project and will benefit greatly from the experience gained. The project covers many areas of science, from device physics to fast digital electronics, and will provide an excellent training ground for a new generation of scientists.