A Dual-Polarisation Receiver for Multi-Beam Interferometry

Interferometry in astronomy is a technique that employs an array of telescopes to obtain high spatial resolution and extremely detailed observation of stars, galaxies, molecular clouds, proto-planetary disk and other interesting stellar objects. In millimetre (mm) and sub-mm range, each of these telescopes is generally equipped with a heterodyne receiver that down-coverts the detected astronomical signal from several hundred GHz to a few GHz, so that the signal can be processed by standard electronics. To achieve high sensitivity for detecting extremely weak astronomical signals, these receivers require the employment of superconducting quantum mixers (SIS, Superconductor-Insulator-Superconductor detector) that need to be cooled below the transition temperature of the superconducting material (typically 4K). In addition, to recover the full signal strength, dual-polarization detection scheme is required to split the polarization of the incoming signal into two orthogonal polarization states, where each is detected via a separate SIS receiver chain, resulting in bulky and complicated receiver architecture. Due to this complexity, almost all the major mm/sub-mm facilities in the world, such as the Sub-Millimetre Array (SMA, Hawai'i) and the Atacama Large Millimetre/Sub-Millimetre Array (ALMA, Chile), have only one-pixel receiver equipped in each of the telescope. Installing more pixels however at the focal plane of each telescope (namely multiple-beam interferometry) will allow fast mapping of extended object such as nearby galaxies, that otherwise would have to be carried out with mosaicking of individual pointing. This is important as it will allow the astronomers to probe further and faster, improving our understanding of the planet, star and galaxies formation, one of the key research themes outlined by STFC. Dual polarisation receiver is also the key instrument in the B-mode Cosmic Microwave Background (CMB) experiment in search for the understanding of the origin of the Universe, another major research area within the STFC programme.

The main objective of this project is to build a compact dual-polarization receiver that will allow the installation of several channels (pixels) in each telescope receiver without excessive requirements on the cryostat size or cooling power of existing telescopes. The key feature of this design is that the two SIS mixer chains will be integrated on a single chip and located in a single block, using a novel planar orthomode transducer (OMT) to split the polarization.

The project will be carried out in collaboration with the Harvard-Smithsonian Centre for Astrophysics (CfA) at Cambridge, Massachusetts, US, with whom we will collaborate in all aspects of the receiver design and construction. The student will develop the compact dual-polarization SIS receiver operating around 230 GHz to demonstrate this pioneering technology for the first time in heterodyne receivers. The SIS mixer chip will be tested in the THz Detectors Laboratory at Oxford and the receiver will also be assembled here. Whence completed, the receiver will be transferred to CfA for further integration before shipping for installation in the focal plane of the SMA, an interferometer comprising eight movable 6-meter diameter telescope sited on the Mauna Kea of Hawai'i. This receiver will also constitute a building block for the future construction of multi-pixel heterodyne receiver array and hence realizing multiple-beam interferometry for the SMA, and even ALMA once this technology is proven to be feasible.