Quantum-Noise Limited Amplifiers for Astronomy Applications

Ground-based and space-based astronomical telescopes are challenging to build and expensive to operate, so it is imperative that they collect as much scientific data as possible during their lifetimes. The speed with which a telescope can make observations can be increased by improving the sensitivities and pixel counts of the spectroscopic and photometric instruments in the focal plane.

An improvement in sensitivity allows astronomers to detect ever fainter astronomical signals, thereby enabling fledgling objects to be observed at the earliest moments in cosmic time; and increasing the number of pixels in a camera allows astronomers to map large nearby objects with high angular resolution. Both of these advances are needed if we are to extend our understanding of the way in which galaxies, stars and planets form and evolve, physics of the Big Bang, the formation of large-scale structure in the early Universe, and the nature of exotic objects such as black holes.

In this project, we propose to develop a technology that can increase the observing speeds of telescopes considerably.

Almost all astronomical instruments need low noise amplifiers (LNAs) for signal processing e.g., as first-stage front-end amplifiers for radio, microwave and millimetre(mm)-wave applications; and post detection readout amplifiers for sub-mm wave spectrometers and interferometers, far-infrared (FIR) and optical imaging arrays. Cryogenically cooled high electron mobility transistor (HEMT) amplifiers have been the LNA of choice since the 1980s, but these devices fall short of theoretical sensitivity limits, are bulky, difficult to cool, and are expensive.

It is not realistic to build instruments that need more than say 8 HEMT LNAs at most. In this proposal, we aim to replace HEMT LNAs with superconducting travelling wave parametric amplifiers (TWPAs). TWPAs can achieve quantum noise limited sensitivity, have large bandwidths, dissipate only small amounts of heat, and are relatively easy and inexpensive to mass produce.

Crucially, large arrays of TWPAs can be packaged into small volumes to enable readout systems having massive throughput.

All TWPAs demonstrated to date operate at sub-Kelvin temperatures, which is not easy to achieve in microwave mm-wave, nor in sub-mm wave heterodyne instruments, where the primary detector does not require such low temperatures. It is also disadvantageous for infrared and optical receiver, where access to the sub-kelvin cold stage is needed for the imaging arrays themselves.

For example, a non-sub-Kelvin LNA would be hugely important for projects such as SPIAKID [https://cordis.europa.eu/project/id/835087] which requires 40 LNAs to read out an 80,000 pixel kinetic inductance detector array. Even with modern cryogenic engineering, it would be almost impossible to provide a suitable environment for cooling such large number of HEMT LNAs.

Space-based missions, such as the next generation FIR and X-ray Probe missions, would have an even greater challenge. In this project, we focus on operating TWPAs at higher temperatures, say 3.5 K, which can easily be achieved using modern pulse-tube coolers. We will also simplify the operation of TWPAs by using an innovative balanced architecture, enabling multistage amplifiers to be built, where TWPAs having different characteristics are cascaded at different temperature stages.

We will also explore a unique method to achieve an extremely flat gain response over a very wide band. Most importantly, we will demonstrate the use of TWPAs in an astronomical receiver by using it to read out a terahertz superconductor-insulator-superconductor (SIS) mixer (a potentially quantum-noise-limited sensor for 100 GHz to 1 THz), and/or a small optical-KID array.

The core themes of our proposed research are intellectually fruitful, and are of central importance in enabling major areas of astronomy, and other areas of physics e.g., dark matter experiments and absolute measurements of neutrino mass.