Dr. Boon Kok Tan

Abstract:

Travelling wave parametric amplifiers (TWPAs) made from highly nonlinear reactive superconducting thin films have been demonstrated to be a potentially viable quantum-noise-limited amplifier technology for various fundamental physics platforms, including microwave/mm/sub-mm astronomy, dark matter search experiments, neutrino mass experiments and qubit readout. To date, only a limited number of successful kinetic inductance TWPA devices have been reported, with the majority fabricated from the same material, niobium titanium nitride (NbTiN), although in principle any highly nonlinear superconducting film can be used for kinetic inductance TWPA fabrication. Here, we present a detailed analysis of titanium nitride (TiN) transmission lines, to ascertain their suitability for use as kinetic inductance TWPAs. We will experimentally characterise our transmission line structures at cryogenic temperatures and compare the results with electromagnetic simulations. This characterisation and analysis would allow us to understand the advantages and limitations of TiN films, and whether they are suitable for applications as kinetic inductance TWPAs.

Abstract:

We report the development of an on-chip, compact dual-polarisation Superconductor-Insulator-Superconductor (SIS) receiver covering the millimetre frequency band from 190 GHz to 290 GHz. All the required components for the receiver are integrated on-chip using planar circuit technology, except the feedhorn. We use a 4-probe orthomode transducer (OMT) to couple the radio frequency (RF) and local oscillator (LO) and to split the incoming signal into two RF polarisation states before they are routed to the twin junction Nb/A10x/Nb mixers via a planar crossover, branch-line hybrids and bandpass filters. The entire receiver chip, only 4.0 mm by 4.1mm in size, is mounted in a split block where the feedhorn and the magnetic biasing are located on the front side, and the intermediate frequency (IF) and DC biasing connectors are on the rear side, leaving the four adjacent sides of the block unobscured for a 2-dimensional extension into a multi-pixel array. In this paper, we present the receiver and mixer block design and preliminary experimental results obtained from testing the receiver.

Abstract:

In this paper, we present a method to measure the effective physical temperature of the tunnel junction in a terahertz superconductor-insulator-superconductor (SIS) mixer when illuminated with a strong local oscillator (LO) power that results in local hot spot around the tunnel junction. We observed that the gap voltage of the pumped current-voltage (IV) curve is suppressed when the LO pumping level is increased, indicating that the junction physical temperature is increased beyond the mixer block temperature. We quantified this extra heating effect by recovering the effective junction temperature through comparing the gap voltage of the pumped IV curves measured at a fixed block temperature, with the unpumped IV curve measured at varying block temperatures. We found that the heat trapped in the tunnel junction can be as high as 1.7 K when the mixer stabilised at 3.3 K is pumped at only 21% of the gap current.