Dr. Boon Kok Tan

Abstract:

We present the design and performance analysis of waveguide-based superconductor–insulator–superconductor (SIS) mixers employing niobium titanium nitride (NbTiN) transmission lines and tunnel junctions comprising niobium/aluminium oxide (Nb/AlO_x/Nb) or niobium/aluminium nitride (Nb/AlN/Nb) topologies. These superconducting quantum devices are developed to enable wideband heterodyne operation for next-generation far-infrared (FIR) space missions in the sub-terahertz regime. The mixer circuits are optimised for simultaneous broadband radio-frequency (RF) and intermediate-frequency (IF) operation using microstrip architectures that incorporate end-stub, end-loaded, and twin-junction tuning networks. Threedimensional electromagnetic simulations, combined with quantum-mixing analyses, predict receiver noise temperatures approaching four times the quantum limit across 0.50–0.79 THz for the AlO_x-based design. The AlN-based variant further extends the RF coverage to 0.39–0.82 THz, satisfying the requirements of several proposed space missions while overlapping the Atacama Large Millimetre/sub-millimetre Array (ALMA) Bands 8 and extended Band 9. These simulated results demonstrate that NbTiN-based microstrip SIS mixers have the potential to deliver high sensitivity and wide instantaneous bandwidth at sub-terahertz frequencies, establishing them as strong candidates for next-generation space-borne and ground-based FIR observatories.

Abstract:

Travelling wave parametric amplifiers (TWPAs) made from highly nonlinear reactive superconducting thin films have been demonstrated to be a viable technology for various quantum applications, including fundamental physics experiments such as astronomy and axion dark matter searches, as well as commercial applications like quantum computational and communication systems. In this paper, we present the design of a kinetic inductance TWPA comprising a patterned titanium nitride film that can operate at 0.3 K to demonstrate the feasibility of operation closer to 1 K temperature, paving the way to achieve even higher bath temperature operation. We discuss in detail the design of our TWPA, along with the predicted gain-bandwidth product and other characteristics. We perform the preliminary experimental investigation of the thin film properties and compare that with the simulated results. We found that there are several discrepancies between the measured and the predicted behaviour of the thin film. We attribute these differences to the fact that the fabricated thin film has a different gap voltage, resistivity and thickness to what we expected. With a new set of estimated parameters, we successfully reproduce the measured transmission profile. We further show that by utilising bridges to ensure equipotential grounds for the CPW lines, we managed to reduce the rippling effect and achieve higher gain with broader bandwidth. We expect that our TWPA can achieve higher than 20 dB gain from approximately 0–8 GHz.

Abstract:

The Line Emission Terahertz Observatory (LETO) is a candidate European Space Agency (ESA) M8 mission designed to investigate the interstellar medium, star formation, and the evolution of galaxies through velocity-resolved spectroscopy of key far-infrared emission lines. To meet the demanding sensitivity, spectral-resolution, and mapping-speed requirements of the mission, LETO will employ multi-pixel heterodyne receiver arrays operating across four frequency bands between 0.45 and 5.5 THz.

This paper presents proposed concepts for the LETO Band 1 array receiver, which targets the frequency range below 1 THz using superconductor-insulator-superconductor (SIS) mixer technology. A wideband SIS mixer design covering the baseline 450–650 GHz frequency range is described. The design builds upon proven receiver technologies developed for astronomical instruments, thereby reducing technical risk while maintaining near quantum-limited sensitivity.

Two complementary array architectures are investigated. The first employs a conventional single-polarisation mixer configuration that can be reconfigured to support either single- or dual-polarisation observations. The second is based on a fourprobe dual-polarisation mixer concept that eliminates mechanically reconfigurable optical components while providing flexible observing modes. Several operational configurations are presented, demonstrating how the proposed architecture can balance sensitivity, mapping speed, and spacecraft resource constraints. These concepts form the basis for future development of the LETO Band 1 receiver should the mission progress to subsequent study phases.

Abstract:

Oxford Physics Colloquium