Quantum Limited SIS Receiver Technology for the Detection of Water Isotopologue Emission from Comets

NASA's Planetary Science Decadal Survey has concluded that isotopic measurements of cometary water vapor are a means to unraveling the mysteries involving the origin of Earth's water and the evolution of our solar system. To support this, a recent Jet Propulsion Laboratory (JPL) internal research program has developed quantum limited Superconductor Insulator Superconductor (SIS) receivers in the important 500 - 600 GHz submillimeter frequency band. These instruments can be used to detect the HDO ground state ( $1_{10}-1_{01}$ ), $H^{16}_{2}$ O ortho ground-state ( $1_{10}-1_{01}$ ), and the oxygen isotopologues $H^{17}_2$ O, and $H^{18}_{2}$ O with exquisite sensitivity. To achieve the presented results we have investigated Aluminum Oxide ( $\text{AlO}_x$ ) and Aluminum Nitride ( $\text{AlN}_x$ ) barrier SIS tunnel junction mixers on 6μm Silicon-on-Insulator (SOI) substrate. The $\text{AlO}_x$ and $\text{AlN}_x$ junction mixer blocks utilize diagonal and smooth-profile conical horns respectively. In both cases a commercial 4-8 GHz Intermediate Frequeny (IF) low noise amplifier (LNA) has been integrated into the mixer block. The $\text{AlO}_x$ (low current density) barrier SIS junctions were fabricated with 2μm gold beam-lead technology, whereas in the case of the $\text{AlN}_x$ SIS tunnel junction we use capacitive RF decoupling tabs. The latter approach simplifies fabrication, increases yield, eases the mounting process, and facilitates scaling to higher frequencies. For an actual flight mission, with operation ≤ 4.2 K, the allowed heat dissipation of the mixer integrated low noise amplifier needs to be minimized. In this paper we also investigate the receiver sensitivity as a function of LNA DC power consumption. We find that the DC power consumption of the low noise amplifier can be reduced to ∼ 1.6 mW with minimal loss in sensitivity. It is anticipated that continued InP HEMT development for quantum computer applications are likely to reduce the required LNA power dissipation even further.