A Superconducting Phase Shifter and Traveling Wave Kinetic Inductance Parametric Amplifier for W-Band Astronomy

Abstract

The W-Band ($75-110\mathrm{GHz}$) sky contains a plethora of information about star formation, galaxy evolution and the cosmic microwave background. We have designed and fabricated a dual-purpose superconducting circuit to facilitate the next generation of astronomical observations in this regime by providing proof-of-concept for both a millimeter-wave low-loss phase shifter, which can operate as an on-chip Fourier transform spectrometer (FTS) and a traveling wave kinetic inductance parametric amplifier (TKIP). Superconducting transmission lines have a propagation speed that depends on the inductance in the line which is a combination of geometric inductance and kinetic inductance in the superconductor. The kinetic inductance has a non-linear component with a characteristic current, $I_{\ast }$, and can be modulated by applying a DC current, changing the propagation speed and effective path length. Our test circuit is designed to measure the path length difference or phase shift, $\Delta \varphi$, between two symmetric transmission lines when one line is biased with a DC current. To provide a measurement of $\Delta \varphi$, a key parameter for optimizing a high gain W-Band TKIP, and modulate signal path length in FTS operation, our $3.6\times 2.5\mathrm{cm}$ chip employs a pair of $503\mathrm{mm}$ long NbTiN inverted microstrip lines coupled to circular waveguide ports through radial probes. For a line of width $3\mu \mathrm{m}$ and film thickness $20\mathrm{nm}$, we predict $\Delta \varphi \approx 1767\mathrm{rad}$ at $90\mathrm{GHz}$ when biased at close to $I_{\ast }$. We have fabricated a prototype with $200\mathrm{nm}$ thick Nb film and the same line length and width. The predicted phase shift for our prototype is $\Delta \varphi \approx 30\mathrm{rad}$ at $90\mathrm{GHz}$ when biased at close to $I_{\ast }$ for Nb.

arXiv comment8 pages, 3 figures, submitted to Journal of Low Temperature Physics

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