Superconducting On-chip Fourier Transform Spectrometer
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Superconducting On‑chip Fourier Transform Spectrometer R. Basu Thakur1,2 · N. Klimovich1,2 · P. K. Day2 · E. Shirokoff3,4 · P. D. Mauskopf5 · F. Faramarzi5 · P. S. Barry6 Received: 30 August 2019 / Accepted: 7 June 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The kinetic inductance effect is strongly nonlinear with applied current in NbTiN, TiN and NbN thin films. This can be utilized to realize novel devices. We present results from transmission lines made with these materials, where DC (current) control is used to modulate the phase velocity thereby enabling on-chip spectrometers. Utility of such compact spectrometers is discussed, along with their natural connection with parametric amplifiers. Keywords Cosmology · CMB · Kinetic inductance · Parametric amplifier · Interferometer · Spectrometer · FTS
1 Introduction In thin-film superconductors, inductance is the sum of a constant value from device geometry ( Lg ), and a kinetic component ( Lk ) due to quasiparticle motion. Lk nonlinearly depends on the super-current [1, 2] (I), and for superconducting resonators/ transmission lines, the resonance frequency/transmission speed can be adjusted with a DC current [3–7]. The total inductance is expressed as
* R. Basu Thakur [email protected] 1
Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA
2
Jet Propulsion Laboratory (NASA), 4800 Oak Grove Dr., Pasadena, CA 91109, USA
3
Kavli Institute for Cosmological Physics, 5640 S. Ellis Ave., Chicago, IL 60637, USA
4
Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637, USA
5
Arizona State University, Tempe, AZ 85281, USA
6
HEP Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA
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Vol.:(0123456789)
Journal of Low Temperature Physics
Fig. 1 SOFTS schematic: each STL is modeled as current-controlled LC circuit (dashed boxes). STLs are capacitatively coupled (Cc) to an antenna (ANT, left) receiving broadband power, and a summing junction (SUM, right) where the phase-delayed signals are added and sensed by a detector (TES/MKID) (Color figure online)
) ( ( )2 L(I) ≈ (Lg + Lk,0 ) 1 + 𝛼 I∕I∗ + 𝛼 � (I∕I∗ )4
(1)
where I∗ is a current scale for nonlinearity, 𝛼 < 1 is the canonical kinetic inductance fraction and 𝛼 ′ is the material and geometry-dependent constant. The phase velocity (u) in a superconducting transmission line (STL) can thus be current-controlled, see Eq. 2, where the per-unit-length inductance/capacitance is L and C , respectively. Note that C and Lg (geometric) can be designed during fabrication, while kinetic inductance is DC tunable. Therefore, phase velocity is engineerable and controllable around desired values: √ ( )( ) u(I) = 1∕ C Lg + Lk,0 1 + 𝛼(I∕I∗ )2 + 𝛼 � (I∕I∗ )4 (2) Utilizing phase velocity control, we demonstrate a superconducting on-chip Fourier transform spectrometer (SOFTS). Two STLs are fed with identical inputs obtained from a source via a splitter. Each STL is one
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