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Письма в журнал технической физики

Superconducting noise-source for ultra-low temperatures

Kim T. M., S. V. Shitov

Published Jan 1, 2022• 0 citations
Publisher

Abstract

The device operates at temperatures <300 mK and comprises a hafnium microbridge and a superconducting aluminum tunnel junction both integrated into a common coplanar waveguide. The microbridge matches with a planar antenna and operates as an optical blackbody at frequencies 600-700 GHz. The coplanar terminal is the blackbody output in the 1-2 GHz frequency range. The microbridge temperature can be set in the range of 0.4-9 K and calibrated using the shot noise of the tunnel junction. Temperature modulation of each of the sources can be performed independently using a direct current which transit them from the superconducting to the normal state with characteristic times <0.1 ms and heating power ~ 1 μW. Keywords: Superconducting microbridge, superconducting tunnel junction, thermodynamic noise, shot noise, noise thermometry.

DOI
10.21883/tpl.2022.15.54266.18897

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The device operates at temperatures <300 mK and comprises a hafnium microbridge and a superconducting aluminum tunnel junction both integrated into a common coplanar waveguide. The microbridge matches with a planar antenna and operates as an optical blackbody at frequencies 600-700 GHz. The coplanar terminal is the blackbody output in the 1-2 GHz frequency range. The microbridge temperature can be set in the range of 0.4-9 K and calibrated using the shot noise of the tunnel junction. Temperature modulation of each of the sources can be performed independently using a direct current which transit them from the superconducting to the normal state with characteristic times <0.1 ms and heating power ~ 1 μW. Keywords: Superconducting microbridge, superconducting tunnel junction, thermodynamic noise, shot noise, noise thermometry.
The device operates at temperatures <300 mK and comprises a hafnium microbridge and a superconducting aluminum tunnel junction both integrated into a common coplanar waveguide. The microbridge matches with a planar antenna and operates as an optical blackbody at frequencies 600-700 GHz. The coplanar terminal is the blackbody output in the 1-2 GHz frequency range. The microbridge temperature can be set in the range of 0.4-9 K and calibrated using the shot noise of the tunnel junction. Temperature modulation of each of the sources can be performed independently using a direct current which transit them from the superconducting to the normal state with characteristic times <0.1 ms and heating power ~ 1 μW. Keywords: Superconducting microbridge, superconducting tunnel junction, thermodynamic noise, shot noise, noise thermometry.
The device operates at temperatures <300 mK and comprises a hafnium microbridge and a superconducting aluminum tunnel junction both integrated into a common coplanar waveguide. The microbridge is a thermodynamic source and operates as an optical blackbody at frequencies 600–700 GHz. The coplanar terminal is the blackbody output in the 1–2 GHz frequency range. The microbridge temperature can be set in the range of 0.4–9 K and calibrated using the shot noise of the tunnel junction. Activation and temperature modulation of each of the sources can be performed independently using a direct current which transit them from the superconducting to the normal state with characteristic times <0.1 ms and heating power <1 μW.
A high-quality superconducting resonator with a microbridge of hafnium film for use in a circuit for readout a terahertz-band imaging array with frequency division multiplexing is demonstrated experimentally. The variability of the impedance of the bridge at a frequency of 1.5 GHz, which is a key factor in the control of the quality of the resonator, is studied. The bridge, having a thickness of about 50 nm, a critical temperature T C ≈ 380 mK, and a plan size of 2.5 × 2.5 μm, was connected as a load of a resonator made of niobium film with a thickness of about 100 nm ( T C ~ 9 K). It is shown that the bridge smoothly changes its impedance proportionally to the bias power in the entire temperature range. The effective thermal insulation of the bridge was measured in a dilution cryostat at temperatures of 50–300 mK. Thermal conductivity G of the bridge was calculated and found to be ~4 × 10 –13 W/K, which gives an estimate of the sensitivity of the structure in the bolometric mode NEP ≈ 8 × 10 –19 W/Hz 1/2 at a temperature of 150 mK.
This thesis investigates the prospects of some different superconducting mixers integrated with planar antennas for the millimeter and submillimeter regime. Two kinds of Superconductor/Insulator/Superconductor (SIS) tunnel junctions for use in mixers have been fabricated; Nb/PbBi edge junctions and Nb trilayer junctions. One of the very high quality trilayer mixers is currently operating in a 350 GHz radio-telescope heterodyne-receiver. The reduced sensitivity of SIS mixers for frequencies above the gap frequency (?700 GHz) of the superconductors has directed the interest towards another type of superconducting mixer device, the superconducting Hot Electron Bolometer (HEB). Various types of this device have been studied; Nb devices for scale-measurements at 20 GHz to more easily verify some of the theoretical background of the HEB, NbN waveguide devices for 100 GHz, and NbN devices integrated with planar antennas for a quasi-optical mixer at 350 GHz. Two types of planar antennas have been used; the endfire "Broken" Linearly Tapered Slotline Antenna (BLTSA) on a thin dielectric membrane, and the double dipole antenna on a quartz substrate lens. The beam pattern of the BLTSA has been examined theoretically and measured at 40 GHz with antennas supported by Kapton foil, and at 350 and 800 GHz with antennas on thin dielectric SiOxNy membranes. Measurements of the 20 GHz Nb HEB show a mixer conversion loss close to 0 dB for IF frequencies below 40 MHz. The maximum IF bandwidth given by the electron temperature relaxation time corresponds to a frequency of about 80 MHz. A DSB mixer noise temperature of only a few hundred Kelvin has been measured. Corresponding measurements of NbN devices at 100 and 350 GHz give a mixer conversion loss of about 6-10 dB and an IF bandwidth of about 700 to 1000 MHz. The output noise is found to be dominated by electron temperature fluctuations with an equivalent noise temperature of about 100 K, and a frequency dependence equal to that of the IF signal. Phenomenological expressions for conversion loss, noise, and IF impedance have been compared measurements and shows good agreement. The quasioptical NbN HEB mixers are still under development with respect to film quality and coupling to radiation, why the mixer performance of these is expected to improve.
We have developed a low-noise 850 GHz superconductor–insulator–superconductor quasiparticle mixer with NbTiN thin-film microstrip tuning circuits and hybrid Nb/AlN/NbTiN tunnel junctions. The mixer uses a quasioptical configuration with a planar twin-slot antenna feeding a two-junction tuning circuit. At 798 GHz, we measured an uncorrected double-sideband receiver noise temperature of TRX=260 K at 4.2 K bath temperature. This mixer outperforms current Nb SIS mixers by a factor of nearly 2 near 800 GHz. The high-gap frequency and low loss at 800 GHz make NbTiN an attractive material with which to fabricate tuning circuits for SIS mixers. NbTiN mixers can potentially operate up to the gap frequency, 2Δ/h∼1.2 THz.