Single-photon detection using large-scale high-temperature MgB2 sensors at 20 K.

Ultra-fast single-photon detectors with high current density and operating temperature can benefit space and ground applications, including quantum optical communication systems, lightweight cryogenics for space crafts, and medical use. Here we demonstrate magnesium diboride (MgB2) thin-film superconducting microwires capable of single-photon detection at 1.55 µm optical wavelength. We used helium ions to alter the properties of MgB2, resulting in microwire-based detectors exhibiting single-photon sensitivity across a broad temperature range of up to 20 K, and detection efficiency saturation for 1 µm wide microwires at 3.7 K. Linearity of detection rate vs incident power was preserved up to at least 100 Mcps. Despite the large active area of up to 400 × 400 µm2, the reset time was found to be as low as ~ 1 ns. Our research provides possibilities for breaking the operating temperature limit and maximum single-pixel count rate, expanding the detector area, and raises inquiries about the fundamental mechanisms of single-photon detection in high-critical-temperature superconductors.

Hot-electron bolometer terahertz mixers for the Herschel Space Observatory.

We report on low noise terahertz mixers (1.4-1.9 THz) developed for the heterodyne spectrometer onboard the Herschel Space Observatory. The mixers employ double slot antenna integrated superconducting hot-electron bolometers (HEBs) made of thin NbN films. The mixer performance was characterized in terms of detection sensitivity across the entire rf band by using a Fourier transform spectrometer (from 0.5 to 2.5 THz, with 30 GHz resolution) and also by measuring the mixer noise temperature at a limited number of discrete frequencies. The lowest mixer noise temperature recorded was 750 K [double sideband (DSB)] at 1.6 THz and 950 K DSB at 1.9 THz local oscillator (LO) frequencies. Averaged across the intermediate frequency band of 2.4-4.8 GHz, the mixer noise temperature was 1100 K DSB at 1.6 THz and 1450 K DSB at 1.9 THz LO frequencies. The HEB heterodyne receiver stability has been analyzed and compared to the HEB stability in the direct detection mode. The optimal local oscillator power was determined and found to be in a 200-500 nW range.