Kitaygorsky J, Zhang J, Verevkin A, Sergeev A, Korneev A, Matvienko V, et al. Origin of dark counts in nanostructured NbN single-photon detectors. IEEE Trans Appl Supercond. 2005;15(2):545–8.
Abstract: We present our study of dark counts in ultrathin (3.5 to 10 nm thick), narrow (120 to 170 nm wide) NbN superconducting stripes of different lengths. In experiments, where the stripe was completely isolated from the outside world and kept at temperature below the critical temperature Tc, we detected subnanosecond electrical pulses associated with the spontaneous appearance of the temporal resistive state. The resistive state manifested itself as generation of phase-slip centers (PSCs) in our two-dimensional superconducting stripes. Our analysis shows that not far from Tc, PSCs have a thermally activated nature. At lowest temperatures, far below Tc, they are created by quantum fluctuations.
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Hübers H-W, Semenov A, Holldack K, Schade U, Wüstefeld G, Gol’tsman G. Time domain analysis of coherent terahertz synchrotron radiation. Appl Phys Lett. 2005;87(18):184103 (1 to 3).
Abstract: The time structure of coherent terahertz synchrotron radiation at the electron storage ring of the Berliner Elektronensynchrotron und Speicherring Gesellschaft has been analyzed with a fast superconducting hot-electron bolometer. The emission from a single bunch of electrons was found to last ∼1500ps at frequencies around 0.4THz, which is much longer than the length of an electron bunch in the time domain (∼5ps). It is suggested that this is caused by multiple reflections at the walls of the beam line. The quadratic increase of the power with the number of electrons in the bunch as predicted for coherent synchrotron radiation and the transition from stable to bursting radiation were determined from a single storage ring fill pattern of bunches with different populations.
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Milostnaya I, Korneev A, Minaeva O, Rubtsova I, Slepneva S, Seleznev V, et al. Superconducting nanostructured detectors capable of single photon counting of mid-infrared optical radiation. In: Rogalski A, Dereniak EL, Sizov FF, editors. Proc. SPIE. Vol 5957. SPIE; 2005. 59570A (1 to 9).
Abstract: We report on our progress in research and development of ultrafast superconducting single-photon detectors (SSPDs) based on ultrathin NbN nanostructures. Our SSPDs were made of the 4-nm-thick NbN films with Tc 11 K, patterned as meander-shaped, 100-nm-wide strips, and covering an area of 10×10 μm2. The detectors exploit a combined detection mechanism, where upon a single-photon absorption, a hotspot of excited electrons and redistribution of the biasing supercurrent, jointly produce a picosecond voltage transient signal across the superconducting nanostripe. The SSPDs are typically operated at 4.2 K, but their sensitivity in the infrared radiation range can be significantly improved by lowering the operating temperature from 4.2 K to 2 K. When operated at 2 K, the SSPD quantum efficiency (QE) for visible light photons reaches 30-40%, which is the saturation value limited by the optical absorption of our 4-nm-thick NbN film. With the wavelength increase of the incident photons,the QE of SSPDs decreases significantly, but even at the wavelength of 6 μm, the detector is able to count single photons and exhibits QE of about 10-2 %. The dark (false) count rate at 2 K is as low as 2x10-4 s,-1 which makes our detector essentially a background-limited sensor. The very low dark-count rate results in a noise equivalent power (NEP) below 10-18 WHz-1/2 for the mid-infrared range (6 μm). Further improvement of the SSPD performance in the mid-infrared range can be obtained by substituting NbN for another, lower-Tc materials with a narrow superconducting gap and low quasiparticles diffusivity. The use of such superconductors should shift the cutoff wavelength below 10 μm.
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Slysz W, Wegrzecki M, Bar J, Grabiec P, Górska M, Latta C, et al. Fiber-coupled quantum-communications receiver based on two NbN superconducting single-photon detectors. In: Rogalski A, Dereniak EL, Sizov FF, editors. Proc. SPIE. Vol 5957. SPIE; 2005. 59571K (1 to 10).
Abstract: We present the design and performance of a novel, two-channel single-photon receiver, based on two fiber-coupled NbN superconducting single-photon detectors (SSPDs). The SSPDs are nanostructured superconducting meanders covering an area of 100 μm2 and are known for ultrafast and efficient counting of single, visible-to-infrared photons. Their operation has been explained within a phenomenological hot-electron photoresponse model. Our receiver is intended for fiber-based quantum cryptography and communication systems, operational at near-infrared (NIR) telecommunication wavelengths, λ = 1.3 μm and λ = 1.55 μm. Coupling between the NbN detector and a single-mode optical fiber was achieved using a specially designed, micromechanical photoresist ring, positioned directly over the SSPD active area. The positioning accuracy of the ring was below 1 μm. The receiver with SSPDs was placed (immersed) in a standard liquid-helium transport Dewar and kept without interruption for over two months at 4.2 K. At the same time, the optical fiber inputs and electrical outputs were kept at room temperature. Our best system reached a system quantum efficiency of up to 0.3 % in the NIR radiation range, with the detector coupling efficiency of about 30 %. The response time was measured to be about 250 ps and was limited by our read-out electronics. The measured jitter was close to 35 ps. The presented performance parameters show that our NIR single photon detectors are suitable for practical quantum cryptography and for applications in quantum-correlation experiments.
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Bryerton E, Percy R, Bass R, Schultz J, Oluleye O, Lichtenberger A, et al. Receiver measurements of pHEB beam lead mixers on 3-μm silicon. In: Proc. 30th IRMMW / 13th THz.; 2005. p. 271–2.
Abstract: We report on receiver noise measurement results of phonon-cooled HEB beam lead mixers on 3 μm thick silicon. This type of ultra-thin mixer chip with integrated beam leads allows easy assembly into a block and holds great promise for array integration. Receiver measurements from 600-720 GHz are presented with a minimum noise temperature of 500 K at 666 GHz. These results verify the mixer performance of the SOI processing techniques allowing for further design and integration of SOI pHEB mixers in receivers operating above 1 THz.
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