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Słysz, W., Wegrzecki, M., Bar, J., Grabiec, P., Górska, M., Zwiller, V., et al. (2007). Fibre-coupled, single photon detector based on NbN superconducting nanostructures for quantum communications. J. Modern Opt., 54(2-3), 315–326.
Abstract: We present a novel, two-channel, single photon receiver based on two fibre-coupled, NbN, superconducting, single photon detectors (SSPDs). The SSPDs are nanostructured superconducting meanders and are known for ultrafast and efficient detection of visible-to-infrared photons. Coupling between the NbN detector and optical fibre was achieved using a micromechanical photoresist ring placed directly over the SSPD, holding the fibre in place. With this arrangement, we obtained coupling efficiencies up to ∼30%. Our experimental results showed that the best receiver had a near-infrared system quantum efficiency of 0.33% at 4.2 K. The quantum efficiency increased exponentially with the photon energy increase, reaching a few percent level for visible-light photons. The photoresponse pulses of our devices were limited by the meander high kinetic inductance and had the rise and fall times of approximately 250 ps and 5 ns, respectively. The receiver's timing jitter was in the 37 to 58 ps range, approximately 2 to 3 times larger than in our older free-space-coupled SSPDs. We stipulate that this timing jitter is in part due to optical fibre properties. Besides quantum communications, the two-detector arrangement should also find applications in quantum correlation experiments.
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Koch, M. (2007). Terahertz communications: a 2020 vision. In NATO Security through Science Series (Vol. 2007, pp. 325–338).
Abstract: We discuss basic considerations for potential short-range THz communication systems which may replace or supplement present WLAN systems in 10–15 years from now. On the basis of a few fundamental estimations we show that such a system will need a line-of-sight connection between receiver and emitter. To circumvent the blocking of the direct line-of-sight connection indoor THz communication systems will also have to rely on non-line-of-sight paths which involve reflections off the walls. The reflectivity of the walls can be enhanced by dielectric mirrors. This new scheme makes steerable high-gain antennas a necessity. Hence, a wireless THz communication system can not be a simple extension of the existing technology of today's local area networks. Instead it involves completely new concepts and ideas that have not yet been worked upon.
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Koshelets, V. P., Ermakov, A. B., Filippenko, L. V., Khudchenko, A. V., Kiselev, O. S., Sobolev, A. S., et al. (2007). Superconducting integrated submillimeter receiver for TELIS. IEEE Trans. Appl. Supercond., 17(2), 336–342.
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Takesue, H., Nam, S. W., Zhang, Q., Hadfield, R. H., Honjo, T., Tamaki, K., et al. (2007). Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors. Nat. Photon., 1, 343–348.
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Torgashin, M. Y., Koshelets, V. P., Dmitriev, P. N., Ermakov, A. B., Filippenko, L. V., & Yagoubov, P. A. (2007). Superconducting integrated receivers based on Nb-AlN-NbN circuits. IEEE Trans. Appl. Supercond., 17(2), 379–382.
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Torgashin, M. Y., Koshelets, V. P., Dmitriev, P. N., Ermakov, A. B., Filippenko, L. V., & Yagoubov, P. A. (2007). Superconducting Integrated Receiver Based on Nb-AlN-NbN-Nb Circuits. IEEE Trans. Appl. Supercond., 17(2), 379–382.
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Ryabchun, S., Tong, C. - Y. E., Blundell, R., Kimberk, R., & Gol'tsman, G. (2007). Study of the effect of microwave radiation on the operation of HEB mixers in the terahertz frequency range. IEEE Trans. Appl. Supercond., 17(2), 391–394.
Abstract: We have investigated the effect of injecting microwave radiation, with a frequency much lower than that corresponding to the energy gap of the superconductor, on the performance of the hot-electron bolometer mixer incorporated into a THz heterodyne receiver. More specifically, we show that exposing the mixer to microwave radiation does not cause a significant rise of the receiver noise temperature and fall of the mixer conversion gain so long as the microwave power is a small fraction of local oscillator power. The injection of a small, but controlled amount of microwave power therefore enables active compensation of local oscillator power and coupling fluctuations which can significantly degrade the gain stability of hot electron bolometer mixer receivers.
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Jiang, L., Antipov, S. V., Voronov, B. M., Gol'tsman, G. N., Zhang, W., Li, N., et al. (2007). Characterization of the performance of a quasi-optical NbN superconducting HEB mixer. IEEE Trans. Appl. Supercond., 17(2), 395–398.
Abstract: In this paper we focus mainly on the investigation of the performance of a quasi-optical (planar log-spiral antenna) phonon-cooled NbN superconducting hot electron bolometer (HEB) mixer, which is cryogenically cooled by a close-cycled 4-K cryocooler, at 500 and 850 GHz frequency bands. The mixer's noise performance, stability of IF output power, and local oscillator (LO) power requirement are characterized for three NbN superconducting HEB devices of different sizes. The transmission characteristics of Mylar and Zitex films with incidence waves of an elliptical polarization are also examined by measuring the mixer's noise temperature. The lowest receiver noise temperatures (with no corrections) of 750 and 1100 K are measured at 500 and 850 GHz, respectively. Experimental results also demonstrate that the bigger the HEB device is, the higher the stability of IF output power becomes.
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Hajenius, M., Yang, Z. Q., Gao, J. R., Baselmans, J. J. A., Klapwijk, T. M., Voronov, B., et al. (2007). Optimized sensitivity of NbN hot electron bolometer mixers by annealing. IEEE Trans. Appl. Supercond., 17(2), 399–402.
Abstract: We report that the heterodyne sensitivity of superconducting hot-electron bolometers (HEBs) increases by 25-30% after annealing at 85degC in high vacuum. The devices studied are twin-slot antenna coupled mixers with a small area NbN bridge of 1 mum times 0.15 mum, above which there is a SiO 2 passivation layer. The mixer noise temperature, gain, and resistance versus temperature curve of a HEB before and after annealing are compared and analysed. We show that the annealing reduces the intrinsic noise of the mixer by 37% and makes the superconducting transition of the bridge and the contacts sharper. We argue that the reduction ofthe noise is mainly due to the improvement of the transparency of the contact/film interface. The lowest receiver noise temperature of 700 K is measured at a local oscillator frequency of 1.63 THz and at a bath temperature of 4.2 K.
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Ursin, R., Tiefenbacher, F., Schmitt-Manderbach, T., Weier, H., Scheidl, T., Lindenthal, M., et al. (2007). Entanglement-based quantum communication over 144km. Nat. Phys., 3(7), 481–486.
Abstract: Quantum entanglement is the main resource to endow the field of quantum information processing with powers that exceed those of classical communication and computation. In view of applications such as quantum cryptography or quantum teleportation, extension of quantum-entanglement-based protocols to global distances is of considerable practical interest. Here we experimentally demonstrate entanglement-based quantum key distribution over 144km. One photon is measured locally at the Canary Island of La Palma, whereas the other is sent over an optical free-space link to Tenerife, where the Optical Ground Station of the European Space Agency acts as the receiver. This exceeds previous free-space experiments by more than an order of magnitude in distance, and is an essential step towards future satellite-based quantum communication and experimental tests on quantum physics in space.
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