Pirandola S, Mancini S, Lloyd S, Braunstein SL. Continuous-variable quantum cryptography using two-way quantum communication. Nat Phys. 2008;4(9):726–30.
Abstract: Quantum cryptography has recently been extended to continuous-variable systems, such as the bosonic modes of the electromagnetic field possessing continuous degrees of freedom. In particular, several cryptographic protocols have been proposed and experimentally implemented using bosonic modes with Gaussian statistics. These protocols have shown the possibility of reaching very high secret key rates, even in the presence of strong losses in the quantum communication channel. Despite this robustness to loss, their security can be affected by more general attacks where extra Gaussian noise is introduced by the eavesdropper. Here, we show a `hardware solution' for enhancing the security thresholds of these protocols. This is possible by extending them to two-way quantum communication where subsequent uses of the quantum channel are suitably combined. In the resulting two-way schemes, one of the honest parties assists the secret encoding of the other, with the chance of a non-trivial superadditive enhancement of the security thresholds. These results should enable the extension of quantum cryptography to more complex quantum communications.
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Meledin D., Desmaris V., Ferm S.-E., Fredrixon M., Henke D., Lapkin I., et al. APEX Band T2: A 1.25 – 1.39 THz Waveguide Balanced HEB Receiver.. 2008:181–5.
Abstract: A waveguide 1.25–1.39 THz Hot Electron Bolometer (HEB) balanced receiver was successfully developed, characterized and installed at the Atacama Pathfinder EXperiment (APEX) telescope. The receiver employs a quadrature balanced scheme using a waveguide 90-degree 3 dB RF hybrid, HEB mixers and a 180-degree IF hybrid. The HEB mixers are based on ultrathin NbN film deposited on crystalline quartz with a MgO buffer layer. Integrated into the multi-channel APEX facility receiver (SHeFI), the results presented here demonstrate exceptional performance; a receiver noise temperature of 1000 K measured at the telescope at the center of the receiver IF band 2-4 GHz, and at an LO frequency of 1294 GHz. Stability of the receiver is fully in line with the SIS mixer bands of the SHeFI, and gives a spectroscopic Allan time of more than 200 s with a noise bandwidth of 1 MHz.
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Marsili F, Bitauld D, Divochiy A, Gaggero A, Leoni R, Mattioli F, et al. Superconducting nanowire photon number resolving detector at telecom wavelength. In: CLEO/QELS. Optical Society of America; 2008. Qmj1 (1 to 2).
Abstract: We demonstrate a photon-number-resolving (PNR) detector, based on parallel superconducting nanowires, capable of resolving up to 5 photons in the telecommunication wavelength range, with sensitivity and speed far exceeding existing approaches.
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Korneev A, Divochiy A, Marsili F, Bitauld D, Fiore A, Seleznev V, et al. Superconducting photon number resolving counter for near infrared applications. In: Tománek P, Senderáková D, Hrabovský M, editors. Proc. SPIE. Vol 7138. Spie; 2008. 713828 (1 to 5).
Abstract: We present a novel concept of photon number resolving detector based on 120-nm-wide superconducting stripes made of 4-nm-thick NbN film and connected in parallel (PNR-SSPD). The detector consisting of 5 strips demonstrate a capability to resolve up to 4 photons absorbed simultaneously with the single-photon quantum efficiency of 2.5% and negligibly low dark count rate.
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Ryabchun SA, Tretyakov IV, Finkel MI, Maslennikov SN, Kaurova NS, Seleznev VA, et al. Fabrication and characterisation of NbN HEB mixers with in situ gold contacts. In: Proc. 19th Int. Symp. Space Terahertz Technol. Groningen, Netherlands; 2008. p. 62–7.
Abstract: We present our recent results of the fabrication and testing of NbN hot-electron bolometer mixers with in situ gold contacts. An intermediate frequency bandwidth of about 6 GHz has been measured for the mixers made of a 3.5-nm NbN film on a plane Si substrate with in situ gold contacts, compared to 3.5 GHz for devices made of the same film with ex situ gold contacts. The increase in the intermediate frequency bandwidth is attributed to additional diffusion cooling through the improved contacts, which is further supported by the its dependence on the bridge length: intermediate frequency bandwidths of 3.5 GHz and 6 GHz have been measured for devices with lengths of 0.35 μm and 0.16 μm respectively at a local oscillator frequency of 300 GHz near the superconducting transition. At a local oscillator frequency of 2.5 THz the receiver has offered a DSB noise temperature of 950 K. When compared to the previous result of 1300 K obtained at the same local oscillator frequency for devices fabricated with an ex situ route, such a low value of the noise temperature may also be attributed to the improved gold contacts.
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