Developments towards practical free-space quantum cryptography

We describe a free space quantum cryptography system which is designed to allow continuous unattended key exchanges for periods of several days, and over ranges of a few kilometres. The system uses a four-laser faint-pulse transmission system running at a pulse rate of 10MHz to generate the required four alternative polarization states. The receiver module similarly automatically selects a measurement basis and performs polarization measurements with four avalanche photodiodes. The controlling software can implement the full key exchange including sifting, error correction, and privacy amplification required to generate a secure key.

Influence of the generated power, measurement bandwidth, and noise level on intensity statistics of a quasi-CW Raman fiber laser

In the present paper we numerically study instrumental impact on statistical properties of quasi-CW Raman fiber laser using a simple model of multimode laser radiation. Effects, that have the most influence, are limited electrical bandwidth of measurement equipment and noise. To check this influence, we developed a simple model of the multimode quasi- CW generation with exponential statistics (i.e. uncorrelated modes). We found that the area near zero intensity in probability density function (PDF) is strongly affected by both factors, for example both lead to formation of a negative wing of intensity distribution. But far wing slope of PDF is not affected by noise and, for moderate mismatch between optical and electrical bandwidth, is only slightly affected by bandwidth limitation. The generation spectrum often becomes broader at higher power in experiments, so the spectral/electrical bandwidth mismatch factor increases over the power that can lead to artificial dependence of the PDF slope over the power. It was also found that both effects influence the ACF background level: noise impact decreases it, while limited bandwidth leads to its increase. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Quantum dot-based terahertz photoconductive antennas

We present novel Terahertz (THz) emitting optically pumped Quantum Dot (QD) photoconductive (PC) materials and antenna structures on their basis both for pulsed and CW pumping regimes. Full text Quantum dot and microantenna design - Presented here are design considerations for the semiconductor materials in our novel QD-based photoconductive antenna (PCA) structures, metallic microantenna designs, and their implementation as part of a complete THz source or transceiver system. Layers of implanted QDs can be used for the photocarrier lifetime shortening mechanism[1,2]. In our research we use InAs:GaAs QD structures of varying dot layer number and distributed Bragg reflector(DBR)reflectivity range. According to the observed dependence of carrier lifetimes on QD layer periodicity [3], it is reasonable to assume that electron lifetimes can be potentially reduced down to 0.45ps in such structures. Both of these features; long excitation wavelength and short carriers lifetime predict possible feasibility of QD antennas for THz generation and detection. In general, relatively simple antenna configurations were used here, including: coplanar stripline (CPS); Hertzian-type dipoles; bow-ties for broadband and log-spiral(LS)or log-periodic(LP)‘toothed’ geometriesfor a CW operation regime. Experimental results - Several lasers are used for antenna pumping: Ti:Sapphire femtosecond laser, as well as single-[4], double-[5] wavelength, and pulsed [6] QD lasers. For detection of the THz signal different schemes and devices were used, e.g. helium-cooled bolometer, Golay cell and a second PCA for coherent THz detection in a traditional time-domain measurement scheme.Fig.1shows the typical THz output power trend from a 5 um-gap LPQD PCA pumped using a tunable QD LD with optical pump spectrum shown in (b). Summary - QD-based THz systems have been demonstrated as a feasible and highly versatile solution. The implementation of QD LDs as pump sources could be a major step towards ultra-compact, electrically controllable transceiver system that would increase the scope of data analysis due to the high pulse repetition rates of such LDs [3], allowing real-time THz TDS and data acquisition. Future steps in development of such systems now lie in the further investigation of QD-based THz PCA structures and devices, particularly with regards to their compatibilitywith QD LDs as pump sources. [1]E. U. Rafailov et al., “Fast quantum-dot saturable absorber for passive mode-locking of solid-State lasers,”Photon.Tech.Lett., IEEE, vol. 16 pp. 2439-2441(2004) [2]E. Estacio, “Strong enhancement of terahertz emission from GaAs in InAs/GaAs quantum dot structures. Appl.Phys.Lett., vol. 94 pp. 232104 (2009) [3]C. Kadow et al., “Self-assembled ErAs islands in GaAs: Growth and subpicosecond carrier dynamics,” Appl. Phys. Lett., vol. 75 pp. 3548-3550 (1999) [4]T. Kruczek, R. Leyman, D. Carnegie, N. Bazieva, G. Erbert, S. Schulz, C. Reardon, and E. U. Rafailov, “Continuous wave terahertz radiation from an InAs/GaAs quantum-dot photomixer device,” Appl. Phys. Lett., vol. 101(2012) [5]R. Leyman, D. I. Nikitichev, N. Bazieva, and E. U. Rafailov, “Multimodal spectral control of a quantum-dot diode laser for THz difference frequency generation,” Appl. Phys. Lett., vol. 99 (2011) [6]K.G. Wilcox, M. Butkus, I. Farrer, D.A. Ritchie, A. Tropper, E.U. Rafailov, “Subpicosecond quantum dot saturable absorber mode-locked semiconductor disk laser, ” Appl. Phys. Lett. Vol 94, 2511 © 2014 IEEE.