An improved quantum secure direct communication （QSDC） protocol is proposed in this paper. Blocks of entangled photon pairs are transmitted in two steps in which secret messages are transmitted directly. The single logical qubits and unitary operations under decoherence free subspaces are presented and the generalized Bell states are constructed which are immune to the collective noise. Two steps of qubit transmission are used in this protocol to guarantee the security of communication. The security of the protocol against various attacks are discussed.
Stimulated Brillouin scattering(SBS)in optical fibers has long been used in frequency-selective optical signal processing,including in the realization of microwave-photonic(MWP)filters.In this work,we report a significant enhancement in the selectivity of SBS-based MWP filters.Filters having a single passband of 250 MHz–1 GHz bandwidth are demonstrated,with selectivity of up to 44 dB.The selectivity of the filters is better than that of the corresponding previous arrangements by about 15 dB.The shape factor of the filters,defined as the ratio between their−20 dB bandwidth and their−3 dB bandwidth,is between 1.35 and 1.5.The central transmission frequency,bandwidth,and spectral shape of the passband are all independently adjusted.Performance enhancement is based on two advances,compared with previous demonstrations of tunable SBS-basedMWPfilters:(a)the polarization attributes of SBS in standard,weakly birefringent fibers are used to discriminate between in-band and out-of-band components and(b)a sharp and uniform power spectral density of the SBS pump waves is synthesized through external modulation of an optical carrier by broadband,frequency-swept waveforms.The signal-to-noise ratio of filtered radio-frequency waveforms and the linear dynamic range of the filters are estimated analytically and quantified experimentally.Lastly,a figure of merit for the performance of the filters is proposed and discussed.The filters are applicable to radio-over-fiber transmission systems.
The effects of bismuth doping content and temperature on the absorption property and near-infrared （NIR） luminescence of Bi-doped La2O3-Al2O3-SiO2 （LAS） glasses are presented. The emission intensity reaches the maximum when the Bi2O3 content in 3.0Bi-LAS is 1.83%. The emission spectra reach their peaks at 1 190 and 1 117 nm, with full-width at half-maximum （FWHM） values of 330 and 228 nm under 500 and 700-nm excitations, respectively. As the Bi203 content increases, the peak wavelengths and FWHMs of emission bands increase, but their lifetimes decrease. The lifetime of 2.0Bi-LAS is 460 μs at 9 K, and is almost temperature independent until 350 K. The NIR emission of Bi in the system has strong resistance to thermal quenching from 9 to 350 K.
