Coherent-state continuous variable quantum key distribution (CVQKD) protocols based on quadratures of the quantized electromagnetic field have seen significant progress during the last decade.Various kinds of protocols have been proposed,[1-17] some of which have been implemented experimentally and field testing was also performed recently.[18-25] Compared with its counterpart discrete variable quantum key distribution (DVQKD),CVQKD has some technological advantages such as better efficiency of homodyne detection over single photon counting at the telecom wavelength.