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In the present work, we report experimental realization of an optical fiberbased COW protocol for QKD in the telecom wavelength (1550 nm) where theattenuation in the optical fiber is minimum. A laser of 1550 nm wavelength,attenuator and intensity modulator is used for the generation of pulses havingaverage photon number 0.5 and repetition rate of 500 MHz. The experiment isperformed over 40 km, 80 km and 120 km of optical fiber and severalexperimental parameters like disclose rate, compression ratio, dead time andexcess bias voltage of the detector are varied for all the cases (i.e., for 40km, 80 km and 120 km distances) to observe their impact on the final key rate.Specifically, It is observed that there is a linear increase in the key rate aswe decrease compression ratio or disclose rate. The key rate obtains itsmaximum value for least permitted values of disclose rate, compression ratioand dead time. It seems to remain stable for various values of excess biasvoltage. While changing various parameters, we have maintained the quantum biterror rate (QBER) below 6%. The key rate obtained is also found to remainstable over time. Experimental results obtained here are also compared with theearlier realizations of the COW QKD protocol. Further, to emulate key rate atintermediate distances and at a distance larger than 120 km, an attenuator of 5dB loss is used which can be treated as equivalent to 25 km of the opticalfiber used in the present implementation. This has made the presentimplementation equivalent to the realization of COW QKD upto 145 km.

Implementation of coherent one way protocol for quantum key distribution up to an effective distance of 145 km