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

It is well known that many of the traditional cryptographic schemes (like RSA and DH) will be vulnerable when scalable quantum computers will be available. However, quantum cryptographic protocols can provide unconditional security. One such protocol for quantum key distribution (QKD) is coherent one way (COW) protocol which is realized often because of ease of implementation, though the security proof is still an open problem. In the present work, we report experimental realization of an optical fiber based COW protocol for QKD in the telecom wavelength (1550 nm) where the attenuation in the optical fiber is minimum. A laser of 1550 nm wavelength, attenuator and intensity modulator is used for the generation of pulses having average photon number 0.5 and repetition rate of 500 MHz. The experiment is performed over 40, 80 and 120 km of optical fiber and several experimental parameters like disclose rate, compression ratio, dead time and excess bias voltage of the detector are varied for all the cases (i.e., for 40, 80 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 as we decrease compression ratio or disclose rate. The key rate obtains its maximum value for least permitted values of disclose rate, compression ratio and dead time. It seems to remain stable for various values of excess bias voltage. While changing various parameters, we have maintained the quantum bit error rate (QBER) below 6% (5%) for distance \(\ge 120\) $\ge 120$  km ( \(\le 120\) $\le 120$  km). The key rate obtained is also found to remain stable over time. Experimental results obtained here are also compared with the earlier realizations of the COW QKD protocol. Further, to emulate key rate at intermediate distances and at a distance larger than 120 km, an attenuator of 5 dB loss is used which can be treated as equivalent to \(\thicksim 25\) $\sim 25$  km of the optical fiber used in the present implementation. This has made the present implementation equivalent to the realization of COW QKD upto \(\thicksim 145\) $\sim 145$  km.