NTT Research and Development 2008 Review of Activities

Quantum cryptography, or quantum key distribution (QKD), is a method to provide secret keys to two distant parties. Using keys distributed by QKD, we can realize unconditionally secure communication. Therefore, QKD is expected to be a technology that can ensure complete network security.

In our recent QKD experiment, we used a QKD system with the world's fastest clock frequency of 10 GHz. This system was jointly developed by NTT Laboratories and Stanford University, and the superconducting single photon detectors (SSPD) were developed by the National Institute of Standards and Technology (NIST) of the United States (see figure). We employed differential phase shift (DPS) protocol, which is based on a transmission of attenuated coherent pulses whose phase is modulated randomly by 0 or π. The DPS protocol is robust against eavesdropping attacks even without special photon sources. In addition, the simple configuration of this scheme makes it relatively easy to increase the key rate. The SSPD counts photons utilizing a phenomenon in which an incident photon breaks the superconducting state of a nanowire made of NbN, and features a low dark count rate and a high timing resolution. We performed a QKD experiment over 200 km of fiber, and successfully generated secure keys at a rate of 12 bit/s. This experiment set the world record of key distribution distance.

We are now trying to increase the key generation rate and to develop repeater systems for quantum information using entangled photon pairs.