Entanglement Based Quantum Key Distribution Using a Bright Sagnac Entangled Photon Source

Abstract

We report on improvements in an entangled free-space quantum key distribution (QKD) system by replacing the original non-collinear type-II spontaneous parametric down-conversion (SPDC) polarization entangled photon source with a new brighter Sagnac interferometric entangled photon source. While the SPDC source was integral to the initial setup of the system, it was limited in photon pair production rate and entanglement quality. Initial experiments with the new Sagnac source have already yielded substantially higher entangled photon rates and improved visibilities. In order to examine the integration of the new source with the QKD system, a local QKD experiment is performed where the source is pumped with 5 mW of power yielding an average raw key rate of 9,423 bits/s and an average final secret key rate of 2,695 bits/s, with an observed average QBER of 2.48%. Initial experiments distributing entangled photons over a single 1,305 m free-space link have seen entangled photon pair coincident detection rates as high as 3,000 cps. Extrapolating based on these initial numbers and previous experiments, we hope to obtain an average secret key rate of 715 bits/s for a two free-space link QKD experiment running the source at full power which will represent an order of magnitude increase over our previous experiments. An additional benefit of the new source is that it has a much narrower bandwidth which will aid in making the system compatible with daylight experiments. However, one drawback of the source is an appreciable double pair emission rate which initial experiments indicate.