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arXiv:0807.2289 · Optics Express

Entangled quantum key distribution over two free-space optical links

Chris Erven, Christophe Couteau, Raymond Laflamme, Gregor Weihs

Published Oct 7, 2008• 96 citations
arXivHTMLPDF

Abstract

We report on the first real-time implementation of a quantum key distribution (QKD) system using entangled photon pairs that are sent over two free-space optical telescope links. The entangled photon pairs are produced with a type-II spontaneous parametric down-conversion source placed in a central, potentially untrusted, location. The two free-space links cover a distance of 435 m and 1,325 m respectively, producing a total separation of 1,575 m. The system relies on passive polarization analysis units, GPS timing receivers for synchronization, and custom written software to perform the complete QKD protocol including error correction and privacy amplification. Over 6.5 hours during the night, we observed an average raw key generation rate of 565 bits/s, an average quantum bit error rate (QBER) of 4.92%, and an average secure key generation rate of 85 bits/s.

DOI
10.1364/oe.16.016840

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We report on the first real-time implementation of a quantum key distribution (QKD) system using entangled photon pairs that are sent over two free-space optical telescope links. The entangled photon pairs are produced with a type-II spontaneous parametric down-conversion source placed in a central, potentially untrusted, location. The two free-space links cover a distance of 435 m and 1,325 m respectively, producing a total separation of 1,575 m. The system relies on passive polarization analysis units, GPS timing receivers for synchronization, and custom written software to perform the complete QKD protocol including error correction and privacy amplification. Over 6.5 hours during the night, we observed an average raw key generation rate of 565 bits/s, an average quantum bit error rate (QBER) of 4.92%, and an average secure key generation rate of 85 bits/s.
We report on the progress of our real-time entanglement based free-space quantum key distribution (QKD) system which uses polarization entangled photon pairs sent over a variety of free-space optical telescope links to distribute the key. An experiment with one photon from each pair sent over a 1,325 m long free-space link and the other photon detected locally next to the source is described. The system performs the complete QKD protocol including all error correction and privacy amplification algorithms. Over the course of 6.5 hours of communication at night an average raw key rate of 1,398 bits/s with an average quantum bit error rate (QBER) of 4.58% was observed producing an average final key rate of 244 bits/s. We also performed a Bell inequality experiment over two free-space links of 435 m and 1,325 m respectively, producing a total separation of 1,575 m between the two detectors in the experiment. During the Bell inequality experiment we observed an average Bell parameter of 2.51 ± 0.11.
We report on a complete experimental implementation of a quantum key distribution protocol through a free space link using polarization-entangled photon pairs from a compact parametric down-conversion source. Over 10 hours of uninterrupted communication between sites 1.5 km apart, we observe average key generation rates of 630 per second after error correction and privacy amplification. Our scheme requires no specific hardware channel for synchronization apart from a classical wireless link, and no explicit random number generator.
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.
The authors report on a complete experimental implementation of a quantum key distribution protocol through a free-space link using polarization-entangled photon pairs from a compact parametric downconversion source. Over 10h of uninterrupted communication between sites 1.5km apart, they observe average key generation rates of 630∕s after error correction and privacy amplification. Their scheme requires no specific hardware channel for synchronization apart from a classical wireless link, and no explicit random number generator.
We report a free-space entanglement-based quantum key distribution experiment, implementing the biased basis protocol between two sites which are 15.3 km apart. Photon pairs from a polarization-entangled source are distributed through two 7.8-km free-space optical links. An optimal bias 20:80 between the X and Z basis is used. A post-processing scheme with finite-key analysis is applied to extract the final secure key. After three-hour continuous operation at night, a 4293-bit secure key is obtained, with a final key rate of 0.124 bit per raw key bit which increases the final key rate by 14.8% comparing to the standard BB84 case. Our results experimentally demonstrate that the efficient BB84 protocol, which increases key generation efficiency by biasing Alice and Bob's basis choices, is potentially useful for the ground-satellite quantum communication.
Abstract Quantum key distribution (QKD) [1] is the first method of quantum information science that will find its way into our everyday life. It employs fundamental laws of quantum physics to ensure provably secure symmetric key generation between two parties. The key can then be used to encrypt and decrypt sensitive data with unconditional security. Here, we report on a free space QKD implementation using strongly attenuated laser pulses over a distance of 480 m. It is designed to work continuously without human interaction. Until now, it produces quantum keys unattended at night for more than 12 hours with a sifted key rate of more than 50 kbit/s and a quantum bit error rate between 3% and 5%.