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New progress on experimental quantum cryptography——experimental free-space distribution of entangled photon pairs over 13km

Peng Cheng

Published Jan 1, 2005• 1 citation
OpenAlex

Abstract

We have experimentally realised free-space distribution of entangled photon pairs through a noisy ground atmosphere of 13km. It is shown that the desired entanglement can still survive after both photons have traversed a distance beyond the effective thickness of the aerosphere. We observe a spacelike separated violation of the Bell-Clauser-Horne-Shimony inequality of 2.45±0.09. With this source we have demonstrated the BB84-Ekert91 quantum cryptography protocol. Our experiment has shown for the first time the feasibility of using entangled photons for ground-to-satellite quantum communication, and presents a significant step towards satellite-based global quantum communication in the future. We first review the rapid progress in quantum cryptography over recent years and then describe our experiment in detail.

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The ability to distribute quantum resources over long distances is a key \nelement for the development of several practical applications in the field of \nquantum information. In this way, major efforts have been made in the \npast decade to extend the maximum distances for the observation of entanglement \nor for the realization of quantum cryptography. Using optical fibers \nas the quantum channel, entanglement has already been achieved between \nphotons separated by 50 km of fiber, and quantum cryptography is now a \ncommercial technology that allows quantum key distributon over distances \nup to around 100 km. Progress has also been made towards entanglement \ndistribution in free space, with the achieved distances of about 10 km \non ground atmosphere being a substantial step toward the development \nof satellite-based quantum communication.

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We have experimentally realised free-space distribution of entangled photon pairs through a noisy ground atmosphere of 13km. It is shown that the desired entanglement can still survive after both photons have traversed a distance beyond the effective thickness of the aerosphere. We observe a spacelike separated violation of the Bell-Clauser-Horne-Shimony inequality of 2.45±0.09. With this source we have demonstrated the BB84-Ekert91 quantum cryptography protocol. Our experiment has shown for the first time the feasibility of using entangled photons for ground-to-satellite quantum communication, and presents a significant step towards satellite-based global quantum communication in the future. We first review the rapid progress in quantum cryptography over recent years and then describe our experiment in detail.
We report free-space distribution of entangled photon pairs over a noisy ground atmosphere of 13 km. It is shown that the desired entanglement can still survive after both entangled photons have passed through the noisy ground atmosphere with a distance beyond the effective thickness of the aerosphere. This is confirmed by observing a spacelike separated violation of Bell inequality of 2.45+/-0.09. On this basis, we exploit the distributed entangled photon source to demonstrate the Bennett-Brassard 1984 quantum cryptography scheme. The distribution distance of entangled photon pairs achieved in the experiment is for the first time well beyond the effective thickness of the aerosphere, hence presenting a significant step towards satellite-based global quantum communication.
Quantum Entanglement is the essence of quantum physics and inspires fundamental questions about the principles of nature. Moreover it is also the basis for emerging technologies of quantum information processing such as quantum cryptography, quantum teleportation and quantum computation. Bell's discovery, that correlations measured on entangled quantum systems are at variance with a local realistic picture led to a flurry of experiments confirming the quantum predictions. However, it is still experimentally undecided whether quantum entanglement can survive global distances, as predicted by quantum theory. Here we report the violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality measured by two observers separated by 144 km between the Canary Islands of La Palma and Tenerife via an optical free-space link using the Optical Ground Station (OGS) of the European Space Agency (ESA). Furthermore we used the entangled pairs to generate a quantum cryptographic key under experimental conditions and constraints characteristic for a Space-to-ground experiment. The distance in our experiment exceeds all previous free-space experiments by more than one order of magnitude and exploits the limit for ground-based free-space communication; significantly longer distances can only be reached using air- or space-based platforms. The range achieved thereby demonstrates the feasibility of quantum communication in space, involving satellites or the International Space Station (ISS).
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.
The ability to distribute quantum resources over long distances is a key \nelement for the development of several practical applications in the field of \nquantum information. In this way, major efforts have been made in the \npast decade to extend the maximum distances for the observation of entanglement \nor for the realization of quantum cryptography. Using optical fibers \nas the quantum channel, entanglement has already been achieved between \nphotons separated by 50 km of fiber, and quantum cryptography is now a \ncommercial technology that allows quantum key distributon over distances \nup to around 100 km. Progress has also been made towards entanglement \ndistribution in free space, with the achieved distances of about 10 km \non ground atmosphere being a substantial step toward the development \nof satellite-based quantum communication.
Photonic entanglement is a fundamental resource in quantum information processing and its distribution between distant parties is a key challenge in quantum communications. Optical satellite links allow transmitting entangled photons over longer distances than currently possible on ground and could provide a path towards global quantum communication networks. The majority of current photonic free-space quantum communication systems, including two recent quantum satellite experiments, use two-dimensional polarization encoding, where each photon can carry at most a single bit of quantum information. Encoding additional qubits in the high-dimensional spatio-temporal degrees of freedom could not only increase channel capacities but also improve robustness with respect to noise and eavesdropping in quantum cryptography. Here, we give an overview of our efforts towards exploiting high-dimensional entanglement in long-distance free-space quantum communications. We report on the results of a first feasibility study, in which we used polarization / energy-time hyperentanglement to increase the dimensionality of the state space and transmit genuine 4-dimensional entanglement via an intra-city free-space link. We discuss how we intend to extend this approach to applications in noise-tolerant large-alphabet quantum key distribution via free-space links, and, ultimately, experiments with satellites.
Significant work has been done to develop quantum satellites, which generate entangled pairs in space and distribute them to ground stations separated some distance away. The reverse “uplink” case, where pairs are generated on the ground and swapped on the satellite using an optical Bell measurement, has not been seriously considered due to a prevailing assumption that it is practically infeasible. In this paper, we illustrate the feasibility of performing Discrete Variable photonic Bell measurements in space by conducting a detailed numerical analysis to estimate the channel efficiency and attainable pair fidelity for various satellite-station configurations. Our model accounts for a wide range of physical effects such as atmospheric effects, stray photons, and mode mismatch. Our findings show promise toward the feasibility of photonic Bell measurements in space, which motivates future research toward large-scale satellite-based uplink entanglement distribution.