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Science

Getting Entangled in Free Space

John G. Rarity

Published Aug 1, 2003• 3 citations
Publisher

Abstract

Entanglement is a counterintuitive property of quantum mechanics that allows particles such as photons to be much more strongly correlated than is possible in a classical world. In his Perspective, [Rarity][1] explains how this property may be exploited in long-distance communication. He highlights the report by [ Aspelmeyer et al .][2], who demonstrate that photons can remain entangled over distances of 600 m in free space. The results indicate that satellites in low-Earth orbit may be used for quantum communication based on entanglement. [1]: http://www.sciencemag.org/cgi/content/full/301/5633/604 [2]: http://www.sciencemag.org/cgi/content/short/301/5633/621

DOI
10.1126/science.1087850

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•Jan 1, 2009•0 citations
Entanglement is a counterintuitive property of quantum mechanics that allows particles such as photons to be much more strongly correlated than is possible in a classical world. In his Perspective, Rarity explains how this property may be exploited in long-distance communication. He highlights the report by Aspelmeyer et al ., who demonstrate that photons can remain entangled over distances of 600 m in free space. The results indicate that satellites in low-Earth orbit may be used for quantum communication based on entanglement.
Academia, governments, and industry around the world are on a quest to build long-distance quantum communication networks for a future quantum internet. Using air and fiber channels, quantum communication quickly faced the daunting challenge of exponential photon loss with distance. Quantum repeaters were invented to solve the loss problem by probabilistically establishing entanglement over short distances and using quantum memories to synchronize the teleportation of such entanglement to long distances. However, due to imperfections and complexities of quantum memories, ground-based proof-of-concept repeater demonstrations have yet been restricted to metropolitan-scale distances. In contrast, direct photon transmission from satellites through empty space faces almost no exponential absorption loss and only quadratic beam divergence loss. A single satellite successfully distributed entanglement over more than 1,200 km. It is becoming increasingly clear that quantum communication over large intercontinental distances (e.g., 4,000–20,000 km) will likely employ a satellite-based architecture. This could involve quantum memories and repeater protocols in satellites, or memory-less satellite-chains through which photons are simply reflected, or some combination thereof. Rapid advancements in the space launch and classical satellite communications industry provide a strong tailwind for satellite quantum communication, promising economical and easier deployment of quantum communication satellites.
The sharing of qubits along links in Space emerged as a novel arena for realizing experiments on the fundamental aspects of Quantum Mechanics as well as a tool for implementing the protocols of Quantum Communications. The test of Bell's inequalities [1] and the wave-particle duality [2] are some of the most recent examples of the first aspect, while the intercontinental QKD [3] and teleportation on a satellite [4] are remarkable findings of the second one. All these results were achieved using satellites in Low-Earth-Orbit (LEO), orbiting at an altitude of about 500 km.
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.
Quantum entanglement, in which the physical properties of particles become correlated, is an essential property of quantum information systems. In their Perspective, [Cory and Havel][1] discuss results reported in the same issue by [ Roos et al .][2] and by [ Leibfried et al .][3] in which trapped ions were put into entangled quantum states. The two complementary studies show that substantial progress is being made in quantum information processing and point to practical applications of entangled states. [1]: http://www.sciencemag.org/cgi/content/full/304/5676/1456 [2]: http://www.sciencemag.org/cgi/content/short/304/5676/1478 [3]: http://www.sciencemag.org/cgi/content/short/304/5676/1476
Entanglement is an essential phenomenon of quantum mechanics. Two entangled particles, photons for example, will individually yield random results upon being measured, but these results will always be perfectly correlated, no matter how far the two particles are separated from each other. Entanglement has been proven to be at the heart of a wide range of fundamental quantum effects and it drives exciting practical applications, such as quantum cryptography, quantum teleportation and quantum computing [1], which would be impossible in a world limited to classical physics. A team of researchers from the Institute for Quantum Optics and Quantum Information (IQOQI) in Vienna and the University of Vienna, led by Anton Zeilinger, has now reported the successful transmission of entangled photon pairs between two Canary Islands, bridging a distance of 144 km and a two-photon attenuation of almost ten million to one. The result, published in Nature Physics [2], is so far the most convincing demonstration to perform experiments with entangled photons in space.