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Quantum cryptography. Eye of an observer reflected in a mirror in quantum cryptography apparatus. Quantum cryptography is based on the principle of entanglement, a property of a pair of particles in which a change to one has an instantaneous effect on the other, no matter how far apart they are. Cryptography in this case refers to the encoding of data so that only specified targets can access it. If the sender and the receiver of the data share entanglement then any data is only readable by them alone. This is more secure than existing encryption methods, most of which rely on mathematical algorithms. Photographed at the lab of Anton Zeilinger at Vienna University, Austria. (KEYSTONE/SCIENCE PHOTO LIBRARY/VOLKER STEGER)