Quantum computing offers revolutionary solutions in the field of computer sciences, applying the opportunities of quantum physics which are incomparably richer than classical physics. Although quantum computers are going to be the tools of the far future, there are already a few algorithms to solve problems which are very difficult to handle with traditional computers. Perhaps the easiest example of a structure of a quantum system is a quantum channel. Typically, one is interested in some basis in the Hilbert space representing the input of a channel, which is entangled with a second Hilbert space representing the environment, and then another (possibly the same) basis for the first space at a later time. Free-space quantum key distribution (QKD)—over an optical path of about 30 cm—was first introduced in 1991, and recent advances have led to demonstrations. Indeed there are certain key distribution problems in this category for which free-space QKD has practical advantages (for example it is not practical to send a courier to a satellite). Quantum computing algorithms can be used to affirm our communication in several ways (open-air communication, satellite communications, satellite broadcast, satellite-satellite communication). We set up a free-space quantum-channel-model at the uni-versity and made several simulations. The main aim is to trace some adoptable algorithms in the communication between Earth and the satellite and also between satellites. This paper is a theoretical study to compare the simulation results of the three models. © 2007 Elsevier Ltd. All rights reserved
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