Quantum Turing machine
The quantum Turing machine (QTM) is the quantum analogon of a Turing machine (TM). For many aspects, a close analogy to the classical counterpart exists. Though a quantum Turing machine can be defined more or less canonically, several conceptional problems associated with it and concerning the notion of 'quantum computation' exist and are still unsolved. Other properties of the quantum Turing machine like the question, whether universality does hold or not, are unknown as well.
Quantum Aspects of Computation
The Church-Turing thesis defines an 'algorithm' as a description of a calculation. But is this the whole story? A calculation is not only a platonic object, usually it is intended to execute the actual calculation in one way or the other. Put in other words, the calculation should be realized physically. Then, the device executing the calculation has to be considered as physical system. A calculation is the execution of a physical experiment, and its result is provided by the observation of the experiment.
Today, many devices executing calculations are known. They are not limited to digital computers. Other types are e.g. analogous computers, soap-bubble based computers, the DNA-computing approach, and the Antikythera mechanism. This leads to the question, whether the restriction to classical models of computation like Turing machines is really adequate.
The idea of the Turing machine dates back to the year 1936. At this time, the physical world seemed to be dominated by mechanical forces; correspondingly, the definition of a Turing machine is based on the ideas of classical mechanics. And though the physical realization of a Turing machine, the digital computer, actually uses quantum mechanics, its construction principles aims at the suppression of any effect associated with the quantum world. With ever-tighter package density, however, this is not achievable anymore in a perfect way. The effects of quantum theory may begin to have an influence on the outcome of the calculation.
Considered from another perspective, computations executed by humans are mental processes. In this way, the Church-Turing thesis is also a statement about the human mind. The Platonic world (including computations) is a mental construction. The brain, which is generating the Platonic world, is a physical system in turn; and at the microscopic level, this system has a quantum mechanical nature.
Consequently it seems to be questionable whether the Turing machine provides a 'natural' model of computation. Searching for alternatives and taking the quantum nature of the world into consideration, Feynman has the idea of quantum computation in 1982. As model executing such a quantum computation, he proposes the quantum Turing machine as quantum theoretical analogon to the Turing machine. Similar ideas were developed independently by Yuri Manin in 1980. Accordingly, David Deutsch generalized the Church-Turing thesis to the Church-Turing-Deutsch thesis in 1985, which states that every computation, which can be realized physically, can be executed using a quantum Turing machine.
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Quantum Turing machine. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Quantum_Turing_machine&oldid=31907