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Quantum Computer

What does "quantum" mean?

The word "quantum", in quantum computer, originates from "quantum mechanics," a basic theory in physics. In brief, a quantum is the smallest unit of a physical quantity expressing a property of matter having both a particle and wave nature. On the scale of atoms and molecules, matter behaves in a quantum manner. The idea that computation might be performed more efficiently by making clever use of the fascinating properties of quantum mechanics is nothing other than the quantum computer.

In actuality, everything that happens in our daily lives conforms to the principals of quantum mechanics if we were to observe them on a microscopic scale, that is, on the scale of atoms and molecules. But because a great many degrees of freedom (such as a huge number of atomic movements) contribute to phenomena that we as human beings can perceive, this quantum mechanical behavior is normally hidden from us.

Yet, if we were to look into the world of individual atoms, we would find that an electron moving about the atomic nucleus can only take on energy having specific (discrete) values. In other words, an electron may enter only a fixed number of set states. This resembles the way in which strings on a guitar can only resonate at set frequencies, and reflects the wave nature of quantum states. This electron, moreover, may take on a "superposition state" that combines different energy states simultaneously. As described later, this superposition state is an extremely important concept in quantum computing. Applying a strong electric field to an atom can also make the electrons circulating around it tunnel through a wall created by strong nuclear binding energy and become unbound. Although the tunneling of say a soccer ball through a wall does not occur in reality, this kind of phenomenon can occur in the microscopic world. Such quantum mechanical behavior must be artificially controlled and measured to achieve a quantum computer.

The idea of using quantum states for a computer first came about in the 1980s. In 1985, David Deutsch, a professor at Oxford University and a proponent of quantum computers, wrote a paper titled "Quantum theory, the Church-Turing principle, and the universal quantum computer" that touched upon the possibility of quantum computers. And three years earlier, in 1982, Richard Feynman, of "Surely You're Joking, Mr. Feynman!" fame, made references to the possibility of quantum computing in his lectures and papers.

The possibility of applying quantum states to fields other than computing has also been considered. NEC, for example, is researching quantum cryptographic communications in addition to quantum computers. This form of communication deals with quantum states configured through the use of photons, the basic unit of light. Quantum cryptographic communications are similar to quantum computing in that they use the superposition state of quantum mechanics. The use of photons in quantum cryptographic communications is reasonable because information must traverse long distances for communication purposes. On the other hand, the utilization of various quantum media is considered for quantum computers (described later).

Is a "quantum" similar to an "atom" or "molecule"?

"Does the 'quantum' of 'quantum computer' consist of matter like an atom or molecule"? - I was asked this question a number of times by visitors at last year's iEXPO event sponsored by NEC. In Japanese, the word for "quantum:ryousi" and the words for "atom:gensi" and "molecule:bunsi" are all suffixed by the same Chinese character (: meaning something small), and for Japanese speakers, this may be misleading. The concept behind "quantum," however, differs from that of atoms and molecules? the former cannot be compared with the latter. In the microworld of atoms and molecules, physical quantities that express various properties of matter take on discrete values even though such quantities appear to be continuous in daily life. The smallest unit of such a jump in value is called a "quantum." Energy, as well, takes on discrete values called "energy levels" in the microworld. The smallest value of this jump in energy is called an "energy quantum." The physical laws that govern this microworld are dictated by quantum mechanics, and a "quantum computer" is a computer whose principles of operation are governed by quantum mechanics.

Richard Feynman

Cowinner of the 1965 Nobel Prize for Physics with Tomonaga Shinichiro and Julian Schwinger for his work on quantum-mechanical renormalization theory.