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Classical physics is adequate for the description of macroscopic objects. It applies to systems larger than one micron (1 micron = 1 millionth of a meter). It was developed gradually and was basically complete by the end of the 19th century.

A radically new set of theories – quantum physics – was developed by physicists such as Max Planck and Albert Einstein during the first thirty years of the 20th century. Quantum physics describes the microscopic world (molecules, atoms, elementary particles), while classical physics remains accurate for macroscopic objects. The predictions of quantum physics drastically differ from those of classical physics.

Although quantum physics had a strong influence on the technological development of the 20th century – it allowed for example the invention of the transistor or the laser – its impact on the processing of information has only been understood more recently.

The first applications of quantum information processing have already been commercialized by ID Quantique (IDQ). The first one, the generation of random numbers, will only be briefly mentioned in this paper. It exploits the fundamentally random nature of quantum physics to produce high quality random numbers. IDQ’s Quantis random number generator was the first commercial product based on this principle. It has been used in security, online gaming and other applications since 2001.

The second application – the main focus of this paper – is called quantum cryptography. It exploits Heisenberg’s uncertainty principle, which prevent an eavesdropper from discovering the exact quantum state of a system, to allow two remote parties to exchange a cryptographic key in a provably secure manner.

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