## Introduction to Quantum Physics and its Applications

Thursday 11 June 2009

Classical physics is adequate for the description of macroscopic objects. It applies basically 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 XIXth century.

At that time, the fact that classical physics did not always provide an adequate description of physical phenomena became clear. A radically new set of theories — quantum physics — was consequently developed by physicists such as Max Planck and Albert Einstein, during the first thirty years of the XXth century. Quantum physics describes adequately 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. For instance quantum physics is intrinsically random, while classical physics is deterministic. It also imposes limitation on the accuracy of the measurements that can be performed on a system (Heisenberg’s uncertainty principle).

Although quantum physics had a strong influence on the technological development of the XXth century — it allowed for example the invention of the transistor or the laser — its impact on the processing of information has only been understood recently. “*Quantum information processing*” is a new and dynamic research field at the crossroads of quantum physics and computer science. It looks at the consequence of encoding digital bits — the elementary units of information — on quantum objects. Does it make a difference if a bit is written on a piece of paper, stored in an electronic chip, or encoded on a single electron? Applying quantum physics to information processing yields revolutionary properties and possibilities, without any equivalent in conventional information theory. In order to emphasize this difference, a digital bit is called a quantum bit or a "qubit" in this context. With the miniaturization of microprocessors, which will reach the quantum limit in the next fifteen to twenty years, this new field will necessarily gain prominence. Its ultimate goal is the development of a fully quantum computer, featuring massively parallel processing capabilities.

Although this goal is still quite distant, the first applications of quantum information processing are already commercialized by companies including id Quantique:

Quantum Random Number Generators (QRNG)

Quantum Key Distribution

The first one exploits the fundamentally random nature of quantum physics to produce high quality random numbers, for cryptographic applications for example. id Quantique’s Quantis QRNG is the first commercial product based on this principle.

The second application, called quantum cryptography, exploits Heisenberg’s uncertainty principle to allow two remote parties to exchange a cryptographic key. It is the main focus of these articles.

Readers with no cryptographic knowledge, should read the Cryptography introduction. Other readers should continue with the Key Distribution article, that discusses the problem of securely distributing keys.