# Definitions of Terms in Quantum Science

**Bit:** Smallest possible information unit. It is mostly represented by the two values “0” and “1”.

**Photon:**

Light consists of myriads of massless particles which are also called photons. Each photon has a particular color which is determined by the wavelength. Although they have no mass, photons can, when they encounter matter, cause it to move through their impulse. Photons are therefore revealed as a wave or as particles depending on the situation. Photons never collide with other photons. They do not see each other. Isolating or generating a single photon is an experimental challenge.

**Quantum cryptography:**

The use of quantum physics effects for innovative encryption techniques. In this way, thanks to superposition a link can be set up between sender and recipient in which any eavesdropping operation is inevitably noticed.

**Quantum computer:** A computer that processes quantum information. Thanks to superposition and entanglement, it can test lots of possibilities for solving a problem at the same time and thereby arrive at the result faster than a conventional computer. Admittedly, this does not work for all types of computation. The quantum computer will therefore probably not be used as a universal computer (as today’s computers are).

**Quantum dot:** A tiny piece of semiconductor (a few nanometers in diameter), which is surrounded by a shell made from another semiconductor material, comparable to a cherry stone and the fruit flesh surrounding it. The inner semiconductor forms, in a similar way to an atom, discrete energy levels for electrons. Consequently, quantum dots are also regarded as “artificial atoms”. In quantum technology, they are to be used as miniaturized sources for single photons as are of interest, for example, for quantum cryptography.

**Quantum information:** Information which is stored in a quantum physical system. Such information cannot be described with the laws of conventional information theory. It forms the basis of applications such as the quantum computer or quantum cryptography. The unit of quantum information is the qubit.

**Quantum coherence:** A relationship between the parts of a quantum system. Without quantum coherence, for example, the qubits in a quantum computer cannot perform any calculations. Quantum coherence is comparable with the interference capability of beams of light: only if the phases of two light sources have a permanent and constant (over time) relationship with one another can the two waves be constructively or destructively superimposed, so-called quantum interference.

**Coherence time:** The length of time for which the quantum coherence is maintained. Only within the coherence time are calculations possible on a quantum computer. Individual computation steps should therefore only last for tiny fractions of what already is mostly a short coherence time, so that more complex tasks become solvable. The coherence time varies for different types of qubits between a few millionths of a second through to several seconds.

**Qubit:** Smallest storage unit for quantum information. It is brought about through two-state quantum systems, e.g. two polarization directions of photons. Unlike the conventional bit, which can either assume the value 0 or the value 1, a qubit can store “mixtures” of the values 0 and 1, i.e. 70% “0” and 30% “1”, for example. It can therefore assume an infinite number of values. This capacity for parallel processing of two diametrical information units forms the basis for the potential capacity of quantum computers.

**Spin:** The English expression for “turning” or “twisting”. What is meant is a type of precession of particles. The spin can act as a carrier of quantum information. Superposition A particle or some other quantum system, e.g. the electrons in a superconductor, exists simultaneously in all its possible physical states. Only the measurement of a variable, e.g. the position, singles out a particular state out of all those possible. The selection here is purely random. Superconduction: If electrons move through a conductor, they are slowed down due to friction. If one cools special materials to very low temperatures, electrons move in pairs through the system without friction.

**Entanglement:** According to the rules of quantum physics, particles can be so closely connected to each other that the measurement of one instantly influences the state of the other, even if the distance between the two particles is very large. This effect has already been proven for photons situated 150 km away from one another.