# Quantum information science

The quantum computer science or quantum information processing is the science of information processing with information carriers, make use of quantum mechanical phenomena. These differ in essential properties of classical information carriers, creating new perspectives. So can thus be carried out much faster some calculations, as it is possible by means of conventional computers.

The classical information processing always uses macroscopic number of particles for representing a state. Although subject to the individual particles quantum mechanical laws, but their quantum mechanical nature can be at macroscopically many particles neglected due to the correspondence principle.

## Quantum Information

In quantum computer science, quantum information replaces the classical information. Analogous to the bits of classical information is available in the quantum information is also a smallest unit, the qubit. This is a quantum mechanical two-level system.

In quantum computer science the quantum properties of a system of qubits can be exploited. In addition to the superposition of this particular entanglement, which can be interpreted as interference of various basis states.

Due to the complementarity and the associated uncertainty of the quantum state of the qubit can not be completely read out. Rather, each reading of a qubit leads to a collapse of the wave function, so that ultimately only one classical bit is read. For this reason, work quantum algorithms generally probabilistic, that is, a run provides only with a certain ( high as possible ) probability of the desired result.

## Quantum communication

An important application of quantum computer science is quantum communications. In this quantum information is sent via quantum channels between nodes of a quantum network. One way to quantum information transfer is the use of quantum teleportation. Quantum communication in particular allows the secure encryption of messages sent by quantum cryptography, but could also be networked quantum computers (see next section ) can be used.

## Quantum computer

The most ambitious goal of quantum computer science is the development of a quantum computer, which can be used for practical tasks. Such could be due to the quantum parallelism certain tasks for which a classical computer takes a long time to calculate in a much shorter time. An example of the extreme acceleration of the solution of certain problems is the Shor algorithm for the decomposition of the product of two primes into its factors. This algorithm has a special relevance, since the security of widely used RSA encryption method just depends on the difficulty of this decomposition.

Similar to classical computers work even quantum computers with discrete operations that affect only a limited number of qubits. Such operations are called quantum gates.

One problem with the development of quantum computers is decoherence, transferred the quantum states into classical random distributions. For the compensation you need special error correction methods that do not involve the measurement of the qubits, as this measurement would in turn destroy the quantum state. These methods are referred to as a quantum error correction.