In quantum mechanics, **quantum information** is physical information that is held in the "state" of a quantum system. The most popular unit of quantum information is the qubit, a two-level quantum system. However, unlike classical digital states (which are discrete), a two-state quantum system can actually be in a superposition of the two states at any given time.

Quantum information differs from classical information in several respects, among which we note the following:

- It cannot be read without the state becoming the measured value,
- An arbitrary state cannot be cloned,
- The state may be in a superposition of basis values.

However, despite this, the amount of information that can be retrieved in a single qubit is equal to one bit. It is in the *processing* of information (quantum computation) that the differentiation occurs.

The ability to manipulate quantum information enables us to perform tasks that would be unachievable in a classical context, such as unconditionally secure transmission of information. Quantum information processing is the most general field that is concerned with quantum information. There are certain tasks which classical computers cannot perform "efficiently" (that is, in polynomial time) according to any known algorithm. However, a quantum computer can compute the answer to some of these problems in polynomial time; one well-known example of this is Shor's factoring algorithm. Other algorithms can speed up a task less dramaticallyâ€”for example, Grover's search algorithm which gives a quadratic speed-up over the best possible classical algorithm.

Quantum information, and changes in quantum information, can be quantitatively measured by using an analogue of Shannon entropy, called the von Neumann entropy. Given a statistical ensemble of quantum mechanical systems with the density matrix, it is given by

Many of the same entropy measures in classical information theory can also be generalized to the quantum case, such as Holevo entropy and the conditional quantum entropy.

Read more about Quantum Information: Quantum Information Theory

### Other articles related to "quantum, quantum information, information":

... and the co-founder and current director of the Institute for

**Quantum**Computing at the University of Waterloo ... Laflamme is currently a Canada Research Chair in

**Quantum Information**... significant scientific works have been his work in

**quantum**computing and

**quantum information**theory ...

... the inherent difficulties of simulating general

**quantum**many-body systems, the exponential increase in parameters with the size of the system, and correspondingly, the high computational costs, one solution would be ... parameters used to fully characterize a

**quantum**many-body system is seriously impeded by the lavishly exponential buildup with the system size of the amount of variables needed for simulation, which leads ... and put into practice in the course of time, one of the most successful ones being the

**quantum**Monte Carlo method (QMC) ...

...

**Quantum**coherence with a single Cooper pair "

**Quantum**Coherence with a Single Cooper Pair" ... "Coherent control of macroscopic

**quantum**states in a single-Cooper-pair box" ... "

**Quantum**-state engineering with Josephson-junction devices" ...

**Quantum Information**Theory

... The theory of

**quantum information**is a result of the effort to generalize classical

**information**theory to the

**quantum**world ...

**Quantum information**theory aims to investigate the following question What happens if

**information**is stored in a state of a

**quantum**system? One of the strengths of classical

**information**... because it is always possible to efficiently transform

**information**from one representation to another ...

... Markus Aspelmeyer is an Austrian

**quantum**physicist ... and then, Junior, later Senior Researcher, at the Institute for

**Quantum**Optics and

**Quantum Information**(IQOQI) of the Austrian Academy of Sciences where he is leading a research team working on

**quantum**... His research interests are

**quantum**entanglement and

**quantum**optics ...

### Famous quotes containing the words information and/or quantum:

“Phenomenal nature shadows him wherever he goes. Clouds in the staring sky transmit to one another, by means of slow signs, incredibly detailed *information* regarding him. His inmost thoughts are discussed at nightfall, in manual alphabet, by darkly gesticulating trees. Pebbles or stains or sunflecks form patterns representing in some awful way messages which he must intercept. Everything is a cipher and of everything he is the theme.”

—Vladimir Nabokov (1899–1977)

“A personality is an indefinite *quantum* of traits which is subject to constant flux, change, and growth from the birth of the individual in the world to his death. A character, on the other hand, is a fixed and definite *quantum* of traits which, though it may be interpreted with slight differences from age to age and actor to actor, is nevertheless in its essentials forever fixed.”

—Hubert C. Heffner (1901–1985)