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 quantum superposition|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 no cloning theorem|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 algorithm|Shor's factoring algorithm. Other algorithms can speed up a task less dramatically—for example, Grover's algorithm|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 Claude Shannon|Shannon Information entropy|entropy, called the John von Neumann|von Neumann Von Neumann entropy|entropy. Given a statistical ensemble of quantum mechanical systems with the density matrix \rho, it is given by: S(\rho) = -\operatorname{Tr}(\rho \ln \rho). \,Many of the same entropy measures in classical information theory can also be generalized to the quantum case, such as and the conditional quantum entropy.

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 quantum superposition|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 no cloning theorem|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...