A question of quantum reality

Physicist Reinhold Bertlmann of the University of Vienna, Austria has published a review of the work of his late long-term collaborator John Stewart Bell of CERN, Geneva in EPJ H. This review, "Real or Not Real: that is the ...

The return of the spin echo

A research team from Garching and Vienna discovered a remarkable echo effect that offers exciting new possibilities for working with quantum information.

Anti-resonant hollow-core optical fiber reduces 'noise'

A new hollow optical fiber greatly reduces the "noise" interfering with the signals it transmits compared to the single-mode fibers now widely used, researchers at the University of Rochester report.

Beating noise via superposition of order

Information can successfully be transmitted through noisy channels using quantum mechanics, according to new research from The University of Queensland and Griffith University.

Quantum researchers create an error-correcting cat

Yale physicists have developed an error-correcting cat—a new device that combines the Schrödinger's cat concept of superposition (a physical system existing in two states at once) with the ability to fix some of the trickiest ...

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Quantum information

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:

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 a difference 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 dramatically - 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. Given a statistical ensemble of quantum mechanical systems with the density matrix S, it is given by

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

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