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Blueprint for a robust quantum future

Claiming that something has a defect normally suggests an undesirable feature. That's not the case in solid-state systems, such as the semiconductors at the heart of modern classical electronic devices. They work because ...

Quantum steering for more precise measurements

Quantum systems consisting of several particles can be used to measure magnetic or electric fields more precisely. A young physicist at the University of Basel has now proposed a new scheme for such measurements that uses ...

Researchers establish the first entanglement-based quantum network

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Researchers realize coherent storage of light over one hour

Remote quantum distribution on the ground is limited because of the loss of photons in optical fibers. One solution for remote quantum communication lies in quantum memories: photons are stored in long-lived quantum memory ...

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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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