Editing Quantum information (section)

== Applications ==
===Quantum communication===

[[Quantum information science|Quantum communication]] is one of the applications of quantum physics and quantum information. There are some famous theorems such as the no-cloning theorem that illustrate some important properties in quantum communication. [[Superdense coding|Dense coding]] and [[quantum teleportation]] are also applications of quantum communication. They are two opposite ways to communicate using qubits. While teleportation transfers one qubit from Alice and Bob by communicating two classical bits under the assumption that Alice and Bob have a pre-shared [[Bell state]], dense coding transfers two classical bits from Alice to Bob by using one qubit, again under the same assumption, that Alice and Bob have a pre-shared Bell state.

===Quantum key distribution===

{{Main|Quantum key distribution|l1 = Quantum key distribution}}

One of the best known applications of quantum cryptography is [[quantum key distribution]] which provide a theoretical solution to the security issue of a classical key. The advantage of quantum key distribution is that it is impossible to copy a quantum key because of the [[no-cloning theorem]]. If someone tries to read encoded data, the quantum state being transmitted will change. This could be used to detect eavesdropping.

====BB84====

The first quantum key distribution scheme, [[BB84]], was developed by Charles Bennett and [[Gilles Brassard]] in 1984. It is usually explained as a method of securely communicating a private key from a third party to another for use in one-time pad encryption.<ref name="Nielsen2010" />

====E91====

[[E91 protocol|E91]] was made by [[Artur Ekert]] in 1991. His scheme uses entangled pairs of photons. These two photons can be created by Alice, Bob, or by a third party including eavesdropper Eve. One of the photons is distributed to Alice and the other to Bob so that each one ends up with one photon from the pair.

This scheme relies on two properties of quantum entanglement:

# The entangled states are perfectly correlated which means that if Alice and Bob both measure their particles having either a vertical or horizontal polarization, they always get the same answer with 100% probability. The same is true if they both measure any other pair of complementary (orthogonal) polarizations. This necessitates that the two distant parties have exact directionality synchronization. However, from quantum mechanics theory the quantum state is completely random so that it is impossible for Alice to predict if she will get vertical polarization or horizontal polarization results.
# Any attempt at eavesdropping by Eve destroys this quantum entanglement such that Alice and Bob can detect.

====B92====

B92 is a simpler version of BB84.<ref name="Bennett1992"/>

The main difference between B92 and BB84:

* B92 only needs two states
* BB84 needs 4 polarization states

Like the BB84, Alice transmits to Bob a string of photons encoded with randomly chosen bits but this time the bits Alice chooses the bases she must use. Bob still randomly chooses a basis by which to measure but if he chooses the wrong basis, he will not measure anything which is guaranteed by quantum mechanics theories. Bob can simply tell Alice after each bit she sends whether he measured it correctly.<ref name="Haitjema2007"/>

===Quantum computation===

{{Main|Quantum computing}}

The most widely used model in quantum computation is the [[quantum circuit]], which are based on the quantum bit "[[qubit]]". Qubit is somewhat analogous to the [[bit]] in classical computation. Qubits can be in a 1 or 0 [[quantum state]], or they can be in a [[Quantum superposition|superposition]] of the 1 and 0 states. However, when qubits are measured, the result of the measurement is always either a 0 or a 1; the [[Probability|probabilities]] of these two outcomes depend on the [[quantum state]] that the qubits were in immediately prior to the measurement.

Any quantum computation algorithm can be represented as a network of [[quantum logic gate]]s.

===Quantum decoherence===

{{Main|Quantum decoherence}}

If a quantum system were perfectly isolated, it would maintain coherence perfectly, but it would be impossible to test the entire system. If it is not perfectly isolated, for example during a measurement, coherence is shared with the environment and appears to be lost with time; this process is called quantum decoherence. As a result of this process, quantum behavior is apparently lost, just as energy appears to be lost by friction in classical mechanics.

===Quantum error correction===

{{Main|Quantum error correction}}

'''QEC''' is used in [[Quantum computer|quantum computing]] to protect quantum information from errors due to [[decoherence]] and other [[quantum noise]]. Quantum error correction is essential if one is to achieve fault-tolerant quantum computation that can deal not only with noise on stored quantum information, but also with faulty quantum gates, faulty quantum preparation, and faulty measurements.

[[Peter Shor]] first discovered this method of formulating a ''quantum error correcting code'' by storing the information of one qubit onto a highly entangled state of [[Ancilla bit|ancilla qubits]]. A quantum error correcting code protects quantum information against errors.