No teleportation theorem

No teleportation theorem

In quantum information theory, the no teleportation theorem states that quantum information cannot be measured with complete accuracy.

Formulation

The term "quantum information" refers to information stored in the state of a quantum system. Two quantum states "ρ"1 and "ρ"2 are identical if the measurement results of any physical observable have the same expectation value for "ρ"1 and "ρ"2. Thus measurement can be viewed as an information channel with quantum input and classical output, that is, performing measurement on a quantum system transforms quantum information into classical information. On the other hand, preparing a quantum state takes classical information to quantum information.

In general, a quantum state is described by a density matrix. Suppose one has a quantum system in some mixed state "ρ". Prepare an ensemble, of the same system, as follows:

#Perform a measurement on "ρ".
#According to the measurement outcome, prepare a system in some pre-specified state.

The no-teleportation theorem states that the result will be different from "ρ", irrespective of how the preparation procedure is related to measurement outcome. A quantum state cannot be determined via a single measurement. In other words, if a quantum channel is measurement followed by preparation, it cannot be the identity channel. Once converted to classical information, quantum information cannot be recovered.

In contrast, perfect transmission is possible if one wishes to convert classical information to quantum information then back to classical information. For classical bits, this can be done by encoding them in orthogonal quantum states, which can always be distinguished.

See also

Among other "no-go" theorems in quantum information are:
*No communication theorem. Entangled states cannot be used to transmit classical information superluminally.

*No cloning theorem. Quantum states cannot be copied.

*No broadcast theorem. A generalization of no cloning theorem (in the case of pure states).

With the aid of shared entanglement, quantum states can be teleported, see

*Quantum teleportation


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