Quantum channel
In quantum information theory, a quantum channel is a communication channel which can transmit quantum information, as well as classical information. An example of quantum information is the state of a qubit. An example of classical information is a text document transmitted over the Internet. More formally, quantum channels are completely positive (CP) trace-preserving maps between spaces of operators. In other words, a quantum channel is just a quantum operation viewed not merely as the reduced dynamics of a system but as a pipeline intended to carry quantum information. (Some authors use the term "quantum operation" to  also include trace-decreasing maps while reserving "quantum channel" for strictly trace-preserving maps.) Memoryless quantum channel. We will assume for the moment that all state spaces of the systems considered, classical or quantum, are finite-dimensional. The memoryless in the section title carries the same meaning as in classical information theory: the output of a channel at a given time depends only upon the corresponding input and not any previous ones. Schrödinger picture. Consider quantum channels that transmit only quantum information. This is precisely a quantum operation, whose properties we now summarize. Let formula_1 and formula_2 be the state  spaces (finite-dimensional Hilbert spaces) of the sending and receiving ends, respectively, of a channel. formula_3 will denote the family of operators on formula_1. In the Schrödinger picture, a purely quantum channel is a map formula_5 between density matrices acting on formula_1 and formula_2 with the following properties: The adjectives completely positive and trace preserving used to describe a map are sometimes abbreviated CPTP. In the literature, sometimes the fourth property is weakened so that formula_5 is only required to be not trace-increasing. In this article, it will be assumed that all channels are CPTP. Heisenberg picture. Density matrices  acting on "H" only constitute a proper subset of the operators on "H" and same can be said for system "B". However, once a linear map formula_5 between the density matrices is specified, a standard linearity argument, together with the finite-dimensional assumption, allow us to extend formula_5 uniquely to the full space of operators. This leads to the adjoint map formula_5, which describes the action of formula_5 in the Heisenberg picture: The spaces of operators "L"("H") and "L"("H") are Hilbert spaces with the Hilbert–Schmidt inner product. Therefore, viewing formula_19 as a map between Hilbert spaces, we obtain its  adjoint formula_5 given by While formula_5 takes states on "A" to those on "B", formula_5 maps observables on system "B" to observables on "A". This relationship is same as that between the Schrödinger and Heisenberg descriptions of dynamics. The measurement statistics remain unchanged whether the observables are considered fixed while the states undergo operation or vice versa. It can be directly checked that if formula_5 is assumed to be trace preserving, formula_5 is unital, that is,formula_5("I") = "I". Physically speaking, this means that, in the Heisenberg picture, the trivial observable remains trivial after applying the channel. Classical information. So  far we have only defined quantum channel that transmits only quantum information. As stated in the introduction, the input and output of a channel can include classical information as well. To describe this, the formulation given so far needs to be generalized somewhat. A purely quantum channel, in the Heisenberg picture, is a linear map Ψ between spaces of operators: that is unital and completely positive (CP). The operator spaces can be viewed as finite-dimensional C*-algebras. Therefore, we can say a channel is a unital CP map between C*-algebras: Classical information can then be included in this formulation. The  observables of a classical system can be assumed to be a commutative C*-algebra, i.e. the space of continuous functions formula_29 on some set formula_30. We assume formula_30 is finite so formula_29 can be identified with the "n"-dimensional Euclidean space formula_33 with entry-wise multiplication. Therefore, in the Heisenberg picture, if the classical information is part of, say, the input, we would define formula_34 to include the relevant classical observables. An example of this would be a channel Notice formula_36 is still a C*-algebra. An element formula_37 of a C*-algebra formula_38 is called positive if formula_39 for some formula_40. Positivity of  a map is defined accordingly. This characterization is not universally accepted; the quantum instrument is sometimes given as the generalized mathematical framework for conveying both quantum and classical information. In axiomatizations of quantum mechanics, the classical information is carried in a Frobenius algebra or Frobenius category. Examples. States. A state, viewed as a mapping from observables to their expectation values, is an immediate example of a channel. Time evolution. For a purely quantum system, the time evolution, at certain time "t", is given by where formula_42 and "H" is the Hamiltonian and "t" is the time. Clearly this gives a  CPTP map in the Schrödinger picture and is therefore a channel. The dual map in the Heisenberg picture is Restriction. Consider a composite quantum system with state space formula_44 For a state the reduced state of "ρ" on system "A", "ρ", is obtained by taking the partial trace of "ρ" with respect to the "B" system: The partial trace operation is a CPTP map, therefore a quantum channel in the Schrödinger picture. In the Heisenberg picture, the dual map of this channel is where "A" is an observable of system "A". Observable. An observable associates a numerical value formula_48 to  a quantum mechanical "effect" formula_49. formula_49's are assumed to be positive operators acting on appropriate state space and formula_51. (Such a collection is called a POVM.) In the Heisenberg picture, the corresponding "observable map" formula_52 maps a classical observable to the quantum mechanical one In other words, one integrate "f" against the POVM to obtain the quantum mechanical observable. It can be easily checked that formula_52 is CP and unital. The corresponding Schrödinger map formula_52 takes density matrices to classical states: where the inner product is the Hilbert–Schmidt inner product. Furthermore, viewing states as normalized functionals, and invoking  the Riesz representation theorem, we can put Instrument. The observable map, in the Schrödinger picture, has a purely classical output algebra and therefore only describes measurement statistics. To take the state change into account as well, we define what is called a quantum instrument. Let formula_59 be the effects (POVM) associated to an observable. In the Schrödinger picture, an instrument is a map formula_60 with pure quantum input formula_61 and with output space formula_62: Let The dual map in the Heisenberg picture is where formula_66 is defined in the following way: Factor formula_67 (this can always be done since elements  of a POVM are positive) then formula_68. We see that formula_52 is CP and unital. Notice that formula_70 gives precisely the observable map. The map describes the overall state change. Measure-and-prepare channel. Suppose two parties "A" and "B" wish to communicate in the following manner: "A" performs the measurement of an observable and communicates the measurement outcome to "B" classically. According to the message he receives, "B" prepares his (quantum) system in a specific state. In the Schrödinger picture, the first part of the channel formula_5 simply consists of "A" making a measurement, i.e. it is the observable  map: If, in the event of the "i"-th measurement outcome, "B" prepares his system in state "R", the second part of the channel formula_5 takes the above classical state to the density matrix The total operation is the composition Channels of this form are called "measure-and-prepare" or in Holevo form. In the Heisenberg picture, the dual map formula_77 is defined by A measure-and-prepare channel can not be the identity map. This is precisely the statement of the no teleportation theorem, which says classical teleportation (not to be confused with entanglement-assisted teleportation) is impossible. In other  words, a quantum state can not be measured reliably. In the channel-state duality, a channel is measure-and-prepare if and only if the corresponding state is separable. Actually, all the states that result from the partial action of a measure-and-prepare channel are separable, and for this reason measure-and-prepare channels are also known as entanglement-breaking channels. Pure channel. Consider the case of a purely quantum channel formula_52 in the Heisenberg picture. With the assumption that everything is finite-dimensional, formula_52 is a unital CP map between spaces of matrices By Choi's theorem on  completely positive maps, formula_52 must take the form where "N" ≤ "nm". The matrices "K" are called Kraus operators of formula_52 (after the German physicist Karl Kraus, who introduced them). The minimum number of Kraus operators is called the Kraus rank of formula_52. A channel with Kraus rank 1 is called pure. The time evolution is one example of a pure channel. This terminology again comes from the channel-state duality. A channel is pure if and only if its dual state is a pure state. Teleportation. In quantum teleportation, a sender wishes to transmit an arbitrary quantum state of  a particle to a possibly distant receiver. Consequently, the teleportation process is a quantum channel. The apparatus for the process itself requires a quantum channel for the transmission of one particle of an entangled-state to the receiver. Teleportation occurs by a joint measurement of the sent particle and the remaining entangled particle. This measurement results in classical information which must be sent to the receiver to complete the teleportation. Importantly, the classical information can be sent after the quantum channel has ceased to exist. In the experimental setting. Experimentally, a simple implementation of a quantum channel is fiber optic  (or free-space for that matter) transmission of single photons. Single photons can be transmitted up to 100 km in standard fiber optics before losses dominate. The photon's time-of-arrival ("time-bin entanglement") or polarization are used as a basis to encode quantum information for purposes such as quantum cryptography. The channel is capable of transmitting not only basis states (e.g. |0>, |1>) but also superpositions of them (e.g. |0>+|1>). The coherence of the state is maintained during transmission through the channel. Contrast this with the transmission of electrical pulses through wires (a classical channel ), where only classical information (e.g. 0s and 1s) can be sent. Channel capacity. The cb-norm of a channel. Before giving the definition of channel capacity, the preliminary notion of the norm of complete boundedness, or cb-norm of a channel needs to be discussed. When considering the capacity of a channel formula_60, we need to compare it with an "ideal channel" formula_87 . For instance, when the input and output algebras are identical, we can choose formula_87 to be the identity map. Such a comparison requires a metric between channels. Since a channel can be viewed as a linear operator , it is tempting to use the natural operator norm. In other words, the closeness of formula_60 to the ideal channel formula_87 can be defined by However, the operator norm may increase when we tensor formula_60 with the identity map on some ancilla. To make the operator norm even a more undesirable candidate, the quantity may increase without bound as formula_94 The solution is to introduce, for any linear map formula_60 between C*-algebras, the cb-norm Definition of channel capacity. The mathematical model of a channel used here is same as the classical one. Let formula_97 be a channel in the  Heisenberg picture and formula_98 be a chosen ideal channel. To make the comparison possible, one needs to encode and decode Φ via appropriate devices, i.e. we consider the composition where "E" is an encoder and "D" is a decoder. In this context, "E" and "D" are unital CP maps with appropriate domains. The quantity of interest is the "best case scenario": with the infimum being taken over all possible encoders and decoders. To transmit words of length "n", the ideal channel is to be applied "n" times, so we consider the tensor power The formula_102 operation describes "n" inputs undergoing  the operation formula_103 independently and is the quantum mechanical counterpart of concatenation. Similarly, "m invocations of the channel" corresponds to formula_104. The quantity is therefore a measure of the ability of the channel to transmit words of length "n" faithfully by being invoked "m" times. This leads to the following definition: A sequence formula_112 can be viewed as representing a message consisting of possibly infinite number of words. The limit supremum condition in the definition says that, in the limit, faithful transmission can be achieved by invoking the channel no more than "r" times the length of a word. One  can also say that "r" is the number of letters per invocation of the channel that can be sent without error. The channel capacity of formula_52 with respect to formula_103, denoted by formula_115 is the supremum of all achievable rates. From the definition, it is vacuously true that 0 is an achievable rate for any channel. Important examples. As stated before, for a system with observable algebra formula_34, the ideal channel formula_103 is by definition the identity map formula_118. Thus for a purely "n" dimensional quantum system, the ideal channel is the identity map on the space of "n" ×  "n" matrices formula_119. As a slight abuse of notation, this ideal quantum channel will be also denoted by formula_119. Similarly, a classical system with output algebra formula_121 will have an ideal channel denoted by the same symbol. We can now state some fundamental channel capacities. The channel capacity of the classical ideal channel formula_121 with respect to a quantum ideal channel formula_119 is This is equivalent to the no-teleportation theorem: it is impossible to transmit quantum information via a classical channel. Moreover, the following equalities hold: The above says, for instance, an ideal quantum channel is no more efficient  at transmitting classical information than an ideal classical channel. When "n" = "m", the best one can achieve is "one bit per qubit". It is relevant to note here that both of the above bounds on capacities can be broken, with the aid of entanglement. The entanglement-assisted teleportation scheme allows one to transmit quantum information using a classical channel. Superdense coding. achieves "two bit per qubit". These results indicate the significant role played by entanglement in quantum communication. Classical and quantum channel capacities. Using the same notation as the previous subsection, the classical capacity of a channel Ψ is  that is, it is the capacity of Ψ with respect to the ideal channel on the classical one-bit system formula_127. Similarly the quantum capacity of Ψ is where the reference system is now the one qubit system formula_129. Channel fidelity. Another measure of how well a quantum channel preserves information is called channel fidelity, and it arises from fidelity of quantum states. Bistochastic quantum channel. A bistochastic quantum channel is a quantum channel formula_130 which is unital, i.e. formula_131.