Considering extensive studies on network coding and quantum information theory in this century, it was just a matter of time that the study of quantum network coding was initiated. In some sense, it is rather important to ask if the amount of quantum bits communicated on a network can be reduced by allowing encoding at intermediate nodes on the network since quantum communication is much more expensive than classical communication in principle. The first result by Hayashi et al. in 2006 was basically negative for this question - quantum network coding does not give any benefit for reducing the amount of qubits sent on the butterfly network. However, this was just the first step to explore the theory of quantum network coding. After their work, it has been shown that the amount of communicated qubits can be reduced in several changes of the settings such as by allowing extra classical communication, while there has been further negative results. In this paper, we review the basic concept of quantum network coding, and report the currently known positive and negative results on quantum network coding in various settings.
Considering extensive studies on network coding and quantum information theory in this century, it was just a matter of time that the study of quantum network coding was initiated. In some sense, it is rather important to ask if the amount of quantum bits communicated on a network can be reduced by allowing encoding at intermediate nodes on the network since quantum communication is much more expensive than classical communication in principle. The first result by Hayashi et al. in 2006 was basically negative for this question - quantum network coding does not give any benefit for reducing the amount of qubits sent on the butterfly network. However, this was just the first step to explore the theory of quantum network coding. After their work, it has been shown that the amount of communicated qubits can be reduced in several changes of the settings such as by allowing extra classical communication, while there has been further negative results. In this paper, we review the basic concept of quantum network coding, and report the currently known positive and negative results on quantum network coding in various settings.
Nowadays, network coding has been a very successful topic in information theory including many applications such as wireless networks while one of the simplest benefits is the efficient transmission in networks by allowing us to encode the transmitted information at every intermediate node. Since quantum information is much more expensive for communication than classical information, it was natural that this benefit made quantum researchers motivate the study of “quantum network coding.” This paper reports the current status of quantum network coding. At present, quantum network coding mostly means sending quantum information (rather than classical information) in a quantum network. Since quantum information cannot be cloned (the quantum no-cloning theorem), multiple unicast networks have been well-studied (in several settings). We present some of the known possibilities and limitations, and future works of quantum network coding, focusing on multiple unicast networks.
Network coding is often explained by using a small network model called Butterfly. In this network, there are two flow paths, s_1 to t_1 and s_2 to t_2, which share a single bottleneck channel of capacity one. So, if we consider conventional flow (of liquid, for instance), then the total amount of flow must be at most one in total, say 1/2 for each path. However, if we consider information flow, then we can send two bits (one for each path) at the same time by exploiting two side links, which are of no use for the liquid-type flow, and encoding/decoding operations at each node. This is known as network coding and has been quite popular since its introduction by Ahlswede, Cai, Li and Yeung in 2000. In QIP 2006, Hayashi et al showed that quantum network coding is possible for Butterfly, namely we can send two qubits simultaneously with keeping their fidelity strictly greater than 1/2. In this paper, we show that the result can be extended to a large class of general graphs by using a completely different approach. The underlying technique is a new cloning method called entanglement-free cloning which does not produce any entanglement at all. This seems interesting on its own and to show its possibility is an even more important purpose of this paper. Combining this new cloning with approximation of general quantum states by a small number of fixed ones, we can design a quantum network coding protocol which ``simulates'' its classical counterpart for the same graph.
Since quantum information is continuous, its handling is sometimes surprisingly harder than the classical counterpart.A typical example is cloning; making a copy of digital information is straightforward but it is not possible exactly for quantum information.The question in this paper is whether or not quantum network coding is possible.Its classical counterpart is another good example to show that digital information flow can be done much more efficiently than conventional (say, liquid) flow.Our answer to the question is similar to the case of cloning, namely, it is shown that quantum network coding is possible if approximation is allowed, by using a simple network model called Butterfly.In this network, there are two flow paths, s1 to t1 and s2 to t2 , which shares a single bottleneck channel of capacity one.In the classical case, we can send two bits simultaneously, one for each path, in spite of the bottleneck.Our results for quantum network coding include: (i) We can send any quantum state ∣ψ1⟩ from s1 to t1 and ∣ψ2⟩ from s2 to t2 simultaneously with a fidelity strictly greater than 1/2.(ii) If one of ∣ψk⟩ and ∣ψ2⟩ is classical, then the fidelity can be improved to 2/3.(iii) Similar improvement is also possible if Cbi⟩ and 1Cb2⟩ are restricted to only a finite number of (previously known) states.This allows us to design an interesting protocol which can send two classical bits from s1 to t1 (similarly from s2 to t2 ) but only one of them should be recovered.
This paper considers the problem of efficiently transmitting quantum states through a network. It has been known for some time that without additional assumptions it is impossible to achieve this task perfectly in general — indeed, it is impossible even for the simple butterfly network. As additional resource we allow free classical communication between any pair of network nodes. It is shown that perfect quantum network coding is achievable in this model whenever classical network coding is possible over the same network when replacing all quantum capacities by classical capacities. More precisely, it is proved that perfect quantum network coding using free classical communication is possible over a network with k source-target pairs if there exists a classical linear (or even vector-linear) coding scheme over a finite ring. Our proof is constructive in that we give explicit quantum coding operations for each network node. This paper also gives an upper bound on the number of classical communication required in terms of k, the maximal fan-in of any network node, and the size of the network. x1 x2 s1 ♠ s2 ♠ ❍❍❍❍ ❥ x1 x2 ✟ n1 x1 x1 ⊕ x2 x2 n2 ♠ x1 ⊕ x2✟ ❍❍❍❍ ❥ x1 ⊕ x2
The k-pair problem in network coding theory asks to send k messages simultaneously between k source-target pairs over a directed acyclic graph. In a previous paper [ICALP 2009, Part I, pages 622-633] the present authors showed that if a classical k-pair problem is solvable by means of a linear coding scheme, then the quantum k-pair problem over the same graph is also solvable, provided that classical communication can be sent for free between any pair of nodes of the graph. Here we address the main case that remained open in our previous work, namely whether nonlinear classical network coding schemes can also give rise to quantum network coding schemes. This question is motivated by the fact that there are networks for which no linear solutions exist to the k-pair problem, whereas nonlinear solutions exist. In the present paper we overcome the limitation to linear protocols and describe a new communication protocol for perfect quantum network coding that improves over the previous one as follows: (i) the new protocol does not put any condition on the underlying classical coding scheme, that is, it can simulate nonlinear communication protocols as well, and (ii) the amount of classical communication sent in the protocol is significantly reduced.
The k-pair problem in network coding theory asks to send k messages simultaneously between k source-target pairs over a directed acyclic graph. In a previous paper [ICALP 2009, Part I, pages 622--633] the present authors showed that if a classical k-pair problem is solvable by means of a linear coding scheme, then the quantum k-pair problem over the same graph is also solvable, provided that classical communication can be sent for free between any pair of nodes of the graph. Here we address the main case that remained open in our previous work, namely whether nonlinear classical network coding schemes can also give rise to quantum network coding schemes. This question is motivated by the fact that there are networks for which there are no linear solutions to the k-pair problem, whereas nonlinear solutions exist. In the present paper we overcome the limitation to linear protocols and describe a new communication protocol for perfect quantum network coding that improves over the previous one as follows: (i) the new protocol does not put any condition on the underlying classical coding scheme, that is, it can simulate nonlinear communication protocols as well, and (ii) the amount of classical communication sent in the protocol is significantly reduced.