It is natural in a quantum network system that multiple users intend to send their quantum message to their respective receivers, which is called a multiple unicast quantum network. We propose a canonical method to derive a secure quantum network code over a multiple unicast quantum network from a secure classical network code. Our code correctly transmits quantum states when there is no attack. It also guarantees the secrecy of the transmitted quantum state even with the existence of an attack when the attack satisfies a certain natural condition. In our security proof, the eavesdropper is allowed to modify wiretapped information dependently on the previously wiretapped messages. Our protocol guarantees the secrecy by utilizing one-way classical information transmission (public communication) in the same direction as the quantum network although the verification of quantum information transmission requires two-way classical communication. Our secure network code can be applied to several networks including the butterfly network.Index Terms secrecy, quantum state, network coding, multiple unicast, general network, one-way public communication secrecy, quantum state, network coding, multiple unicast, general network, one-way public communication
I. INTRODUCTIONIn order to realize quantum information processing protocols to overwhelm the conventional information technologies among multiple users, it is needed to build up a quantum network system among multiple users. For example, various quantum protocols, e.g., quantum blind computation [2], [3], quantum public key cryptography [1], and quantum money [4] require the transmission of quantum states. To meet the demand, the paper [5] initiated the study of quantum network coding with the butterfly network as a typical example. Under this example, the paper [6] clarified the importance of prior entanglement in a quantum network code by proposing a network code, which was experimentally implemented recently [7]. Kobayashi et al. [8] discussed a method for generating GHZ-type states via quantum network coding. Leung et al. [9] investigated several types of networks when classical communication is allowed. Based on these studies, Kobayashi et al. [10] made a code to transmit quantum states based on a linear classical network code. Then, Kobayashi et al. [11] generalized the result to the case with non-linear network codes. These studies [8], [9], [10], [11], [12] clarified that quantum network coding is needed among multiple users for efficient transmission of the quantum states over a quantum network. However, these existing studies did not discuss the security for quantum network codes when an adversary attacks the quantum network.Since the improvement of the security is one of the most essential requirements for developing quantum networks, the security analysis is strongly required for quantum network codes. Indeed, it is possible to check the security in these existing methods by verifying the non-existence of the eavesdropper. However, the verification requires us...