Retrieving and Routing Quantum Information in a Quantum Network

In extant quantum secret sharing protocols, once the secret is shared in a quantum network textsc{qnet} it can not be retrieved back, even if the dealer wishes that her secret no longer be available in the network. For instance, if the dealer is part of two \textsc{qnet}s, say mathcal{Q}₁ and mathcal{Q}₂ and subsequently finds that mathcal{Q}₂ is more reliable than mathcal{Q}₁, the dealer may wish to transfer all her secrets from mathcal{Q}₁ to mathcal{Q}₂. In this work we address this problem by designing a protocol that enables the source/dealer to bring back the information shared in the network, if desired. Unlike classical revocation, no-cloning-theorem automatically ensures that the secret is no longer shared in the network. The implications of our results are multi-fold. One interesting implication of our technique is the possibility of routing qubits in asynchronous \textsc{qnets}. By asynchrony we mean that the requisite data/resources are intermittently available (but not necessarily simultaneously) in the \textsc{qnet}. For example, we show that a source S can send quantum information to a destination R even though (a) S and R share no quantum resource, (b) R's identity is {\em unknown}\/ to S initially, (c) S herself can be R at a later date and/or in a different location to bequeath her information and (d) the path chosen for routing the secret may hit a dead-end due to resource constraints. Another implication of our technique is the possibility of using {\em insecure}\/ resources. For instance, it may safely store its private information with a neighboring organization without revealing data to the host and losing control over retrieving the data. Putting the two implications together, namely routing and secure storage, it is possible to envision applications like quantum mail (qmail) as an outsourced service.