Delayed-choice entanglement swapping for robust quantum key generation between users without quantum memory

Quantum key distribution offers the potential for provably secure communications. However, the establishment of quantum keys between geographically separated parties requires a quantum channel between them (for example optical line of sight or optical fiber), which restricts the generation of ad hoc keys between participants who may not initially know that they need to share a quantum key. Here we show that delayed-choice entanglement swapping provides a means for participants with solely quantum measurement and classical memory to generate quantum secure keys. To achieve this requires a central participant, which we term Charlie, that has access to long-lived quantum memory and the ability to perform arbitrary Bell state measurements between their stored qubits. Such a protocol would be one way for Charlie to provide quantum keys as a service between participants who never need a direct quantum channel, nor quantum resources beyond single qubit measurements. We illustrate our approach considering the case of multiuser delayed-choice entanglement swapping for large-scale quantum secure communication. This protocol suggests a possible business model for a quantum memory provider distributing entanglement via quantum sneakernet to consumers who desire quantum security.