Optimized attacks on twin-field quantum key distribution

In twin-field quantum key distribution the two communicating parties, Alice and Bob, each send a weak coherent pulse to a third party stationed between them. The key bits are generated by interference between these pulses, with the results communicated to Alice and Bob. We consider optimized strategies for eavesdropping on the communication built upon state discrimination and quantum non-demolition measurements. We find that the best strategy comprises a two-step process but that even this does not compromise the security of the protocol.