  We experimentally demonstrate that we can detect correlated errors in a
twin-field quantum key distribution (TFQKD) system by using a technique that is
related to self-consistent tomography. We implement a TFQKD system based on a
fiber-Sagnac loop, in which Alice and Bob encode information in the phase of
weak coherent states that propagate in opposite directions around the loop.
These states interfere as they exit the loop and are detected by a third party,
Charlie, who reports the results of their measurements to Alice and Bob. We
find that it is possible for Alice and Bob to detect correlated
state-preparation and measurement errors while trusting only their own
individual states, and without trusting Charlie's measurements.
