Countermeasure for security loophole caused by asymmetric correlations of reference frame independent quantum key distribution with fewer quantum states.

One of the challenging issues in free-space quantum key distribution (QKD) is the requirement of active compensation of the reference frame between the transmitter and receiver. Reference frame independent (RFI) QKD removes active compensation, but it requires more quantum states. A recent proposal can effectively reduce the required quantum states, but this can be achieved assuming the correlations defined in RFI QKD are symmetric.