  Measurement-device-independent quantum key distribution (MDI QKD) offers
great security in practice because it removes all detector side channels.
However, conducting MDI QKD over free-space channels is challenging. One of the
largest culprits is the mismatched transmittance of the two independent
turbulent channels causing a reduced Hong-Ou-Mandel visibility and thus a lower
secret key rate. Here we introduce a dynamic attenuation scheme, where the
transmittance of each of the two channels is monitored in real time by
transmitting bright light pulses from each users to the measurement device.
Based on the measured channel transmittance, a suitable amount of attenuation
is introduced to the low-loss channel at the measurement device. Our simulation
results show a significant improvement of QKD performance, especially when
using short raw keys.
