A weakness in typical QKD systems is the current lack of reliable commercial single-photon sources. Very weak laser pulses are used instead, and they often produce more than one photon per pulse, all with the same orientation and bit value (0 or 1). This introduces vulnerability: An eavesdropper could intercept a photon and "read" it accurately without its loss being detected by the intended receiver, because the same laser pulse may still contain another photon.
The LANL/NIST absolute distance record of 184.6 km is secure against reasonable attacks, that is, the laser adjustments used in this case have only a moderate probability of generating more than one photon per pulse. The team also used slightly different adjustments to set other QKD distance records, including absolutely secure transmission of secret keys over 67.5 km, surpassing the previous record of 50.6 km. This method generated so few multi-photon pulses that some of the photons detected at the receiver must have originated in single-photon pulses, enabling the creation of secret key.