  The security of real-world quantum key distribution (QKD) critically depends
on the number of data points the system can collect in a fixed time interval.
To date, state-of-the-art finite-key security analyses require block lengths in
the order of 1E4 bits to obtain positive secret keys. This requirement,
however, can be very difficult to achieve in practice, especially in the case
of entanglement-based satellite QKD systems, where the overall channel loss can
go up to 70 dB or more. Here, we provide an improved finite-key security
analysis which can reduce the block length requirement by 14% to 17% for
standard channel and protocol settings. In practical terms, this reduction
could save entanglement-based satellite QKD weeks of measurement time and
resources, thereby bringing space-based QKD technology closer to reality. As an
application, we use the improved analysis to show that the recently reported
Micius QKD satellite is capable of generating positive secret keys with a
$1E-5$ security level.

Quantum Physics