The secret key rate is the fraction of secure key bits produced per protocol round, where a round is the transmission of a quantum state through the quantum channel [Gra21, p. 38].

The secret key rate generally depends on the total number of rounds $M$ performed.

The asymptotic secret key rate (often just asymptotic key rate) is the secret key rate when $M\to\infty$ is assumed to simplify analysis.

• Assuming direct reconciliation, the asymptotic key rate of any quantum key distribution (QKD) protocol with one-way error correction is lower-bounded by the Devetak-Winter rate [DW05]:

  $r_\text{DW} = I(A;B) - I(A;E),$

  where $I$ denotes quantum mutual information, $A$, $B$ and $E$ are the random variables representing Alice’s, Bob’s and Eve’s raw key bits.

• An intuitive interpretation of the Devetak-Winter rate: the fraction of secret bits generated per round of using the protocol is equal to the amount of information shared by Alice and Bob, $I(A; B)$, minus the amount of information that Eve has on Alice’s part of the key, $I(A;E)$ [Wol21, p.145].

Assuming $M\to\infty$ is not realistic, and the security of a QKD protocol has to be analysed assuming a finite $M$ and generally finite resources.

Analysis of the secret key rate and associated security properties of a QKD protocol is called finite-key analysis [TLGR12], to be covered in the future.