Many papers on Quantum Key Distribution protocols discuss the protocols upper and lower bounds (on quantum bit error rate QBER).
For example, BB84 has a lower bound of 11% and an upper bound of ~14.6%.
What is the meaning of these bounds?
I believe that if the QBER is less than 11%, a secure key can be established, and if the QBER exceeds ~14.6% a secure key cannot be established, but what if the QBER is 12%? Does this mean that it is possible to create a secure key after error correction and privacy amplification, but prior to these being applied a secure key cannot be garuanteed?