The classical capacity of quantum thermal noise channels to within 1.45 bits

We find a tight upper bound for the classical capacity of quantum thermal noise channels that is within $1\mathrm{/}\ln 2$ bits of Holevo’s lower bound. This lower bound is achievable using unentangled, classical signal states, namely displaced coherent states. Thus, we find that while quantum tricks might offer benefits, when it comes to classical communication they can only help a bit.