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/ln2 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. © 2013 American Physical Society.