  Quantum key distribution (QKD) provides information-theoretic security based
on the laws of quantum mechanics. The desire to reduce costs and increase
robustness in real-world applications has motivated the study of coexistence
between QKD and intense classical data traffic in a single fiber. Previous
works on coexistence in metropolitan areas have used wavelength-division
multiplexing, however, coexistence in backbone fiber networks remains a great
experimental challenge, as Tbps data of up to 20 dBm optical power is
transferred, and much more noise is generated for QKD. Here we present for the
first time, to the best of our knowledge, the integration of QKD with a
commercial backbone network of 3.6 Tbps classical data at 21 dBm launch power
over 66 km fiber. With 20 GHz pass-band filtering and large effective core area
fibers, real-time secure key rates can reach 4.5 kbps and 5.1 kbps for
co-propagation and counter-propagation at the maximum launch power,
respectively. This demonstrates feasibility and represents an important step
towards building a quantum network that coexists with the current backbone
fiber infrastructure of classical communications.
