  The feasibility of coexisting a quantum channel with carrier-grade classical
optical channels over Hollow Core Nested Antiresonant Nodeless Fibre (HC-NANF)
is experimentally explored for the first time in terms of achievable quantum
bit error rate (QBER), secret key rate (SKR) as well as classical signal bit
error rates (BER). A coexistence transmission of 1.6 Tbps is achieved for the
classical channels simultaneously with a quantum channel over a 2 km-long
HC-NANF with a total coexistence power of 0 dBm. To find the best and worst
wavelength position for the classical channels, we simulated different
classical channels bands with different spacing between the quantum and
classical channels considering the crosstalk generated from both Raman
scattering and four-wave-mixing (FWM) on the quantum channel. Following our
simulation, we numerically estimate the best (Raman spectrum dip) and worst
locations (Raman spectrum peak) of the classical channel with respect to its
impact on the performance on the quantum channel in terms of SKR and QBER. We
further implemented a testbed to experimentally test both single mode fibre
(SMF) and HC-NANF in the best and worst-case scenarios. In the best-case
scenario, the spacing between quantum and classical is 200 GHz (1.6 nm) with 50
GHz (0.4 nm) spacing between each classical channel. The SKR was preserved
without any noticeable changes when coexisting the quantum channel with eight
classical channels at 0 dBm total coexistence power in HC-NANF compared to a
significant drop of 73% when using SMF at -24 dBm total coexistence power which
is 250 times lower than the power used in HC-NANF. In the worst-case scenario
using the same powers, and with 1 THz (8 nm) spacing between quantum and
classical channels, the SKR dropped 10% using the HC-NANF, whereas in the SMF
the SKR plummeted to zero.
