  The secure transfer of information is critical for the ever-increasing
demands of the digital world. Continuous-variable quantum key distribution
(CV-QKD) is a technology that can potentially provide information-theoretic
security to cryptographic systems. CV-QKD is highly attractive thanks to high
secret key rate generation over metropolitan distances and integration with the
current classical communication networks. Here we demonstrate CV-QKD across a
15 km multi-core fiber (MCF) where we take advantage of one core to remotely
frequency lock Bob's local oscillator with Alice's transmitter. We also
demonstrate the capacity of the MCF to boost the secret key rate by
parallelizing CV-QKD across multiple cores. Our results point out that MCFs are
promising for metropolitan deployment of QKD systems.
