  Quantum key distribution (QKD) facilitates the distribution of
information-theoretic secure encryption keys by utilizing the fundamental laws
of physics. However, the security of QKD systems in practice can be undermined
due to implementation flaws, also known as side channels.
Measurement-device-independent (MDI) QKD offers a solution to these type of
attacks by eliminating the vulnerability of the detector. Among different
flavors of MDI-QKD, continuous variable (CV) MDI-QKD based on coherent states
is a promising candidate for integration into existing telecom infrastructure.
Despite previous demonstrations of the concept and the potential for secure
communication offered by CV MDI-QKD, a practical implementation of the system
for real-world data encryption has yet to be achieved. Here, we introduce a
simple and practical CV MDI-QKD system design that can coexist with classical
telecommunications channels. This is achieved through the use of a new relay
structure, a real-time frequency and phase locking system and a well-designed
digital signal-processing pipeline. Our design demonstrates the first practical
CV MDI-QKD system, operating at a symbol rate of 20 MBaud and generating keys
that are secure against collective attacks in both the finite-size and
asymptotic regimes. This sets an important milestone towards in-field
implementation and integration of high-performance CV MDI-QKD into telecom
networks.
