Measurement Device-Independent Quantum Key Distribution

Measurement-Device-Independent Quantum Key Distribution (MDI-QKD) was developed to eliminate one of the most significant practical vulnerabilities in conventional QKD systems: attacks targeting the measurement devices. In MDI-QKD, the detectors are treated as completely untrusted and are operated by a third party, Charlie, who performs Bell-state measurements on quantum signals sent by the communicating parties, Alice and Bob. By removing trust from the measurement apparatus, MDI-QKD closes the entire class of detector side-channel attacks, including detector-efficiency mismatch, time-shift, and detector-blinding attacks, while preserving the information-theoretic security guarantees of quantum key distribution.

At QC2, we will develop an MDI-QKD testbed in which Alice and Bob prepare decoy-state weak coherent pulses encoded using time-bin states, a format well suited for deployment over telecom fibre due to its robustness against polarization drift. The quantum signals are transmitted through independent channels to Charlie, who performs partial Bell-state measurements using two-photon Hong–Ou–Mandel interference at a beam splitter followed by time-resolved single-photon detection. The publicly announced measurement outcomes reveal only correlations between Alice’s and Bob’s data and provide no information about the final secret key. Following the basis reconciliation and decoy-state analysis, which bounds information leakage associated with multi-photon emissions and photon-number-splitting attacks, Alice and Bob extract a shared secret key whose security is independent of the measurement devices.

The system will initially be validated under controlled laboratory conditions before being potentially deployed over commercial-grade dark-fibre infrastructure. The field testbed will operate across fibre links of varying lengths, with a target range of up to 100 km, providing a realistic platform for evaluating secure quantum communications in operational telecom environments. This testbed will provide a foundation for future metropolitan-scale quantum networks that balance practical deployability with immunity to detector-side-channel attacks.