Measurement Device Independent Qkd

Short answer: Measurement-device-independent QKD (MDI-QKD) is a quantum key distribution protocol in which the two users send signals to an untrusted middle node that performs the measurement. Because the detectors sit at a node the eavesdropper may control, attacks on detectors no longer work against the protocol. It is related to, but different from, fully device-independent QKD, because it still makes assumptions about the light emitters. Papers from 2017 to 2023 describe both its advantages and its practical costs.

The idea

A 2023 paper in JETP Letters describes QKD systems with an untrusted intermediate node. Two nodes of the network obtain a common key through an intermediate node whose equipment does not need protection, since the eavesdropper sees all of its operation, including photodetector results. The paper says this idea was proposed in earlier work and the protocol was called measurement device independent QKD. It contrasts this with most QKD systems, where the key is formed from photocounts in a pair of detectors and the detector results must be inaccessible to the eavesdropper.

A 2017 Scientific Reports paper says that, in the effort to build loophole-free QKD, device-independent protocols can remove all loopholes from imperfect devices, and that MDI-QKD is the more practical of them. It says MDI-QKD removes all side channels from the measurement apparatus, which the authors call the most critical part of QKD systems, and that several groups have demonstrated it experimentally with current technology.

The costs

The same 2017 paper says MDI-QKD setups, which need two-photon interference from two independent lasers, are rather complicated and give a lower key rate than conventional QKD such as decoy-state BB84. It describes a later proposal, detector-device-independent QKD (DDI-QKD), which uses an untrusted single-photon Bell state measurement instead, aiming at comparable performance to conventional QKD while staying immune to detector side-channel attacks. It reports that another researcher showed two attacks on DDI-QKD, indicating that additional assumptions about the Bell state measurement setup are needed to reach the claimed security level.

What MDI does not remove

A 2022 Optics Letters paper states that MDI-QKD can remove all detection side channels but still makes additional assumptions about the light emitters, which can be compromised through uncharacterized side channels in practice. The authors combine a recently proposed reference technique to prove MDI-QKD security against source imperfections or side channels. It requires reference states and an upper bound on a parameter describing source quality. Investigating the asymptotic performance of single-photon sources, they find the side channels have a great impact on the key rates.

What this does not tell you

These papers do not say MDI-QKD has been deployed in networks, and they do not compare products. Key rates and security depend on the assumptions about sources, and the results given are theory or laboratory results.

FAQ

What does measurement device independent mean?

The measurement is done at an untrusted node, so detector side channels do not compromise the key.

Is it the same as device-independent QKD?

No. The 2022 paper notes MDI-QKD still makes assumptions about the light emitters.

What is the drawback?

Setups are complicated and give a lower key rate than conventional QKD, per the 2017 paper.

What is DDI-QKD?

A later proposal using an untrusted single-photon Bell state measurement. The 2017 paper reports attacks showing extra assumptions are needed.

Do source side channels matter?

In the 2022 analysis, they have a great impact on the key rates.