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Experimental Demonstration of Polarization Encoding Measurement-Device-Independent Quantum Key Distribution
Zhiyuan Tang, Zhongfa Liao, Feihu Xu, Bing Qi, Li Qian, Hoi-Kwong Lo
TL;DR
The paper addresses practical security and deployment challenges in QKD caused by imperfect sources and detectors. It demonstrates polarization-encoding MDI-QKD using phase-randomized weak coherent pulses and decoy states, generating a 1600-bit secure key over 10 km and indicating a network architecture with centralized detection.
Problem
Practical QKD uses imperfect sources and detectors, leaving systems vulnerable to attacks such as photon number splitting and detector side channels.
Method
The experiment implements polarization-encoding MDI-QKD, with Alice and Bob preparing phase-randomized weak coherent decoy-state BB84 pulses for Bell-state measurement by an untrusted Charlie.
Results
A 1600-bit secure key was generated between Alice and Bob over 10 km of optical fibers, with an estimated secure key rate of RL = 9.8 × 10^-9.
Takeaways & Limitations
Polarization encoding with compact optoelectronic preparation equipment supports the potential construction of detector-side-channel-free MDI-QKD networks with expensive detection systems centralized at a service centre.
Takeaways & Limitations
Compared with conventional BB84 QKD, MDI-QKD has a lower key rate because it relies on coincidence rather than single detection events.
Abstract
from arXiv · showhide
We demonstrate the first implementation of polarization encoding measurement-device-independent quantum key distribution (MDI-QKD), which is immune to all detector side-channel attacks. Active phase randomization of each individual pulse is implemented to protect against attacks on imperfect sources. By optimizing the parameters in the decoy state protocol, we show that it is feasible to implement polarization encoding MDI-QKD over large optical fiber distances. A 1600-bit secure key is generated between two parties separated by 10 km of telecom fibers. Our work suggests the possibility of building a MDI-QKD network, in which complicated and expensive detection system is placed in a central node and users connected to it can perform confidential communication by preparing polarization qubits with compact and low-cost equipment. Since MDI-QKD is highly compatible with the quantum network, our work brings the realization of quantum internet one step closer.
Appendix A: Polarization alignment in the experiment
The experiment aligns Alice’s and Bob’s polarization reference frames using fiber-based controllers and a three-squeezer electrical polarization controller. Alice’s diagonal basis is rotated on the Poincaré sphere until it matches Bob’s without disturbing rectilinear-basis alignment.
- The electrical polarization controller applies unitary transformations to Alice’s polarization states so Alice and Bob share a polarization reference frame.The controller uses three fiber squeezers driven by piezoelectric actuators, oriented at 0°, 45°, and 0°.
- Alice’s diagonal basis is rotated about the H-V axis on the Poincaré sphere by changing phase retardation with an applied voltage.The diagonal basis is defined as |±⟩ = 1/√2(|H⟩ ± |V⟩).
- The voltage is adjusted until Alice’s diagonal basis aligns with Bob’s.This adjustment completes the reference-frame alignment required for successful Bell state measurement.
- The diagonal-basis alignment procedure does not disturb the previously established rectilinear-basis alignment.