Experimental quantum cryptography with single photons and imperfect devices

Quantum Key Distribution (QKD) protocols are often analyzed under the assumption of perfect hardware, which creates security loopholes when real-world devices exhibit imperfections. This paper addresses the challenge of implementing the BB84 protocol using a semiconductor quantum dot (QD) source while rigorously accounting for uncertainties in both the source (multiphoton emission) and the receiver (detector efficiency, dark counts, and beam-splitter ratios).

The researchers implemented a prepare-and-measure BB84 protocol using a deterministic single-photon source based on an InAs quantum dot embedded in a microcavity. To ensure high-quality polarization encoding, they employed dynamic polarization modulation with a custom-built arbitrary waveform generator. The receiver (Bob) utilized a passive four-state polarization analyzer equipped with superconducting nanowire single-photon detectors (SNSPDs). Crucially, the security analysis was conducted within the Entropic Uncertainty Relation (EUR) framework, which allows for the inclusion of device imperfections characterized by uncertainty margins rather than fixed, idealized values.

The experimental setup achieved a basis-averaged Quantum Bit Error Rate (QBER) of 3.51% over a 20-minute accumulation time. By incorporating the measured device imperfections and their associated uncertainties into the security proof, the authors calculated a secure key length of approximately 250 kilobytes. The results demonstrate that while accounting for hardware imperfections significantly reduces the theoretical secret key rate compared to idealized models, it provides a more robust and realistic foundation for practical, loophole-free quantum cryptography.

This work bridges the gap between theoretical security proofs and experimental reality. By demonstrating that high-performance QKD is possible even when devices are incompletely characterized, the authors provide a pathway toward more secure, practical quantum communication systems that do not rely on the unrealistic assumption of perfect hardware.