Li Li

Recent advances in loophole-free Bell tests have profoundly impacted quantum cryptography, yet their security assumes trusted random number generators (RNGs) for measurement choices—a vulnerability termed the freedom-of-choice loophole. Here, we demonstrate that classical systems can spoof Bell violations under ostensibly loophole-free conditions using compromised RNGs. By synchronizing laser-generated separable states with imperfect RNG outputs in an optical setup, we simulate a CHSH test closing locality and detection loopholes. With full RNG access, we achieve a near-maximal CHSH value of 3.99, exceeding quantum limits. Crucially, partial RNG knowledge suffices: predetermining 10.6% of bits reproduces our “loophole free” optical system's CHSH value of 2.007, while Santha-Vazirani generators with 0.38-biased bits enable optimal spoofing. Even weakly correlated RNGs coordinated via entangled states—deviating by 0.04 from independence—allow violations. Prediction-based ratio analysis gives a P-value upper bound of 10^(-18266), misleadingly implying non-classicality if RNG flaws are ignored. Strikingly, we extract "device-independent" random bits from simulated outcomes, mirroring cryptographic protocols. This exposes a critical flaw: compromised input randomness invalidates security guarantees in Bell-inequality-based cryptography. Our findings mandate rigorous verification of both RNG integrity and Bell violations to ensure quantum cryptographic security.

In this presentation, I will introduce two of our recent works: experimental realization of measurement-device-independent (MDI) quantum key distribution (QKD) [arXiv:1209.6178] and unambiguous-state-discrimination (USD) attack on a decoy-state QKD system without phase randomization [arXiv:1304.2541]. On one hand, the MDI-QKD is able to shield all practical attacks realized so far. We experimentally demonstrate the MDI-QKD protocol by implementing high-speed and low-noise up-conversion single photon detectors. The security of MDI-QKD relies on a trusted source scenario, where the decoy-state method is assumed. On the other hand, phase randomization is commonly ignored from the decoy-state method. We demonstrate a USD attack on a decoy-state QKD system when the phase randomization is ignored.