QUEST: A Quantum Key Distribution Protocol Employing Eight-State Time-Bin Modulation and Homodyne-Heterodyne Hybridization

Quantum Key Distribution (QKD) is a technology that uses the laws of quantum physics to create secret encryption keys between two people. Its main advantage is that any attempt to spy on the communication automatically disturbs the quantum signals, making eavesdropping detectable. In this work, we introduce a new QKD method called ModPhase-8 (QUEST). Instead of using only a few quantum signal types, our system uses eight carefully designed signal variations created by adjusting the phase between two very short light pulses. These eight signal patterns increase the uncertainty for an eavesdropper and allow more information to be securely encoded in each transmission. On the receiving side, the system adaptively switches between two measurement techniques based on the prevailing channel conditions. This adaptive detection mechanism enhances reliability and helps maintain low error rates even when the communication channel is affected by noise. We provide a mathematical proof showing that the protocol achieves a high level of security, restricting an attacker’s probability of success to less than one in ten billion. Even when practical imperfections such as signal loss, channel noise, or detector inaccuracies are present, the system continues to preserve strong security guarantees. Simulation studies further demonstrate that the proposed protocol maintains low error rates (approximately 1–2% under low-noise conditions, and below 4.1% in standard operating regimes) while efficiently generating secure cryptographic keys across a variety of channel conditions. A rigorous nine-experiment benchmarking study establishes that ModPhase-8 achieves 6.75× noise tolerance, surpassing even the Gaussian-modulated GG02 protocol (4.01×), together with 10× faster finite-key convergence, and exclusive key generation in high-noise regimes where six out of eight benchmark protocols fail entirely. Three additional detector-parameter experiments demonstrate that ModPhase-8 tolerates up to 20.6% higher timing jitter than BB84 (158.8 ps vs. 131.7 ps), sustains key generation at afterpulsing probabilities exceeding 15%, and maintains the highest composite detector figure of merit across six detector-quality axes. The reported secret-key-rate values are computed under a heuristic d-ary entropy approximation applied uniformly to all protocols; a fully rigorous discrete-modulation continuous-variable QKD numerical security analysis, justified here through the explicit construction of the equivalent entanglement-based protocol and source-replacement scheme, is identified as essential follow-up work. Overall, ModPhase-8 offers a practical, scalable, and highly secure framework for next-generation quantum communication networks.