Surpassing the loss-noise robustness trade-off in quantum key distribution

Quantum key distribution (QKD) offers a theoretically secure method to share secret keys, yet practical implementations face challenges due to noise and loss over long-distance channels. Traditional QKD protocols require extensive noise compensation, hindering their industrial scalability and lowering the achievable key rates. Alternative protocols encode logical qubits in noise-resilient states, but at the cost of using many physical qubits, increasing susceptibility to loss and limiting transmission distance. In this work, we introduce a logical qubit encoding that uses antisymmetric Bell-states in the continuous photonic degrees of freedom, frequency and time. By leveraging the continuous space, we overcome this noise-loss robustness trade-off by minimising the number of photons per logical qubit, whilst optimising the encoding resilience over noise fluctuations. We analyse the security of our encoding and demonstrate its robustness compared to existing state-of-the-art protocols. This approach provides a path towards scalable, efficient QKD implementations under realistic noise conditions.