Continuous-Variable QKD: Gaussian-Modulated Coherent States

Most quantum key distribution demos, like BB84 or E91, send single photons in discrete polarization states. Continuous-variable QKD (CV-QKD) instead sends ordinary laser pulses — coherent states — modulated with a continuous Gaussian spread across their amplitude and phase quadratures, and Bob reads them out with a shot-noise-limited homodyne detector borrowed straight from telecom optics. This simulator renders that phase-space picture in 3D: Alice's plane on the left scatters Gaussian-modulated points, each pulse streams down a lossy fiber channel toward Bob's plane on the right, and the received constellation visibly shrinks and spreads as fiber length and excess noise increase. Live readouts track the channel transmittance, the Shannon mutual information Alice and Bob actually share, and a security bound on what a collective-attack eavesdropper (Eve) could learn — collapsing the secret key rate to zero exactly where real CV-QKD systems lose their security margin.