Continuous variable QKD inspired analog encryption for classical PAM links

Abstract We present a continuous-variable (CV), quantum-key-distribution (QKD)—inspired, keyed physical-layer masking method for classical M-level pulse-amplitude-modulation (M-PAM) links. A transmitter adds a per-symbol Gaussian dither $$d[n]\!\sim \!\mathcal {N}(\mu ,\sigma ^{2})$$ , generated by a seeded pseudorandom number generator (PRNG), directly to the analog waveform, and an authorized receiver that shares the PRNG, its seed, and $$(\mu ,\sigma )$$ regenerates and subtracts the same sequence prior to slicing. Because masking and demasking act on the data-carrying optical signal, the scheme operates over conventional, amplifier-compatible fiber without reserving a separate quantum channel. First, we analyze an idealized continuous-valued baseband model in which the PAM symbols pass through an additive-white-Gaussian-noise (AWGN) channel and no extra amplitude quantization or link impairments are present. In this setting we show that, when the seed and $$(\mu ,\sigma )$$ are matched, subtractive cancellation is essentially ideal and the Gray-coded 4-PAM bit-error-rate (BER) versus signal-to-noise ratio (SNR) curve coincides with the standard AWGN benchmark, whereas seed or parameter mismatches act as additional Gaussian noise and produce several-decibel SNR penalties or high, SNR-independent BER floors. We then implement the same masking mechanism in a system-level OptiSystem model of an intensity-modulation/direct-detection (IM/DD) link and enable a $$4\rightarrow 8\rightarrow 4$$ double-quantization mapping, in which dithered 4-PAM symbols are passed through a mid-rise 8-level quantizer at the transmitter and a mid-tread 4-level quantizer at the receiver. In this OptiSystem realization the resulting pseudo-constellation exhibits amplitude statistics reminiscent of probabilistic shaping and an intrinsic, SNR-independent algorithmic BER floor, typically $$\log _{10}\textrm{BER}\!\approx \!-2.8$$ in back-to-back simulations. By choosing forward-error-correction (FEC) codes whose waterfall threshold lies near this floor and by tuning the dither variance at the transmitter and receiver, the link can be operated in a deliberately fragile edge-of-FEC regime in which small excess disturbances (for example, seed or variance mismatch) push the BER out of the decodable region and strongly obfuscate the payload. The proposed masking is modulation-agnostic and is intended as a classical protection layer rather than a full QKD protocol; keying material can be supplied over an authenticated supervisory channel or by an independent QKD system.