Fig. 1: Scheme of quantum noise stream cipher (QNSC) with delta-sigma modulation (DSM).
a Flow chart of the quadrature amplitude modulation (QAM) QNSC encryption and decryption with DSM. The QAM plaintext signal is encrypted into high-order QAM ciphertext signal by QNSC and converted into low-order QAM signal by DSM. After transmission, the received low-order QAM signal is restored to the high-order QAM ciphertext signal after a low-pass filter. Then, the plaintext signal is obtained by decrypting the ciphertext signal. b Two components of plaintext (SI, SQ) are encrypted by the random key (RI, RQ) and followed by bases (BI, BQ), respectively. Considering the quantum noise, sparse plaintext levels are easy to distinguish, but dense ciphertext levels are hard to distinguish. c High-order QAM ciphertext signal is converted into low-order QAM signal for transmission by DSM. d Ciphertext signal is sampled according to the Nyquist–Shannon sampling theorem. The half value of the sampling rate fs/2 is equal to or larger than the signal band fb for the baseband signal. e Oversample the ciphertext signal can expand the sampling zone resulting in a lower floor of noise. f Noise shaping will push the quantization noise from low frequency to high frequency, leading to an uneven distribution of quantization noise. g Filtering out the out-of-band noise will get the ciphertext signal and in-band noise (IBN).
