Demonstration of superadditive communication and nonlocality without entanglement with the Green Machine temporal mode sorter.

The ultimate limit of optical communications capacity entails superadditive communications, a higher bit rate than that of any symbol-by-symbol detection. It is a special case of the celebrated nonlocality without entanglement [1] and has yet to be realized. We propose a practical design of the Green Machine [2], a joint-detection receiver that can attain superadditive capacity with a binary-phase-shift-keying (BPSK) modulated Hadamard code [3, 4]. We demonstrate this receiver [5] and show that its achieved capacity, after backing out losses within the receiver, surpasses that of any symbol-by-symbol receiver permissible by quantum physics, in the low received photon-flux regime. In addition to reducing the transmitter peak power needed compared with the conventional pulse-position modulation used for deep-space laser communications, we show the Green Machine's self-referenced phase makes it far more immune to channel phase noise, e.g., due to atmospheric turbulence or platform vibrations, by orders of magnitude compared with other BPSK-compatible receivers. These advantages make the Green Machine a promising candidate for next-generation deep-space laser communications.

[1] C. H. Bennett, et al., PRA 59, 1070 (1999).

[2] S. Guha, PRL 106, 240502 (2011).

[3] T. Rambo, Ph.D. Dissertation, Northwestern University (advisor: P. Kumar), (2016).

[4] K. Banaszek, et al., J. Light. Technol. 38, 2741 (2020).

[5] C. Cui, et al., arXiv: 2310.05889 (2023).