Fig. 3: Private communication with chaos anti-synchronization.

a 2D and 1D (insets) correlation diagrams for the filtered intensity of the slave and the master QCL. The filtered signal of the slave laser was flipped so that the 1D correlation diagram has a maximum value of 1 and the 2D correlation diagram shows a positive correlation (instead of −1 and negative correlation, respectively). b Bit series for the initial message (IM in green), the difference (in purple), the master’s signal (in red), and the slave’s signal (in blue). Except for the initial message, the number of errors is written on the right side of the bit series, with the corresponding color. The translation of the difference bit signal is also displayed by comparison with the initial message. The private mid-infrared transmission achieves a BER of 6%, which corresponds to 12 errors out of 191 bits. c Experimental eye diagrams for the four time traces displayed in Fig. 1d. From these diagrams, one can see that it is impossible to recover the message only from the signal of the master QCL but it becomes possible for most of the bits from the difference signal. Bits deciphered as “0” are drawn with a light color while bits deciphered as “1” are drawn with a stressed color (particularly visible in the difference eye diagram where the “1” and “0” are well separated).