In principle, therefore, it should be possible to transmit six bits of information using four light waves, 28 bits using eight light waves, and so forth. The value “1” of a bit can then be represented by a positive correlation (synchronised noise), and the value “0” by a negative correlation.
Remote controlled experiment
On paper this type of “correlation coding” worked perfectly. To make sure, however, that it could also be realized in practice, Divitt also wanted to test in an experiment. There was one problem, though: Divitt is a U.S. citizen, and his visa expired towards the end of his doctorate. So, he took a rather unusual approach. Before returning to the U.S. he set up an experiment in Novotny’s lab in which the encoding of information in an optical fibre bundle is simulated using a so-called spatial light modulator. The correlations between the light waves are manipulated and later read out with the help of an interference pattern. Back in the U.S., Divitt started the experiment – by remote control from his computer. In the meantime, colleagues in Zurich made sure that the experimental setup was always in good shape.
Afterwards, Divitt analysed the results in his “home office” and found that his method did actually work. He and his PhD advisor have since submitted a patent application for it. “Of course, doing research like that is somewhat unusual”, Novotny comments. “Moreover, it was only possible because ETH provides people with the necessary freedom to test wild ideas now and then – if necessary, even from far away.”
Possible security advantages
Divitt and Novotny hope that, on the one hand, their method will be able to increase the data capacity of fibre optic cables even more. As their method does not require coherent laser light, it should also be cheaper than conventional technologies. On the other hand, correlation coding could also contribute to data safety. Since the oscillations of light waves cannot be recorded in “real time” due to their high frequency, a possible eavesdropper would have to divert a considerable part of the optical power in order to obtain an interference pattern and hence intercept the information. That, in turn, would be noticed immediately, which would expose the eavesdropper.