Absolute Security with Diffraction Grating in Terahertz Communication Links

We realize a new approach to absolute security by constructing and testing a point-to-point line-of-sight (LOS) outdoor link at 200 GHz. With a diffraction grating placed at an intermediate focus immediately after an emitter horn antenna we can engineer frequency-varying nulls that create blind regions in space where an eavesdropper is unable to detect at least one of the frequency channels, satisfying the security condition for absolute security. By measuring power and bit error rate at 41 different frequencies, we demonstrate a shift in nulls with each frequency and verify a bit error rate greater than 0.5 on the nulls.