Limited-Diffraction Beams for Secure Fast Data Communications

X wave is one of the limited-diffraction beams and was developed in 1992. It is an exact solution to the linear isotropic/homogeneous or free-space scalar wave equation. X wave can be highly localized with a sharp center peak in both transverse (perpendicular to the wave propagation) and axial (along the wave propagation) directions in the three-dimensional (3D) space, and in theory, can propagate to an infinite distance without changing its shape. In addition, both the group and phase velocities of the X wave are always larger than the speed of sound (supersonic) or the speed of light (superluminal).Because of the localization property and the supersonic group velocity, in this paper, the X wave was used to transmit digital information (or energy) at a supersonic speed for secure and high-speed data communications in multiple channels. Both computer simulations and experiment were conducted using a 2.5-MHz ultrasound transducer in water. The results show that even with a finite transducer aperture, the X wave has a very large depth of field and is nondispersive (the wave speed does not change with the temporal frequency). As a comparison, Bessel pulses also were used for multi-channel data communications. However, Bessel pulses are dispersive, which lowers the data transmission rate. Despite of this disadvantage, the Bessel pulses have an adjustable group velocity that is always slower than the speed of sound (subsonic) in the isotropic/homogeneous media or the speed of light (subluminal) in free space (or vacuum). Thus, the Bessel pulses can be used as a delay line in a way similar to the surface acoustical wave (SAW) devices but with an adjustable delay time.Since the ultrasound experiments show that X wave can transmit digital information at a supersonic speed and the ultrasound X wave shares the same wave equation as the light or electromagnetic waves, X wave also can transmit information at a speed that is greater than the speed of light in free-space or vacuum. In 2023, the author has developed X wave solutions to the free-particle Dirac, Klein-Gordon, and Weyl equations in quantum mechanics. Given the superluminal group velocity of these X waves, it is possible that both the massive and massless particles can be used to transmit information (energy) at a speed that is larger than the speed of light in free space or vacuum.