A quantum-enabled beam steering micro-antenna based on spin injection from a topological insulator

Antennas are the quintessential means to communicate information wirelessly over long distances via electromagnetic waves. Traditional antennas have two shortcomings that have stymied their miniaturization: (1) their radiation efficiencies plummet and (2) they radiate isotropically (no directivity), when their dimensions are shrunk to small fractions of the radiated wavelength. Here, we report a new genre of quantum-enabled antennas that overcome these limitations by employing topological insulators. An alternating charge current of frequency 1–10 GHz injected into a thin film of a three-dimensional topological insulator (3D-TI) injects an out-of-plane spin current of alternating spin polarization into a periodic array of cobalt nanomagnets deposited on the surface of the 3D-TI. This generates spin waves in the nanomagnets, which radiate electromagnetic waves in space, thereby implementing an antenna. The frequency of the radiated electromagnetic wave is the same as that of the current. The antenna dimension is only 0.6%–1.8% of the free-space wavelength, and it radiates with an efficiency several orders of magnitude larger than the theoretical limit for conventional antennas of that size. Furthermore, it radiates anisotropically (despite being effectively a “point source”). One can change the anisotropic radiation pattern by changing the direction of the injected alternating charge current in the plane parallel to the surface because it changes the spin wave patterns within the nanomagnets. That happens because of spin-momentum locking of surface electrons in the 3D-TI. This enables beam steering without the use of a phased array, resulting in extreme miniaturization. Thus, we have overcome several limitations of classical antennas by harnessing the quantum mechanical attributes of a quantum material, namely, a 3D-TI.