Your car radio converts electromagnetic radiation—radio waves—into electric currents that the speakers use to produce sound. The same process should be able to convert visible or infrared radiation to electricity. Scientists have now demonstrated a metal-insulator-metal (MIM) diode “rectenna” (rectifying antenna) that works via quantum tunneling and can harvest electricity from heat (Nat. Commun. 2021: DOI: 10.1038/s41467-021-23182-0). Amina Belkadi and colleagues at the University of Colorado, Boulder tuned the materials and construction of their rectenna, thereby designing an electron tunneling device that they demonstrate can turn heat from radiation into current. Other researchers have achieved this result only by applying voltage to the device. MIM diodes are metal electrodes sandwiching oxide insulators. The group found that certain thicknesses in one oxide layer enable electrons to tunnel through efficiently, making an electrical current. Belkadi produced a few tens of µV with an array of 250,000 rectennas. The group says their devices have 100 times the efficiency of the next-best rectennas. If perfected, they could make electricity on walls of bakeries or spacecraft.