The Max Planck Institute for Radio Astronomy's press release Something is Lurking in the Heart of Quasar 3C 279; First Event Horizon Telescope Images of a Black-Hole Powered Jet shows a stunning montage of three Event Horizon telescope images at 7, 3 and 1.3 mm wavelengths (43, 86 and 230 GHz) demonstrating how the highest frequency in combination with the planet-sized baselines work together to produce observations at "an extreme 20 microarcsecond resolution", quoting the title of the April 5 2020 Astronomy and Astrophysics paper Kim et al. 2020 Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution.
From How does ALMA produce stable, mutually coherent ~THz local oscillators for all of their dishes? I know that ALMA's receivers can go as far as about 950 GHz. Is there work underway to increase the number of radiotelescope sites around the Earth with circa 1 THz receivers to push the long baseline capabilities of the Event Horizon Telescope to sub-millimeter wavelengths?