Is there any work underway to push the long baseline capabilities of the Event Horizon Telescope to sub-millimeter wavelengths?

Question

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.

Question: 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?

Answer 1

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?

Yes, regarding the Event Horizon Telescope network itself, this recent paper evaluates the efficacy of 40 new sites for observing submilimeter wavelengths at $\sim$ THz (really $\approx$ 300 GHz). They conclude that "A group of new sites with favorable transmittance and geographic placement leads to greatly enhanced imaging and science on horizon scales.

On a longer developmental timescale, there's also this white paper which proposes a space-based observatory TeraHertz Exploration and Zooming-in for Astrophysics (THEZA), which would aim to observe the THz regime. From the abstract:

The concept will open up a sizeable range of hitherto unreachable parameters of observational astrophysics. It unifies two major lines of development of space-borne radio astronomy of the past decades: Space VLBI (Very Long Baseline Interferometry) and mm- and sub-mm astrophysical studies with "single dish" instruments. It also builds upon the recent success of the Earth-based Event Horizon Telescope (EHT) -- the first-ever direct image of a shadow of the super-massive black hole in the centre of the galaxy M87. As an amalgam of these three major areas of modern observational astrophysics, THEZA aims at facilitating a breakthrough in high-resolution high image quality studies in the millimetre and sub-millimetre domain of the electromagnetic spectrum.