It's not an antenna like I thought at first. It's a tuner with 4 MHz instantaneous bandwidth and a very wide tuning range, https://arxiv.org/abs/2009.14383 - the actual (laser intensity) measurements are done within the RF electrical field above a coplanar waveguide track on a PCB.

It's a cool RF tuner. But it still need a real, full size, antenna. It's also not clear to me if they can get the phase informations out of the laser intensity measurements. I suppose it's all a matter of how fast they can sample the light.