Comment by glumreaper

The 50GHz or 20ps figure is actually the limit of the gear the scientists had available - from the paper at https://iopscience.iop.org/article/10.1088/1361-6463/ad9284/...

"the signal state transitions were established at less than 20 ps, which is comparable to the bandwidth limit of the oscilloscope utilized at 50 GHz. [...] the initial rise time constant may be shorter than the measurement limits of the oscilloscope."