So imagine you could use this neural dust, one mote per neuron, to "read out" a brain. How much data would this be?
Get out your envelope backs!
How much is that?
It's about 1 optical fiber. The maximum conceivable bandwidth of a single-mode optical fiber is set by the frequency range where it's clear enough to transmit light without melting. That number is about 1 petabit (10^15)/s, depending on the transmission distance. (see this discussion on StackExchange). Bit rates of about 10% of that have been achieved in the laboratory using "Space Division Multiplexing" (see this news article or this keynote from a real expert), while the current generation of optical networking products use multiple channels of 100 Gigabit Ethernet to achieve as much as 10Tb/s on a fiber, about 1% of the theoretical limit. A petabit per second is a ways in the future, but so is our neuro-dust.) Even now, we could probably fit a brain dump on 16 of the laboratory fiber systems.
So we can imagine putting the bandwidth of a brain onto a cable we can hold in our hands.
But how much is THAT?
Cisco puts out a report every year estimating the total traffic on the internet. This year, they're estimating that the total IP traffic in the world is 62,476 petabytes per month. That's about 190 terabits/second. So a brain readout would be about 8 times the internet's total data rate.
Right now, powering that much dust would be impractical. Currently a neural dust mote uses a half a milliwatt of power, which means 10 megawatts to read the whole brain. So it gets fried, but it was just watching Netflix.