Including classical controls in bloq decompositions

[kjm538]

I am interested in implementing decompositions that involve measurements and classical controls using qualtran. I see that And is treated as a leaf bloq, so it's usage of these is hidden in a sense. However, one can imagine more complicated structures, such as the uncompute of the data loading primitives, where the measurements and classical controls can't necessarily be contained by a leaf bloq. I see there is a shim bloq for handling measurements, but I don't see a way to handle classically controlled operations. Is there a recommended way to go about this? Based on the status of #445 and #1458 it looks like the infrastructure for this is not in place yet, but I would be interested to know if there are thoughts on how to approach this.

[mpharrigan]

Yes, #445 is the relevant bug here.

I imagine the infrastructure will look something like: introduce additional data types for classical data; but otherwise use the same Signature and CompositeBloq way of routing information around. A measurement gate would therefore take in a qubit as an input/left register and output a classical bit. Classical bits can be used in places that make sense: control bit for controlled operations; classical operations. The resource counting code will need to be updated to ignore classical costs. The simulation infrastructure would need to be updated -- either with density matrix simulation and/or random trajectories

I don't have any timelines to share at this moment from our side but if you or anyone you know are interested in contributing, let me know