[BUG] ADAPT ansatz not removing superfluous qubits

[KrzysztofB-1qbit]

Issue: Bug Report

Expected Behavior
When generating an ADAPT ansatz with scbk, there should be 2 qubits removed

Current Behavior
ADAPT returns a circuit with superfluous quits which are not actually used for anything. These can be removed using the trim_qubits method of the Circuit class.

Steps to Reproduce (minimal example)
The ansatz in the project Neon should be a 2 qubit circuit, but it clearly indicates a width of 4 qubits, even though there are only gates on the first 2.

Environment
MacOS 11.6.1

[ValentinS4t1qbit]

Thank you for reporting this ! I think you raise an important point we may not have looked into: meta VQEs or complex algorithms in general may not have a constant number of qubits. Maybe it can grow, maybe it can go down as well. When going to a different step or concatenating circuits, maybe we should try trimming once in a while and updating qubit number if relevant to ensure we keep resources low during runtime.

I think it's probably a small surgery we can do inside the code of ADAPT itself, unless we foresee VQE itself may have fancy ansatze whose number of qubits-in-use may change because... stuff.

[alexfleury-sb]

Closed with #214.