[Feature]: QuantumSystem that stores composite system and level info for subspace embeddings

[aarontrowbridge]

Feature Description

I want to have the functionality to create, e.g., a two transmon system by first creating two individual transmon systems and combining them into one composite system, so for example

sys1 = Transmon(levels=4)
sys2 = Transmon(levels=4)
coupling = Coupling(sys1, sys2)
sys = QuantumSystem(sys1, sys2, coupling)

this will make some useful functionality possibility like, e.g.,

op = gate(:X)
embedded_op = embed_operator(op, system; component=1)

to embed gates in the qubit subspace of the multilevel system.

Importance

3

What does this feature affect?

• [X] quantum system construction
• [X] problem setup
• [ ] problem solution
• [ ] problem performance
• [X] solution analysis
• [ ] plotting
• [ ] documentation
• [ ] tests
• [ ] other (please specify below)

Other information

It should be noted that this will also affect the way leakage suppression is implemented.

[aarontrowbridge]

merge issues resulted in a forced push to main.