equilibrium physics

[Roger-luo]

• getting a ground state from sparse matrix
• measure observables wrap up some common observables from Yao #8
• entanglement entropy (support entropies QuantumBFS/YaoArrayRegister.jl#88)

[fanglifl]

Here is an written example for the ground state:
https://github.com/Happy-Diode/RydbergEmulatorExamples.jl/blob/main/Ground_States_Ladder_1.ipynb

The goal would be writting some wrapper functions to simplify the code.

[Roger-luo]

I think we can just close this now, since we mention how to use Krylov.jl and other solver on our hamiltonian now. We may want to support converting our hamiltonian expression to ITensor definition in the future, but let's do that in separate issue