[Bug report] Missing external fluxes from split-transmon

[QuantumWitness]

Documentation check

If the bug manifests in unexpected behavior (as opposed to a crash), confirm that you have consulted the
API documentation

• I have checked the API documentation.
• I could not locate relevant information in the documentation or information is missing.

Describe the bug

When constructing a split-junction transmon from explicit circuit elements (not using the pre-made class), no external flux variables are created. Thus, flux biasing the transmon is impossible.

Expected behavior

Expected one external flux variable to be able to bias the split-junction transmon.

To Reproduce

Here's the code I use to create the transmon:

import scqubits as scqub

tmon_yaml = """# transmon circuit
branches:
- ["JJ", 1, 0, EJ=12,.01]
- ["JJ", 1, 0, EJ,0.01]
- ["C", 0, 1, EC=0.2]
"""

tmon = scqub.Circuit.from_yaml(tmon_yaml, from_file=False,
                               ext_basis="discretized")

tmon.sym_external_fluxes()

OS and version used (please complete the following information):

• OS: macOS 12.1
• scqubits version 3.0.0
• Python version 3.8

Additional context

I tried this both with a floating and with a grounded transmon and received the same result. When I replace one junction with an inductor with $E_L = E_J$, I get the expected external flux variable, but not the expected eigenspectrum. I was hoping I could just define this using only junctions and a capacitor, to avoid making extra nodes, but perhaps that is just not possible?

EDIT: I guess I'm not sure if this is a bug, it's just not what I expected. What's the correct work-around?

Also do you want me to keep posting issues to get help figuring out scQubits or is there a discord/slack/somethingelse channel?

[jkochNU]

I can confirm that issue, and we are looking into a bug fix.

There's a temporary workaround: switch the order of nodes in one of the two JJ branches:

tmon_yaml = """# transmon circuit
branches:
- ["JJ", 1, 0, EJ=12,.01]
- ["JJ", 0, 1, EJ,0.01]
- ["C", 0, 1, EC=0.2]
"""

[QuantumWitness]

EDIT: The workaround for the YAML definition did work to get me the external fluxes I wanted, thank you.

Updated to latest version and offset charges are appearing correctly now, too.

[ZhaoTianPu]

This issue has been addressed with 1dab4f1 and released with v3.0.2.