0-qubit MPS circuits

[vincentmr]

The Circuit constructor allows me to create a 0-qubit circuit as follows

circ = qtn.Circuit(0)

If I would like to use the MPS_computational_state constructor

tn = qtn.MPS_computational_state("")

it fails as

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vincent.michaud-rioux/Documents/repos/pennylane/venv/lib/python3.10/site-packages/quimb/tensor/tensor_builder.py", line 2934, in MPS_computational_state
    return MPS_product_state(tuple(gen_arrays()), cyclic=cyclic, **mps_opts)
  File "/home/vincent.michaud-rioux/Documents/repos/pennylane/venv/lib/python3.10/site-packages/quimb/tensor/tensor_builder.py", line 2904, in MPS_product_state
    return MatrixProductState(mps_arrays, shape="lrp", **mps_opts)
  File "/home/vincent.michaud-rioux/Documents/repos/pennylane/venv/lib/python3.10/site-packages/quimb/tensor/tensor_1d.py", line 1360, in __init__
    self.cyclic = (ops.ndim(arrays[0]) == 3)
IndexError: tuple index out of range

Is that expected behaviour? Is there a way to create empty circuits with MPS_computational_state?

[jcmgray]

I would suggest that both should error! but yes it appears this is not implemented yet.

[vincentmr]

I would suggest that both should error!

This is really an edge case. I would say it depends whether one can do anything with a 0-qubit object (e.g. list available backends for example). I'm not shocked that it errors ;)