Hamiltoninan variational ansatz not working when considering Pauli-strings with only identity operators

[axz60]

Environment

• Qiskit version: 1.3.1
• Python version: 3.12.7
• Operating system: macOS 14.6.1 (23G93) Darwin 23.6.0

What is happening?
The hamiltonian variational ansatz can't produce circuits for pauli strings only containing identity operators.

How can we reproduce the issue?

Code to reproduce the error:

from qiskit.quantum_info import SparsePauliOp
from qiskit.circuit.library import hamiltonian_variational_ansatz

# this Hamiltonian will be split into the two terms [III, IZZ] and [IXI]

hamiltonian = SparsePauliOp(["III", "IZZ", "IXI"])
ansatz = hamiltonian_variational_ansatz(hamiltonian, reps=2)
ansatz.draw("mpl")

Error Generated:

	"name": "PanicException",
	"message": "internal error: entered unreachable code",
	"stack": "---------------------------------------------------------------------------
PanicException                            Traceback (most recent call last)
Cell In[110], line 6
      4 # this Hamiltonian will be split into the two terms [III, IZZ] and [IXI]
      5 hamiltonian = SparsePauliOp([\"III\", \"IZZ\", \"IXI\"])
----> 6 ansatz = hamiltonian_variational_ansatz(hamiltonian, reps=2)
      7 ansatz.draw(\"mpl\")

File /opt/anaconda3/envs/hva_test/lib/python3.12/site-packages/qiskit/circuit/library/n_local/evolved_operator_ansatz.py:265, in hamiltonian_variational_ansatz(hamiltonian, reps, insert_barriers, name, parameter_prefix)
    262 if isinstance(hamiltonian, SparsePauliOp):
    263     hamiltonian = hamiltonian.group_commuting()
--> 265 return evolved_operator_ansatz(
    266     hamiltonian,
    267     reps=reps,
    268     evolution=None,
    269     insert_barriers=insert_barriers,
    270     name=name,
    271     parameter_prefix=parameter_prefix,
    272     flatten=True,
    273 )

File /opt/anaconda3/envs/hva_test/lib/python3.12/site-packages/qiskit/circuit/library/n_local/evolved_operator_ansatz.py:141, in evolved_operator_ansatz(operators, reps, evolution, insert_barriers, name, parameter_prefix, remove_identities, flatten)
    136         param = next(param_iter)
    137         expanded_paulis += [
    138             (pauli, indices, 2 * coeff * param) for pauli, indices, coeff in term
    139         ]
--> 141 data = pauli_evolution(num_qubits, expanded_paulis, insert_barriers, False)
    142 circuit = QuantumCircuit._from_circuit_data(data, add_regs=True)
    143 circuit.name = name

PanicException: internal error: entered unreachable code"
}

Why does the error occur?

pauli_evolution(...) function takes a sparse list of pauli operators like so,

sparse_list = hamiltonian.to_sparse_list()
print(sparse_list)

#output
[(' ', [ ], np.complex128(1+0j)), ('ZZ', [0, 1], np.complex128(1+0j)), ('X', [1], np.complex128(1+0j))]

However if a pauli string only contains identity operators, they are defined by an empty string and array, which 'pauli_evolution(...)' can't handle.

The question
Is there anyway of circumventing this issue?

PS: Apologies if my syntaxing of this issue is incorrect, this is the first time I have raised an issue on github :)

[nihaocami]

Right I believe the wrong parameters are being passed here:

qiskit/qiskit/circuit/library/n_local/evolved_operator_ansatz.py

Line 142 in b872e88

data = pauli_evolution(num_qubits, expanded_paulis, insert_barriers, False)

As the pauli_evolution function in Rust expects different ones ?

qiskit/crates/accelerate/src/circuit_library/pauli_evolution.rs

Lines 47 to 52 in b872e88

	pub fn pauli_evolution<'a>(
	pauli: &'a str,
	indices: Vec<u32>,
	time: Param,
	phase_gate: bool,
	do_fountain: bool,

Not a 100% sure. Maybe someone from the team has a better explanation.

[Cryoris]

This is a duplicate of #13625 and will be fixed by #13634.

[axz60]

Thank very much @nihaocami and @Cryoris for having a look!

[axz60]

To implement the new changes, do I pip install qiskit again?

[Cryoris]

We have a bugfix release planned next week, once that is released you will be able to install qiskit==1.3.2. Until then, you'd have to install Qiskit from source to have the changes locally. See https://github.com/Qiskit/qiskit/blob/main/CONTRIBUTING.md#installing-qiskit-from-source for instructions on how to do that.

[axz60]

Cheers! Thanks again!