ProcessTomography fails when the number of prepared and measured qubits differ

[chriseclectic]

Informations

• Qiskit Experiments version: 0.2
• Python version: All
• Operating system: All

What is the current behavior?

If process tomography is performed on a subset of qubits in a circuit with a different number of preparation and measurement qubits analysis will raise an exception.

Steps to reproduce the problem

Run an experiment like

qc = QuantumCircuit(3)
qc.h(0)
qc.x(1)
qc.s(2)

qptexp = ProcessTomography(
    qc, preparation_qubits=[0], measurement_qubits=[1, 2])
qptdata = qptexp.run(AerSimulator()).block_for_results()

Raises ValueError: cannot reshape array of size 64 into shape (8,2,2)

What is the expected behavior?

Experiment returns a fitted channel with 1 input qubit and 2 output qubits

Suggested solutions

This failure is because the analysis condition for checking the trace-preserving constraint assumes the input and output dimension of the channel are equal.