Set Initial Layout for State Tomography

Question

I am trying to do some state tomography to generate GHZ states and calculate fidelity. For running the code, I want to execute my code with the qubits with the lowest readout error using initial layout in the transpilation process. However, I arrive at this error and I can not specify the qubits .

Traceback (most recent call last):
  Cell In[28], line 2
    best_qc = transpile(st, backend, initial_layout=[90,94,95])
  File /opt/conda/lib/python3.10/site-packages/qiskit/compiler/transpiler.py:338 in transpile
    output_name = _parse_output_name(output_name, circuits)
  File /opt/conda/lib/python3.10/site-packages/qiskit/compiler/transpiler.py:593 in _parse_output_name
    return [circuit.name for circuit in circuits]
  File /opt/conda/lib/python3.10/site-packages/qiskit/compiler/transpiler.py:593 in <listcomp>
    return [circuit.name for circuit in circuits]
AttributeError: 'StateTomography' object has no attribute 'name' 




from qiskit import *
from qiskit.visualization import array_to_latex
from qiskit_experiments.library import StateTomography
q1 = QuantumRegister(1)
q2 = QuantumRegister(1)
q3 = QuantumRegister(1)
c = ClassicalRegister(3)
qc = QuantumCircuit(q1, q2, q3, c)
qc.h(q1)
qc.cx(q1, q2)
qc.ccx(q1, q2, q3)
st = StateTomography(qc)
best_qc = transpile(st, backend, initial_layout=[90,94,95])
stdata = st.run(backend).block_for_results()
state_result = stdata.analysis_results("state")
array_to_latex(state_result.value)
fid_result = stdata.analysis_results("state_fidelity")
print(f"state Fidelity = {fid_result.value}") 




                                                                                                                                                                                                                                                                                                                                                                                              

Answer 1

You can't use transpile with an instance of StateTomography because it is not a circuit. The proper way to achieve your goal is to use StateTomography parameter physical_qubits.

So, you should replace these two lines:

st = StateTomography(qc)
best_qc = transpile(st, backend, initial_layout=[90,94,95])

with

st = StateTomography(qc, physical_qubits=[90,94,95])

Internally, StateTomography.run() method will transpile the experiment circuits and use physical_qubits as initial_layout.

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Note, you can use the following code snippet if you want to check that the specified qubits are actually used:

from qiskit.visualization import plot_circuit_layout

plot_circuit_layout(st._transpiled_circuits()[0], backend=backend, view='physical')