Qiskit experiments state tomography maximum allowed dimension exceeded for pulse gates

Question

I'm trying to run state tomography on a custom pulse gate (currently just a CR(pi/4) pulse) and any attempt to run it on an IBM backend gives a maximum allowed dimension exceeded error. The code runs fine for any standard gates on the backend.

All the potentially relevant code is:

from qiskit_experiments.library import StateTomography
from qiskit import pulse

service = QiskitRuntimeService()
backend = service.get_backend(name='ibm_kyoto')

#Extracts the CR(pi/4) pulse
from qiskit.pulse import Play

backend_defaults = backend.defaults()
inst_sched_map = backend_defaults.instruction_schedule_map 
cr_pulse = inst_sched_map.get('ecr', (1,0)).filter(channels = [ControlChannel(1)], instruction_types=[Play]).instructions[0][1].pulse

with pulse.build(backend, name="Default CR") as cr_sched_default:
        pulse.play(cr_pulse, ControlChannel(1))


from qiskit.circuit import Gate
from qiskit.pulse import builder
from qiskit.transpiler import InstructionProperties

# Makes the circuit, turns it into a schedule
circ = QuantumCircuit(4)
custom_gate = Gate('cr', num_qubits=2, params=[])
circ.add_calibration('cr', [0,1], cr_sched_default)
circ.append(custom_gate, [0,1])


backend.target.add_instruction(
    custom_gate, 
    {(0,1): InstructionProperties(calibration=cr_sched_default)},
)
circ = transpile(circ, backend)

#runs the state tomography and returns the results
from qiskit_ibm_provider import IBMProvider

provider = IBMProvider()
backend = provider.get_backend(name='ibm_kyoto')

with backend.open_session() as session:
    qstexp = StateTomography(circ, measurement_indices=[0,1])
    qstdata = qstexp.run(backend).block_for_results()

    for result in qstdata.analysis_results():
        print(result)
    state_result = qstdata.analysis_results("state")
    print(state_result.value)

Much of the code comes from the qiskit textbook: https://github.com/Qiskit/textbook/blob/main/notebooks/quantum-hardware-pulses/hamiltonian-tomography.ipynb

The error is:

ValueError                                Traceback (most recent call last)
Cell In[97], line 10
      6 backend = provider.get_backend(name='ibm_kyoto')
      9 with backend.open_session() as session:
---> 10     qstexp = StateTomography(circ, measurement_indices=[0,1])
     11     qstdata = qstexp.run(backend).block_for_results()
     13     for result in qstdata.analysis_results():

File ~\AppData\Local\Programs\Python\Python312\Lib\site-packages\qiskit_experiments\warnings.py:169, in deprecate_arguments.<locals>.decorator.<locals>.wrapper(*args, **kwargs)
    160 if kwargs:
    161     _rename_kwargs(
    162         args[0].__class__.__name__ + "." + func.__name__,
    163         kwargs,
   (...)
    167         stacklevel,
    168     )
--> 169 return func(*args, **kwargs)

File ~\AppData\Local\Programs\Python\Python312\Lib\site-packages\qiskit_experiments\library\tomography\qst_experiment.py:122, in StateTomography.__init__(self, circuit, backend, physical_qubits, measurement_basis, measurement_indices, basis_indices, conditional_circuit_clbits, analysis, target)
    120 if isinstance(self.analysis, TomographyAnalysis):
    121     if target == "default":
--> 122         target = self._target_quantum_state()
    123     self.analysis.set_options(target=target)

File ~\AppData\Local\Programs\Python\Python312\Lib\site-packages\qiskit_experiments\library\tomography\qst_experiment.py:138, in StateTomography._target_quantum_state(self)
    136         state = DensityMatrix(circuit)
    137     else:
--> 138         state = Statevector(circuit)
    139 except QiskitError:
    140     # Circuit couldn't be simulated
    141     return None

File ~\AppData\Local\Programs\Python\Python312\Lib\site-packages\qiskit\quantum_info\states\statevector.py:91, in Statevector.__init__(self, data, dims)
     89     self._data = np.ravel(data.data)
     90 elif isinstance(data, (QuantumCircuit, Instruction)):
---> 91     self._data = Statevector.from_instruction(data).data
     92 else:
     93     raise QiskitError("Invalid input data format for Statevector")

File ~\AppData\Local\Programs\Python\Python312\Lib\site-packages\qiskit\quantum_info\states\statevector.py:765, in Statevector.from_instruction(cls, instruction)
    763     instruction = instruction.to_instruction()
    764 # Initialize an the statevector in the all |0> state
--> 765 init = np.zeros(2**instruction.num_qubits, dtype=complex)
    766 init[0] = 1.0
    767 vec = Statevector(init, dims=instruction.num_qubits * (2,))

ValueError: Maximum allowed dimension exceeded

Does anyone know what am I missing?

Answer 1

Had the same problem and managed to solve it: There is an optional argument you can pass to the StateTomography function called target. If you do not pass it, the default is to simulate the circuit. Since your backend have a lot of qubits, that fills the machine RAM and it collapses. You can fix it by passing None instead:

qstexp = StateTomography(circuit, measurement_indices = [0,1], target = None )