Avoid dependency on storing tomography target in metadata

qiskit_experiments/library/tomography/qpt_analysis.py

@@ -83,11 +83,9 @@ def _default_options(cls) -> Options:
             rescale_trace (bool): If True rescale the state returned by the fitter have either
                 trace 1 (Default: True).
             target (Union[str, :class:`~qiskit.quantum_info.operators.channel.quantum_channel`,
-                :class:`~qiskit.quantum_info.Operator`]): Set a custom target quantum
+                :class:`~qiskit.quantum_info.Operator`]): Optional, Set a custom target quantum
                 channel for computing the :func:~qiskit.quantum_info.process_fidelity` of the
-                fitted process against. If ``"default"`` the ideal process corresponding for
-                the input circuit will be used. If ``None`` no fidelity will be computed
-                (Default: "default").
+                fitted process against (Default: None).
         """
         options = super()._default_options()
         options.measurement_basis = PauliMeasurementBasis()

qiskit_experiments/library/tomography/qpt_experiment.py

@@ -14,8 +14,11 @@
 """
 
 from typing import Union, Optional, Iterable, List, Tuple, Sequence
+import numpy as np
 from qiskit.circuit import QuantumCircuit, Instruction
 from qiskit.quantum_info.operators.base_operator import BaseOperator
+from qiskit.quantum_info import Choi, Operator, Statevector, partial_trace
+from qiskit_experiments.exceptions import QiskitError
 from .tomography_experiment import TomographyExperiment
 from .qpt_analysis import ProcessTomographyAnalysis
 from . import basis
@@ -88,5 +91,50 @@ def __init__(
             preparation_qubits=preparation_qubits,
             basis_indices=basis_indices,
             qubits=qubits,
+            analysis=ProcessTomographyAnalysis(),
         )
-        self.analysis = ProcessTomographyAnalysis()
+
+        # Set target quantum channel
+        self.analysis.set_options(target=self._target_quantum_channel())
+
+    def _target_quantum_channel(self) -> Union[Choi, Operator]:
+        """Return the process tomography target"""
+        # Check if circuit contains measure instructions
+        # If so we cannot return target state
+        circuit_ops = self._circuit.count_ops()
+        if "measure" in circuit_ops:
+            return None
+        perm_circ = self._permute_circuit()
+        try:
+            if "reset" in circuit_ops or "kraus" in circuit_ops or "superop" in circuit_ops:
+                channel = Choi(perm_circ)
+            else:
+                channel = Operator(perm_circ)
+        except QiskitError:
+            # Circuit couldn't be simulated
+            return None
+
+        total_qubits = self._circuit.num_qubits
+        if self._meas_qubits:
+            num_meas = len(self._meas_qubits)
+        else:
+            num_meas = total_qubits
+        if self._prep_qubits:
+            num_prep = len(self._prep_qubits)
+        else:
+            num_prep = total_qubits
+
+        if num_prep == total_qubits and num_meas == total_qubits:
+            return channel
+
+        # Trace out non-measurement subsystems
+        tr_qargs = []
+        if self._prep_qubits:
+            tr_qargs += list(range(num_prep, total_qubits))
+        if self._meas_qubits:
+            tr_qargs += list(range(total_qubits + num_meas, 2 * total_qubits))
+
+        chan_state = Statevector(np.ravel(channel, order="F"))
+        chan_state = partial_trace(chan_state, tr_qargs) / 2 ** (total_qubits - num_meas)
+        channel = Choi(chan_state.data, input_dims=[2] * num_prep, output_dims=[2] * num_meas)
+        return channel

qiskit_experiments/library/tomography/qst_analysis.py

@@ -79,11 +79,9 @@ def _default_options(cls) -> Options:
             rescale_trace (bool): If True rescale the state returned by the fitter have either
                 trace 1 (Default: True).
             target (Union[str, :class:`~qiskit.quantum_info.DensityMatrix`,
-                :class:`~qiskit.quantum_info.Statevector`]): Set a custom target quantum
-                state for computing the :func:~qiskit.quantum_info.state_fidelity` of the
-                fitted state against. If ``"default"`` the ideal state corresponding for
-                the input circuit will be used. If ``None`` no fidelity will be computed
-                (Default: "default").
+                :class:`~qiskit.quantum_info.Statevector`]): Optional, et a custom target
+                quantum state for computing the :func:~qiskit.quantum_info.state_fidelity`
+                of the fitted state against (Default: None).
         """
         options = super()._default_options()
         options.measurement_basis = PauliMeasurementBasis()

qiskit_experiments/library/tomography/qst_experiment.py

@@ -16,7 +16,8 @@
 from typing import Union, Optional, Iterable, List, Sequence
 from qiskit.circuit import QuantumCircuit, Instruction
 from qiskit.quantum_info.operators.base_operator import BaseOperator
-from qiskit.quantum_info import Statevector
+from qiskit.quantum_info import Statevector, DensityMatrix, partial_trace
+from qiskit_experiments.exceptions import QiskitError
 from .tomography_experiment import TomographyExperiment
 from .qst_analysis import StateTomographyAnalysis
 from . import basis
@@ -87,5 +88,38 @@ def __init__(
             measurement_qubits=measurement_qubits,
             basis_indices=basis_indices,
             qubits=qubits,
+            analysis=StateTomographyAnalysis(),
         )
-        self.analysis = StateTomographyAnalysis()
+
+        # Set target quantum state
+        self.analysis.set_options(target=self._target_quantum_state())
+
+    def _target_quantum_state(self) -> Union[Statevector, DensityMatrix]:
+        """Return the state tomography target"""
+        # Check if circuit contains measure instructions
+        # If so we cannot return target state
+        circuit_ops = self._circuit.count_ops()
+        if "measure" in circuit_ops:
+            return None
+
+        perm_circ = self._permute_circuit()
+        try:
+            if "reset" in circuit_ops or "kraus" in circuit_ops or "superop" in circuit_ops:
+                state = DensityMatrix(perm_circ)
+            else:
+                state = Statevector(perm_circ)
+        except QiskitError:
+            # Circuit couldn't be simulated
+            return None
+
+        total_qubits = self._circuit.num_qubits
+        if self._meas_qubits:
+            num_meas = len(self._meas_qubits)
+        else:
+            num_meas = total_qubits
+        if num_meas == total_qubits:
+            return state
+
+        # Trace out non-measurement qubits
+        tr_qargs = range(num_meas, total_qubits)
+        return partial_trace(state, tr_qargs)

qiskit_experiments/library/tomography/tomography_analysis.py

@@ -77,7 +77,8 @@ def _default_options(cls) -> Options:
             rescale_trace (bool): If True rescale the state returned by the fitter
                 have either trace 1 for :class:`~qiskit.quantum_info.DensityMatrix`,
                 or trace dim for :class:`~qiskit.quantum_info.Choi` matrices (Default: True).
-            target (Any): depends on subclass.
+            target (Any): Optional, target object for fidelity comparison of the fit
+                (Default: None).
         """
         options = super()._default_options()
 
@@ -87,7 +88,7 @@ def _default_options(cls) -> Options:
         options.fitter_options = {}
         options.rescale_positive = True
         options.rescale_trace = True
-        options.target = "default"
+        options.target = None
         return options
 
     @classmethod
@@ -107,12 +108,6 @@ def _run_analysis(self, experiment_data):
             experiment_data.data()
         )
 
-        # Get target state
-        target_state = self.options.target
-        if target_state == "default":
-            metadata = experiment_data.metadata
-            target_state = metadata.get("target", None)
-
         # Get tomography fitter function
         fitter = self._get_fitter(self.options.fitter)
         try:
@@ -136,7 +131,7 @@ def _run_analysis(self, experiment_data):
             analysis_results = self._postprocess_fit(
                 state,
                 metadata=fitter_metadata,
-                target_state=target_state,
+                target_state=self.options.target,
                 rescale_positive=self.options.rescale_positive,
                 rescale_trace=self.options.rescale_trace,
                 qpt=bool(self.options.preparation_basis),