Fix QuantumVolumeAnalysis (#711)

Fixes QuantumVolumeAnalysis to remove unnecessary dependence on the QuantumVolume experiment instance being stored in the ExperimentData, since this is not saved to the result DB and will be missing from loaded data.

Author: chriseclectic

qiskit_experiments/library/quantum_volume/qv_analysis.py

@@ -19,6 +19,7 @@
 
 import numpy as np
 import uncertainties
+from qiskit_experiments.exceptions import AnalysisError
 from qiskit_experiments.curve_analysis import plot_scatter, plot_errorbar
 from qiskit_experiments.framework import (
     BaseAnalysis,
@@ -55,14 +56,19 @@ def _default_options(cls) -> Options:
         return options
 
     def _run_analysis(self, experiment_data):
-        depth = experiment_data.experiment.num_qubits
         data = experiment_data.data()
         num_trials = len(data)
+        depth = None
         heavy_output_prob_exp = []
 
         for data_trial in data:
+            trial_depth = data_trial["metadata"]["depth"]
+            if depth is None:
+                depth = trial_depth
+            elif trial_depth != depth:
+                raise AnalysisError("QuantumVolume circuits do not all have the same depth.")
             heavy_output = self._calc_ideal_heavy_output(
-                data_trial["metadata"]["ideal_probabilities"], data_trial["metadata"]["depth"]
+                data_trial["metadata"]["ideal_probabilities"], trial_depth
             )
             heavy_output_prob_exp.append(
                 self._calc_exp_heavy_output_probability(data_trial, heavy_output)