Rewrite the compatibility helper function

doc/pulse-paper/dj_algorithm.py

@@ -18,7 +18,7 @@
     OptPulseProcessor, LinearSpinChain, SCQubits, SpinChainModel)
 from qutip_qip.circuit import QubitCircuit
 from qutip import sigmaz, sigmax, identity, tensor, basis
-from qutip_qip.utilities import _qobj_compatibility_helper
+from qutip_qip.compat import to_scalar
 
 
 # Deutsch-Josza algorithm
@@ -146,7 +146,7 @@
 for state in result1.states:
     tmp = state.ptrace([0,1])
     tmp = basis([2,2], [0,0]).dag() * tmp * basis([2,2], [0,0])
-    expect.append(np.real(_qobj_compatibility_helper(tmp)[0, 0]))
+    expect.append(np.real(to_scalar(tmp)))
 
 fig5, ax5 = plt.subplots(figsize=(LINEWIDTH, LINEWIDTH*0.7), dpi=200)
 ax5.plot(t_record, expect, color="slategrey")

src/qutip_qip/compat.py

@@ -0,0 +1,15 @@
+"""
+For the compatibility between qutip-v5 and v4.
+"""
+from itertools import chain
+from functools import reduce
+from packaging.version import parse as parse_version
+import numpy as np
+import qutip
+
+
+def to_scalar(qobj_or_scalar):
+    if isinstance(qobj_or_scalar, qutip.Qobj):
+        if qobj_or_scalar.dims == [[1], [1]]:
+            return qobj_or_scalar[0, 0]
+    return qobj_or_scalar

src/qutip_qip/vqa.py

@@ -7,7 +7,7 @@
 from scipy.optimize import minimize
 from scipy.linalg import expm_frechet
 from .operations import gate_sequence_product
-from .utilities import _qobj_compatibility_helper
+from .compat import to_scalar
 
 
 class VQA:
@@ -430,8 +430,8 @@ def cost_derivative(self, U, dU):
         dCost = (init.dag() * dU.dag()) * obs * (U * init) + (
             init.dag() * U.dag()
         ) * obs * (dU * init)
-        dCost = _qobj_compatibility_helper(dCost)
-        return np.real(dCost.full().item())
+        dCost = to_scalar(dCost)
+        return np.real(dCost)
 
     def compute_jac(self, angles, indices_to_compute=None):
         """