Add a helper for qobj compatibility

In qutip-v5, a bra ket multiplication returns a number instead of a Qobj. This makes it hard to keep the code clean and compatible for both version. This helper function changes the returned value so that it is identical to the one returned by qutip-v4.

doc/pulse-paper/customize.py

@@ -15,7 +15,8 @@
 import numpy as np
 from qutip import (
     fidelity, sigmax, sigmay, sigmaz, basis, qeye, tensor, Qobj, fock_dm)
-from qutip_qip.circuit import QubitCircuit, Gate
+from qutip_qip.circuit import QubitCircuit
+from qutip_qip.operations import Gate
 from qutip_qip.device import ModelProcessor, Model
 from qutip_qip.compiler import GateCompiler, Instruction
 import qutip

doc/pulse-paper/dj_algorithm.py

@@ -18,6 +18,8 @@
     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
+
 
 # Deutsch-Josza algorithm
 dj_circuit = QubitCircuit(num_qubits)
@@ -79,7 +81,6 @@
     ax[i].fill_between([full_tlist[point2], full_tlist[point3]], [vmin ,vmin],  [vmax, vmax], color="lightgray", alpha=0.5)
     ax[i].vlines([full_tlist[point2], full_tlist[point3]], vmin, vmax, "gray", "--", linewidth=0.8, alpha=0.5)
 
-fig.tight_layout()
 fig.savefig("optimal_control_pulse.pdf")
 fig.show()
 
@@ -124,7 +125,6 @@
 
 full_tlist = scqubits_processor.get_full_tlist()
 
-fig3.tight_layout()
 fig3.savefig("transmon_pulse.pdf")
 fig3.show()
 
@@ -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(tmp[0, 0]))
+    expect.append(np.real(_qobj_compatibility_helper(tmp)[0, 0]))
 
 fig5, ax5 = plt.subplots(figsize=(LINEWIDTH, LINEWIDTH*0.7), dpi=200)
 ax5.plot(t_record, expect, color="slategrey")

src/qutip_qip/utilities.py

@@ -0,0 +1,16 @@
+from itertools import chain
+from functools import reduce
+from packaging.version import parse as parse_version
+import numpy as np
+import qutip
+
+
+def _qobj_compatibility_helper(qobj):
+    """
+    Make qobj generated by operations in qutip-v5 identical to the one by qutip-v4.
+    Here, if `qobj` is a number (e.g. generated by a bra-ket multiplication), it is replaced by a Qobj. 
+    """
+    if parse_version(qutip.__version__) >= parse_version("5.dev"):
+        if np.isscalar(qobj):
+            return qutip.Qobj([[qobj]], dims=[[1], [1]])
+    return qobj

src/qutip_qip/vqa.py

@@ -7,6 +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
 
 
 class VQA:
@@ -429,10 +430,8 @@ def cost_derivative(self, U, dU):
         dCost = (init.dag() * dU.dag()) * obs * (U * init) + (
             init.dag() * U.dag()
         ) * obs * (dU * init)
-        if isinstance(dCost, Qobj):  # qutip version < 5
-            return np.real(dCost.full().item())
-        else:  # qutip version >= 5
-            return np.real(dCost)
+        dCost = _qobj_compatibility_helper(dCost)
+        return np.real(dCost.full().item())
 
     def compute_jac(self, angles, indices_to_compute=None):
         """