Creating a Parameterised Gate Using Unitary Matrix

[vandit2209]

Environment

• Qiskit Terra version: 0.24.0
• Python version: 3.8.5
• Operating system: Linux-3.10.0-957.el7.x86_64-x86_64-with-glibc2.10
  OS printed using platform.platform

What is happening?

Error

RuntimeError                              Traceback (most recent call last)
File [~/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:474]/~/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:474, in ParameterExpression.__float__(self)
/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:473 try:
/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:474     return float(self._symbol_expr)
/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:475 # TypeError is for sympy, RuntimeError for symengine

File symengine_wrapper.pyx:1151, in symengine.lib.symengine_wrapper.Basic.__float__()

File symengine_wrapper.pyx:976, in symengine.lib.symengine_wrapper.Basic.n()

File symengine_wrapper.pyx:4346, in symengine.lib.symengine_wrapper.evalf()

RuntimeError: Symbol cannot be evaluated.

The above exception was the direct cause of the following exception:

TypeError                                 Traceback (most recent call last)
[/qrl/test.ipynb/qrl/test.ipynb) Cell 19 line 1
line=9 theta = Parameter("theta")
line=11 circuit.x(0)
line=12 cos_val = cos(float(theta) [/] 2)
line=13 sin_val = -1j * sin(float(theta) [/] 2)
line=14 circuit.data.insert(
line=15     0,
line=16     CircuitInstruction(
   (...)
line=26     ),
line=27 )

File [~/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:477/qrl/~/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:477), in ParameterExpression.__float__(self)
    /qrl/~/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:475) # TypeError is for sympy, RuntimeError for symengine
    /qrl/~/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:476) except (TypeError, RuntimeError) as exc:
--> /qrl/~/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:477)     raise TypeError(
    /qrl/~/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:478)         "ParameterExpression with unbound parameters ({}) "
    /qrl/~/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:479)         "cannot be cast to a float.".format(self.parameters)
    /qrl/~/.conda/envs/ibm_q/lib/python3.8/site-packages/qiskit/circuit/parameterexpression.py:480)     ) from exc

TypeError: ParameterExpression with unbound parameters ({Parameter(theta)}) cannot be cast to a float.

How can we reproduce the issue?

Code:

from qiskit import QuantumCircuit, QuantumRegister
from qiskit.quantum_info.operators import Operator
from qiskit.circuit import Parameter, CircuitInstruction
from math import cos, sin, pi

controls = QuantumRegister(2)
circuit = QuantumCircuit(controls)
# circuit.unitary()
# cx = Operator([[1, 0, 0, 0], [0, 0, 0, 1], [0, 0, 1, 0], [0, 1, 0, 0]])
theta = Parameter("theta")

circuit.x(0)
cos_val = cos(float(theta) / 2)
sin_val = -1j * sin(float(theta) / 2)
circuit.data.insert(
    0,
    CircuitInstruction(
        operation=Operator(
            [
                [1, 0, 0, 0],
                [0, cos_val, 0, sin_val],
                [0, 0, 1, 0],
                [0, sin_val, 0, cos_val],
            ]
        ),
        qubits=[0, 1],
    ),
)
print(circuit)

What should happen?

Unbound Parameter error, originally initiated due to symengine

Any suggestions?

How do I create a Parameterised Gate from unitary matrix

[jakelishman]

We do not support symbolic manipulation of dense matrices in Qiskit and have no plans to add such support. You can look at community-driven packages such as qiskit-symb for that kind of capability.

I would note that the gate you're trying to add is a controlled X rotation, which is (in theory) supported by the RXGate(theta).control() object or the QuantumCircuit.crx method. I think at the moment that there's a potential bug in that (see #10311 and linked issues). Your code is also using several low-level internal objects, and using them incorrectly:

• You should not attempt to modify QuantumCircuit.data directly like that.
• CircuitInstruction is a low-level object that you largely should not need to use.
• The operation field of CircuitInstruction must be an instance of qiskit.circuit.Operation, not qiskit.quantum_info.Operator which is something entirely different.

[vandit2209]

Thanks a lot for the suggestion and the explanation

[jakelishman]

I'll close this issue as "solved" now, since I don't think there's anything more to do from the Qiskit side. Please feel free to re-open if there's more to discuss.