parameters in RXXGate

[Muzhou-Ma]

Environment

python version 3.10.11

What is happening?

I'm trying to give RXXGate a parameter phi when I encounter this error:
ParameterExpression with unbound parameters ({Parameter(a)}) cannot be cast to a float.

Here is my codes:

phi = Parameter('a')
qc = QuantumCircuit(4)
a = RXXGate(phi).control(2)
qc = qc.compose(a)
qc.draw()

However, it's interesting that everything works fine if the control method isn't used:

phi_1 = Parameter('a')
qc = QuantumCircuit(4)
a = RXXGate(theta = phi_1)
qc = qc.compose(a, [2, 3])
qc.draw()

which can give the result:

q_0: ───────────
                
q_1: ───────────
     ┌─────────┐
q_2: ┤0        ├
     │  Rxx(a) │
q_3: ┤1        ├
     └─────────┘

I'm wondering what I've done wrong and how to fix it?

Besides, is it possible for me to make this parameter trainable for optimization tasks like using gradient descend?

How can we reproduce the issue?

Perhaps making the Parameter class more flexible can help.

What should happen?

I suppose the gate should take this parameter and give it a random(or zero) initialization.

Any suggestions?

No response

[Raghav-Bell]

You can stop the error even without assigning any value in control()

[Raghav-Bell]

This error is raised from the following method.

qiskit/qiskit/circuit/parameterexpression.py

Lines 474 to 482 in 57137ff

	def __float__(self):
	try:
	return float(self._symbol_expr)
	# TypeError is for sympy, RuntimeError for symengine
	except (TypeError, RuntimeError) as exc:
	if self.parameters:
	raise TypeError(
	"ParameterExpression with unbound parameters ({}) "
	"cannot be cast to a float.".format(self.parameters)

Check PR #10244 .

[jakelishman]

I strongly suspect that this is the same root as #7326, because the internal definition of the multi-controlled $R_{xx}$ gate will use CU, which has issues in Qiskit at the moment. There's a potential fix in #9118 that I've just bumped up the priority list, because it keeps getting forgotten (by me - sorry!).

[Muzhou-Ma]

Thanks for pointing out, hope to see the update soon!

[jakelishman]

Sorry to come back to this after a long time with somewhat bad news: this isn't related to #9118, and the problem is that some of our controlled-gate synthesis code can't cope with parametrised gates in certain circumstances. In particular, we fail if asked to control a parametric RZGate with more than a single control state:

from qiskit.circuit import QuantumCircuit, Parameter

a = Parameter("a")
qc = QuantumCircuit(1)
qc.rz(a, 0)
gate = qc.to_gate()

gate.control(2)

<...>

File ~/code/qiskit/terra/qiskit/circuit/add_control.py:148, in control(operation, num_ctrl_qubits, label, ctrl_state)
    139     controlled_circ.mcry(
    140         gate.definition.data[0].operation.params[0],
    141         q_control,
   (...)
    145         use_basis_gates=True,
    146     )
    147 elif gate.name == "rz":
--> 148     controlled_circ.mcrz(
    149         gate.definition.data[0].operation.params[0],
    150         q_control,
    151         q_target[bit_indices[qargs[0]]],
    152         use_basis_gates=True,
    153     )
    154     continue
    155 elif gate.name == "p":

File ~/code/qiskit/terra/qiskit/circuit/library/standard_gates/multi_control_rotation_gates.py:381, in mcrz(self, lam, q_controls, q_target, use_basis_gates)
    378         self.append(CRZGate(lam), control_qubits + [target_qubit])
    379 else:
    380     cgate = _mcsu2_real_diagonal(
--> 381         RZGate(lam).to_matrix(),
    382         num_controls=len(control_qubits),
    383         use_basis_gates=use_basis_gates,
    384     )
    385     self.compose(cgate, control_qubits + [target_qubit], inplace=True)

<...>

TypeError: ParameterExpression with unbound parameters (dict_keys([Parameter(a)])) cannot be cast to a float.

That example is the root cause of your bug.

@ewinston, @alexanderivrii: do you two have any ideas of how we might go about modifying the synthesis to allow this?

[SimoneGasperini]

This is related to issue #10311.

[ShellyGarion]

closing this issue as #12752 is merged