Circuit parameter binding does not bind global_phase in instruction definitions #10283
Description
Environment
- Qiskit Terra version: 0.24.1
- Python version: 3.10
- Operating system: macOS 13.3.1
What is happening?
When a custom gate with a set definition contains a parametrised global phase, any circuit that contains that custom gate will fail to bind the global phase during parameter assignment.
How can we reproduce the issue?
import math
from qiskit.circuit import Parameter, QuantumCircuit
from qiskit.quantum_info import Operator
x = Parameter("x")
custom = QuantumCircuit(1, global_phase=x).to_gate()
base = QuantumCircuit(1)
base.append(custom, [0], [])
Operator(base.assign_parameters({x: math.pi}))TypeError Traceback (most recent call last)
Cell In[1], line 12
9 base = QuantumCircuit(1)
10 base.append(custom, [0], [])
---> 12 Operator(base.assign_parameters({x: math.pi}))
File ~/code/qiskit/terra/qiskit/quantum_info/operators/operator.py:85, in Operator.__init__(self, data, input_dims, output_dims)
76 self._data = np.asarray(data, dtype=complex)
77 elif isinstance(data, (QuantumCircuit, Operation)):
78 # If the input is a Terra QuantumCircuit or Operation we
79 # perform a simulation to construct the unitary operator.
(...)
83 # conditional gates, measure, or reset will cause an
84 # exception to be raised.
---> 85 self._data = self._init_instruction(data).data
86 elif hasattr(data, "to_operator"):
87 # If the data object has a 'to_operator' attribute this is given
88 # higher preference than the 'to_matrix' method for initializing
89 # an Operator object.
90 data = data.to_operator()
File ~/code/qiskit/terra/qiskit/quantum_info/operators/operator.py:614, in Operator._init_instruction(cls, instruction)
612 if isinstance(instruction, QuantumCircuit):
613 instruction = instruction.to_instruction()
--> 614 op._append_instruction(instruction)
615 return op
File ~/code/qiskit/terra/qiskit/quantum_info/operators/operator.py:691, in Operator._append_instruction(self, obj, qargs)
689 else:
690 new_qargs = [qargs[bit_indices[tup]] for tup in instruction.qubits]
--> 691 self._append_instruction(instruction.operation, qargs=new_qargs)
File ~/code/qiskit/terra/qiskit/quantum_info/operators/operator.py:669, in Operator._append_instruction(self, obj, qargs)
666 if obj.definition.global_phase:
667 dimension = 2**obj.num_qubits
668 op = self.compose(
--> 669 ScalarOp(dimension, np.exp(1j * float(obj.definition.global_phase))),
670 qargs=qargs,
671 )
672 self._data = op.data
673 flat_instr = obj.definition
File ~/code/qiskit/terra/qiskit/circuit/parameterexpression.py:480, in ParameterExpression.__float__(self)
478 except (TypeError, RuntimeError) as exc:
479 if self.parameters:
--> 480 raise TypeError(
481 "ParameterExpression with unbound parameters ({}) "
482 "cannot be cast to a float.".format(self.parameters)
483 ) from None
484 try:
485 # In symengine, if an expression was complex at any time, its type is likely to have
486 # stayed "complex" even when the imaginary part symbolically (i.e. exactly)
487 # cancelled out. Sympy tends to more aggressively recognise these as symbolically
488 # real. This second attempt at a cast is a way of unifying the behaviour to the
489 # more expected form for our users.
490 cval = complex(self)
TypeError: ParameterExpression with unbound parameters ({Parameter(x)}) cannot be cast to a float.
What should happen?
All parameter usage in custom definitions should be bound.
Any suggestions?
This is due to ad-hoc recursion/iteration in QuantumCircuit._rebind_definition. It treats Instruction._definition like a list[tuple[Instruction, qubits, clbits]], which (a long time ago) was a possibility. Since Instruction._definition is now required to be a QuantumCircuit, the simplest fix is to make the definition-rebinding an explicit recursion via QuantumCircuit.assign_parameters.