Unable to Create Multi-Controlled Gates with More Than 3 Controls in Qiskit

[Augusto12]

Environment

• Qiskit version: 1.0.2
• Python version: 3.10
• Operating system: Ubuntu 22.04

What is happening?

I am encountering an issue where I cannot create multi-controlled gates with more than 3 controls in Qiskit.

How can we reproduce the issue?

Code Example:

from qiskit import QuantumCircuit
from qiskit.circuit import Parameter

theta1 = Parameter('θ1')

qc = QuantumCircuit(5)
qc.ry(theta1,4)
qc.control(4,ctrl_state=0)

qc.draw("mpl")

What should happen?

I got the error: TypeError: ParameterExpression with unbound parameters (dict_keys([Parameter(θ1)])) cannot be cast to a float.

Any suggestions?

No response

[ShellyGarion]

Thanks for finding this bug. This issue should be solved after #12752 will be merged.

[ShellyGarion]

closing this issue since #12752 was merged

[Augusto12]

Were you able to run the code from this issue after the merge? @ShellyGarion

[ShellyGarion]

@Augusto12 - you should slightly update your code with the new parameter annotated as follows:

        from qiskit import QuantumCircuit
        from qiskit.circuit import Parameter

        theta1 = Parameter('θ1')

        qc = QuantumCircuit(5)
        qc.ry(theta1, 4)
        qc.control(4, ctrl_state=0, annotated=True)
        qc.draw() 

gives the circuit:

q_0: ──────────
               
q_1: ──────────
               
q_2: ──────────
               
q_3: ──────────
     ┌────────┐
q_4: ┤ Ry(θ1) ├
     └────────┘

[Cryoris]

@ShellyGarion's solution works, alternatively you could create the RYGate directly which doesn't require you arguing about the annotation. This is also a cleaner solution as it doesn't create an intermediate gate called "ry" which is then controlled later on.

from qiskit.circuit import QuantumCircuit, Parameter
from qiskit.circuit.library import RYGate

theta1 = Parameter('θ1')
mcry = RYGate(theta1).control(4, ctrl_state=0)

qc = QuantumCircuit(5)
qc.append(mcry, range(5))
qc.draw()

giving

q_0: ────o─────
         │
q_1: ────o─────
         │
q_2: ────o─────
         │
q_3: ────o─────
     ┌───┴────┐
q_4: ┤ Ry(θ1) ├
     └────────┘

[Augusto12]

Thanks!

[Augusto12]

Hi again.

With this approach, I can no longer access the parameters.

The code:

qc.parameters

Always returns ParameterView([])

[ShellyGarion]

This code:

        from qiskit import QuantumCircuit
        from qiskit.circuit import Parameter

        theta1 = Parameter('θ1')

        qc = QuantumCircuit(5)
        qc.ry(theta1, 4)
        qc.control(4, ctrl_state=0, annotated=True)
        qc.parameters 

outputs:
ParameterView([Parameter(θ1)])

[Augusto12]

Ok. @ShellyGarion , could you share your qc.data? When you shared qc.draw(), the printed circuit was without controls. I'm confused.

[ShellyGarion]

@Augusto12 - you should slightly update your code as follows (note that in your original code you have printed the circuit qc which has only ry, and not the updated controlled circuit). Please make sure that you have the updated version of Qiskit.

        from qiskit import QuantumCircuit
        from qiskit.circuit import Parameter

        theta1 = Parameter('θ1')

        qc = QuantumCircuit(5, name="my_ry")
        qc.ry(theta1, 4)
        qc1 = qc.control(4, ctrl_state=0, annotated=True)
        print (qc1.parameters)
        print (qc1.data)
        print (qc1)

outputs:

ParameterView([Parameter(θ1)])
[CircuitInstruction(operation=<qiskit.circuit.annotated_operation.AnnotatedOperation object at 0x7f8a5e94c380>, qubits=(Qubit(QuantumRegister(4, 'q0'), 0), Qubit(QuantumRegister(4, 'q0'), 1), Qubit(QuantumRegister(4, 'q0'), 2), Qubit(QuantumRegister(4, 'q0'), 3), Qubit(QuantumRegister(5, 'q'), 0), Qubit(QuantumRegister(5, 'q'), 1), Qubit(QuantumRegister(5, 'q'), 2), Qubit(QuantumRegister(5, 'q'), 3), Qubit(QuantumRegister(5, 'q'), 4)), clbits=())]
                    
q0_0: ──────o───────
            │       
q0_1: ──────o───────
            │       
q0_2: ──────o───────
            │       
q0_3: ──────o───────
      ┌─────┴──────┐
 q_0: ┤0           ├
      │            │
 q_1: ┤1           ├
      │            │
 q_2: ┤2 My_ry(θ1) ├
      │            │
 q_3: ┤3           ├
      │            │
 q_4: ┤4           ├
      └────────────┘

[ShellyGarion]

In addition, the code that @Cryoris suggested above produces the following output (with the latest version of Qiskit).

        from qiskit.circuit import QuantumCircuit, Parameter
        from qiskit.circuit.library import RYGate

        theta1 = Parameter('θ1')
        mcry = RYGate(theta1).control(4, ctrl_state=0)

        qc = QuantumCircuit(5)
        qc.append(mcry, range(5))
        print (qc.parameters)
        print (qc.data)
        print (qc)

outputs:

ParameterView([Parameter(θ1)])
[CircuitInstruction(operation=<qiskit.circuit.annotated_operation.AnnotatedOperation object at 0x7f8a5e859430>, qubits=(Qubit(QuantumRegister(5, 'q'), 0), Qubit(QuantumRegister(5, 'q'), 1), Qubit(QuantumRegister(5, 'q'), 2), Qubit(QuantumRegister(5, 'q'), 3), Qubit(QuantumRegister(5, 'q'), 4)), clbits=())]
               
q_0: ────o─────
         │     
q_1: ────o─────
         │     
q_2: ────o─────
         │     
q_3: ────o─────
     ┌───┴────┐
q_4: ┤ Ry(θ1) ├
     └────────┘