Classical feedforward and control flow

qiskit[all]~=2.0.0

# returns True if backend supports dynamic circuits
"dynamic_circuits" in backend.configuration().supported_features

This guide demonstrates the functionality available in the Qiskit SDK for performing classical feedforward and control flow. These features are sometimes referred to collectively as "dynamic circuits." Classical feedforward refers to the ability to measure qubits in the middle of a circuit and perform additional quantum operations that depend on the measurement outcome. Qiskit supports four control flow constructs for classical feedforward, each implemented as a method on QuantumCircuit. The constructs and their corresponding methods are:

• If statement - QuantumCircuit.if_test
• If statement - QuantumCircuit.if_else

Each of these methods returns a context manager and is typically used in a with statement. In the rest of this guide, we will explain each of these constructs and how to use them.

If statement

The if statement is used to conditionally perform operations based on the value of a classical bit or register.

In the example below, we apply a Hadamard gate to a qubit and measure it. If the result is 1, then we apply an X gate on the qubit, which has the effect of flipping it back to the 0 state. We then measure the qubit again. The resulting measurement outcome should be 0 with 100% probability.

from qiskit.circuit import QuantumCircuit, QuantumRegister, ClassicalRegister
 
qubits = QuantumRegister(1)
clbits = ClassicalRegister(1)
circuit = QuantumCircuit(qubits, clbits)
(q0,) = qubits
(c0,) = clbits
 
circuit.h(q0)
circuit.measure(q0, c0)
with circuit.if_test((c0, 1)):
    circuit.x(q0)
circuit.measure(q0, c0)
circuit.draw("mpl")
 
# example output counts: {'0': 1024}

Output:

The with statement can be given an assignment target which is itself a context manager that can be stored and subsequently used to create an else block, which is executed whenever the contents of the if block are not executed.

In the example below, we initialize registers with two qubits and two classical bits. We apply a Hadamard gate to the first qubit and measure it. If the result is 1, then we apply a Hadamard gate on the second qubit; otherwise, we apply an X gate on the second qubit. Finally, we measure the second qubit as well.

qubits = QuantumRegister(2)
clbits = ClassicalRegister(2)
circuit = QuantumCircuit(qubits, clbits)
(q0, q1) = qubits
(c0, c1) = clbits
 
circuit.h(q0)
circuit.measure(q0, c0)
with circuit.if_test((c0, 1)) as else_:
    circuit.h(q1)
with else_:
    circuit.x(q1)
circuit.measure(q1, c1)
 
circuit.draw("mpl")
 
# example output counts: {'01': 260, '11': 272, '10': 492}

Output:

In addition to conditioning on a single classical bit, it's also possible to condition on the value of a classical register composed of multiple bits.

In the example below, we apply Hadamard gates to two qubits and measure them. If the result is 01, that is, the first qubit is 1 and the second qubit is 0, then we apply an X gate to a third qubit. Finally, we measure the third qubit. Note that for clarity, we chose to specify the state of the third classical bit, which is 0, in the if condition. In the circuit drawing, the condition is indicated by the circles on the classical bits being conditioned on. A black circle indicates conditioning on 1, while a white circle indicates conditioning on 0.

qubits = QuantumRegister(3)
clbits = ClassicalRegister(3)
circuit = QuantumCircuit(qubits, clbits)
(q0, q1, q2) = qubits
(c0, c1, c2) = clbits
 
circuit.h([q0, q1])
circuit.measure(q0, c0)
circuit.measure(q1, c1)
with circuit.if_test((clbits, 0b001)):
    circuit.x(q2)
circuit.measure(q2, c2)
 
circuit.draw("mpl")
 
# example output counts: {'101': 269, '011': 260, '000': 252, '010': 243}

Output:

Limitations

Be aware of the following constraints when using dynamic circuits:

• Whenever you do a measure followed by an if (c), the values that are measured and subsequently used in the if condition has a limit of 60 bits.
• Nested conditionals are not allowed.
• Having reset inside conditionals is not supported.

Next steps