I would like to build a quantum circuit that executes a gate based on mid-circuit measurements. In particular I want to create a $n$ qubit GHZ state using the scheme formulated to this question:https://quantumcomputing.stackexchange.com/a/26678/26583

How can I use qiskit classical expressions: https://docs.quantum.ibm.com/api/qiskit/circuit_classical to compute a sum mod 2 of $a + b + c,$ where $a$, $b$, and $c$ are mid-circuit measurement results? I don't need to use qiskit but anything that can access IBM backends.

Thank you!


1 Answer 1


Assume that your classical register is cr, then using Qiskit you can XOR the first three bits then apply an $X$-gate based on the expression value as follows:

from qiskit.circuit.classical import expr

_condition = expr.bit_xor(expr.bit_xor(cr[0], cr[1]), cr[2])
with circ.if_test(_condition):

Note that, you can get a full implementation for the mentioned circuit from this blog post.

Edit: here is a full example tested with both Qiskit 0.46 and Qiskit 1.0:

from qiskit.circuit import QuantumCircuit, QuantumRegister, ClassicalRegister
from qiskit.circuit.classical import expr
from qiskit.providers.fake_provider import GenericBackendV2
from qiskit import transpile

qr = QuantumRegister(4, 'q')
cr = ClassicalRegister(3, 'c')
cr2 = ClassicalRegister(1, 'c2')
circ = QuantumCircuit(qr, cr, cr2)

circ.measure(qr[0:3], cr)

_condition = expr.bit_xor(expr.bit_xor(cr[0], cr[1]), cr[2])
with circ.if_test(_condition):

circ.measure(qr[3], cr2)

# Execute the circuit:
backend = GenericBackendV2(num_qubits=len(circ.qubits), control_flow=True)
tr_circ = transpile(circ, backend)
result = backend.run(tr_circ).result()
  • $\begingroup$ When I try to use such a block in my circuit I get a TypeError: cannot unpack non-iterable Binary object. I am using the following code to simulate the circuit: python backend = GenericBackendV2(num_qubits=len(qc.qubits), control_flow=True) qc_trans = transpile(qc, backend) result = backend.run(qc_trans).result() $\endgroup$
    – ty.
    Commented Apr 22 at 20:31
  • $\begingroup$ I added a full example to my answer. Try it on your machine. Or, add your circuit-building code to the question so that I can examine it. $\endgroup$ Commented Apr 23 at 5:38
  • $\begingroup$ I didn't have Aer installed. pip install qiskit-aer solved it. Thank you for the help! $\endgroup$
    – ty.
    Commented Apr 23 at 6:33

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