Dynamic Circuits

qc/hardware qc/dynamic

A traditional circuit is a fixed pipeline: Preparation → Quantum Operations → Final Measurement. Every operation is decided in advance; measurement only happens at the very end.

A dynamic circuit breaks that rule: it allows mid-circuit measurement, followed by classical logic that decides what happens next — a gate applied conditionally based on a measurement outcome. This is often called feedforward (or feedback).

Two building blocks make this possible:

• Mid-circuit measurement — measuring a qubit before the circuit ends, storing the result in a classical bit.
• Reset + reuse — qc.reset(q) reinitializes a measured qubit back to $|0⟩$ so it can be reused later in the same circuit. This matters a lot when physical qubits are scarce.
• Conditional operations — with qc.if_test((cr[i], 1)): ... applies gates only if a classical bit matches a given value.

qc.measure(qr[i], cr[0])
qc.reset(qr[i])
with qc.if_test((cr[0], 1)):
    qc.x(qr[j])

The headline use case explored in this lab is Dynamic GHZ via Qubit Reuse — using this mechanism to build long-range entanglement without needing every qubit to be directly, physically connected to every other qubit.

Related

• Dynamic GHZ via Qubit Reuse
• GHZ States
• Coupling Map and Topology

Self-Check

• Could you explain “feedforward” to someone who’s only seen fixed-pipeline circuits before?
• What are the three building blocks that make a dynamic circuit possible?
• Why does reset-and-reuse matter when physical qubits are scarce?