QSeqSim: A Symbolic Simulator for Qiskit While Loops Using Sequential Quantum Circuits (Long Tool Paper)

Abstract We present a tool QSeqSim , a Qiskit-integrated symbolic backend that fills the current gap of having no Qiskit-native support for simulating -loop quantum programs and their induced sequential quantum circuits. QSeqSim takes Qiskit objects, translates them into OpenQASM 3 code, and organises the resulting program into a combination of combinational, dynamic, and sequential circuits, thereby assigning -loops a precise sequential circuit semantics with explicit internal and external qubits. Building on this semantics, QSeqSim adopts a Binary Decision Diagram (BDD)-based symbolic representation and integrates weighted model counting to compute measurement probabilities efficiently by exploiting sharing in structured and sparse BDDs. On top of this Boolean backbone, it introduces dedicated symbolic operators for quantum state composition and state retention, thereby enabling efficient symbolic execution of sequential quantum circuits. Our experiments demonstrate that QSeqSim scales to substantial -induced sequential circuits; in particular, in the quantum random walk benchmark we successfully simulate circuits with over 1000 qubits for more than 10 loop iterations. QSeqSim is available at https://github.com/Veri-Q/QSeqSim .