QSeqSim: A Symbolic Simulator for Qiskit While Loops Using Sequential Quantum Circuits

2026-05-14

AI Summary

This work addresses the lack of native simulation support in Qiskit for quantum programs containing while loops and their resulting sequential quantum circuits. The authors propose a transformation-based approach grounded in OpenQASM 3 that establishes a unified model encompassing combinatorial, dynamic, and sequential circuits, thereby endowing while loops with precise sequential semantics. They introduce specialized symbolic operators tailored for state composition and preservation, integrating Binary Decision Diagrams (BDDs) with weighted model counting to enable efficient symbolic simulation. This method provides, for the first time in Qiskit, exact sequential circuit semantics for while loops and demonstrates strong scalability by successfully simulating quantum random walk benchmarks involving over 1,000 qubits and more than ten loop iterations.

Abstract

We present a tool QSeqSim, a Qiskit-integrated symbolic backend that fills the current gap of having no Qiskit-native support for simulating while-loop quantum programs and their induced sequential quantum circuits. QSeqSim takes Qiskit QuantumCircuit objects, translates them into OpenQASM 3 code, and organises the resulting program into a combination of combinational, dynamic, and sequential circuits, thereby assigning while-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 state composition and state retention, thereby enabling efficient symbolic execution of sequential quantum circuits. Our experiments demonstrate that QSeqSim scales to substantial while-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.

Problem

• quantum simulation
• while loops
• sequential quantum circuits
• Qiskit
• symbolic execution

Innovation

• symbolic simulation
• sequential quantum circuits
• while loops
• Binary Decision Diagrams (BDD)
• weighted model counting