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

TL;DR

QSeqSim is a symbolic backend for Qiskit that enables efficient simulation of while-loop quantum programs and sequential circuits using BDDs and symbolic operators.

Contribution

It introduces a Qiskit-integrated symbolic simulator supporting while-loops with precise semantics and scalable simulation capabilities.

Findings

Successfully simulates circuits with over 1000 qubits and 10 loop iterations

Uses BDD-based symbolic representation for efficient probability computation

Integrates with Qiskit and translates circuits into OpenQASM 3

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.