SynDebug: A Framework to Debug Logic Bugs in Fault-Tolerant Quantum Computing Programs

The gist SynDebug introduces a systems-level debugging framework for FTQC programs that enables lightweight assertion-based validation directly within encoded execution flows, demonstrating that practical FTQC debugging can be achieved with a low additional circuit-depth overhead (1.8% on average) and no measurable increase in the logical error rate>

The Problem

Debugging fault-tolerant quantum computing programs is substantially more difficult than debugging conventional software or even noisy intermediate-scale quantum (NISQ) programs because existing approaches do not naturally extend to the complex execution flows of FTQC systems> Many quantum software bugs arise from mistakes in program logic rather than failures in the underlying hardware, including reversed gate ordering, incorrect gate operands, and swapped control and target qubits> Existing debugging approaches primarily target logical circuits in the NISQ regime, but FTQC hardware requires these logical circuits to be translated into complex FTQC implementations for execution> Consequently, existing approaches do not naturally support debugging in this regime>

The Gap

The gap identified is that existing approaches provide important foundations but introduce constraints when extended to FTQC systems> These constraints include the need for frameworks that remain generalizable across diverse, rapidly evolving hardware technologies and QEC methodologies> Furthermore, FTQC systems already incur extremely large physical qubit overheads due to QEC encoding, making it undesirable for debugging infrastructures to require additional logical surfaces or dedicated debugging qubits> Additional debugging gates and measurements may increase circuit depth and runtime> Finally, debugging techniques must remain scalable to large encoded systems without relying on infeasible approaches such as full-state tomography or exponentially scaling simulation>

Design Principles

SynDebug follows five core principles guiding its design across hardware and QEC methods> These include: (1) generality across hardware and QEC methods through stabilizer-code abstractions, (2) no debuggingspecific logical surface, (3) minimal temporal perturbation, (4) scalability through lightweight logical observables, and (5) a developer-facing logical API> More broadly, developers specify logical invariants while SynDebug handles their physical realization> Probabilistic behavior is assessed by comparing observed and expected outcome distributions using sound statistical tests>

The Solution: SynDebug Primitives

SynDebug introduces three user-facing debugging primitives to validate logical states, superposition structure, and entanglement relationships during FTQC execution> These primitives are: (1) assert classical, (2) assert superposition, and (3) assert entanglement> The framework is designed at the stabilizer-code abstraction level to remain compatible with diverse QEC methodologies while avoiding additional qubit overhead and minimizing the execution cost of debugging>

Implementation Details

The implementation architecture involves a four-level abstraction hierarchy for managing multi-patch CSS code circuits, layered atop the MQT QECC library’s code-theoretic primitives> The design introduces two coexisting addressing modes: physical level and logical level, which reference the same underlying physical register> The CodePatch class serves as both a physical-layout descriptor and a logical-qubit handle, with the offset acting as the single point of coupling between the two views> Logical Pauli operators are obtained by querying the patch and are returned as lists of physical indices already translated into the global register’s coordinate system>

Performance Metrics

SynDebug is designed to meet specific requirements, including R1 Generalizability Across Hardware+QEC Paradigms, R2 Zero Spatial Overhead, R3 Minimal Temporal Overhead, R4 Scalability to Large-Scale FTQC Systems, and R5 Practicality and Developer Usability> The evaluation across multiple QEC codes (Color, Surface, Tetrahedral, Golay, Steane) and code distances demonstrates that SynDebug enables practical FTQC debugging with an average 1.8% additional circuit depth overhead (0% in most cases) and no measurable increase in the logical error rate> This low overhead is achieved because the measurements required for assertion checking can be performed during the QEC syndrome extraction by design, without incurring any additional depth overhead>

Case Studies

SynDebug's utility is demonstrated through multiple case studies:

1. State Preparation Debugging: Using assert classical verifies that the state matches a specified computational basis (e.g., initializing a register to the integer 2) during initialization phase, which is useful for catching program bugs during the initialization phase>

2. Entanglement Checking: Using assert entanglement checks whether an expected entanglement relationship exists between logical qubit registers, utilizing a contingency table chi-squared test to determine consistency with expected correlations> This assertion acts as a checkpoint if execution must proceed beyond that location>

3. Shor’s Algorithm Debugging: This case study shows how assertions can be composed to support the debugging of complex quantum programs, using assert classical to verify arithmetic outputs and assert entanglement to validate control register correlation prior to the inverse quantum Fourier transform>

SynDebug is designed at the system-level abstraction for generic Stabilizer Codes, ensuring that no additional ancilla are required for debugging> The framework focuses on detecting logic-level bugs in FTQC programs through lightweight statistical assertions and observable-based debugging primitives> It does not attempt to solve formal verification or detect every possible class of quantum software bug, but it targets fundamental software bugs like incorrect gate ordering and missing operations>

The work concludes that SynDebug represents an important first step toward practical software reliability infrastructures for large-scale fault-tolerant quantum computing systems> This framework is generalizable to devices across superconducting, neutral-atom, and trapped-ion architectures while avoiding additional qubit overhead and minimizing the execution cost of debugging> It is open-sourced to support reproducibility and future FTQC debugging research>

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SynDebug’s name reflects how debugging is performed through QEC syndromes and encoded execution structures in FTQC implementations.

1. Bold assertion-based validation during FTQC execution: SynDebug introduces assert classical, assert superposition, and assert entanglement primitives to allow developers to validate logical states, superposition structure, and entanglement relationships during FTQC execution. This enables debugging by checking invariants like a register being in a known basis state or verifying entanglement relationships.

2. Zero spatial overhead design: The framework is designed to avoid increasing the physical qubit footprint because it avoids requiring additional logical surface area or dedicated debugging qubits, ensuring compatibility with existing hardware constraints where practical FTQC systems already have large physical qubit overheads for logical encoding.

3. Minimal temporal perturbation: SynDebug minimizes runtime cost by designing assertions to be performed during computation, resulting in an average 1.8% increase in circuit depth and 0% [increase] in most cases, which is demonstrated across various QEC codes and workloads.

4. Generalizability across hardware and QEC methods: By focusing debugging at the stabilizer-code abstraction level, SynDebug ensures it remains compatible with evolving QEC methodologies (like Surface, Color, or Steane codes) and diverse hardware architectures (superconducting, neutral-atom, trapped-ion).

5. Logic bug detection in complex algorithms: The framework supports debugging of intricate quantum routines by using a combination of assertions; for example, Shor's algorithm debugging uses assert classical to verify initialization states and assert entanglement to confirm correlations between control and working registers.

Fault-tolerant quantum computing (FTQC) introduces substantial systems-level complexity through quantum error correction (QEC), making debugging FTQC programs challenging. Existing quantum debugging approaches primarily target logical-level circuits and do not naturally support debugging within realistic FTQC execution flows. We present SynDebug, the first systems-level debugging framework for FTQC programs. SynDebug introduces lightweight assertion-based debugging primitives that operate directly within encoded FTQC infrastructures while avoiding additional logical surface area and minimizing temporal overhead. We evaluate SynDebug across multiple QEC code families, code distances, and encoded workloads, including state preparation, entanglement validation, and Shor-style programs. Our results demonstrate that SynDebug enables practical FTQC debugging with an average 1.8% additional circuit depth overhead (0% in most cases) and no measurable increase in the logical error rate, showing that systems-level debugging for large-scale FTQC applications is both feasible and practical.