What did I do this week?
Over the past week, I’ve been working on analyzing two open issues in the QuTiP-QOC GitHub repository, as discussed in our weekly meeting. These issues highlight important discrepancies in the behavior of optimization algorithms for open quantum systems and represent a significant opportunity to improve reliability and correctness.
📌 Issue #47: JOPT Fails for Open-System State Transfer
Summary:
When using the JOPT algorithm to solve a simple open-system state transfer problem (qubit from ground to excited state), the optimization crashes with a ValueError related to a dimension mismatch in the Liouville representation.
Findings: - The issue arises during the construction of the adjoint of a matrix difference in the JOPT fidelity computation. - The mismatch in shapes ((4, 1) vs (1, 4)) suggests a bug in how the density matrix difference is reshaped and handled in Liouville space. - Notably, the exact same control setup works fine with GOAT, indicating that this is a JOPT-specific bug.
📌 Issue #46: GRAPE Infidelity Mismatch
Summary:
The GRAPE algorithm reports a final infidelity below the target threshold (0.0008), but manual simulation using the optimized control returns a significantly worse infidelity (0.0038).
Findings: - Manual evolution was independently verified using both exponential propagators and mesolve, and the results are consistent. - This suggests that GRAPE either overestimates fidelity internally or there’s a bug in its cost function or propagation logic. - A ~300% mismatch raises concerns about the reliability of convergence metrics, especially in open-system scenarios.
Plan for next week?
Next week, I’ll focus on:
- Digging deeper into the JOPT dimension mismatch in open-system Liouville space dynamics (Issue #47) and verifying reshaping logic and adjoint consistency
- Investigating GRAPE’s fidelity evaluation pathway (Issue #46) to locate possible discrepancies in how cost values are tracked or computed internally
- Prototyping potential fixes in a sandboxed environment without affecting core modules
- Depending on the outcome of these investigations, I plan to raise a PR addressing one or both issues if the proposed fixes prove reliable and non-breaking
“I’m motivated by the opportunity to resolve foundational bugs and improve reliability in QuTiP-QOC’s optimization backend — each fix brings us closer to a more robust and trustworthy control framework.”