Quantum
Robust Quantum Gate Preparation in Open Environments
Accepted in 2025 American Control Conference (ArXiv)
In our research, we developed an optimal control algorithm aimed at enhancing the robustness of quantum gate operations within open quantum systems—those that interact with their surrounding environments. Utilizing the Lindblad master equation to represent the quantum state's evolution, our method employs adaptive linearization coupled with iterative quadratic programming. This approach incrementally refines control signals to achieve optimal performance. To address uncertainties inherent in quantum systems, we introduced uncertain parameters into the master equation and expanded the parameterized state using Legendre polynomials, facilitating exponential convergence rates. Notably, when robustness considerations are omitted and signal constraints are relaxed, our algorithm simplifies to the well-known GRadient Ascent Pulse Engineering (GRAPE) method.
We applied our algorithm to the preparation of Controlled NOT (CNOT) and SWAP quantum gates, achieving remarkable precision. By incorporating just second-order Legendre polynomials, our method demonstrated unprecedented resilience to 100% uncertainty in the interaction strength between qubits and effectively compensated for 20% uncertainty in signal intensity. These results suggest that our approach could significantly advance the implementation of robust quantum gates and circuits, even in challenging environments with substantial hardware limitations.
The significance of this work lies in its potential to enhance the reliability and accuracy of quantum computations. By effectively managing environmental interactions and parameter uncertainties, our algorithm contributes to the development of more stable and dependable quantum information processing systems. This advancement is crucial for the practical realization of quantum computing technologies, where precise control over quantum gate operations is essential for achieving computational advantages over classical systems.
Correlated Noise Enhancement of Coherence and Fidelity in Coupled Qubits
Philosophical Magazine, Volume 104, Issue 13-14, 2024 (ArXiv)
Our research investigates how correlated environmental noise affects open quantum systems. We modelled a quantum communication scenario using two interacting qubits where each qubit experiences a different local environment with different levels of correlation between these environments. We found that noise correlation can have profound effects on the fidelity and purity of entangled (Bell) states. By observing how different initial Bell states evolve, we can determine the correlation between two local environments. This could be useful for designing high-fidelity quantum gates and communication protocols.
Specifically, anticorrelated longitudinal noise led to sharper and more intense transitions in the absorption spectrum, as local fluctuations cancelled out. We also showed that anticorrelated noise helps the Φ+ Bell state maintain purity for longer, while correlated noise helps preserve the purity and fidelity of the Ψ+ Bell state.
This is important for quantum information because maintaining the fidelity of entangled resources is crucial for tasks like super-dense coding and quantum teleportation. Our results suggest that understanding and manipulating noise correlations could be key to preserving quantum resources.
For quantum error modelling, our work highlights that correlations within the noise are important and that simplified models assuming independent noise might be insufficient. The fact that certain correlated noise can enhance coherence offers a potential new direction for error management, possibly through engineering environments that are more resistant to decoherence.
In essence, our paper demonstrates that noise correlations play a significant and sometimes beneficial role in quantum systems, with important implications for developing robust quantum technologies.
