Quantum Computing Overcomes Disorder, Achieving 99.9% Fidelity with Pulse Optimization

Disorder, inherent in the fabrication of solid-state devices, presents a significant obstacle to building reliable quantum computers. Riccardo Aiudi, Julien Despres, and Roberto Menta, from Planckian, alongside Guido Menichetti and Vittorio Giovannetti from the Dipartimento di Fisica dell’Universit`a di Pisa, investigate how imperfections in the physical structure of a superconducting quantum computer affect its ability to perform calculations. Their work focuses on a globally-controlled architecture and reveals that even small amounts of disorder degrade the performance of essential quantum operations and information transfer. However, the team demonstrates that carefully designed pulse sequences, optimised using a technique called GRAPE, can overcome these limitations, achieving exceptionally high fidelity, exceeding 99. 9%, and paving the way for robust and reliable quantum computation in real-world devices.

Disorder Mitigation in Globally Controlled Superconducting Qubits

This research addresses a significant challenge in building larger quantum computers: the impact of imperfections on globally-controlled superconducting qubits. Global control, where all qubits are addressed simultaneously, simplifies wiring but is highly sensitive to variations in qubit properties. Scientists investigated a ladder-like architecture, a two-dimensional arrangement of qubits, to understand and mitigate these effects. The team employed a powerful optimization algorithm called GRAPE, which finds the best control pulses to maximize fidelity, even with imperfections present. To scale up the optimization, researchers developed a new method based on Matrix Product States, a technique for efficiently representing complex quantum states.

This advanced method allowed the team to optimize control pulses for a 15-qubit ladder, demonstrating the feasibility of scaling up optimization for larger systems. The optimized pulse sequences cleverly exploit small fluctuations to overcome the effects of disorder, achieving 96% fidelity for a Hadamard gate on the 15-qubit ladder in 160 nanoseconds with 2% disorder. The research reveals that variations in qubit frequencies are more detrimental to performance than variations in coupling strength, demonstrating a pathway to building more robust and scalable quantum computers by addressing a critical bottleneck in scaling up superconducting quantum computers.

Fabrication Imperfections in Superconducting Qubit Ladders

Scientists have investigated how fabrication imperfections impact a globally-controlled superconducting quantum computing architecture, specifically a ladder-based design utilizing three distinct types of superconducting qubits. Researchers employed numerical simulations to quantify how variations in qubit resonant frequencies and coupling strengths affect quantum state propagation and the fidelity of fundamental quantum operations. The simulations assessed the detrimental effects of static disorder on single-qubit rotations, two-qubit entangling gates, and overall quantum information transport. To counteract these effects, the team pioneered the application of pulse optimization techniques, specifically the Gradient Ascent Pulse Engineering (GRAPE) algorithm, tailoring the control signals to minimize errors caused by imperfections.

Optimized pulse sequences achieved high-fidelity operations, exceeding 99. 9% for the three quantum operations examined, restoring reliable universal quantum logic and robust information flow. This highlights pulse optimization as a powerful strategy for enhancing the resilience of solid-state, globally-driven quantum computing platforms, supporting the viability of this architecture by demonstrating that disorder can be effectively mitigated.

Robust Quantum Control Amidst Fabrication Disorder

Scientists have demonstrated a pathway to robust quantum computation using a globally-driven superconducting qubit architecture, achieving high-fidelity operations despite the presence of fabrication-induced disorder. The research focuses on a two-dimensional ladder geometry comprising three types of superconducting qubits, each driven by a shared control signal, and systematically investigates how imperfections in qubit parameters impact performance. Numerical simulations revealed that static disorder significantly affects single-qubit rotations, two-qubit entangling gates, and quantum information transport within the system. To counteract these detrimental effects, the team implemented pulse optimization techniques, specifically the GRAPE algorithm, to fine-tune the control signals.

Results demonstrate that optimized pulse sequences can achieve remarkably high-fidelity operations, exceeding 99. 9% for all three quantum operations examined. This breakthrough restores reliable universal quantum logic and robust information flow, even when realistic levels of disorder are present, validating the viability of globally-controlled architectures and supporting the development of scalable and resilient quantum processors.

Disorder Mitigation via Optimal Control Strategies

This research demonstrates that globally-controlled superconducting quantum computing architectures, specifically a ladder-based design, can effectively overcome the detrimental effects of fabrication-induced static disorder through optimized control strategies. Simulations reveal that even modest levels of disorder in qubit resonance frequencies significantly reduce the fidelity of quantum operations. However, the team successfully employed the GRAPE algorithm, a quantum optimal control technique, to identify robust pulse sequences tailored to specific disorder realizations, restoring high-fidelity performance exceeding 99% for both single- and two-qubit gates. Notably, these optimized sequences not only recover performance but also enable faster gate implementations, reducing operation times by more than a factor of two while maintaining high fidelity. While the simulations were limited to a small system, the approach is scalable in principle, supporting the viability of globally-controlled superconducting quantum architectures as a scalable alternative to fully local control, offering reduced wiring complexity and improved hardware efficiency.