Contributed Session: Quantum Control
Paper ID: 25
Authors: Sofiane Chalal, Nina H. Amini, Hamed Amini and Mathieu Laurière
Title: Dynamic Programming Principle and Stabilization for Mean-Field Quantum Filtering Systems
Abstract: Working within the quantum filtering framework, we establish a dynamic programming principle in an infinitedimensional setting by embedding the state space into the Hilbert–Schmidt space. We then study a stabilization problem for continuously monitored Ising-coupled qubits and, in the mean-field limit, demonstrate quantum state reduction together with exponential convergence toward prescribed eigenstates under suitable feedback laws.
Paper ID: 39
Authors: Re-Bing Wu
Title: Dense Encoding of Single-Flux-Quantum Pulse Sequences for High-Performance Qubit Control
Abstract: Superconducting quantum computing faces scalability challenges due to the complexity and thermal load of microwave control systems. Single-Flux-Quantum (SFQ) technology offers a low-power alternative using picosecond voltage pulses. However, storing SFQ pulse sequences on-chip introduces a trade-off among storage, fidelity, and gate speed. We propose a dense encoding scheme—inspired by microwave control—that reduces storage needs and enhances effective control power, thereby speeding up operations. Our results show that dense encoding accelerates gates via constructive pulse interference. This work highlights encoding as a key design principle for scalable quantum control.
Paper ID: 47
Authors: Yuan-Hung Kuan and Jr-Shin Li
Title: Quantum Ensemble Control in Bose–Einstein Condensates
Abstract: In this paper, we investigate the quantum ensemble control problem of Bose-Einstein condensates (BECs) formulated within the Gross–Pitaevskii framework. Starting from the mean-field description of a dilute, weakly interacting condensate, we formulate inhomogeneities in the external potential and interaction strength as a BEC ensemble, whose dynamics are described by an infinite-dimensional ensemble Gross–Pitaevskii equation. To address this challenging quantum ensemble control problem, we introduce a unitary transformation referred to as the moment operator, which maps the ensemble condensate state into a hierarchy of moment quantum states evolving in a separable Hilbert space. We leverage the established moment representation to address state transfer problems in BECs through moment truncations. This reduction results in both analytically and numerically tractable control scenarios, bridging the gap between the complex ensemble Gross–Pitaevskii dynamics and the practical control design for infinite-dimensional inhomogeneous quantum systems.
Paper ID: 71
Authors: Thomas Schulte-Herbrueggen, Gunther Dirr and Emanuel Malvetti
Title: How to Break Symmetries for Tomographying Coherently Controlled Bilinear Quantum Systems
Abstract: In quantum engineering one wants to know “what one can do” with a given controlled dynamical system when starting with given initial conditions. The degree to which such a system can be controlled, accessed, observed, or tomographied is readily assessed by symmetries entailing invariant subspaces. Key are those symmetries of the system’s infinitesimal generators (in the adjoint representation acting on states in Liouville space) that are shared by projections onto an observable or POVMs or onto an initial state. — We can thus give guidelines how to break symmetries to bring quantum systems under control or observation or tomography. They are part of an overarching Lie-frame for quantum systems theory.
Paper ID: 76
Authors: Amit Devra, Emanuel Malvetti, Niklas J. Glaser, Abhishek Agarwal, Santana Lujan, Max Werninghaus, Stefan Filipp, Leo Van Damme and Steffen J. Glaser
Title: Dynamical Decoupling using Universal Optimal Tracking
Abstract: Dynamical decoupling (DD) is a widely used and resource‑efficient technique for error suppression, but conventional DD relies on periodically repeating a short pulse block to refocus the qubit state during idle periods. Imperfections in this block cause residual errors to accumulate, ultimately degrading state recovery over long idle times. Here, we introduce a universal optimal tracking approach that extends the original tracking concept to a fully state‑independent setting for designing DD sequences. By monitoring the qubit’s evolution at predefined waypoints during optimization, the method dynamically compensates residual errors while preserving regular refocusing. Experimental demonstrations on a superconducting‑qubit platform confirm the suppression of error accumulation under static control imperfections, in agreement with numerical predictions. Complementary simulations further show that optimal‑tracking‑based sequences maintain strong performance under time‑dependent noise, outperforming standard DD protocols in relevant regimes. These results establish optimal tracking as a practical and hardware‑agnostic approach to designing short, robust DD sequences suitable for noisy quantum devices.
Paper ID: 99
Authors: Junkai Zeng and Xiu-Hao Deng
Title: Fundamental Costs of Noise-Robust Quantum Control: Speed Limits and Complexity
Abstract: Noise is ubiquitous in quantum systems and is a major obstacle for the advancement of quantum information science. Noise-robust quantum control achieves high-fidelity operations by engineering the evolution path so that first-order noise contributions cancel at the final time. Such dynamical error correction typically incurs a time overhead beyond standard quantum speed limits. We derive general lower bounds on control complexity that quantify this overhead for quasi-static coherent noise under bounded control amplitude. For a single noise source, we prove a universal time lower bound for first-order robustness and give a constructive scheme that implements any target gate robustly in time 4T plus a constant time. For robustness against an entire noise space, we show dimension lower bounds on the number M of segments in any mixed-unitary schedule from two mechanism: (i) a coherent dimension bound when the error subspace contains an irreducible block isomorphic to su(q), and (ii) a projection dimension bound when the noise space contains the trace-zero span of orthogonal projectors. Under bounded speed, these bounds on number of segments imply time lower bounds. With only local controls robust against noise space defined on a graph, we obtain a graph-orthogonality time bound scales linear with graph chromatic number. We illustrate the bounds through examples. Collectively, these results establish quantitative limitations on the feasibility of first-order noise-resilient operations.