Scalable quantum computing in frequency-crowded nuclear spin registers promises long coherence times and native many-body logic. However, as system size increases, spectral crowding introduces two fundamentally competing challenges: coherent-control-induced spectral crosstalk and fragility to frequency fluctuations arising from charge and magnetic noise. Conventional pulse shaping optimized for crosstalk suppression is intrinsically incompatible with robustness against frequency fluctuations. In this work, we show that this incompatibility reflects a fundamental breakdown of single-qubit robust control principles in many-body entangling operations, manifested as a robustness inversion under single-tone ESR control: locally robust pulses disrupt intersubspace phase synchronization and degrade the robustness of many-body operations. We further uncover a Hilbert-space dilution effect, whereby $C$ Z gate robustness decays exponentially with system size, scaling as $1/2^M$. Leveraging these insights, we establish a phase-synchronized global multitone control paradigm that simultaneously suppresses frequency fluctuations and spectral crosstalk, in which square pulses—traditionally avoided due to spectral leakage—induce an intersubspace dynamical decoupling mechanism that converts unstructured crosstalk into effectively static, geometrically neutralized frequency noise. Full Hamiltonian simulations demonstrate programmable quantum logic units achieving the fidelities of many-body entangling operations approaching 99.99%, representing an order-of-magnitude improvement over conventional strategies, and establishing a scalable route toward fault-tolerant modular quantum computing.

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• Fine & hyperfine structure
• Quantum circuits
• Quantum communication, protocols & technology
• Quantum control
• Quantum information with solid state qubits
• Spin optoelectronics
• Spin qubits
• Electron spin resonance