Optimally Driven Dressed Qubits

TL;DR

This work addresses CRT-induced limits in dressed qubits by introducing a CRT-free control protocol that uses a rotating-frame circularly polarized drive, implemented with a single lab-frame axis. The resulting exact Hamiltonian H_{II} = 1/2 [ (\Omega_1-\widetilde{\Omega}_1)\sigma_x + \Omega_2(\cos(\phi)\sigma_y + \sin(\phi)\sigma_z) ] removes the need for the dressed RWA and enables dressed-qubit gates to match bare-qubit speeds with enhanced noise robustness. Across three applications—high-fidelity two-qubit gates, extended-range quantum sensing, and stabilized atomic clocks—the protocol yields ~27× improvement in two-qubit gate fidelity, >10× sensing-range enhancement, and ~5× coherence-time gains, under realistic noise models. A Floquet-based coherence-time expression and a scalable approximation for T_2 guide experimental design and parameter optimization, demonstrating broad potential for dressed-qubit architectures in quantum technologies.

Abstract

The applicability and performance of qubits dressed by classical fields are limited because their control protocols give rise to an undesired counter-rotating term (CRT). This in turn forces operation in a regime where a (dressed) rotating-wave approximation (RWA) is valid, thereby restricting key aspects of their operation. Here, using only a single coupling axis in the laboratory frame, we introduce a dressed-qubit control protocol that optimally removes the CRT, eliminating the need for the RWA and delivering substantial improvements in multiple performance metrics, including single-qubit gate speed, two-qubit gate fidelity, spectroscopic range, clock stability, and coherence preservation. In addition, we provide a general parameterization together with a Floquet-based coherence-time expression, which elucidates the protocol's working principles and lowers the barrier to adoption. Collectively, these advances position our scheme as the state-of-the-art strategy for qubit control, paving the way for a wider class of quantum technologies to be realized using dressed-qubit architectures.

Figures (5)

• Figure 1: Concept and applications of optimally driven dressed qubits. (a) A bare qubit is controlled to obtain an arbitrarily controlled dressed qubit (b), which can then be driven by a rotating-frame circularly polarized field that optimally removes the counter-rotating term (CRT). This leads to substantially extended coherence times over a broad bandwidth. (c) Our scheme improves performance in several key applications, namely, single-qubit gate speed, two-qubit gate fidelity, extended range high-sensitivity quantum sensing, and magic-angle (MA) $\And$ correlated-noise (CN) decoupled multi-ion frequency references. The performance is benchmarked against recent experiments reported in PhysRevX2025Danielhermann2024extendingMulti-ionPRL2024.
• Figure 2: Two-qubit gate infidelity as a function of $\Omega_2$. Our new scheme using the optimal dressed drive (blue circles), at its optimal point, improves the infidelity by a factor of $\sim27$ compared to the optimum of the previous scheme in PhysRevX2025Daniel which uses the phase-modulated (PM) drive (brown squares). Furthermore, the new optimal infidelity is below the QEC threshold (gray line), by an order-of-magnitude.
• Figure 3: Sensitivity gain of our new protocol (blue circles) compared to the Hartmann-Hahn (HH) sensing protocol (green diamond) hartmann1962nuclearhermann2024extending. At the optimum ($\Omega_1/2\pi=100$ MHz), the gain exceeds an order of magnitude, which translates to over two orders-of-magnitude reduction in integration time for a given signal-to-noise ratio. The sensing scheme in PhysRevLett.132.223601, adapted to the signal matching condition (see main text), provides a more modest gain (brown squares) despite accounting for the Bloch-Siegert shift. This demonstrates that the residual effects of the counter-rotating terms compromise the gain significantly. Finally, the gain provided by the new scheme is more robust, as it spans across a wider $\Omega_1$ range.
• Figure 4: Zeeman coherence times as a function of $\Omega_1$ ($T_2^\ast$-scaled). At the optima, our new protocol (blue circles) extends coherence time by a factor of $\sim5$ compared to the magic-angle detuned double-drive used in Multi-ionPRL2024 (brown squares). Scalings are expected to depend on control noise strength $\sigma_\epsilon$ ($=0.5\%$ here, see discussion around Eq. \ref{['eq:T2app']}).
• Figure 5: Coherence time as a function of $\Omega_1$ ($T^\ast_2$-scaled, normalized), demonstrating the validity of our approximation in Eq. \ref{['eq:T2app']}. The full simulation results (blue traingles) compare well with the Floquet $4^{th}$-order perturbation theory derived $\overline{T}_2=\frac{\sqrt{2}}{\sqrt{Var(\Delta E)}}$ (red stars), and its scaling ansatz approximation $\overline{T}^{app}_2$ (green pentagons, Eq. \ref{['eq:T2app']}). The optimization of our protocol's control parameters can, therefore, begin with our simple $\overline{T}^{app}_2$ formula, followed by more fine-grained numerical simulation and validation.