Quantum control papers on arXiv, selected and classified automatically by QOC. https://llorebaga.github.io/QOC/topics/composite-pulses/ 2026-09-25T00:00:00Z QOC https://arxiv.org/abs/2609.31913 2026-09-25T00:00:00Z 2026-09-25T00:00:00Z Ankita ChakravartyArnaud Carignan-DugasEva Dupont-FerrierYves Bérubé-Lauzière

Ensemble nitrogen-vacancy (NV) centers are widely used for quantum sensing and magnetometry, where performance depends on the accurate implementation of microwave pulse sequences. In these systems, inhomogeneous broadening and short coherence times limit reliable multi-pulse control, while hardware imperfections and off-resonant excitation distort implemented rotations, making direct calibration of arbitrary gates tedious. To address this, we implement a control framework with a ZXZXZ gate decomposition which uses fixed-angle, phase-parameterized pulses, enabling universal single-qubit control from a single calibrated $ π/2$ primitive ($X_{π/2}$) along with virtual Z operations. We compare a standard pulse with a composite pulse designed for high-power operation in a regime where the Rabi frequency is comparable to the hyperfine splitting, leading to non-uniform evolution across hyperfine levels. Using pre-determined combinations of $π/2$ pulse sequences, we observe that standard pulses rapidly deviate from ideal behavior under repeated application, while the composite pulse suppresses this error accumulation by more than 70$\%$ for longer sequences despite its longer duration. In contrast, randomized benchmarking yields similar decay factors for both implementations, highlighting its limited sensitivity to coherent control errors. These results establish a practical route to robust control in short-coherence ensemble systems and emphasize the importance of pulse-level characterization.

https://arxiv.org/abs/2609.22728 2026-09-19T00:00:00Z 2026-09-19T00:00:00Z Masamitsu Bando

Residual-error preservation (REP) provides a sufficient rule for constructing concatenated composite pulses that compensate multiple systematic errors. We show that local REP is not necessary: deviations from REP can cancel across the full sequence. Using first-order error generators, we derive a necessary and sufficient condition for concatenation to retain the robustness of an outer sequence. A simple sufficient condition is that all inner pulses rescale the corresponding elementary error generators by a common real factor, not necessarily unity. We illustrate this principle using pulse-length and off-resonance errors, two systematic error models commonly considered in magnetic resonance. Short CORPSE and SCROFULOUS each compensate one error without preserving the other locally. Combining these inner pulses with suitable equal-rotation-angle outer sequences yields simultaneous first-order compensation of both errors, extending concatenated composite-pulse design beyond local REP.

https://arxiv.org/abs/2609.03045 2026-09-02T00:00:00Z 2026-09-20T00:00:00Z Wenjie GongMatteo VottoSoonwon Choi

We present a robust protocol to reshape interactions in a spin ensemble based on global control pulse sequences applied to multiple subensembles in parallel. This setting arises naturally from ensembles of solid-state defects or multi-species atomic arrays. We show that it is provably computationally hard to find pulse sequences that simultaneously engineer interactions both within and between subensembles. Despite its formal hardness, we identify a set of necessary or sufficient conditions under which one can synthesize a target Hamiltonian from the native one. Moreover, we introduce efficient numerical strategies for designing pulse sequences that engineer target Hamiltonians robust against common control imperfections. As a specific application, we discuss the robust generation of two-mode spin squeezing in dual-species atomic ensembles, which is a challenging task without subensemble control. Our results provide a practical toolbox to design novel quantum simulation and sensing experiments with minimal control overhead.

https://arxiv.org/abs/2608.16702 2026-08-17T00:00:00Z 2026-08-17T00:00:00Z Valentín ReparazSantiago FerreyraMaría José SánchezDaniel DominguezLeandro Tosi

We determine the optimal quantum manipulation protocols for implementing high-fidelity, fast single-qubit gates. We demonstrate that the time-optimal pulse sequence is a ``bang-bang'' sequence: discrete pulses of either positive or negative maximum amplitude or zero. The non-adiabatic bang-bang pulse sequence minimizes gate duration providing a speedup for low-frequency architectures. We first derive the protocol for a transversally driven two-level system, extracting exact analytic expressions for minimum gate times. We then extend this framework to general multilevel architectures, identifying conditions that enable the coherent suppression of leakage errors. Using the fluxonium circuit as a representative case study, we optimize $X/2$ and $Y/2$ gates through a combination of discrete bang sequences and continuous waveform smoothing. This approach preserves near-optimal execution speeds while mitigating transitions outside the computational subspace. Open-system simulations demonstrate that these sequences outperform commensurate and resonant pulse schemes across different fluxonium regimes, achieving low-error manipulation significantly faster than standard resonant control, even in the presence of $1/f$ flux noise and dissipation.

https://arxiv.org/abs/2608.03702 2026-08-04T00:00:00Z 2026-08-06T00:00:00Z Anastasia PipiValentin DuruisseauxEmily M. BeenXuecheng TaoTaylor L. PattiAnima AnandkumarPrineha Narang

Quantum optimal control is a key tool for steering quantum dynamics, but its computational cost grows rapidly with the Hilbert space dimension. Here, we introduce a Fourier Neural Operator (FNO)-based framework for learning high dimensional molecular quantum dynamics and accelerating the inverse design of control protocols. Given an initial molecular population distribution, laser frequency, and polarization, the FNO predicts molecular-motional population dynamics up to $10^7$ times faster than GPU-accelerated numerical propagation with CUDA-Q Dynamics. Using this fast and differentiable surrogate, we develop the FNO stochastic pulse-measurement planner (FNO-SPMP), which constructs pulse sequences to purify an initially mixed Boltzmann distribution. We demonstrate the protocol in an 888-dimensional subspace of the hydronium molecule at 20 K, achieving a target-state population of 0.98 with a sequence success rate of up to 86.2%. In a shared discrete control space, FNO-SPMP achieves nearly twice the success rate of a reinforcement-learning baseline while using roughly half as many quantum control pulses and reducing pulse-sequence generation time from approximately 10 hours to 10-20 minutes. These results show that operator-learning surrogates can enable inverse design in quantum systems whose Hilbert spaces are too large for conventional direct optimization.

https://arxiv.org/abs/2607.13422 2026-07-15T00:00:00Z 2026-07-15T00:00:00Z Shingo KukitaKento NawataYuichiro Matsuzaki

Quantum sensing estimates a physical parameter encoded in the state of a probe; with independent spin probes the precision follows the standard quantum limit. Studies of sensing precision often assume that the parameters entering the model, such as the noise, are known. In practice these parameters are not always known, and a mismatch between the assumed and actual values induces a systematic error. Here we study single-qubit Ramsey magnetometry of a DC magnetic field under an unknown detuning between the actual and nominal spin frequencies: A first pulse puts the qubit into a superposition of its two states, the field to be sensed then adds a relative phase during an exposure stage, and a second pulse enables the readout. In our setting, the field acts only during the exposure stage, whereas the detuning acts throughout the whole protocol. We analyze how the detuning biases the estimate, preventing the total estimation error from following the standard quantum limit. We then construct a composite-pulse preparation and readout that exploits the difference in the intervals over which the field and the detuning act to cancel the detuning to first order. We evaluate the performance of this composite-pulse protocol and show that it suppresses the detuning-induced bias.

https://arxiv.org/abs/2606.24507 2026-06-23T00:00:00Z 2026-06-23T00:00:00Z Tobias KiermeyerThomas HeydenreichLéo Van DammeSebastian HohenemserFlorian MarquardtSteffen J. Glaser

Real-time adaptive control of quantum systems requires rapid generation of robust, high-fidelity pulses across a continuous range of operating conditions. Standard optimization algorithms such as gradient-ascent pulse engineering (GRAPE) solve each instance independently, discarding information between runs and requiring costly reinitialization when parameters change. We present an approach to robust optimal quantum control based on model-based reinforcement learning, in which a single neural network -- embedding the Hamiltonian directly into the training pipeline -- generates robust gates across an entire family of gate configurations, without pre-computed training data. Demonstrated on a single-spin (two-level) system, the trained networks produce pulses for arbitrary rotation angles over a range of pulse durations, detunings, and field inhomogeneities in milliseconds, at fidelities comparable to multi-seed GRAPE. The framework is inherently adaptable: any parameter entering the Hamiltonian can serve as a network input, extending the approach to different systems and control settings. Beyond speed, the network reveals structure in the control landscape: it discovers the same structured phase profiles that appear in GRAPE solutions -- made identifiable through fidelity-invariant symmetry transformations -- but more consistently than independent optimization. This consistency enables smooth interpolation across the entire trained parameter space.

https://arxiv.org/abs/2605.03056 2026-05-04T00:00:00Z 2026-05-04T00:00:00Z Rajdeep Rameshchandra DwivediAmitoj Singh MiglaniVishvendra Singh Poonia

Exchange-only (EO) spin qubits in silicon realise all-electrical qubit control through pairwise Heisenberg exchange interactions, making them attractive for scalable quantum computation. Their principal vulnerability is charge noise, which couples multiplicatively to the exchange coupling and degrades gate fidelity. We present a \emph{two-stage} Physics-Informed Neural Network (PINN) framework for per-gate pulse optimisation. In \textbf{Stage~I} (iterations~1--100) the PINN maximises the noise-averaged gate fidelity toward a threshold of $\Fth=0.99$; the pulse duration is held fixed at its nominal hardware value. Once the threshold is crossed, \textbf{Stage~II} (iterations~101--250) progressively compresses the total pulse time while maintaining $F\geq\Fth$ via continuous fine-tuning of the pulse-shape parameters. The cost function is a Monte-Carlo ensemble mean-squared error (MSE) averaged over $N_{\rm real}=2000$ quasi-static Gaussian noise realisations drawn fresh at every iteration. We benchmark the framework on the single-qubit gate set $\{X,Y,Z,H\}$ and the two-qubit set $\{X,Y,Z,H,\mathrm{CX}\}$ at noise levels $\sigmaJ/J\in\{1\%,5\%,10\%\}$. All single-qubit gates cross $\Fth$ within the first 100 iterations across all noise levels; Stage~II then reduces pulse durations by 20--40\% from their nominal values. The two-qubit gates follow the same two-phase behaviour, with the CX gate compressing from its nominal \SI{31}{\nano\second} to $\approx\SI{22}{\nano\second}$ at 1\% noise.

https://arxiv.org/abs/2604.21594 2026-04-23T00:00:00Z 2026-04-30T00:00:00Z Hristo G. TonchevNikolay V. Vitanov

Systematic control errors remain a primary obstacle to realizing high-fidelity single-qubit gates. We introduce composite pulse sequences that implement X and Hadamard gates while simultaneously compensating amplitude (Rabi-frequency), detuning (frequency), and duration errors. Our construction uses two complementary strategies: (i) derivative-based cancellation of error terms in the full unitary (not just the transition probability), formulated via the Cayley-Klein parametrization, and (ii) direct minimization of the average gate infidelity over prescribed error ranges. We derive symmetric five-pulse solutions with closed-form phases that cancel all first-order terms (including the mixed derivative), and numerically optimize longer sequences -- up to 15 pulses -- to achieve higher-order suppression. We also show that standard ``universal'' five-pulse sequences (U5a/U5b) emerge as simple phase-shifted instances of our symmetric solutions, yielding broad robustness to both detuning and amplitude errors. Finally, we construct variable-area sequences for $R_x(π/2)$, which, up to virtual Z rotations, benchmark the Hadamard gate. Across all families we observe the expected trade-off between sequence length and robustness window, with substantial boosts in fidelity over large error domains.

https://arxiv.org/abs/2604.03629 2026-04-04T00:00:00Z 2026-04-04T00:00:00Z Sheng-Zhe WangWei-Chen JiaYue XinQian-Lan CaiYingpeng ZhaoYan-Ying Feng

The performance of atom interferometers is commonly limited by the finite spectral acceptance of atomic beam splitters and mirrors, which restricts efficient coupling to atoms with large Doppler shifts and reduces the usable atomic flux. Here, we demonstrate spectral-domain coherent control of Raman coupling by engineering its effective two-photon spectrum. By synthesizing multiple frequency components, the Raman interaction simultaneously addresses a broad range of atomic velocities, effectively overcoming the conventional transit-time-limited linewidth. Implemented in a continuous atomic-beam Mach-Zehnder interferometer, where the transverse Doppler broadening is 17 times larger than the intrinsic Raman linewidth, this approach enhances the fringe contrast from 5.9(2)% to 15.1(2)%, indicating a substantial increase in effective atomic participation. Our results establish spectral-domain coherent control as a general strategy for achieving spectrally robust atom interferometry and open new opportunities for quantum sensing in systems with strong inhomogeneous broadening.

https://arxiv.org/abs/2603.21057 2026-03-22T00:00:00Z 2026-03-22T00:00:00Z Enrico Daniel RichterRyan J. SmithBrayden GlockzinEmanuel DrugaThomas SchenkelAshok Ajoy

Practical performance of quantum sensors is often curtailed by uncontrolled environmental drift (bias-field instability, temperature fluctuations, mechanical vibration), background fields, and imperfect control pulses. This motivates developing physical mechanisms that intrinsically compensate for such perturbations while retaining high sensitivity to target fields. We introduce an interaction-protected magnetometry scheme where periodic driving steers the collective magnetization onto two long-lived, prethermal Floquet "orbit" axes well-separated on the Bloch sphere. Rapid toggling between these axes encodes target fields as a differential signal, whereas background fields appear as common-mode motion that is strongly rejected, achieving >1000-fold suppression while canceling prethermal transients. This enables accurate reconstruction of rapidly varying audio-band magnetic signals without predictive filtering or spectral tuning. We provide an experimental proof-of-principle using a dense ensemble of coupled nuclear spins, operated here as a broadband (0-1 kHz) magnetometer. The protocol is remarkably tolerant to imperfections, operating robustly across millions of pulses under pulse-angle (~10°) and pulse frequency (>1 kHz) errors, large bias-field drifts (>50 $\mathrmμ$T), temperature variations over 150 K, and harsh mechanical vibrations. These results establish Floquet prethermalization as a resource for robust quantum sensors that combines broadband magnetic-field sensitivity with intrinsic immunity to diverse environmental and control perturbations, opening a path toward stable quantum metrology beyond controlled laboratory conditions.

https://arxiv.org/abs/2603.16784 2026-03-17T00:00:00Z 2026-03-17T00:00:00Z Naoya EgawaKaoru MizutaJoji Nasu

Quantum signal processing (QSP), originally developed for composite pulse sequences in nuclear magnetic resonance systems, has recently attracted attention as a unified framework for quantum algorithms. A pioneering study applied QSP to nonequilibrium control in integrable many-body systems, enabling the realization of nonequilibrium dynamics with greater flexibility than Floquet engineering. However, extending QSP to nonintegrable systems faces fundamental obstacles arising from the limited number of conserved quantities and thermalization. In this work, we propose a protocol that leverages QSP in systems exhibiting Hilbert space fragmentation (HSF). Specifically, we consider a pair-hopping model with four-fold periodic potentials that exhibits an HSF structure, thereby providing integrable and nonintegrable sectors within a single system. We analytically show that nonequilibrium dynamics can be flexibly designed through QSP engineered by these potentials in the integrable sectors. In contrast, we numerically identify signatures of thermalization in the nonintegrable sectors. Remarkably, by inserting domain walls, we achieve parallel control of multiple quantum dynamics within a single system. This approach sheds light on the control of nonequilibrium dynamics from the perspective of quantum computation by extending the scope of QSP to nonintegrable systems.

https://arxiv.org/abs/2603.08522 2026-03-09T00:00:00Z 2026-03-09T00:00:00Z Jonathan BerkheimDavid J. Tannor

In nuclear magnetic resonance (NMR), Composite Pulses (CPs) are widely used to correct for pulse imperfections, e.g., RF field inhomogeneity and resonance offset. Although robust pulse sequences have been developed throughout the years, the imperfection of the initial state has not been widely discussed in the literature as an additional systematic error. In previous work, we developed a classical canonical framework to perform stability analysis and used this as a measure of CP robustness. In that work, a single initial condition was allowed to evolve under various pulse imperfections. The current work extends this approach to $2D$ distributions of initial conditions on the Bloch Sphere; the objective is to minimize the area in order to preserve coherence, while maximizing population inversion of the entire distribution. As a case study, we investigate Levitt's $90(x)180(y)90(x)$ pulse sequence, when there is a spread in initial conditions. The canonical framework enables us to assess the robustness of Levitt's pulse sequence, and we find that it is maintained to a great extent even when considering a spread of initial conditions. Nevertheless, by conducting a numerical optimization, we have identified several variants of Levitt's pulse sequence that produce a larger coherent population inversion when there is a spread in initial conditions.

https://arxiv.org/abs/2602.19443 2026-02-23T00:00:00Z 2026-02-23T00:00:00Z L. B. A. MéloDaniel FelintoMarcio H. G. de Miranda

By combining genetic algorithm and a spatial light modulator we theoretically analyse how to improve a two-photon cascade absorption in atomic ensembles, inspecting the impact of various configurations and parameters in the optimized phase mask. At low atomic densities, we compare the cases of sequential transitions with the two photons coming from the same pulse or from two different pulses. For the former, we predict an enhancement by a factor of $9.5$, similar to what was previously reported in the literature [Phys. Rev. Lett. {\bf 86}, 47 (2002)]. For the later, on the other hand, we obtain an enhancement factor of $26$ times. This absorption of two photons by different pulses is of particular interest for the storage of ultra-broadband single photons by atomic ensembles, in which case the second photon would come from a control pulse. We investigate this process as a function of the atomic density, demonstrating enhancements by factors up to 3 for the two-photon absorption after propagating through large optical depths. However, for the experimental conditions considered in the previous work by Carvalho {et al.} [Phys. Rev. A {\bf 101}, 053426 (2020)], in terms of control power and optical depths, we show that this enhancement in two-photon absorption would still result in just a modest increase of the absorption of a weak probe pulse.

https://arxiv.org/abs/2602.05343 2026-02-05T00:00:00Z 2026-09-14T00:00:00Z Leeseok KimMilad Marvian

We introduce a high-order dynamical decoupling (DD) scheme for arbitrary bounded system-bath interactions in the weak-coupling regime. Given any decoupling group $\mathcal G$ that averages the interaction to zero, our construction {guarantees the existence of pulse sequences with at most $(|\mathcal G|-1)K$ pulses}, while canceling all error terms linear in the system-bath coupling strength up to order $K$ in the total evolution time. As a corollary, for an $n$-qubit system with $k$-local system-bath interactions, we obtain an $\mathcal{O}(n^{k-1}K)$-pulse sequence, a significant improvement over existing schemes with $\mathcal{O}(\exp(n))$ pulses (for $k=\mathcal{O}(1)$). The construction is obtained via a mapping to the continuous necklace-splitting problem, which asks how to cut a multi-colored interval into pieces that give each party the same share of every color. We provide explicit pulse sequences for suppressing general single-qubit decoherence, prove that the pulse count is asymptotically optimal, and verify the predicted error scaling in numerical simulations. For the same number of pulses, we observe that our sequences outperform the state-of-the-art Quadratic DD in the weak-coupling regime. We also construct explicit high-order sequences for suppressing representative 2-local noise models. Finally, the same construction extends to suppress slow, time-dependent classical noise and to filter-function design.

https://arxiv.org/abs/2602.05199 2026-02-05T00:00:00Z 2026-02-05T00:00:00Z Jiaming LiXi-Wang LuoGuang-Can GuoZheng-Wei Zhou

High-fidelity coherent population transfer plays a vital role in the realization of quantum memories. However, population transfer with high performance across a broad frequency range is still challenging due to the finite Rabi coupling strength limited by laser powers. Here we propose a novel population-transfer scheme by suturing adiabatic control pulses with each pulse covering certain frequency interval, which are connected in a way that neighboring adiabatic pulses have opposite chirping directions. Taking the widely utilized hyperbolic-square-hyperbolic pulse as an example, we demonstrate that rapid and robust population transfer can be achieved. The transfer bandwidth scales linearly with the number of suture pulses while maintaining high fidelity, even at the suture points where adiabaticity breaks down. Crucially, these pulses can be realized by a single laser by means of temporal multiplexing. For a given bandwidth, this strategy substantially reduces the operational time which is necessary for on demand read-out and suppressing decoherence effects. Our scheme enables a dramatic increase in multimode storage capacity and paves the way for realizing practical quantum networks.

https://arxiv.org/abs/2601.10435 2026-01-15T00:00:00Z 2026-01-15T00:00:00Z Benoît VermerschOscar GravierNathan MiscopeinJulia GuignonCarlos Ramos MarimónJonathan DurandauMatthieu DartiailhTristan MeunierValentin Savin

We introduce SpinPulse, an open-source python package for simulating spin qubit-based quantum computers at the pulse-level. SpinPulse models the specific physics of spin qubits, particularly through the inclusion of classical non-Markovian noise. This enables realistic simulations of native gates and quantum circuits, in order to support hardware development. In SpinPulse, a quantum circuit is first transpiled into the native gate set of our model and then converted to a pulse sequence. This pulse sequence is subsequently integrated numerically in the presence of a simulated noisy experimental environment. We showcase workflows including transpilation, pulse-level compilation, hardware benchmarking, quantum error mitigation, and large-scale simulations via integration with the tensor-network library quimb. We expect SpinPulse to be a valuable open-source tool for the quantum computing community, fostering efforts to devise high-fidelity quantum circuits and improved strategies for quantum error mitigation and correction.

https://arxiv.org/abs/2601.09014 2026-01-13T00:00:00Z 2026-08-01T00:00:00Z Agata BarsottiPaolo MarconciniGregorio ProcissiMassimo Macucci

As qubit decoherence times are increased and readout technologies are improved, nonidealities in the drive signals, such as phase noise, are going to represent a crucial limitation to the fidelity achievable at the end of complex control pulse sequences. Although the effect of phase noise of reference oscillators on qubit performance has been studied previously, its interaction with realistic time-dependent control pulses and its contribution to fidelity degradation have not yet been investigated in sufficient detail, and remains a critical challenge. Here we study the impact on fidelity of phase noise affecting reference oscillators with the help of numerical simulations, which allow us to directly take into account the interaction between the phase fluctuations in the control signals and the evolution of the qubit state, thereby achieving a comprehensive understanding of the actual role played by the different spectral components of phase noise. In particular, we perform an analysis of the effect of the individual noise frequency contributions, providing a clear identification of the spectral regions that most critically impact fidelity and establishing their relative weight in the overall fidelity degradation. Our method is based on the generation of phase noise realizations consistent with a given power spectral density, that are then applied to the pulse carrier in simulations, with Qiskit-Dynamics, of the qubit temporal evolution. By comparing the final state obtained at the end of a noisy pulse sequence with that in the ideal case and averaging over multiple noise realizations, we estimate the resulting degradation in fidelity, and, exploiting an approximate analytical representation of a carrier affected by phase fluctuations, we shed new light on the nature of the different contributions, and provide an intuitive physical picture.

https://arxiv.org/abs/2601.01521 2026-01-04T00:00:00Z 2026-01-04T00:00:00Z Ignacio R. SoláSebastian C. CarrascoVladimir S. MalinovskySeokmin ShinBo Y. Chang

Stark shifts introduce additional phases that constrain the set of entangling gates that can be prepared via two-photon transitions in the strong Rydberg blockade limit. For non-independently addressed qubits, by controlling the absolute phases and the local amplitudes of the pulses at each qubit, we show that any two-qubit phase gate can be prepared with high fidelity using a three-pulse sequence. Based on these insights, we introduce two robust control schemes tailored to different phase gates that yield better results with pulse sequences of either even or odd length.

https://arxiv.org/abs/2512.22998 2025-12-28T00:00:00Z 2025-12-28T00:00:00Z Dionisis StefanatosIoannis ThanopulosEmmanuel Paspalakis

In this work, we formulate the problem of achieving in minimum-time perfect chiral resolution with bounded control fields, as an optimal control problem on two non-interacting spins-$1/2$. We assume the same control bound for the two Raman fields (pump and Stokes) and a different bound for the field connecting directly the two lower-energy states. Using control theory, we show that the optimal fields can only take the boundary values or be zero, the latter corresponding to singular control. Subsequently, using numerical optimal control and intuitive arguments, we identify some three-stage symmetric optimal pulse-sequences, for relatively larger values of the ratio between the two control bounds, and analytically calculate the corresponding pulse timings as functions of this ratio. For smaller values of the bounds ratio, numerical optimal control indicates that the optimal pulse-sequence loses its symmetry and the number of stages increases in general. In all cases, the analytical or numerical optimal protocol achieves a faster perfect chiral resolution than other pulsed protocols, mainly because of the simultaneous action of the control fields. The present work is expected to be useful in the wide spectrum of applications across the natural sciences where enantiomer separation is a crucial task.

https://arxiv.org/abs/2512.13890 2025-12-15T00:00:00Z 2025-12-15T00:00:00Z Charles MarrderShuo SunMurray J. Holland

Dynamical decoupling seeks to mitigate phase decoherence in qubits by applying a carefully designed sequence of effectively instantaneous electromagnetic pulses. Although analytic solutions exist for pulse timings that are optimal under specific noise regimes, identifying the optimal timings for a realistic noise spectrum remains challenging. We propose a reinforcement learning (RL)-based method for designing pulse sequences on qubits. Our novel action set enables the RL agent to efficiently navigate this inherently non-convex optimization landscape. The action set, derived from Thompson's group $F$, is applicable to a broad class of sequential decision problems whose states can be represented as bounded sequences. We demonstrate that our RL agent can learn pulse sequences that minimize dephasing without requiring explicit knowledge of the underlying noise spectrum. This work opens the possibility for real-time learning of optimal dynamical decoupling sequences on qubits which are dephasing-limited. The model-free nature of our algorithm suggests that the agent may ultimately learn optimal pulse sequences even in the presence of unmodeled physical effects, such as pulse errors or non-Gaussian noise.

https://arxiv.org/abs/2512.05934 2025-12-05T00:00:00Z 2025-12-05T00:00:00Z Qianxu WangJuan S. Salcedo-GalloSalil BedkihalTian XiaMaciej W. OlszewskiValla FatemiMattias Fitzpatrick

Defects in solid-state materials play a central role in determining coherence, stability, and performance in quantum technologies. Although narrowband techniques can probe specific resonances with high precision, a broadband spectroscopic approach captures the full spectrum of defect properties and dynamics. Two-level system (TLS) defects in amorphous dielectrics are a particularly important example because they are major sources of decoherence and energy loss in superconducting quantum devices. However, accessing and characterizing their collective dynamics remains far more challenging than probing individual TLS defects. Building on our previously developed Broadband Cryogenic Transient Dielectric Spectroscopy (BCTDS) technique, we study the coherent control and time-resolved dynamics of TLS defect ensembles over a wide frequency range of 3-5 GHz without requiring full device fabrication, revealing quantum interference effects, memory-dependent dynamics, and dressed-state evolution within the TLS defect bath. The spectral response reveals distinct V-shaped structures corresponding to the bare eigenmode frequencies. Using these features, we extract a TLS defect spectral density of 84 GHz^-1 for a silicon sample, across a 4.1-4.6 GHz span. Furthermore, we systematically investigate amplitude- and phase-controlled interference fringes for multiple temperatures and inter-pulse delays, providing direct evidence of coherent dynamics and control. A driven minimal spin model with dipole-dipole interactions that qualitatively capture the observed behavior is presented. Our results establish BCTDS as a versatile platform for broadband defect spectroscopy, offering new capabilities for diagnosing and mitigating sources of decoherence, engineering many-body dynamics, and exploring non-equilibrium phenomena in disordered quantum systems.

https://arxiv.org/abs/2511.19244 2025-11-24T00:00:00Z 2025-11-24T00:00:00Z José P. CarvalhoAnders Bodholt NielsenDavid L. GoodwinNino WiliNiels Chr. Nielsen

Taking inspiration from NMR spectroscopy, periodic irradiation schemes have recently shown remarkable performance when implemented into pulsed dynamic nuclear polarization (DNP) sequences. This has prompted considerable interest in development of broadband pulsed DNP sequences utilizing such schemes. On this background, most efforts have focused on solid-state NMR like transverse spin-locked pulse sequences whose performance in DNP applications may be compromised by the broadband capabilities of the initial excitation pulse. Leveraging the flexibility and robustness of optimal control theory combined with underlying insights from effective Hamiltonian theory, we present a new family of broadband DNP pulse sequences, termed LOOP (Longitudinally Optimized with Overarching Periodicity), that alleviates the excitation-pulse challenge by accomplishing longitudinal polarization transfer. These sequences define robust single-spin effective $z$ rotations, with impressive compensation towards microwave field inhomogeneity, and are capable of delivering DNP transfer with bandwidths exceeding 100 MHz, while employing a peak microwave field amplitude of only 32 MHz, at an external magnetic field of 0.35 T.

https://arxiv.org/abs/2510.07802 2025-10-09T00:00:00Z 2026-03-03T00:00:00Z Jixing ZhangBo PengYang WangCheuk Kit CheungGuodong BianHualuo PangAndrew M. EdmondsMatthew MarkhamZhe ZhaoYuan HouDurga DasariRuoming PengYe WeiJörg Wrachtrup

As quantum control approaches hardware-imposed performance limits, weak effects omitted by reduced models become consequential. Assumptions required for analytic tractability then cease to guide control design and instead constrain further improvement. Here, we relax such assumptions and use simulation-guided stochastic tree search to navigate combinatorially large, discrete pulse-sequence spaces for robust many-body spin control. Experimentally, in a solid-state spin ensemble, the resulting computationally discovered pulse sequences substantially outperform analytically optimized baselines, despite being excluded by construction from analytic design criteria. Importantly, these unconventional sequences expose predictive structural features that enable rapid neural network--based performance evaluation. This efficiency gain makes the combinatorial scaling tractable and expands the control alphabet from 8 symmetry-restricted pulses to over 26,000 hardware-resolved options. The resulting fine-grained design freedom provides the control resolution required to reliably address weak, performance-limiting effects, unlocking qualitatively different spin-control capabilities beyond decades of traditional sequence design. Together, these results show that near performance limits, simplifying assumptions can become a primary constraint on quantum control in realistic hardware, and must be repurposed to guide computational discovery.

https://arxiv.org/abs/2510.04462 2025-10-06T00:00:00Z 2025-10-06T00:00:00Z Qi-Pei LiuZheng-Yuan Xue

Scalable quantum computation demands high-fidelity two-qubit gates. However, decoherence and control errors are inevitable, which can decrease the quality of implemented quantum operations. We propose a robust iSWAP gate protocol for semiconductor spin qubits, which is a promising platform for scalable quantum computing. Our scheme uses only local microwave drives on conventional exchange-coupled spin qubits. This approach simultaneously addresses two critical challenges on semiconductor quantum computing: it suppresses low-frequency noise via continuous dynamical decoupling, and it circumvents the control difficulties associated with the ac modulation of the exchange interaction. We further develop a composite pulse sequence to remove drive-strength constraints and a dynamically corrected method to provide first-order immunity to microwave amplitude errors.Numerical simulations confirm that our scheme can achieve fidelity above the fault-tolerance threshold under current experimental conditions, offering a building block for practical quantum processors.

https://arxiv.org/abs/2510.00804 2025-10-01T00:00:00Z 2026-05-25T00:00:00Z Muhammad ErewMoshe GoldsteinYaron OzHaim Suchowski

Magic state distillation (MSD) is a cornerstone of fault-tolerant quantum computing, enabling non-Clifford gates via state injection into stabilizer circuits. However, the substantial overhead of current MSD protocols remains a major obstacle to scalable implementations. We propose a general framework for pre-distillation, based on composite pulse sequences that suppress systematic errors in the generation of magic states. While most composite designs target simple gates such as X, Z, or Hadamard, our schemes directly implement the non-Clifford T gate with enhanced robustness to the targeted systematic errors. We develop composite sequences tailored to the dominant control imperfections in superconducting, trapped-ion, neutral-atom, and integrated photonic platforms. To quantify improvement in the implementation, we introduce an operationally motivated fidelity measure specifically tailored to the T gate: the T-magic error, which captures the gate's effectiveness in preparing high-fidelity magic states. We further show that the error in the channel arising from the injection of faulty magic states scales linearly with the leading-order error of the states. Within the systematic-error-dominated regimes considered, this approach lowers the number of required distillation levels by up to three across the platforms we study, translating to substantial qubit-overhead savings and offering a practical route toward more resource-efficient universal quantum computation.

https://arxiv.org/abs/2507.13714 2025-07-18T00:00:00Z 2025-07-18T00:00:00Z Guanchu ChenBrendan C. SheehanIlija NikolovJames W. LoganCharles A. CollettGajadhar JoshiGrigore A. TimcoJillian E. DenhardtKevin R. KittilstvedRichard E. P. WinpennyJonathan R. Friedman

Molecular magnets are attractive as spin qubits due to their chemical tunability, addressability through electron-spin resonance techniques, and long coherence times. Clock transitions (CTs), for which the system is immune to the effect of magnetic-field fluctuations to first order, provide a method to enhance the coherence time $T_2$, and to reveal mechanisms of decoherence that are not due to such fluctuations. Here we investigate two variants of Cr$_7$Mn, a spin-1 molecular nanomagnet, at fields near a zero-field CT. We find that at temperatures $\le$2 K, $T_2\sim1$ $μ$s at the CT using a Hahn-echo pulse sequence. Away from the CT, electron-spin-echo envelope modulation (ESEEM) oscillations due to coupling to nuclear spins are observed and have a $T_2$ as high as $1.35$ $μ$s, indicating a distinct mechanism of coherence preservation. Dynamical decoupling with the CPMG pulse sequence yields $T_2\sim\!2.8$ $μ$s at the CT and up to $\sim\!3.6$ $μ$s in the ESEEM regime along with a demodulation of the oscillatory behavior. The experimental values of $T_2$ are largely independent of the degree of dilution of the molecules in solvent or whether the solvent is deuterated, indicating that much of the decoherence and ESEEM arises from sources within the molecules themselves. To account for decoherence, we develop a model that includes not only field fluctuations but also fluctuations in the CT transition frequency itself. Our results can be well explained by treating the environment as a combination of noise at the nuclear Larmor precession frequency and $1/f$ noise in the transverse anisotropy parameter $E$. Such information about the microscopic origins of decoherence can aid the rational design of molecular-based spin qubits.

https://arxiv.org/abs/2507.07875 2025-07-10T00:00:00Z 2025-11-28T00:00:00Z Edison S. CarreraHarold ErbinGrégoire Misguich

We present a quantum optimal control protocol to generate highly spin-squeezed states in Rydberg atom arrays coupled via Ising-type van der Waals interactions. Using gradient-based optimization techniques, we construct time-dependent pulse sequences that steer an initial product state toward highly entangled, spin-squeezed states with predefined magnetization and squeezing axes. We focus on the Wineland parameter $ξ_W^2$ to measure spin squeezing, and our approach achieves near-optimal spin squeezing in one-dimensional ring arrays of up to $N=8$ spins, significantly outperforming conventional quench dynamics for all system sizes studied. Remarkably, optimized pulse sequences can be directly scaled to larger arrays without additional optimization, achieving a squeezing parameter as low as $ξ_W^2 = 0.227$ in systems containing $N=50$ spins. This work demonstrates the potential of quantum optimal control methods for preparing highly spin-squeezed states, opening pathways to enhanced quantum metrology.

https://arxiv.org/abs/2507.01364 2025-07-02T00:00:00Z 2025-07-02T00:00:00Z Jonathan BerkheimDavid J. Tannor

Composite Pulses (CPs) are widely used in Nuclear Magnetic Resonance (NMR), optical spectroscopy, optimal control experiments and quantum computing to manipulate systems that are well-described by a two-level Hamiltonian. A careful design of these pulses can allow the refocusing of an ensemble at a desired state, even if the ensemble experiences imperfections in the magnitude of the external field or resonance offsets. Since the introduction of CPs, several theoretical justifications for their robustness have been suggested. In this work, we suggest another justification based on the classical mechanical concept of a stability matrix. The motion on the Bloch Sphere is mapped to a canonical system of coordinates and the focusing of an ensemble corresponds to caustics, or the vanishing of an appropriate stability matrix element in the canonical coordinates. Our approach highlights the directionality of the refocusing of the ensemble on the Bloch Sphere, revealing how different ensembles refocus along different directions. The approach also clarifies when CPs can induce a change in the width of the ensemble as opposed to simply a rotation of the axes. As a case study, we investigate the $90(x)180(y)90(x)$ CP introduced by Levitt, where the approach provides a new perspective into why this CP is effective.

https://arxiv.org/abs/2506.22767 2025-06-28T00:00:00Z 2025-06-28T00:00:00Z Hao WuGrant D. MittsClayton Z. C. HoJoshua A. RabinowitzEric R. Hudson

Due to quantum fluctuations, non-orthogonal quantum states cannot be distinguished with complete certainty, making their underlying physical parameters difficult to resolve. Traditionally, it has been believed that the linewidth of a system behaves like these quantum fluctuations to set the ultimate limit on frequency resolution as two oscillating electromagnetic fields are applied. Consequently, the measurement time required to resolve a frequency difference $Δω$ was assumed to diverge as $Δω\rightarrow 0$. Here, we show that linewidth does not play a defining role in resolving two closely spaced frequencies. Instead, the ultimate limit is set by parameter-independent quantum fluctuations, such as shot noise in our case. We propose and experimentally demonstrate the first general broadband protocol for super-resolution spectroscopy. Specifically, our protocol uses a Walsh-modulated dynamical decoupling (WMDD) sequence to encode $Δω$ between two unknown tones into a quantum state. This leverages phase information to suppress parameter-independent shot noise, thereby enhancing the signal-to-noise ratio and enabling super-resolution spectroscopy. With this approach, we resolve two randomly chosen oscillating electric fields of order 100 MHz separated by 5 Hz, with a measured frequency difference of 5.0(1.6) Hz using a measurement time per run of just 1 ms, representing an improvement of 200 beyond the traditional resolution limit. As such, our technique accelerates data acquisition by more than $10^5$ magnitude compared to conventional methods. Crucially, as our protocol is rooted in the motional Raman (quantum vector signal analyzer) framework, it is effective across an arbitrary frequency range and thus promises to enhance broadband sensing of electromagnetic fields and improve spectral efficiency of next-generation communication systems.