arXiv subjects

Ressa S. Said

Publications and source records attributed to Ressa S. Said.

14 recordsLinked to original sources

arXiv2026

Observation of period doubling and higher multiplicities in a driven single-spin system

One of the prime features of quantum systems strongly driven by external time-periodic fields is the subharmonic response with integer multiples of the drive period $k\, T_d$ due to long-lived interference. Here, we demonstrate experimentally, based on a careful theoretical analysis, period doubling and higher multiplicities ($k=2,\ldots 5$) for one of the most fundamental systems, namely, an individual spin $1/2$. Nitrogen-vacancy centers in diamond support sufficiently stable coherent dynamics owing to long coherence times and allow for optical addressability of their spin states. This allows to monitor coherent period $k$-tupling oscillations over a broad set of driving parameters in the vicinity of the ideal manifolds. In this domain, superimposed low-frequency modulations serve as unique proxy for the approach toward period $k$-tupling.

quant-ph↗

arXiv2026

Efficient time-evolution of matrix product states using average Hamiltonians

Simulating quantum many-body systems (QMBS) is one of the long-standing, highly non-trivial challenges in condensed matter physics and quantum information due to the exponentially growing size of the system's Hilbert space. To date, tensor networks have been an essential tool for studying such quantum systems, owing to their ability to efficiently capture the entanglement properties of the systems they represent. One of the well-known tensor network architectures, namely matrix product states (MPS), is the standard method for simulating one-dimensional QMBS. Here, we propose a simple, yet efficient, method to augment the already available MPS algorithms to simulate the dynamics of time-dependent Hamiltonians with better accuracy and a faster convergence rate, giving a second-order convergence compared to the first-order convergence of the standard method. We apply our proposed method to simulate the dynamics of a chain of single spins associated with nitrogen-vacancy color centers in diamonds, which has potential applications for practical and scalable quantum technologies, and find that our method improves the average error for a system of few NV centers by a factor of about 1000 for moderate step sizes. Our work paves the way for efficient simulation of QMBS under the influence of time-dependent Hamiltonians.

quant-ph↗

arXiv2026

High-resolution wide-field magnetic imaging with sparse sampling using nitrogen-vacancy centers

Nitrogen-vacancy (NV) centers in diamond enable quantitative magnetic imaging, yet practical implementations must balance spatial resolution against acquisition time (and thus per-pixel sensitivity). Single-NV scanning magnetometry achieves genuine nanoscale resolution, nonetheless requires typically a slow pixel-by-pixel acquisition. Meanwhile, wide-field NV-ensemble microscopy provides parallel readout over a large field of view, however is jointly limited by the optical diffraction limit and the sensor-sample standoff. Here, we present a sparse-sampling strategy for reconstructing high-resolution wide-field images from only a small number of measurements. Using simulated NV-ensemble detection of ac magnetic fields, we show that a mean-adjusted Bayesian estimation (MABE) framework can reconstruct 10000-pixel images from only 25 sampling points, achieving SSIM values exceeding 0.999 for representative smooth field distributions, while optimized dynamical-decoupling pulse sequences yield an approximately twofold improvement in magnetic-field sensitivity. The method further clarifies how sampling patterns and sampling density affect reconstruction accuracy and suggests a route toward faster and more scalable magnetic-imaging architectures that may extend to point-scanning NV sensors and other magnetometry platforms, such as SQUIDs, Hall probes, and magnetic tunnel junctions.

quant-ph↗

arXiv2025

High-Fidelity Electron Spin Gates for Scaling Diamond Quantum Register

Diamond is a promising platform for quantum information processing as it can host highly coherent qubits that could allow for the construction of large quantum registers. A prerequisite for such devices is a coherent interaction between nitrogen vacancy (NV) electron spins. Entanglement between dipolar-coupled NV spin pairs has been demonstrated, but with a limited entanglement fidelity and its error sources have not been characterized. Here, we design and implement a robust, easy to implement entangling gate between NV spins in diamond and quantify the influence of multiple error sources on the gate performance. Experimentally, we demonstrate a record gate fidelity of $F=(96.0 \pm 2.5)$ % under ambient conditions. Our identification of the dominant errors paves the way towards NV-NV gates beyond the error correction threshold.

quant-ph↗

arXiv2024

Efficiency of optimal control for noisy spin qubits in diamond

Decoherence is a major challenge for quantum technologies. A way to mitigate its negative impact is by employing quantum optimal control. The decoherence dynamics varies significantly based on the characteristics of the surrounding environment of qubits, consequently affecting the outcome of the control optimization. In this work, we investigate the dependence of the shape of a spin inversion control pulse on the correlation time of the environment noise. Furthermore, we analyze the effects of constraints and optimization options on the optimization outcome and identify a set of strategies that improve the optimization performance. Finally, we present an experimental realization of the numerically-optimized pulses validating the optimization feasibility. Our work serves as a generic yet essential guide to implementing optimal control in the presence of realistic noise, e.g., in nitrogen-vacancy centers in diamond.

quant-ph↗

arXiv2024

Rapid transform optimisation strategy for decoherence-protected quantum register in diamond

Decoherence-protected spins associated with nitrogen-vacancy color centers in diamond possess remarkable long coherence time, which make them one of the most promising and robust quantum registers. The current demand is to explore practical rapid control strategies for preparing and manipulating the such register. Our work provides all-microwave control strategies optimized using multiple optimization methods to significantly reduce the processing time by $80\%$ with a set of smooth near-zero-endpoints control fields that are shown to be experimentally realizable. Furthermore, we optimize and analyze the robustness of these strategies under frequency and amplitude imperfections of the control fields, during which process we use only $16$ samples to give a fair estimation of the robustness map with $2500$ pixels. Overall, we provide a ready-to-implement recipe to facilitate high-performance information processing via decoherence-protected quantum register for future quantum technology applications.

quant-ph↗

arXiv2023

Bayesian-based hybrid method for rapid optimization of NV center sensors

NV center is one of the most promising platforms in the field of quantum sensing. Magnetometry based on NV center, especially, has achieved a concrete development in regions of biomedicine and medical diagnostics. Improving the sensitivity of NV center sensor under wide inhomogeneous broadening and filed amplitude drift is one crucial issue of continuous concern, which relies on the coherent control of NV center with higher average fidelity. Quantum optimal control (QOC) methods provide access to this target, nevertheless the high time consumption of current methods due to the large number of needful sample points as well as the complexity of the parameter space has hindered their usability. In this paper we propose the Bayesian estimation phase-modulated (B-PM) method to tackle this problem. In the case of state transforming of NV center ensemble, the B-PM method reduces the time consumption by more than $90\%$ compared to the conventional standard Fourier base (SFB) method while increasing the average fidelity from $0.894$ to $0.905$. In AC magnetometry scenery, the optimized control pulse given by B-PM method achieves a eight-fold extension of the coherence time $T_2$ compared to rectangular $π$ pulse. Similar application can be made in other sensing situations. As a general algorithm, the B-PM method can be further extended to open- and closed-loop optimization of complex systems based on a variety of quantum platforms.

quant-ph↗

arXiv2022

QuOCS: The Quantum Optimal Control Suite

Quantum optimal control includes the family of pulse-shaping algorithms that aim to unlock the full potential of a variety of quantum technologies. Our Quantum Optimal Control Suite (QuOCS) unites experimental focus and model-based approaches in a unified framework. The easy usage and installation of QuOCS and the availability of various combinable optimization strategies is designed to improve the performance of many quantum technology platforms, such as color defects in diamond, superconducting qubits, atom- or ion-based quantum computers. It can also be applied to the study of more general phenomena in physics. In this paper, we describe the software and the main toolbox of gradient-free and gradient-based algorithms. We then show how the user can connect it to their experiment. In addition, we provide illustrative examples where our optimization suite solves typical quantum optimal control problems, in both open- and closed-loop settings. Integration into existing experimental control software is already provided for the experiment control software Qudi [J. M. Binder et al., SoftwareX, 6, 85-90, (2017)], and further extensions are investigated and highly encouraged. QuOCS is available from GitHub, under Apache License 2.0, and can be found on the PyPI repository.

quant-ph↗

arXiv2022

Optimal quantum optical control of spin in diamond

The nitrogen-vacancy (NV) center spin represents an appealing candidate for quantum information processing. Besides the widely used microwave control, its coherent manipulation may also be achieved using laser as mediated by the excited energy levels. Nevertheless, the multiple levels of the excited state of NV center spin make the coherent transition process become complex and may affect the fidelity of coherent manipulation. Here, we adopt the strategy of optimal quantum control to accelerate coherent state transfer in the ground state manifold of NV center spin using laser. The results demonstrate improved performance in both the speed and the fidelity of coherent state transfer which will be useful for optical control of NV center spin in diamond.

quant-ph↗

arXiv2021

One decade of quantum optimal control in the chopped random basis

The Chopped RAndom Basis (CRAB) ansatz for quantum optimal control has been proven to be a versatile tool to enable quantum technology applications, quantum computing, quantum simulation, quantum sensing, and quantum communication. Its capability to encompass experimental constraints -- while maintaining an access to the usually trap-free control landscape -- and to switch from open-loop to closed-loop optimization (including with remote access -- or RedCRAB) is contributing to the development of quantum technology on many different physical platforms. In this review article we present the development, the theoretical basis and the toolbox for this optimization algorithm, as well as an overview of the broad range of different theoretical and experimental applications that exploit this powerful technique.

quant-ph↗

arXiv2020

Quantum optimal control using phase-modulated driving fields

Quantum optimal control represents a powerful technique to enhance the performance of quantum experiments by engineering the controllable parameters of the Hamiltonian. However, the computational overhead for the necessary optimization of these control parameters drastically increases as their number grows. We devise a novel variant of a gradient-free optimal-control method by introducing the idea of phase-modulated driving fields, which allows us to find optimal control fields efficiently. We numerically evaluate its performance and demonstrate the advantages over standard Fourier-basis methods in controlling an ensemble of two-level systems showing an inhomogeneous broadening. The control fields optimized with the phase-modulated method provide an increased robustness against such ensemble inhomogeneities as well as control-field fluctuations and environmental noise, with one order of magnitude less of average search time. Robustness enhancement of single quantum gates is also achieved by the phase-modulated method. Under environmental noise, an XY-8 sequence constituted by optimized gates prolongs the coherence time by $50\%$ compared with standard rectangular pulses in our numerical simulations, showing the application potential of our phase-modulated method in improving the precision of signal detection in the field of quantum sensing.

quant-ph↗

arXiv2017

Autonomous Calibration of Single Spin Qubit Operations

Fully autonomous precise control of qubits is crucial for quantum information processing, quantum communication, and quantum sensing applications. It requires minimal human intervention on the ability to model, to predict and to anticipate the quantum dynamics [1,2], as well as to precisely control and calibrate single qubit operations. Here, we demonstrate single qubit autonomous calibrations via closed-loop optimisations of electron spin quantum operations in diamond. The operations are examined by quantum state and process tomographic measurements at room temperature, and their performances against systematic errors are iteratively rectified by an optimal pulse engineering algorithm. We achieve an autonomous calibrated fidelity up to 1.00 on a time scale of minutes for a spin population inversion and up to 0.98 on a time scale of hours for a Hadamard gate within the experimental error of 2%. These results manifest a full potential for versatile quantum nanotechnologies.

quant-ph↗

arXiv2015

Quantum Speed Limit and Optimal Control of Many-Boson Dynamics

We extend the concept of quantum speed limit -- the minimal time needed to perform a driven evolution -- to complex interacting many-body systems. We investigate a prototypical many-body system, a bosonic Josephson junction, at increasing levels of complexity: (a) within the two-mode approximation {corresponding to} a nonlinear two-level system, (b) at the mean-field level by solving the nonlinear Gross-Pitaevskii equation in a double well potential, and (c) at an exact many-body level by solving the time-dependent many-body Schrödinger equation. We propose a control protocol to transfer atoms from the ground state of a well to the ground state of the neighbouring well. Furthermore, we show that the detrimental effects of the inter-particle repulsion can be eliminated by means of a compensating control pulse, yielding, quite surprisingly, an enhancement of the transfer speed because of the particle interaction -- in contrast to the self-trapping scenario. Finally, we perform numerical optimisations of both the nonlinear and the (exact) many-body quantum dynamics in order to further enhance the transfer efficiency close to the quantum speed limit.

quant-ph↗

arXiv2014

Precise ultra fast single qubit control using optimal control pulses

Ultra fast and accurate quantum operations are required in many modern scientific areas - for instance quantum information, quantum metrology and magnetometry. However the accuracy is limited if the Rabi frequency is comparable with the transition frequency due to the breakdown of the rotating wave approximation (RWA). Here we report the experimental implementation of a method based on optimal control theory, which does not suffer these restrictions. We realised the most commonly used quantum gates - the Hadamard (π/2 pulse) and NOT (πpulse) gates with fidelities ($F^{\mathrm{exp}}_{π/2}=0.9472\pm0.01$ and $F^{\mathrm{exp}}_π=0.993\pm0.016$), in an excellent agreement with the theoretical predictions ($F^{\mathrm{theory}}_{π/2}=0.9545$ and $F^{\mathrm{theory}}_π=0.9986$). Moreover, we demonstrate magnetic resonance experiments both in the rotating and lab frames and we can deliberately "switch" between these two frames. Since our technique is general, it could find a wide application in magnetic resonance, quantum computing, quantum optics and broadband magnetometry.

quant-ph↗