arXiv subjects

Servaas Kokkelmans

Publications and source records attributed to Servaas Kokkelmans.

At least 19 recordsLinked to original sources

arXiv2026

A Robust Strontium Tweezer Apparatus for Quantum Computing

Neutral atoms for quantum computing applications show promise in terms of scalability and connectivity. We demonstrate the realization of a versatile apparatus capable of stochastically loading a 5x5 array of optical tweezers with single $^{88}$Sr atoms featuring flexible magnetic field control and excellent optical access. A custom-designed oven, spin-flip Zeeman slower, and deflection stage produce a controlled flux of Sr directed to the science chamber. In the science chamber, featuring a vacuum pressure of $3 \times 10^{-11}$ mbar, the Sr is cooled using two laser cooling stages, resulting in $\sim 3 \times 10^5$ atoms at a temperature of 5(1) $μ$K. The optical tweezers feature a $1/e^2$ waist of 0.81(2) $μ$m, and loaded atoms can be imaged with a fidelity of $\sim 0.997$ and a survival probability of $0.99^{+0.01}_{-0.02}$. The atomic array presented here forms the core of a full-stack quantum computing processor targeted for quantum chemistry computational problems.

physics.atom-ph↗

arXiv2025

Fidelity Relations in an Array of Neutral Atom Qubits -- Experimental Validation of Control Noise

Noise is a hindering factor for current-era quantum computers. In this study, we experimentally validate the theoretical relationships between amplitude noise of the control signal and qubit state fidelity. The experiment comprises a 10x10 site optical tweezer array stochastically loaded with single rubidium-85 atoms. A global microwave field is used to manipulate the state of the hyperfine qubits. With precise control of the time-dependent amplitude of the microwave drive, we apply control signals featuring artificial noise. We systematically analyze the impact of various noise profiles on the fidelity distribution of the quantum states. The measured fidelities are compared against theoretical predictions made using the stochastic Schrödinger equation. Our results show a good agreement between the experimentally measured and theoretically predicted results. This validation is consequential, as the model provides critical information on noise identification and optimal control protocols in NISQ-era quantum systems.

quant-ph↗

arXiv2025

$Λ$-Enhanced Gray Molasses Cooling of $^{85}$Rb Atoms in Tweezers Using the D$_2$ Line

We demonstrate the implementation of $Λ$-enhanced gray molasses cooling on the D$_2$ line of $^{85}$Rb atoms in an optical tweezer array. This technique yields lower atomic temperatures of 4.0(2) $μ$K compared to red-detuned polarization gradient cooling, and consequently extends the $T_2^*$ coherence time of the hyperfine clock qubit by a factor of 1.5. The method is alignment-free and can be readily implemented on laser beams used for magneto-optical trapping, as it only requires frequency and phase modulation control. Our experimental observations are corroborated by a numerical model based on a semi-classical force approach extended to a four-level system, including two hyperfine states of the upper manifold that are 120 MHz apart.

physics.atom-ph↗

arXiv2025

Quantum relative entropy for unravelings of master equations

This work explores connections between the quantum relative entropy of two faithful states $ρ,σ$ (i.e. full-rank density matrices) and the Kullback-Leibler divergences of classical measures $μ,ν$. Here, $μ$ and $ν$ are measures on the space of pure states, realizing $ρ$ and $σ$ respectively. The motivation for this result is to establish a notion of quantum relative entropy in the space of pure state distributions, which are the resulting objects of unravelings of the Lindblad equation, such as the stochastic Schrödinger equation. Our results show that the measures that achieve the minimal KL divergence are those supported on a (possibly non-orthogonal) common basis between $ρ$ and $σ$. Using the classical and quantum data-processing inequalities, our notion of quantum relative entropy is shown to be equivalent to the Belavkin-Staszewski entropy on states, revealing new insights on this quantity. Furthermore, the common basis is used to provide a novel proof of contraction of the relative entropy under Lindblad flow and offers insights into results from large deviation theory.

quant-ph↗

arXiv2025

Parametrized multiqubit gate design for neutral-atom based quantum platforms

A clever choice and design of gate sets can reduce the depth of a quantum circuit, and can improve the quality of the solution one obtains from a quantum algorithm. This is especially important for near-term quantum computers that suffer from various sources of error that propagate with the circuit depth. Parametrized gates in particular have found use in both near-term algorithms and circuit compilation. The one- and two-qubit versions of these gates have been demonstrated on various computing architectures. The neutral atom platform has the capability to implement native $N$-qubit gates (for $N \geq 2$). However, one needs to first find the control functions that implement these gates on the hardware. We study the numerical optimization of neural networks towards obtaining families of controls $-$ laser pulses to excite an atom to Rydberg states $-$ that implement phase gates with one and two controls, the $\mathrm{C_1P}$ and $\mathrm{C_2P}$ gates respectively, on neutral atom hardware. The pulses we obtain have a duration significantly shorter than the loss time scale, set by decay from the Rydberg state. Further, they do not require single-site addressability and are smooth. Hence, we expect our gates to have immediate benefits for quantum algorithms implemented on current neutral atom hardware.

quant-ph↗

arXiv2025

Consensus-based qubit configuration optimization for variational algorithms on neutral atom quantum systems

In this work, we report an algorithm that is able to tailor qubit interactions for individual variational quantum algorithm problems. Here, the algorithm leverages the unique ability of a neutral atom tweezer platform to realize arbitrary qubit position configurations. These configurations determine the degree of entanglement available to a variational quantum algorithm via the interatomic interactions. Good configurations will accelerate pulse optimization convergence and help mitigate barren plateaus. As gradient-based approaches are ineffective for position optimization due to the divergent $R^{-6}$ nature of neutral atom interactions, we opt to use a consensus-based algorithm to optimize the qubit positions. By sampling the configuration space instead of using gradient information, the consensus-based algorithm is able to successfully optimize the positions, yielding adapted variational quantum algorithm ansatzes that lead to both faster convergence and lower errors. In this work, we show that these optimized configurations generally result in large improvements in the system's ability to solve ground state minimization problems for both random Hamiltonians and small molecules.

quant-ph↗

arXiv2025

Fidelity-Enhanced Variational Quantum Optimal Control

Creating robust quantum operations is a major challenge in the current noisy intermediate-scale quantum computing era. Recently, the importance of noise-resilient control methods has become more pronounced in the field. Ordinarily, noisy quantum systems are described by the Lindblad equation. However, minimizing noise susceptibility using this equation has proven challenging because of its irreversibility. In this study, we propose a new method for creating robust pulses based on the stochastic Schrödinger equation. This equation describes individual noise realizations under any colored noise process, contrary to the Lindblad equation, which describes mean system behavior under white noise. Using stochastic optimal control techniques, our method, Fidelity-Enhanced Variational Quantum Optimal Control (F-VQOC), is able to construct higher fidelity paths than its non-stochastic counterpart (VQOC). By accounting for both environmental noise sources as well as noise sources inherent to the control system, highly significant increases in fidelity are noted for both single and multiqubit state preparations.

quant-ph↗

arXiv2024

Pulse family optimization for parametrized quantum gates using spectral clustering

Parametrized gate circuits are used in plentiful applications in the current NISQ era of quantum computing. These parametrized gates are chiefly implemented using analytically found pulse protocols, often yielding suboptimal gate times, and consequently, fidelities. Alternatively, gate optimization algorithms are designed to construct high fidelity pulses for individual, fixed points in continuous parameter space. Gates for intermediate parameters can subsequently be found by some form of interpolation between previously constructed pulses. Nevertheless, it is not guaranteed (as with analytic protocols) that the pulses found by the optimization algorithms belong to the same \textit{family} of solutions and thus show resemblance. Interpolation between two pulses of differing solution families often leads to high infidelities, as the pulse strays away from the minimum in the parameter/fidelity landscape. In this work, we introduce a \textit{spectral clustering} method to sort high-fidelity, optimized pulses in families, and interpolating solely between pulses of the same family. Accordingly, interpolations will always approach maximal fidelity. Furthermore, as more than one pulse family is constructed, the parameter space can be partitioned according to which family prevails fidelity-wise. This work provides a meticulous demonstration of our constitutive continuous gate family construction by applying it to a universal gate set for Rydberg and Cat qubits under noise.

quant-ph↗

arXiv2024

Qubit fidelity under stochastic Schrödinger equations driven by colored noise

Environmental noise on a controlled quantum system is generally modeled by a dissipative Lindblad equation. This equation describes the average state of the system via the density matrix $ρ$. One way of deriving this Lindblad equation is by introducing a stochastic operator evolving under white noise in the Schrödinger equation. However, white noise, where all noise frequencies contribute equally in the power spectral density, is not a realistic noise profile as lower frequencies generally dominate the spectrum. Furthermore, the Lindblad equation does not fully describe the system as a density matrix $ρ$ does not uniquely describe a probabilistic ensemble of pure states $\{ψ_j\}_j$. In this work, we introduce a method for solving for the full distribution of qubit fidelity driven by important stochastic Schrödinger equation cases, where qubits evolve under more realistic noise profiles, e.g. Ornstein-Uhlenbeck noise. This allows for predictions of the mean, variance, and higher-order moments of the fidelities of these qubits, which can be of value when deciding on the allowed noise levels for future quantum computing systems, e.g. deciding what quality of control systems to procure. Furthermore, these methods will prove to be integral in the optimal control of qubit states under (classical) control system noise.

quant-ph↗

arXiv2023

Phonon-assisted coherent transport of excitations in Rydberg-dressed atom arrays

Polarons, which arise from the self-trapping interaction between electrons and lattice distortions in a solid, have been known and extensively investigated for nearly a century. Nevertheless, the study of polarons continues to be an active and evolving field, with ongoing advancements in both fundamental understanding and practical applications. Here, we present a microscopic model that exhibits a diverse range of dynamic behavior, arising from the intricate interplay between two excitation-phonon coupling terms. The derivation of the model is based on an experimentally feasible Rydberg-dressed system with dipole-dipole interactions, making it a promising candidate for realization in a Rydberg atoms quantum simulator. Remarkably, our analysis reveals a growing asymmetry in Bloch oscillations, leading to a macroscopic transport of non-spreading excitations under a constant force. Moreover, we compare the behavior of excitations, when coupled to either acoustic or optical phonons, and demonstrate the robustness of our findings against on-site random potential. Overall, this work contributes to the understanding of polaron dynamics with their potential applications in coherent quantum transport and offers valuable insights for research on Rydberg-based quantum systems.

cond-mat.quant-gas↗

arXiv2023

Robust control and optimal Rydberg states for neutral atom two-qubit gates

We investigate the robustness of two-qubit gates to deviations of experimental controls, on a neutral atom platform utilizing Rydberg states. We construct robust CZ gates - employing techniques from quantum optimal control - that retain high Bell state fidelity $F > 0.999$ in the presence of significant deviations of the coupling strength to the Rydberg state. Such deviations can arise from laser intensity noise and atomic motion in an inhomogeneous coupling field. We also discuss methods to mitigate errors due to deviations of the laser detuning. The designed pulses operate on timescales that are short compared to the fundamental decay timescale set by spontaneous emission and blackbody radiation. We account for the finite lifetime of the Rydberg state in both the optimisation and fidelity calculations - this makes the gates conducive to noisy intermediate-scale quantum experiments, meaning that our protocols can reduce infidelity on near-term quantum computing devices. We calculate physical properties associated with infidelity for strontium-88 atoms - including lifetimes, polarisabilities and blockade strengths - and use these calculations to identify optimal Rydberg states for our protocols, which allows for further minimisation of infidelity.

quant-ph↗

arXiv2023

Resonance-facilitated three-channel p-wave scattering

Feshbach resonances of arbitrary width are typically described in terms of two-channel models. Within these models, one usually considers a single dressed resonance, with the option to extend the analysis by including resonant open-channel features that can drastically change the observed threshold effects. For the strong $^{40}\mathrm{K}$ p-wave resonance studied in Ref. \cite{ahmed2021}, the interplay between an open-channel shape resonance and the Feshbach resonance could explain the unexpected nonlinear variation of the binding energy with magnetic field. However, the presented two-channel treatment relies on the introduction of two independent fitting parameters, whereas the typical Breit-Wigner expression would only account for one. This results in an effective magnetic moment that acquires a nonphysical value, which is an indication of a major shortcoming of the two-channel model treatment. In this study, we observe how the presence of a closed-channel shape resonance explains the physical mechanism behind the observations and demonstrates the need of a three-channel treatment. We introduce our novel model as \textit{resonance facilitated}, where all coupling is mediated by the Feshbach state, while there is no direct coupling between the additional channel and the open channel. Notably, the resonance-facilitated structure greatly reduces the complexity of the full three-channel model. The typical Breit-Wigner form of the two-channel Feshbach formalism is retained and the full effect of the added channel can be captured by a single resonance dressing factor, which describes how the free propagation in the Feshbach state is dressed by the added channel.

cond-mat.quant-gas↗

arXiv2023

Pulse based Variational Quantum Optimal Control for hybrid quantum computing

This work studies pulse based variational quantum algorithms (VQAs), which are designed to determine the ground state of a quantum mechanical system by combining classical and quantum hardware. In contrast to more standard gate based methods, pulse based methods aim to directly optimize the laser pulses interacting with the qubits, instead of using some parametrized gate based circuit. Using the mathematical formalism of optimal control, these laser pulses are optimized. This method has been used in quantum computing to optimize pulses for quantum gate implementations, but has only recently been proposed for full optimization in VQAs. Pulse based methods have several advantages over gate based methods such as faster state preparation, simpler implementation and more freedom in moving through the state space. Based on these ideas, we present the development of a novel adjoint based variational method. This method can be tailored towards and applied in neutral atom quantum computers. This method of pulse based variational quantum optimal control is able to approximate molecular ground states of simple molecules up to chemical accuracy and is able to compete with the gate based variational quantum eigensolver in terms of total number of quantum evaluations. The total evolution time $T$ and the form of the control Hamiltonian $H_c$ are important factors in the convergence behavior to the ground state energy, both having influence on the quantum speed limit and the controllability of the system.

quant-ph↗

arXiv2019

High-precision analysis of Feshbach resonances in a Mott insulator

We show that recent high-precision measurements of relative on-site interaction energies $ΔU$ in a Mott insulator require a theoretical description beyond the standard Hubbard-model interpretation, when combined with an accurate coupled-channels calculation. In contrast to more sophisticated lattice models, which can be elaborate especially for parameter optimization searches, we introduce an easy to use effective description of $U$ valid over a wide range of interaction strengths modeling atomic pairs confined to single lattice sites. This concise model allows for a straightforward combination with a coupled-channels analysis. With this model we perform such a coupled-channels analysis of high-precision $^7$Li spectroscopic data on the on-site interaction energy $U$, which spans over four Feshbach resonances and provide an accurate and consistent determination of the associated resonance positions. Earlier experiments on three of the Feshbach resonances are consistent with this new analysis. Moreover, we verify our model with a more rigorous numerical treatment of the two atom system in an optical lattice.

cond-mat.quant-gas↗

arXiv2019

Finite-range effects in Efimov physics beyond the separable approximation

We study Efimov physics for three identical bosons interacting via a pairwise square-well potential, analyze the validity of the separable approximation as a function of the interaction strength, and investigate what is needed to improve this approximation. We find separable approximations to be accurate for potentials with just one (nearly) bound dimer state. For potentials with more bound or almost bound dimer states, these states need to be included for an accurate determination of the Efimov spectrum and the corresponding three-body observables. We also show that a separable approximation is insufficient to accurately compute the trimer states for energies larger than the finite-range energy even when the two-body T matrix is highly separable in this energy regime. Additionally, we have analyzed three distinct expansion methods for the full potential that give exact results and thus improve on the separable approximation. With these methods, we demonstrate the necessity to include higher partial-wave components of the off-shell two-body T matrix in the three-body calculations. Moreover, we analyze the behavior of the Efimov states near the atom-dimer threshold and observe the formation of non-Efimovian trimer states as the potential depth is increased. Our results can help to elaborate simpler theoretical models that are capable of reproducing the correct three-body physics in atomic systems.

physics.atom-ph↗

arXiv2018

Simulating polaron biophysics with Rydberg atoms

Transport of excitations along proteins can be formulated in a quantum physics context, based on the periodicity and vibrational modes of the structures. Exact solutions are very challenging to obtain on classical computers, however, approximate solutions based on the Davydov ansatz have demonstrated the possibility of stabilized solitonic excitations along the protein. We propose an alternative study based on a chain of ultracold atoms. We investigate the experimental parameters to control such a quantum simulator based on dressed Rydberg atoms. We show that there is a feasible range of parameters where a quantum simulator can directly mimic the Davydov equations and their solutions. Such a quantum simulator opens up new directions for the study of transport phenomena in a biophysical context.

physics.atom-ph↗

arXiv2017

Rydberg dressing of a one-dimensional Bose-Einstein condensate

We study the influence of Rydberg dressed interactions in a one-dimensional (1D) Bose-Einstein Condensate (BEC). We show that a 1D geometry offers several advantages over 3D for observing BEC Rydberg dressing. The effects of dressing are studied by investigating collective BEC dynamics after a rapid switch-off of the Rydberg dressing interaction. The results can be interpreted as an effective modification of the s-wave scattering length. We include this modification in an analytical model for the 1D BEC, and compare it to numerical calculations of Rydberg dressing under realistic experimental conditions.

cond-mat.quant-gas↗

arXiv2017

Atomic Clock Measurements of Quantum Scattering Phase Shifts Spanning Feshbach Resonances at Ultralow Fields

We use an atomic fountain clock to measure quantum scattering phase shifts precisely through a series of narrow, low-field Feshbach resonances at average collision energies below $1\,μ$K. Our low spread in collision energy yields phase variations of order $\pm π/2$ for target atoms in several $F,m_F$ states. We compare them to a theoretical model and establish the accuracy of the measurements and the theoretical uncertainties from the fitted potential. We find overall excellent agreement, with small statistically significant differences that remain unexplained.

physics.atom-ph↗