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arXiv2018

Efimov Physics in Quenched Unitary Bose Gases

We study the impact of three-body physics in quenched unitary Bose gases, focusing on the role of the Efimov effect. Using a local density model, we solve the three-body problem and determine three-body decay rates at unitary, finding density-dependent, log-periodic Efimov oscillations, violating the expected continuous scale-invariance in the system. We find that the breakdown of continuous scale-invariance, due to Efimov physics, manifests also in the earliest stages of evolution after the interaction quench to unitarity, where we find the growth of a substantial population of Efimov states for densities in which the interparticle distance is comparable to the size of an Efimov state. This agrees with the early-time dynamical growth of three-body correlations at unitarity [Colussi {\em et al}., Phys. Rev. Lett. 120, 100401 (2018)]. By varying the sweep rate away from unitarity, we also find a departure from the usual Landau-Zener analysis for state transfer when the system is allowed to evolve at unitarity and develop correlations.

physics.atom-ph↗

arXiv2021

Fast Preparation and Detection of a Rydberg Qubit using Atomic Ensembles

We demonstrate a new approach for fast preparation, manipulation, and collective readout of an atomic Rydberg-state qubit. By making use of Rydberg blockade inside a small atomic ensemble, we prepare a single qubit within 3~$μ$s with a success probability of $F_p=0.93 \pm 0.02$, rotate it, and read out its state in $6$ $μs$ with a single-shot fidelity of $F_d=0.92 \pm 0.04$. The ensemble-assisted detection is $10^3$ times faster than imaging of a single atom with the same optical resolution, and enables fast repeated non-destructive measurement. We observe qubit coherence times of 15~$μ$s, much longer than the $π$ rotation time of 90~ns. Potential applications ranging from faster quantum information processing in atom arrays to efficient implementation of quantum error correction are discussed.

quant-ph↗

arXiv2022

Enhanced coupling of electron and nuclear spins by quantum tunneling resonances

Noble-gas spins feature hours long coherence times owing to their great isolation from the environment, and find practical usage in various applications. However, this isolation leads to extremely slow preparation times, relying on weak spin transfer from an electron-spin ensemble. Here we propose a controllable mechanism to enhance this transfer rate. We analyze the spin dynamics of helium-3 atoms with hot, optically-excited potassium atoms and reveal the formation of quasi-bound states in resonant binary collisions. We find a resonant enhancement of the spin-exchange cross section by up to six orders of magnitude and two orders of magnitude enhancement for the thermally averaged, polarization rate-coefficient. We further examine the effect for various other noble gases and find that the enhancement is universal. We outline feasible conditions under which the enhancement may be experimentally observed and practically utilized.

quant-ph↗

arXiv2024

Interferometry of non-Abelian band singularities and Euler class topology

In systems with a real Bloch Hamiltonian band nodes can be characterised by a non-Abelian frame-rotation charge. The ability of these band nodes to annihilate pairwise is path dependent, since by braiding nodes in adjacent gaps the sign of their charges can be changed. Here, we theoretically construct and numerically confirm two concrete methods to experimentally probe these non-Abelian braiding processes and charges in ultracold atomic systems. We consider a coherent superposition of two bands that can be created by moving atoms through the band singularities at some angle in momentum space. Analyzing the dependency of excitations on the frame charges, we demonstrate an interferometry scheme passing through two band nodes, which reveals the relative frame charges and allows for measuring the multi-gap topological invariant. The second method relies on a single wavepacket probing two nodes sequentially, where the frame charges can be determined from the band populations. Our results present a feasible avenue for measuring non-Abelian charges of band nodes and the direct experimental verification of braiding procedures, which can be applied in a variety of settings including the recently discovered anomalous non-Abelian phases arising under periodic driving.

cond-mat.quant-gas↗

arXiv2021

Two-photon resonance fluorescence of two interacting non-identical quantum emitters

We study a system of two interacting, non-indentical quantum emitters driven by a coherent field. We focus on the particular condition of two-photon resonance and obtain analytical expressions for the stationary density matrix of the system and observables of the fluorescent emission. Importantly, our expressions are valid for the general situation of non-identical emitters with different transition energies. Our results allow us to determine the regime of parameters in which coherent two-photon excitation, enabled by the coherent coupling between emitters, is dominant over competing, first-order processes. Using the formalism of quantum parameter estimation, we show that the features imprinted by the two-photon dynamics into the spectrum of resonance fluorescence are particularly sensitive to changes in the distance between emitters, making two-photon excitation the optimal driving regime for the estimation of inter-emitter distance. This can be exploited for applications such as superresolution imaging of point-like sources.

quant-ph↗

arXiv2013

Probing atom-surface interactions by diffraction of Bose-Einstein condensates

In this article we analyze the Casimir-Polder interaction of atoms with a solid grating and an additional repulsive interaction between the atoms and the grating in the presence of an external laser source. The combined potential landscape above the solid body is probed locally by diffraction of Bose-Einstein condensates. Measured diffraction efficiencies reveal information about the shape of the Casimir-Polder interaction and allow us to discern between models based on a pairwise-summation (Hamaker) approach and Lifshitz theory.

physics.atom-ph↗

arXiv2013

Optimisation of Quantum Trajectories Driven by Strong-field Waveforms

Quasi-free field-driven electron trajectories are a key element of strong-field dynamics. Upon recollision with the parent ion, the energy transferred from the field to the electron may be released as attosecond duration XUV emission in the process of high harmonic generation (HHG). The conventional sinusoidal driver fields set limitations on the maximum value of this energy transfer, and it has been predicted that this limit can be significantly exceeded by an appropriately ramped-up cycleshape. Here, we present an experimental realization of such cycle-shaped waveforms and demonstrate control of the HHG process on the single-atom quantum level via attosecond steering of the electron trajectories. With our optimized optical cycles, we boost the field-ionization launching the electron trajectories, increase the subsequent field-to-electron energy transfer, and reduce the trajectory duration. We demonstrate, in realistic experimental conditions, two orders of magnitude enhancement of the generated XUV flux together with an increased spectral cutoff. This application, which is only one example of what can be achieved with cycle-shaped high-field light-waves, has farreaching implications for attosecond spectroscopy and molecular self-probing.

physics.atom-ph↗

arXiv2014

Quantum-mechanical calculation of ionization potential lowering in dense plasmas

The charged environment within a dense plasma leads to the phenomenon of ionization potential depression (IPD) for ions embedded in the plasma. Accurate predictions of the IPD effect are of crucial importance for modeling atomic processes occurring within dense plasmas. Several theoretical models have been developed to describe the IPD effect, with frequently discrepant predictions. Only recently, first experiments on IPD in Al plasma have been performed with an x-ray free-electron laser (XFEL), where their results were found to be in disagreement with the widely-used IPD model by Stewart and Pyatt. Another experiment on Al, at the Orion laser, showed disagreement with the model by Ecker and Kröll. This controversy shows a strong need for a rigorous and consistent theoretical approach to calculate the IPD effect. Here we propose such an approach: a two-step Hartree-Fock-Slater model. With this parameter-free model we can accurately and efficiently describe the experimental Al data and validate the accuracy of standard IPD models. Our model can be a useful tool for calculating atomic properties within dense plasmas with wide-ranging applications to studies on warm dense matter, shock experiments, planetary science, inertial confinement fusion and studies of non-equilibrium plasmas created with XFELs.

physics.plasm-ph↗

arXiv2017

Probing quench dynamics across a quantum phase transition into a 2D Ising antiferromagnet

Simulating the real-time evolution of quantum spin systems far out of equilibrium poses a major theoretical challenge, especially in more than one dimension. We experimentally explore the dynamics of a two-dimensional Ising spin system with transverse and longitudinal fields as we quench it across a quantum phase transition from a paramagnet to an antiferromagnet. We realize the system with a near unit-occupancy atomic array of over 200 atoms obtained by loading a spin-polarized band insulator of fermionic lithium into an optical lattice and induce short-range interactions by direct excitation to a low-lying Rydberg state. Using site-resolved microscopy, we probe the correlations in the system after a sudden quench from the paramagnetic state and compare our measurements to exact calculations in the regime where it is possible. We achieve many-body states with longer-range antiferromagnetic correlations by implementing a near-adiabatic quench and study the buildup of correlations as we cross the quantum phase transition at different rates.

quant-ph↗

arXiv2011

Observation and cancellation of the dc Stark shift in strontium optical lattice clocks

We report on the observation of a dc Stark frequency shift at the $10^{-13}$ level by comparing two strontium optical lattice clocks. This frequency shift arises from the presence of electric charges trapped on dielectric surfaces placed under vacuum close to the atomic sample. We show that these charges can be eliminated by shining UV light on the dielectric surfaces, and characterize the residual dc Stark frequency shift on the clock transition at the $10^{-18}$ level by applying an external electric field. This study shows that the dc Stark shift can play an important role in the accuracy budget of lattice clocks, and should be duly taken into account.

physics.atom-ph↗

arXiv2013

Sub-Doppler optical resolution by confining a vapour in a nanostructure

We show that a thermal vapor confined in a nanostructure is of spectroscopic interest. We perform reflection spectroscopy on a Cs vapour cell whose window is covered with a thin opal film (typically, 10 or 20 layers of ~ 1μm diameter spheres). Sub-Doppler structures appear in the optical spectrum in a purely linear regime of optical excitation and the signal is shown to originate from the interstitial regions of the opal. These narrow spectral structures, observable for a large range of oblique incidence angles (~ 30-50&deg), are an original feature associated to the 3-D vapor confinement. It remembers a Dicke narrowing, i.e. a Doppler broadening suppression when the atomic motion is sub-wavelength confined. This narrowing, commonly observed in the r.f. domain when a buffer gas ensures a collision confinement effect, had remained elusive in the optical frequency. Also, we describe preliminary experiments performed in a pump-probe technique, intended to elucidate the spatial origin of the narrow contribution. We finally discuss how our results allow envisioning micron-size references for optical frequency clocks, and high resolution spectroscopy of weak and hard-to-saturate molecular lines.

physics.atom-ph↗

arXiv2013

Computational relativistic quantum dynamics and its application to relativistic tunneling and Kapitza-Dirac scattering

Computational methods are indispensable to study the quantum dynamics of relativistic light-matter interactions in parameter regimes where analytical methods become inapplicable. We present numerical methods for solving the time-dependent Dirac equation and the time-dependent Klein-Gordon equation and their implementation on high performance graphics cards. These methods allow us to study tunneling from hydrogen-like highly charged ions in strong laser fields and Kapitza-Dirac scattering in the relativistic regime.

quant-ph↗

arXiv2013

Observation of Zeeman shift in the rubidium D2 line using an optical nanofiber in vapor

We report on the observation of a Zeeman shift (order of 100 MHz) of the Doppler-broadened D2 transition of both 85Rb and 87Rb isotopes via transmission through a tapered optical nanofiber in the presence of a DC magnetic field. Linearly-polarized light propagating in the nanofiber is analyzed as a superposition of two orthogonally circularly-polarized orientations, σ+ and σ-. In the absence of the magnetic field, the absorption of these polarizations by the atomic vapor, via the evanescent field at the waist of the nanofiber, is degenerate. When a weak magnetic field is applied parallel to the propagating light, this degeneracy is lifted and relative shifts in the resonance frequencies are detected. Typical linear shift rates of 1.6 MHz/G and -2.0 MHz/G were observed. We also demonstrate a dichroic atomic vapor laser lock line shape by monitoring the real-time subtraction of the two magnetically-shifted absorption spectra. This is particularly interesting for magneto-optical experiments as it could be directly implemented for diode laser frequency-stabilization.

physics.atom-ph↗

arXiv2013

An atomic clock with $10^{-18}$ instability

Atomic clocks have been transformational in science and technology, leading to innovations such as global positioning, advanced communications, and tests of fundamental constant variation. Next-generation optical atomic clocks can extend the capability of these timekeepers, where researchers have long aspired toward measurement precision at 1 part in $\bm{10^{18}}$. This milestone will enable a second revolution of new timing applications such as relativistic geodesy, enhanced Earth- and space-based navigation and telescopy, and new tests on physics beyond the Standard Model. Here, we describe the development and operation of two optical lattice clocks, both utilizing spin-polarized, ultracold atomic ytterbium. A measurement comparing these systems demonstrates an unprecedented atomic clock instability of $\bm{1.6\times 10^{-18}}$ after only $\bm{7}$ hours of averaging.

physics.atom-ph↗

arXiv2013

Precision Spectroscopy of Polarized Molecules in an Ion Trap

Polar molecules are desirable systems for quantum simulations and cold chemistry. Molecular ions are easily trapped, but a bias electric field applied to polarize them tends to accelerate them out of the trap. We present a general solution to this issue by rotating the bias field slowly enough for the molecular polarization axis to follow but rapidly enough for the ions to stay trapped. We demonstrate Ramsey spectroscopy between Stark-Zeeman sublevels in 180Hf19F+ with a coherence time of 100 ms. Frequency shifts arising from well-controlled topological (Berry) phases are used to determine magnetic g-factors. The rotating-bias-field technique may enable using trapped polar molecules for precision measurement and quantum information science, including the search for an electron electric dipole moment.

physics.atom-ph↗

arXiv2013

Order of Magnitude Smaller Limit on the Electric Dipole Moment of the Electron

The Standard Model (SM) of particle physics fails to explain dark matter and why matter survived annihilation with antimatter following the Big Bang. Extensions to the SM, such as weak-scale Supersymmetry, may explain one or both of these phenomena by positing the existence of new particles and interactions that are asymmetric under time-reversal (T). These theories nearly always predict a small, yet potentially measurable ($10^{-27}$-$10^{-30}$ $e$ cm) electron electric dipole moment (EDM, $d_e$), which is an asymmetric charge distribution along the spin ($\vec{S}$). The EDM is also asymmetric under T. Using the polar molecule thorium monoxide (ThO), we measure $d_e = (-2.1 \pm 3.7_\mathrm{stat} \pm 2.5_\mathrm{syst})\times 10^{-29}$ $e$ cm. This corresponds to an upper limit of $|d_e| < 8.7\times 10^{-29}$ $e$ cm with 90 percent confidence, an order of magnitude improvement in sensitivity compared to the previous best limits. Our result constrains T-violating physics at the TeV energy scale.

physics.atom-ph↗

arXiv2018

Transport of secondary electrons through coatings of ion-irradiated metallic nanoparticles

The transport of low-energy electrons through the coating of a radiosensitizing metallic nanoparticle under fast ion irradiation is analyzed theoretically and numerically. As a case study, we consider a poly(ethylene glycol)-coated gold nanoparticle of diameter 1.6~nm excited by a carbon ion in the Bragg peak region in water as well as by more energetic carbon ions. The diffusion equation for low-energy electrons emitted from a finite-size spherical source representing the surface of the metal core is solved to obtain the electron number density as a function of radial distance and time. Information on the atomistic structure and composition of the coating is obtained from molecular dynamics simulations performed with the MBN Explorer software package. Two mechanisms of low-energy electron production by the metallic core are considered: the relaxation of plasmon excitations and collective excitations of valence $d$ electrons in individual atoms of gold. Diffusion coefficients and characteristic lifetimes of electrons propagating in gold, water, and poly(ethylene glycol) are obtained from relativistic partial wave analysis and the dielectric formalism, respectively. On this basis, the number of electrons released through the organic coating into the surrounding aqueous medium and the number of hydroxyl radicals produced are evaluated. The largest increase of the radical yield due to low-energy electrons is observed when the nanoparticle is excited by an ion with energy significantly exceeding that in the Bragg peak region. It is also shown that the water content of the coating, especially near the surface of the metal core, is crucial for the production of hydroxyl radicals.

physics.atm-clus↗

arXiv2018

One-center calculations of the electron-positron pair creation in low-energy collisions of heavy bare nuclei

The probabilities of bound-free electron-positron pair creation are calculated for head-on collisions of bare uranium nuclei beyond the monopole approximation. The calculations are based on the numerical solving of the time-dependent Dirac equation in the target reference frame with multipole expansion of the projectile potential. In addition, the energy dependence of the pair-creation cross section is studied in the monopole approximation.

physics.atom-ph↗