Laser-cooled Atoms Research Articles

Generalized Einstein relation for aging processes

Physical aging appears in many systems ranging from glassy/granular materials, blinking quantum dots to laser-cooled atoms. Aging is a process with three fingerprints: (i) slow, non-exponential relaxation, (ii) breaking of time-translation-invariance, and (iii) dynamical scaling. Here, we show that all these features are present in our minimal Langevin model for aging. A natural extension of the Einstein relation, which was expected to be true in an equilibrium state, is conjectured to hold in aging processes where both the damping and the temperature decrease with time in power-law forms. The generalized Einstein relation can be used to tackle the difficult problem of determining non-ergodic behaviours. The model shows a power-law-type diffusion away from the critical point and a logarithmic Sinai-type ultra-slow diffusion at the critical point. Application to granular gases is also discussed.

Searching for axion forces with spin precession in atoms and molecules

We propose to use atoms and molecules as quantum sensors of axion-mediated monopole-dipole forces. We show that electron spin precession experiments using atomic and molecular beams are well-suited for axion searches thanks to the presence of co-magnetometer states and single-shot temporal resolution. Experimental strategies to detect axion gradients from localised sources and the earth are presented, taking ACME III as a prototype example. Other possibilities including atomic beams, and laser-cooled atoms and molecules are discussed.

Momentum-exchange interactions in a Bragg atom interferometer suppress Doppler dephasing.

Large ensembles of laser-cooled atoms interacting through infinite-range photon-mediated interactions are powerful platforms for quantum simulation and sensing. Here we realize momentum-exchange interactions in which pairs of atoms exchange their momentum states by collective emission and absorption of photons from a common cavity mode, a process equivalent to a spin-exchange or XX collective Heisenberg interaction. The momentum-exchange interaction leads to an observed all-to-all Ising-like interaction in a matter-wave interferometer. A many-body energy gap also emerges, effectively binding interferometer matter-wave packets together to suppress Doppler dephasing in analogy to Mössbauer spectroscopy. The tunable momentum-exchange interaction expands the capabilities of quantum interaction-enhanced matter-wave interferometry and may enable the realization of exotic behaviors, including simulations of superconductors and dynamical gauge fields.

Quantum Sensing in Tweezer Arrays: Optical Magnetometry on an Individual-Atom Sensor Grid

We implement a scalable platform for quantum sensing comprising hundreds of sites capable of holding individual laser-cooled atoms and demonstrate the applicability of this single-quantum-system sensor array to magnetic field mapping on a two-dimensional grid. With each atom being confined in an optical tweezer within an area of 0.5μm2 at mutual separations of 7.0(2)μm, we obtain micrometer-scale spatial resolution and highly parallelized operation. An additional steerable optical tweezer allows rearrangement of atoms within the grid and enables single-atom scanning microscopy with submicron resolution. This individual-atom sensor platform finds an immediate application in mapping an externally applied dc gradient magnetic field. In a Ramsey-type measurement, we obtain a field resolution of 98(29) nT. We estimate the sensitivity to be 25μT/Hz. Published by the American Physical Society 2024

Spatial magnetic field mapping with Raman spectra of laser-cooled atoms in free-fall

Spatial magnetic field mapping with Raman spectra of laser-cooled atoms in free-fall

Optical levitation in high vacuum using a 0.9-numerical-aperture lens

Nanoparticles levitated in high vacuum are isolated from the surrounding environment and thus can be used in various applications, including quantum physics research. We demonstrated optical levitation of a nanoparticle, which was trapped without cooling at a pressure of 4.5 × 10−3 Pa, using a single aspheric lens with a large numerical aperture (= 0.9) and 1030-nm laser. We also activated parametric feedback cooling to trap the particle at a pressure of 5.8 × 10−4 Pa. This experimental system will be useful for studying nanoparticles in ultrahigh vacuum and for building a mixed system with laser-cooled atoms.

Space charge effect in low-density ultracold ion bunches

We have implemented an approach to investigate the space charge effect (SCE) in the ultracold ion bunch produced through the near-threshold photoionization of laser-cooled rubidium atoms trapped in a magneto-optical trap. The non-linear broadening of spatial profile of the ultracold ion bunch induced by SCE within the initial density range of 3.7 × 106–4.5 × 107/cm3 was explored using a time-of-flight spectrometer coupled with an imaging detector. A charged particle tracing simulation accounting for all pairwise ion–ion Coulomb interactions and an analytical model calculation, which predicts the dependence of the ion bunch density on time evolution and initial density, reproduced the experimental results successfully, indicating that the study could capture the evolution dynamics of ion bunch dominated by SCE. The aim of this work is to extend the investigation on SCE to extreme low-density regions of the order of 106 /cm3 and is expected to be useful in optimizing the performance of ultracold ion/electron sources.

Photoionization-cross-section measurement of the Rb85 5 P3/22 excited state using microwave cavity resonance spectroscopy

We present microwave cavity resonance spectroscopy as a technique to determine excited-state photoionization cross sections of laser-cooled atoms. We demonstrate this technique by measuring the photoionization cross section of the $5^{2}P_{3/2}({F}^{\ensuremath{'}}=4)$ excited state of $^{85}\mathrm{Rb}$ to continuum by creating an ultracold plasma inside a 5-GHz resonant microwave cavity. We find a photoionization cross section of $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}\phantom{\rule{4pt}{0ex}}{\mathrm{m}}^{2}$ for photon excess energies of 50, 100, 200, and 500 K above the ionization threshold. The measurements yield a cross section which is approximately 2 to 3 times smaller than the value provided by existing theory for the $5p$ excited state of $^{85}\mathrm{Rb}$ and two earlier measurements performed with other techniques for similar excess energies.

Temporally ultralong biphotons with a linewidth of 50 kHz

We report the generation of biphotons, with a temporal full width at the half maximum (FWHM) of 13.4 ± 0.3 µs and a spectral FWHM of 50 ± 1 kHz, via the process of spontaneous four-wave mixing with laser-cooled atoms. The temporal width is the longest, and the spectral linewidth is the narrowest to date. This is also the first biphoton result that obtains a linewidth below 100 kHz, reaching a new milestone. The very long biphoton wave packet has a signal-to-background ratio of 3.4, which violates the Cauchy–Schwarz inequality for classical light by 4.8 folds. Furthermore, we demonstrated a highly tunable-linewidth biphoton source and showed that while the biphoton source’s temporal and spectral width were controllably varied by about 24 folds, its generation rate only changed by less than 15%. A spectral brightness or generation rate per pump power per linewidth of 1.2× 106 pairs/(s mW MHz) was achieved at the temporal width of 13.4 µs. The above results were made possible by the low decoherence rate and high optical depth of the experimental system, as well as a novel scheme of classical fields’ and biphotons’ propagation directions in the experiment. This work has demonstrated a high-efficiency ultranarrow-linewidth biphoton source and has made substantial advancements in quantum technology utilizing heralded single photons.

Anatomy of an extreme event: What can we infer about the history of a heavy-tailed random walk?

Extreme events are by nature rare and difficult to predict, yet are often much more important than frequent, typical events. An interesting counterpoint to the prediction of such events is their retrodiction-given a process in an outlier state, how did the events leading up to this endpoint unfold? In particular, was there only a single, massive event, or was the history a composite of multiple, smaller but still significant events? To investigate this problem we take heavy-tailed stochastic processes (specifically, the symmetric, α-stable Lévy processes) as prototypical random walks. A natural and useful characteristic scale arises from the analysis of processes conditioned to arrive in a particular final state (Lévy bridges). For final displacements longer than this scale, the scenario of a single, long jump is most likely, even though it corresponds to a rare, extreme event. On the other hand, for small final displacements, histories involving extreme events tend to be suppressed. To further illustrate the utility of this analysis, we show how it provides an intuitive framework for understanding three problems related to boundary crossings of heavy-tailed processes. These examples illustrate how intuition fails to carry over from diffusive processes, even very close to the Gaussian limit. One example yields a computationally and conceptually useful representation of Lévy bridges that illustrates how conditioning impacts the extreme-event content of a random walk. The other examples involve the conditioned boundary-crossing problem and the ordinary first-escape problem; we discuss the observability of the latter example in experiments with laser-cooled atoms.

Coherent dynamics in a five-level atomic system

The coherent control of multi-partite quantum systems presents one of the central prerequisites in state-of-the-art quantum information processing. With the added benefit of inherent high-fidelity detection capability, atomic quantum systems in high-energy internal states, such as metastable noble gas atoms, promote themselves as ideal candidates for advancing quantum science in fundamental aspects and technological applications. Using laser-cooled neon atoms in the metastable 3P2 state of state 1s 22s 22p 53s (LS-coupling notation) (Racah notation: 2 P 3/23s[3/2]2) with five m J -sublevels, experimental methods for the preparation of all Zeeman sublevels |m J ⟩ = |+2⟩, |+1⟩, |0⟩, |−1⟩, |−2⟩ as well as the coherent control of superposition states in the five-level system |+2⟩, …, |−2⟩, in the three-level system |+2⟩, |+1⟩, |0⟩, and in the two-level system |+2⟩, |+1⟩ are presented. The methods are based on optimized radio frequency and laser pulse sequences. The state evolution is described with a simple, semiclassical model. The coherence properties of the prepared states are studied using Ramsey and spin echo measurements.

Satellite-borne atomic clock based on diffuse laser-cooled atoms

The technique of laser cooling of atoms gives an opportunity to improve the performance of atomic clocks by using laser-cooled atoms. The most successful cold atom clock, called the atomic fountain, is now widely used as the primary frequency standard in many labs. The cold atom clock for satellite applications, however, has not been reported so far due to special requirements of space applications. Here, we report the development of an engineering model of a satellite-borne cold atom clock, which satisfied all requirements of in-orbit operation. The core of the clock’s principle is the laser cooling of atoms by diffuse laser lights inside the microwave cavity. The structure of the physics package is presented, and its main parameters are also given. The principle and design of the optical bench are described. The initial test results are presented, and the possible improvements are also discussed.

Characterization of laser cooling in microgravity via long-term operations in TianGong-2 space lab.

The invention of laser cooling has fundamentally influenced the research frontier of atomic physics and quantum physics, and recently an intense focus has been on the studies of cold atom physics in microgravity environments. Herein, we report the results of our laser cooling experiment in TianGong-2 space lab, which operated for 34 consecutive months in orbit. Over such an extended operation time, the quality of laser cooling did not experience any significant decline, while the properties of laser cooling in orbital microgravity were systematically studied. In particular, we demonstrate magneto-optical trapping and polarization-gradient cooling in orbit and carefully examine their performances. A comparison of the in-orbit and on-ground results indicates that a higher cooling efficiency exists in microgravity, including a smaller loss rate during the trapping and cooling process and lower ultimate temperature of laser-cooled atoms. Our progress has laid the technical foundations for future applications of cold atoms in space missions with operation times of the order of years.

Generalized virial equationfor nonlinear multiplicative Langevin dynamics: Application to laser-cooled atoms.

The virial theorem, and the equipartition theorem in the case of quadratic degrees of freedom, are handy constraints on the statistics of equilibrium systems. Their violation is instrumental in determining how far from equilibrium a driven system might be. We extend the virial theorem to nonequilibrium conditions for Langevin dynamics with nonlinear friction and multiplicative noise. In particular, we generalize it for confined laser-cooled atoms in the semiclassical regime. The resulting relation between the lowest moments of the atom position and velocity allows to measure in experiments how dissipative the cooling mechanism is. Moreover, its violation can reveal the departure from a strictly harmonic confinement or from the semiclassical regime.

Continuous Sub-Doppler-Cooled Atomic Beam Interferometer for Inertial Sensing

In atomic sensors, continuous interrogation of laser-cooled atoms carries the benefits of improved sensitivity and high measurement bandwidth. However, cooling in proximity to coherent atomic state evolution can degrade performance in compact systems, due to decoherence. This study demonstrates an inertially sensitive matter-wave interferometer in a three-dimensionally-cooled atomic beam that mitigates decoherence while operating continuously. The technique could enable compact atom-interferometer sensors that measure continuously and with high sensitivity on dynamic platforms.

Prospects for assembling ultracold radioactive molecules from laser-cooled atoms

Molecules with unstable isotopes often contain heavy and deformed nuclei and thus possess a high sensitivity to parity-violating effects, such as the Schiff moments. Currently the best limits on Schiff moments are set with diamagnetic atoms. Polar molecules with quantum-enhanced sensing capabilities, however, can offer better sensitivity. In this work, we consider the prototypical 223Fr107Ag molecule, as the octupole deformation of the unstable 223Fr francium nucleus amplifies the nuclear Schiff moment of the molecule by two orders of magnitude relative to that of spherical nuclei and as the silver atom has a large electron affinity. To develop a competitive experimental platform based on molecular quantum systems, 223Fr atoms and 107Ag atoms have to be brought together at ultracold temperatures. That is, we explore the prospects of forming 223Fr107Ag from laser-cooled Fr and Ag atoms. We have performed fully relativistic electronic-structure calculations of ground and excited states of FrAg that account for the strong spin-dependent relativistic effects of Fr and the strong ionic bond to Ag. In addition, we predict the nearest-neighbor densities of magnetic-field Feshbach resonances in ultracold 223Fr + 107Ag collisions with coupled-channel calculations. These resonances can be used for magneto-association into ultracold, weakly-bound FrAg. We also determine the conditions for creating 223Fr107Ag molecules in their absolute ground state from these weakly-bound dimers via stimulated Raman adiabatic passage using our calculations of the relativistic transition electric dipole moments.

Non-Reciprocal Amplification of Light Using Cold Atoms Coupled to an Optical Nanofiber

Optical nanofibers realized as the waist of tapered silica fibers can be used to trap and optically interface laser-cooled atoms. Building on this system, we experimentally show a novel scheme for the nonreciprocal Raman amplification of light. While typically either the magneto-optical effect, a temporal modulation or an optical nonlinearity is employed to break reciprocity, in our approach, this results from the spin of the atoms forming the gain medium. By taking advantage of the inherent spin-momentum locking present in optical nanofibers, we perform an experiment in which we set the amplification direction by a suitable preparation of the atomic spin state. Our approach is general and, suitable quantum emitters provided, could also be implemented beyond the optical domain of the electromagnetic spectrum.

Theoretical aspects of radium-containing molecules amenable to assembly from laser-cooled atoms for new physics searches

We explore the possibilities for a next-generation electron-electric-dipole-moment experiment using ultracold heteronuclear diatomic molecules assembled from a combination of radium and another laser-coolable atom. In particular, we calculate their ground state structure and their sensitivity to parity- and time-reversal () violating physics arising from flavor-diagonal charge-parity () violation. Among these species, the largest -violating molecular interaction constants—associated for example with the electron electric dipole moment—are obtained for the combination of radium (Ra) and silver (Ag) atoms. A mechanism for explaining this finding is proposed. We go on to discuss the prospects for an electron EDM search using ultracold, assembled, optically trapped RaAg molecules, and argue that this system is particularly promising for rapid future progress in the search for new sources of violation.

High-performance coherent population trapping clock based on laser-cooled atoms

We present a coherent population trapping clock system based on laser-cooled 87Rb atoms. The clock consists of a frequency-stabilized CPT interrogation laser and a cooling laser as well as a compact magneto-optical trap, a high-performance microwave synthesizer, and a signal detection system. The resonance signal in the continuous wave regime exhibits an absorption contrast of ∼ 50%. In the Ramsey interrogation method, the linewidth of the central fringe is 31.25 Hz. The system achieves fractional frequency stability of , which goes down to 1.8 × 10−13 at 20000 s. The results validate that cold atom interrogation can improve the long-term frequency stability of coherent population trapping clocks and holds the potential for developing compact/miniature cold atoms clocks.

Temporal analog of Fabry-P\xe9rot resonator via coherent population trapping

Ramsey spectroscopy via coherent population trapping (CPT) is essential in precision measurements. The conventional CPT-Ramsey fringes contain numbers of almost identical oscillations and so that it is difficult to identify the central fringe. Here we experimentally demonstrate a temporal analog of Fabry–Pérot resonator via double-Λ CPT of laser-cooled 87Rb atoms. By inserting a periodic CPT pulse train between the two CPT-Ramsey pulses, due to the constructive interference of spin coherence, the transmission spectrum appears as a comb of equidistant peaks in frequency domain and thus the central Ramsey fringe can be easily identified. From the five-level Bloch equations for our double-Λ system, we find that the multi-pulse CPT interference can be regarded as a temporal analog of Fabry–Pérot resonator. Because of the small amplitude difference between the two Landé g factors, each peak splits into two when the external magnetic field is not too weak. This splitting is exactly linear with the magnetic field strength and thus can be used for measuring a magnetic field without involving magneto-sensitive transitions.