Intensity dependence of photoassociation in a quantum degenerate atomic gas

Precise Photoexcitation Measurement of Tan's Contact in the Entire BCS-BEC Crossover

We study two-body correlations in a spin-balanced ultracold harmonically trapped Fermi gas of ^{6}Li atoms in the crossover from the Bardeen-Cooper-Schrieffer (BCS) to the Bose-Einstein-Condensate (BEC) regime. For this, we precisely measure Tan's contact using a novel method based on photoexcitation of atomic pairs, which was recently proposed by Wang et al. [Photoexcitation measurement of Tan's contact for a strongly interacting Fermi gas, Phys. Rev. A 104, 063309 (2021).PLRAAN2469-992610.1103/PhysRevA.104.063309]. We map out the contact in the entire phase diagram of the BCS-BEC crossover for various temperatures and interaction strengths, probing regions in phase space that have not been investigated yet. Our measurements reach an uncertainty of ≈2% at the lowest temperatures and thus represent a precise quantitative benchmark. By comparison to our data, we localize the regions in phase space where theoretical predictions and interpolations give valid results. In regions where the contact is already well known we find excellent agreement with our measurements. Thus, our results demonstrate that photoinduced loss is a precise probe to measure quantum correlations in a strongly interacting Fermi gas.

Nonlinear laser-induced frequency shift in a 23Na spin-1 condensate

Herein, we report on the experimental observations and a quantitative determination of the laser-induced frequency shift (LIFS) in the photoassociation (PA) spectra of spinor Bose-Einstein condensate of sodium. Our investigations revealed a nonlinear dependence of the LIFS on the intensity of PA laser. By developing a model within the quadratic Stark effect, we simulate the experimental results via a theoretical model that confirms the former. The experimental observations and the theoretical analysis can further improve the accuracy of investigations on important molecular properties and on preparation of specific molecular states, with possible applications in various key fields.

Pair Correlations and Photoassociation Dynamics of Two Atoms in an Optical Tweezer

We investigate the photoassociation dynamics of exactly two laser-cooled ^{85}Rb atoms in an optical tweezer and reveal fundamentally different behavior to photoassociation in many-atom ensembles. We observe nonexponential decay in our two-atom experiment that cannot be described by a single rate coefficient and find its origin in our system's pair correlation. This is in stark contrast to many-atom photoassociation dynamics, which are governed by decay with a single rate coefficient. We also investigate photoassociation in a three-atom system, thereby probing the transition from two-atom dynamics to many-atom dynamics. Our experiments reveal additional reaction dynamics that are only accessible through the control of single atoms and suggest photoassociation could measure pair correlations in few-atom systems. It further showcases our complete control over the quantum state of individual atoms and molecules, which provides information unobtainable from many-atom experiments.

The effects of Feshbach resonance on spectral shifts in photoassociation of Cs atoms

We study the effects of magnetic Feshbach resonance on the shifts in photoassociation (PA) spectra of ultracold Cs atoms. A series of atom loss spectra show a linear variation of the frequency shift with the PA laser intensity at different magnetic fields near the d-wave Feshbach resonance of optically trapped Cs atoms. The magnetic field-dependence of the slope of the shift on the PA laser intensity exhibits a dispersive change near the Feshbach resonance. The theoretical formula derived from a model based on Fano resonance fits well with the experimental data. Using a model rectangular potential with tunable well depth and applying the Franck-Condon principle, we obtain numerical results, which are found to be largely in disagreement with the experimental findings.

Probing Energy-Dependent Feshbach Resonances by Optical Control

Optical control enables new high resolution probes of narrow collisional (Feshbach) resonances, which are strongly dependent on the relative momentum of colliding atom pairs, and important for simulating neutron matter with ultracold atomic gases. We demonstrate a two-field optical vernier, which expands kHz (mG) magnetic field detunings near a narrow resonance into MHz optical field detunings, enabling precise control and characterization of the momentum-dependent scattering amplitude. Two-photon loss spectra are measured for the narrow resonance in ^{6}Li, revealing rich structure in very good agreement with our theoretical model. However, anomalous overall frequency shifts between the measured and predicted two-photon spectra are not yet explained.

Manipulation of photoassociation of ultracold Cs atoms with tunable scattering length by external magnetic fields

We demonstrate that for ultracold, optically trapped Cs atoms the photoassociation (PA) can be manipulated by using external uniform magnetic fields due to the alteration of the scattering wavefunction in the region of the free–bound optical transition. We present PA–induced atom loss measurements with the same intensity for PA laser but different external magnetic fields, and analyze main contributions of the PA to the variation of the number of atoms in the trap. The PA rate exhibits a strong dependence on the changing uniform magnetic field. The experimental data are simulated within the model of a single–channel one–well rectangular potential, whose depth is adjusted so as to assure the predicted variation of the scattering length with the magnetic field. The computational and experimental results are in a reasonable agreement to each other. The same model is used to illustrate some general properties of the two–body quantum system in the near–threshold state.

Review of pseudogaps in strongly interacting Fermi gases

A central challenge in modern condensed matter physics is developing the tools for understanding nontrivial yet unordered states of matter. One important idea to emerge in this context is that of a 'pseudogap': the fact that under appropriate circumstances the normal state displays a suppression of the single particle spectral density near the Fermi level, reminiscent of the gaps seen in ordered states of matter. While these concepts arose in a solid state context, they are now being explored in cold gases. This article reviews the current experimental and theoretical understanding of the normal state of strongly interacting Fermi gases, with particular focus on the phenomonology which is traditionally associated with the pseudogap.

Concept of a Contact Spectrum and Its Applications in Atomic Quantum Hall States

A unique feature of ultracold atoms is the separation of length scales, r_{0}≪k_{F}^{-1}, where k_{F} and r_{0} are the Fermi momentum characterizing the average particle distance and the range of interaction between atoms, respectively. For s-wave scattering, Shina Tan discovered that such diluteness leads to universal thermodynamic relations governed by contact. Here, we show that the concept of contact can be generalized to an arbitrary partial-wave scattering. Contact of all partial-wave scatterings forms a contact spectrum, which establishes universal thermodynamic relations with notable differences from those in the presence of s-wave scattering alone. Such a contact spectrum is particularly useful for characterizing many-body correlations in atomic quantum Hall states (QHSs). It has an interesting connection with a special bipartite entanglement spectrum of QHSs and enables an intrinsic probe of atomic QHSs using short-range two-body correlations.

Theory of Long-Range Ultracold Atom-Molecule Photoassociation

The creation of ultracold molecules is currently limited to diatomic species. In this Letter, we present a theoretical description of the photoassociation of ultracold atoms and molecules to create ultracold excited triatomic molecules, thus being a novel example of a light-assisted ultracold chemical reaction. The calculation of the photoassociation rate of an ultracold Cs_{2} molecule in its rovibrational ground state with an ultracold Cs atom at frequencies close to its resonant excitation is reported, based on the solution of the quantum dynamics involving the atom-molecule long-range interactions and assuming a model potential for the short-range physics. The rate for the formation of excited Cs_{3} molecules is predicted to be comparable with currently observed atom-atom photoassociation rates. We formulate an experimental proposal to observe this process relying on the available techniques of optical lattices and standard photoassociation spectroscopy.

Control of laser-induced frequency shift in ultracold cesium molecules by an external magnetic field

We investigate an effective control of the laser-induced frequency shift of ultracold cesium molecules formed by photoassociation (PA) in a magnetically levitated crossed dipole trap via an external magnetic field. A series of molecular PA spectra have been measured with increasing the PA laser intensity at different bias fields. We find that the slope of laser-induced frequency shift is strongly dependent on the external magnetic field: the slope can be increased up to a maximum of ∼6 times compared with a slope without an external magnetic field. A qualitatively theoretical explanation has been offered by considering the external magnetic field that can be used to manipulate the interaction between ultracold atoms.

High sensitive determination of laser-induced frequency shifts of ultracold cesium molecules

We experimentally present a technique for sensitively determining the laser-induced frequency shifts of the long-range molecular vibrational and rotational levels. The scheme relies on an optical frequency shifter, leading to two laser beams with a precise and adjustable frequency interval. A series of photoassociation spectra are recorded with both beams inducing molecular lines, whose peak separation provides an accurate frequency ruler to measure the frequency shifts of cesium molecular levels, which have not yet been observed in previous reports. The data are compared to theoretical predictions and show a good agreement.

The quest for cold and ultracold molecules

Cool molecules: The cooling of molecules to sub-Kelvin temperatures promises to have a great impact in chemistry and physics. Recently, the first experimental realizations of samples of deeply bound molecules that are approaching the ultracold regime were reported. In this contribution, these interesting results are briefly discussed.

Photoassociation of a Bose-Einstein condensate near a Feshbach resonance

We measure the effect of a magnetic Feshbach resonance (FR) on the rate and light-induced frequency shift of a photoassociation resonance in ultracold 7Li. The photoassociation-induced loss-rate coefficient K_{p} depends strongly on magnetic field, varying by more than a factor of 10;{4} for fields near the FR. At sufficiently high laser intensities, K_{p} for a thermal gas decreases with increasing intensity, while saturation is observed for the first time in a Bose-Einstein condensate. The frequency shift is also strongly field dependent and exhibits an anomalous blueshift for fields just below the FR.

Cross-molecular coupling in combined photoassociation and Feshbach resonances

We model combined photoassociation and Feshbach resonances in a Bose-Einstein condensate. When the magnetic field is far-off resonance, cross coupling between the two target molecules--enabled by the shared dissociation continuum--leads to an anomalous dispersive shift in the position of laser resonance, as well as unprecedented elimination and enhancement of resonant photoassociation via quantum interference. For off-resonant lasers, a dispersive shift and quantum interference appear similarly in resonant three-body Feshbach losses, except that the Feshbach node is tunable with intensity.

Two-body transients in coupled atomic-molecular bose-einstein condensates

We discuss the dynamics of an atomic Bose-Einstein condensate when pairs of atoms are converted into molecules by single-color photoassociation. Three main regimes are found, and it is shown that they can be understood on the basis of time-dependent two-body theory. In particular, the so-called rogue dissociation regime [Phys. Rev. Lett. 88, 090403 (2002)10.1103/PhysRevLett.88.090403], which has a density-dependent limit on the photoassociation rate, is identified with a transient regime of the two-atom dynamics exhibiting universal properties. Finally, we illustrate how these regimes could be explored by photoassociating condensates of alkaline-earth atoms.

Pseudopotential analog for zero-range photoassociation and Feshbach resonance

A zero-range approach to atom-molecule coupling is developed in analogy to the Fermi-Huang pseudopotential approach to collisions. It is shown by explicit comparison to an exactly solvable finite-range model that replacing the molecular bound-state wave function with a regularized delta function can reproduce the exact scattering amplitude in the long-wavelength limit. Using this approach, we find an analytical solution to the two-channel Feshbach resonance problem for two atoms in a spherical harmonic trap, highlighting the strong dependence of the effective scattering length and bare-molecule population on the atom-molecule coupling strength.

Molecular probe of pairing in the BEC-BCS crossover

We have used optical molecular spectroscopy to probe the many-body state of paired 6Li atoms near a broad Feshbach resonance. The optical probe projects pairs of atoms onto a vibrational level of an excited molecule. The rate of excitation enables a precise measurement of the closed-channel contribution to the paired state. This contribution is found to be quite small, supporting the concept of universality for the description of broad Feshbach resonances. The dynamics of the excitation provide clear evidence for pairing across the BEC-BCS crossover and into the weakly interacting BCS regime.

Photoassociative spectroscopy at long range in ultracold strontium

We report photoassociative spectroscopy of 88Sr(2) in a magneto-optical trap operating on the 1S0-->3P1 intercombination line at 689 nm. Photoassociative transitions are driven with a laser red detuned by 600-2400 MHz from the 1S0-->1P1 atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the 1P1 atomic lifetime (tau=5.22+/-0.03 ns) and resolves a discrepancy between experiment and recent theoretical calculations.

Photoassociation spectroscopy of ultracold Cs below the 6P(3/2) limit

High precision photoassociation spectroscopy is performed in ultracold cesium gas, with detunings as large as 51 cm(-1) below the Cs(6S(1/2))+Cs(6P(3/2)) asymptote. Trap-loss fluorescence detection is used for detecting the photoassociation to excited state ultracold molecules. Long vibrational progressions are assigned to electronic states of 0(g) (-), 0(u) (+), and 1(g) symmetry. The spectral data are fitted to a LeRoy-Bernstein equation, in order to obtain the effective coefficients of the leading long-range interaction term (C(3)/R(3)) and the relative vibrational quantum numbers measured down from dissociation. Additionally we present evidence for perturbations between the 0(g) (-) state and the dark 2(u) state.

Photoassociation spectroscopy of ultracold Cs below the 6P1/2 limit

We have performed high precision photoassociation spectroscopy of ultracold cesium gas. Using trap-loss fluorescence detection and controlling the background cesium pressure we were able to photoassociate atoms into excited states of ultracold molecules with large detunings up to 56 cm(-1) below the Cs(6S(1/2)) + Cs(6P(1/2)) atomic asymptote. Vibrational progressions are assigned to 0(g)(-), 0(u)(+), and 1(g) long-range states. By fitting the spectral data to the LeRoy-Bernstein expression, the effective coefficients of the leading long-range interactions and the vibrational quantum number at dissociation are obtained. In addition we have observed spectral perturbations between states of the same symmetry belonging to different asymptotes (6P(1/2) and 6P(3/2)). The perturbations are manifested through irregular vibrational level spacings and are especially pronounced in the 0(u)(+) symmetry. Many observed rotational levels indicate d- and higher partial wave contributions to the photoassociation cross section in the presence of trapping laser light, while spectral regions with only weak features suggest nodes in the lower state wave functions corresponding to the two ground state atoms asymptote.

Shortcut to a Fermi-degenerate gas of molecules via cooperative association

We theoretically examine the creation of a Fermi-degenerate gas of molecules by considering a photoassociation or Feshbach resonance applied to a degenerate Bose-Fermi mixture of atoms. This problem raises interest because, unlike bosons, fermions in general do not behave cooperatively, so that the collective conversion of a degenerate gas atoms into a macroscopic number of diatomic molecules is not to be expected. Nevertheless, we find that the coupled Fermi system displays collective Rabi-like oscillations and a rapid adiabatic passage between atoms and molecules, thereby mimicking Bose-Einstein statistics. Cooperative association of a degenerate mixture of Bose and Fermi gases could therefore serve as a shortcut to a degenerate gas of Fermi molecules.

Anomalous frequency shift in the photoassociation spectrum of a Bose-Einstein condensate

We theoretically investigate the effect of anomalous quantum correlations on the light-induced frequency shift in the photoassociation spectrum of a Bose-Einstein condensate. Anomalous quantum correlations arise because, although formed from a pair of zero-momentum condensate atoms, a condensate molecule need not dissociate back to the atomic condensate, but may just as well form a noncondensate atom pair with equal and opposite momentum, i.e., due to rogue photodissociation. The uncorrelated frequency shift of the photoassociation spectrum is to the red and linearly dependent on the laser intensity I. In contrast, anomalous correlations due to rogue dissociation lead to a blueshifted photoassociation spectrum. For sufficiently low light intensities, the rogue blueshift is dominant and proportional to sqrt[I].