Semianalytic treatment of two-color photoassociation spectroscopy and control of cold atoms

Complex Contact Interaction for Systems with Short-Range Two-Body Losses

The concept of contact interaction is fundamental in various areas of physics. It simplifies physical models by replacing the detailed short-range interaction with a zero-range contact potential that reproduces the same low-energy scattering parameter, i.e., the s-wave scattering length. In this Letter, we generalize this concept to open quantum systems with short-range two-body losses. We show that the short-range two-body losses can be effectively described by a complex scattering length. However, in contrast to closed systems, the dynamics of an open quantum system is governed by the Lindblad master equation the includes a non-Hermitian Hamiltonian as well as an extra recycling term. We thus develop proper methods to regularize both terms in the master equation in the contact (zero-range) limit. We then apply our regularized complex contact interaction to study the dynamic problem of a weakly interacting and dissipating Bose-Einstein condensate. It is found that the physics is greatly enriched because the scattering length is continued from the real axis to the complex plane. For example, we show that a strong dissipation may prevent an attractive Bose-Einstein condensate from collapsing. We further calculate the particle decay in this system to the order of (density)^{3/2} which resembles the celebrated Lee-Huang-Yang correction to the ground state energy of interacting Bose gases [Lee and Yang, Phys. Rev. 105, 1119 (1957)PHRVAO0031-899X10.1103/PhysRev.105.1119; Lee, Huang, and Yang, Phys. Rev. 106, 1135 (1957)PHRVAO0031-899X10.1103/PhysRev.106.1135]. Possible methods for tuning the complex scattering length in cold atomic gas experiments are also discussed.

Formation of ultracold 39K133Cs molecules via Feshbach optimized photoassociation

A Feshbach optimized photoassociation (FOPA) process for preparing ultracold excited-state ³⁹K¹³³Cs molecules is studied theoretically. Under the joint action of the magnetic field and short laser pulse, the colliding atoms in a superposition state composed of eight hyperfine components are converted into a molecule in the vibrational level of the excited state via two transition processes, the transition between singlet states and the transition between triplet states. The association efficiency can be significantly enhanced by taking advantage of Feshbach resonance. At different resonance positions, different hyperfine components of the superposition state dominate over the FOPA process, and the quantum interference displays different behaviors. Compared with the FOPA process only including a single hyperfine component, the quantum interference in the FOPA process containing all hyperfine components has a visible effect on the association efficiency.

Universal many-body response of heavy impurities coupled to a Fermi sea: a review of recent progress

In this report we discuss the dynamical response of heavy quantum impurities immersed in a Fermi gas at zero and at finite temperature. Studying both the frequency and the time domain allows one to identify interaction regimes that are characterized by distinct many-body dynamics. From this theoretical study a picture emerges in which impurity dynamics is universal on essentially all time scales, and where the high-frequency few-body response is related to the long-time dynamics of the Anderson orthogonality catastrophe by Tan relations. Our theoretical description relies on different and complementary approaches: functional determinants give an exact numerical solution for time- and frequency-resolved responses, bosonization provides accurate analytical expressions at low temperatures, and the theory of Toeplitz determinants allows one to analytically predict response up to high temperatures. Using these approaches we predict the thermal decoherence rate of the fermionic system and prove that within the considered model the fastest rate of long-time decoherence is given by [Formula: see text]. We show that Feshbach resonances in cold atomic systems give access to new interaction regimes where quantum effects can prevail even in the thermal regime of many-body dynamics. The key signature of this phenomenon is a crossover between different exponential decay rates of the real-time Ramsey signal. It is shown that the physics of the orthogonality catastrophe is experimentally observable up to temperatures [Formula: see text] where it leaves its fingerprint in a power-law temperature dependence of thermal spectral weight and we review how this phenomenon is related to the physics of heavy ions in liquid [Formula: see text]He and the formation of Fermi polarons. The presented results are in excellent agreement with recent experiments on LiK mixtures, and we predict several new phenomena that can be tested using currently available experimental technology.

Fano profiles in two-photon photoassociation spectra

In this article we derive the lineshapes observed in two-photon photoassociation spectrocopy of molecules using an effective Hamiltonian adapted from previous work in atomic physics. The lineshape is decomposed in terms of sums and products of Breit-Wigner and Fano profiles, which we associate with physical absorption and emission processes, and to quantum interferences between transition amplitudes. We emphasize the specific features which do not exist in the atomic case, linked to the dissociation width of the photoassociated molecules in the electronic ground state.

Prospects for making polar molecules with microwave fields

We propose a mechanism to produce ultracold polar molecules with microwave fields. It converts trapped ultracold atoms into vibrationally excited molecules by a single microwave transition and entirely depends on the existence of a permanent dipole moment in the molecules. As opposed to production of molecules by photoassociation or magnetic-field Feshbach resonances, our method does not rely on properties of excited states or existence of Feshbach resonances. We determine conditions for optimal creation of polar molecules in vibrationally excited states of the ground-state potential by changing frequency and intensity of the microwave field. We also explore the possibility to produce vibrationally cold molecules by combining the microwave field with an optical Raman transition or by applying a microwave field to Feshbach molecules. The production mechanism is illustrated for KRb and RbCs.

Accurate determination of the scattering length of metastable helium atoms using dark resonances between atoms and exotic molecules

We present a new measurement of the s-wave scattering length a of spin-polarized helium atoms in the 2(3)S1 metastable state. Using two-photon photoassociation spectroscopy and dark resonances, we measure the energy E(nu)=14= -91.35+/- 0.06 MHz of the least-bound state nu = 14 in the interaction potential of the two atoms. We deduce a value of a=7.512+/-0.005 nm, which is at least 100 times more precise than the best previous determinations and is in disagreement with some of them. This experiment also demonstrates the possibility to create exotic molecules binding two metastable atoms with a lifetime of the order of 1 micros.

Photoassociation of sodium in a Bose-Einstein condensate

We form ultracold Na2 molecules by single-photon photoassociation of a Bose-Einstein condensate, measuring the photoassociation rate, linewidth, and light shift of the J = 1, v = 135 vibrational level of the A1 Sigma (+)(u) molecular state. The photoassociation rate constant increases linearly with intensity, even where it is predicted that many-body effects might limit the rate. Our observations are in good agreement with a two-body theory having no free parameters.

Creating massive entanglement of Bose-Einstein condensed atoms

We propose a direct, coherent coupling scheme that can create massively entangled states of Bose-Einstein condensed atoms. Our idea is based on an effective interaction between two atoms from coherent Raman processes through a (two atom) molecular intermediate state. We compare our scheme with other recent proposals for the generation of massive entanglement of Bose condensed atoms.

Photoassociation of Ultracold Atoms: A New Spectroscopic Technique

The new spectroscopic technique of photoassociation of ultracold atoms is reviewed, with an emphasis on connecting this area to traditional bound-state molecular spectroscopy. In particular, in contrast to photoassociative spectra at thermal energies, which are broad and of low information content, photoassociative spectra of ultracold atoms are high resolution, permitting observation of small vibrational and rotational spacings of long-range molecular levels near dissociation (typically with outer classical turning points >20 Å). The types of detection and theoretical analysis employed are illustrated, primarily using the example of 39K2. Future directions and applications of this field (e.g., to ultracold molecular formation) are also discussed. Copyright 1999 Academic Press.

Photoassociative Spectroscopy and Formation of Cold Molecules in Cold Cesium Vapor: Trap-Loss Spectrum versus Ion Spectrum

We report on the experimental spectra of all the optically accessible long-range attractive molecular states of the Cs2 dimer below the 6s2S1/2 + 6p2P3/2 dissociation limit by molecular photoassociation of cold Cs atoms. The spectra are obtained by the usual trap-loss method as well as by pulsed-laser photoionization of Cs2 molecules into Cs+2 ions. The two spectra present markedly different features. While the 1g, 0(+)u, and 0(-)g vibrational progressions are present in the trap-loss spectrum, the Cs+2 ion spectrum presents only the 0(-)g and 1u vibrational progressions. Those states (0(-)g and 1u) lead to the formation of the translationally cold Cs2 ground state molecules at temperatures in the 100 µK range, to our knowledge the lowest molecular temperature reported up until now. The C3 asymptotic coefficients for the 0(+)u and 1g states are determined through a fit of the experimental energy levels. Copyright 1999 Academic Press.