We theoretically investigate Raman photoassociation of a degenerate Bose-Fermi mixture of atoms and the subsequent prospect for anomalous (Cooper) pairing between atoms and molecules. Stable fermionic molecules are created via free-bound-bound stimulated Raman adiabatic passage which, in contrast to purely bosonic systems, can occur in spite of collisions. With the leftover atomic condensate to enhance intrafermion interactions, the superfluid transition to atom-molecule Cooper pairs occurs at a temperature that is roughly an order of magnitude below what is currently feasible.
We model the formation of stable heteronuclear molecules via pulsed Raman photoassociation of a two-component Bose-Einstein condensate near a strong Feshbach resonance, for both counterintuitive and intuitive pulse sequencing. Compared to lasers alone, weak Raman photoassociation is enhanced by as much as a factor of ten (five) for a counterintuitive (intuitive) pulse sequence, whereas strong Raman photoassociation is barely enhanced at all--regardless of pulse sequence. Stronger intra-atom, molecule, or atom-molecule collisions lead to an expected decrease in conversion efficiency, but strong…
We analyze coherent two-color photoassociation of a Bose-Einstein condensate, focusing on stimulated Raman adiabatic passage (STIRAP) in free-bound-bound transitions from atoms to molecules. This problem is of particular interest since STIRAP is predicted to be necessarily absent in the nondegenerate case [Javanainen and Mackie, Phys. Rev. A 58, R789 (1998)]. However, Bose-stimulation enhances the free-bound dipole matrix element for an atomic condensate, and photoassociative STIRAP turns out to be a viable mechanism for converting an atomic condensate to a molecular condensate with near-unit …
We theoretically examine Raman photoassociation of a Bose-Einstein condensate, revisiting stimulated Raman adiabatic passage (STIRAP). Due to collisional mean-field shifts, efficient molecular conversion requires strong coupling and low density, either of which can bring about rogue photodissociation to noncondensate modes. We demonstrate explicitly that rogue transitions are negligible for low excited-state fractions and photodissociation that is slower than the STIRAP timescale. Moreover, we derive a reduced-parameter model of collisions, and thereby find that a gain in the molecular convers…
Coherent control of reactants remains a longstanding challenge in quantum chemistry. In particular, we have studied laser-induced molecular formation (photoassociation) in a Raman-dressed spin-orbit-coupled 87Rb Bose-Einstein condensate, whose spin quantum state is a superposition of multiple bare spin components. In contrast to the notably different photoassociation-induced fractional atom losses observed for the bare spin components of a statistical mixture, a superposition state with a comparable spin composition displays the same fractional loss on every spin component. We interpret this a…
We discuss the role of Bose enhancement of the dipole matrix element in photoassociation, using stimulated Raman adiabatic passage as an example. In a nondegenerate gas the time scale for coherent optical transients tends to infinity in the thermodynamic limit, whereas Bose enhancement keeps this time scale finite in a condensate. Coherent transients are therefore absent in photoassociation of a thermal nondegenerate gas, but are feasible if the gas is a condensate.
We study coherent photoassociation, phenomena analogous to coherent optical transients in few-level systems, which may take place in photoassociation of an atomic Bose-Einstein condensate but not in a nondegenerate gas. We develop a second-quantized Hamiltonian to describe photoassociation, and apply the Hamiltonian both in the momentum representation and in the position representation (field theory). Solution of the two-mode problem including only one mode each for the atomic and molecular condensates displays analogs of Rabi oscillations and rapid adiabatic passage. A classical version of th…
We report on two-photon photoassociation (PA) spectroscopy of ultracold heteronuclear LiRb molecules. This is used to determine the binding energies of the loosely bound levels of the electronic ground singlet and the lowest triplet states of LiRb. We observe strong two-photon PA lines with power broadened line widths greater than 20 GHz at relatively low laser intensity of 30 W/cm$^{2}$. The implication of this observation on direct atom to molecule conversion using stimulated Raman adiabatic passage (STIRAP) is discussed and the prospect for electronic ground state molecule production is the…
We produce ${^{84}\mathrm{Sr}_2}$ molecules using Bose-enhanced Raman photoassociation. We apply the stimulated Raman adiabatic passage (STIRAP) technique on a Bose-Einstein condensate (BEC) to produce more than $8 \times 10^3$ ultracold molecules. This chemical reaction is only made possible because of the Bose enhancement of the optical transition dipole moment between the initial atomic state and an intermediate molecular state. We study the effect of Bose enhancement by measuring the transition Rabi frequency in a BEC and by comparing it with measurements for two atoms in sites of a Mott i…
We have produced ultracold, polar RbCs* molecules via photoassociation in a laser-cooled mixture of Rb and Cs atoms. Using a model of the RbCs* molecular interaction which reproduces the observed rovibrational structure, we infer decay rates in our experiments into deeply bound singlet ground state RbCs vibrational levels as high as 5 x 10^5 s^-1 per level. Population in such deeply bound levels could be efficiently transferred to the vibrational ground state using a single stimulated Raman transition, opening the possibility to create large samples of stable, ultracold polar molecules.
We report on a resonantly coupled 2(1) - 4 (1) photoassociation resonance in LiRb. This pair of states displays interesting decay physics that hint at interference-like effects caused by two different decay paths. We observe decay to predominantly $X \ ^1Σ^+ \ v=43$, with significant numbers of molecules produced in $X \ ^1Σ^+ \ v=2$ - 12. This photoassociation resonance also produces $\sim$300 $X \ ^1Σ^+ \ v=0 \ J=0$ molecules/second. Finally, we identify a stimulated Raman adiabatic passage transfer pathway from $v=43$ to $v=0$ that has the potential to produce up to $2 \times 10^5$ LiRb mol…
We survey results on the creation of heteronuclear Fermi molecules by tuning a degenerate Bose-Fermi mixture into the neighborhood of an association resonance, either photoassociation or Feshbach, as well as the subsequent prospects for Cooper-like pairing between atoms and molecules. In the simplest case of only one molecular state, corresponding to either a Feshbach resonance or one-color photoassociation, the system displays Rabi oscillations and rapid adiabatic passage between a Bose-Fermi mixture of atoms and fermionic molecules. For two-color photoassociation, the system admits stimulate…
We develop the theory for the Adiabatic Raman Photoassociation (ARPA) of ultracold atoms to form ultracold molecules in the presence of scattering resonances. Based on a computational method in which we replace the continuum with a discrete set of "effective modes", we show that the existence of resonances greatly aids in the formation of deeply bound molecular states. We illustrate our general theory by computationally studying the formation of $^{85}$Rb$_2$ molecules from pairs of colliding ultracold $^{85}$Rb atoms. The single-event transfer yield is shown to have a near-unity value for wid…
We show theoretically that it is possible to form a cold molecular ion from a pair of colliding atom and ion at low energy by photoassociative two-photon Raman processes. We explore the possibility of stimulated Raman adiabatic passage (STIRAP) from the continuum of ion-atom scattering states to an ionic molecular state. We provide physical conditions under which coherent population transfer is possible in stimulated Raman photoassociation. Our results are important for experimental realization of PA in ion-atom cold collisions.
We study non-perturbative effects of a static electric field on two-color photoassociation of different atoms. A static electric field induces anisotropy in scattering between two different atoms and hybridizes field-free rotational states of heteronuclear dimers or polar molecules. In a previous paper [D. Chakraborty $\it {et.}$ $\it {al.}$, J. Phys. B 44, 095201 (2011)], the effects of a static electric field on one-color photoassociation between different atoms has been described through field-modified ground-state scattering states, neglecting electric field effects on heteronuclear diatom…
Collective two-color photoassociation of a freely-interacting 87Rb Bose-Einstein condensate is theoretically examined, focusing on stimulated Raman adiabatic passage (STIRAP) from an atomic to a stable molecular condensate. In particular, Drummond et al. [Phys. Rev. A 65, 063619 (2002); cond-mat/0110578] have predicted that particle-particle interactions can limit the efficiency of collective atom-molecule STIRAP, and that optimizing the laser parameters can partially overcome this limitation. We suggest that the molecular conversion efficiency can be further improved by treating the initial c…
We theoretically examine the neutral atom-molecule analogue of the anomalous quantum correlations between degenerate electrons, i.e., Cooper pairs, that are responsible for superconductivity. Based on rogue dissociation of triatomic molecules (trimers) into opposite-momentum pairs of atoms and diatomic molecules (dimers) via a photoassociation or Feshbach resonance, we find a superfluid transition to a Bose-Einstein condensate of trimers dressed by atom-dimer Cooper pairs, at a critical temperature in reach of present ultracold technology.
State-of-the-art {\em ab initio} techniques have been applied to compute the potential energy curves for the SrYb molecule in the Born-Oppenheimer approximation for the ground state and first fifteen excited singlet and triplet states within the coupled-cluster framework. The leading long-range coefficients describing the dispersion interactions at large interatomic distances are also reported. The electric transition dipole moments have been obtained as the first residue of the polarization propagator computed with the linear response coupled-cluster method restricted to single and double exc…
We theoretically examine the creation of a Fermi-degenerate gas of molecules via a photoassociation or Feshbach resonance applied to a degenerate Bose-Fermi mixture of atoms. This problem raises an interest because, unlike bosons, fermions in general do not behave cooperatively, so that the collective conversion of, say, two million atoms into one million molecules is not to be expected. Nevertheless, we find that the coupled Fermi system displays collective Rabi-like oscillations and adiabatic passage between atoms and molecules, thereby mimicking Bose-Einstein statistics. Cooperative associa…
Photoassociation and the Feshbach resonance are, in principle, feasible means for creating a molecular Bose-Einstein condensate from an already-quantum-degenerate gas of atoms; however, mean-field shifts and irreversible decay place practical constraints on the efficient delivery of stable molecules using either mechanism alone. We therefore propose Feshbach-stimulated Raman photoproduction, i.e., a combination of magnetic and optical methods, as a viable means to collectively convert degenerate atoms into a stable molecular condensate with near-unit efficiency.
Studies on the interactions between an alkali atom and an alkaline earth ion at low energy are important in the field of cold chemistry. In this paper we, extensively, study the structure, interactions, and dynamics of (MgCs)$^+$ molecular ion. We use an ab initio approach based on the formalism of non-empirical pseudo-potential for Mg$^{2+}$ and Cs$^+$ cores, large Gaussian basis sets and full valence configuration interaction. In this context, the (MgCs)$^+$ cation is treated as an effective two-electron system. Potential energy curves and their spectroscopic constants for the ground and the…
Ultracold paramagnetic and polar diatomic molecules are among the promising systems for quantum simulation of lattice-spin models. Unfortunately, their experimental observation is still challenging. Based on our recent \textit{ab-initio} calculations, we analyze the feasibility of all-optical schemes for the formation of ultracold $^{87}$Rb$^{84}$Sr bosonic molecules. First, we have studied the formation by photoassociation followed by spontaneous emission. The photoassociation rates to levels belonging to electronic states converging to the $^{87}$Rb$(5s\,^2S)$+$^{84}$Sr($5s5p\,^3P_{0,1,2}$) …
We report on the direct conversion of laser-cooled 41K and 87Rb atoms into ultracold 41K87Rb molecules in the rovibrational ground state via photoassociation followed by stimulated Raman adiabatic passage. High-resolution spectroscopy based on the coherent transfer revealed the hyperfine structure of weakly bound molecules in an unexplored region. Our results show that a rovibrationally pure sample of ultracold ground-state molecules is achieved via the all-optical association of laser-cooled atoms, opening possibilities to coherently manipulate a wide variety of molecules.
We report efficient all-optical creation of an ultracold gas of alkaline-earth-metal dimers, $^{88}\rm{Sr}_2$, in their absolute ground state. Starting with weakly bound singlet molecules formed by narrow-line photoassociation in an optical lattice, followed by stimulated Raman adiabatic passage (STIRAP) via a singlet-dominant channel in the $(1)0_u^+$ excited potential, we prepare pure samples of more than 5500 molecules in $X^1Σ_g^+(v=0,J=0)$. We observe two-body collisional loss rates close to the universal limit for both the least bound and most bound vibrational states in $X^1Σ_g^+$. We d…
We investigate the possibility of forming deeply bound ultracold RbCs molecules by a two-color photoassociation experiment. We compare the results with those for Rb_2 in order to understand the characteristic differences between heteronuclear and homonuclear molecules. The major differences arise from the different long-range potential for excited states. Ultracold 85Rb and 133Cs atoms colliding on the X^1Sigma+ potential curve are initially photoassociated to form excited RbCs molecules in the region below the Rb(5S) + Cs(6P_1/2) asymptote. We explore the nature of the Omega=0^+ levels in thi…
We spectroscopically investigate a pathway for the conversion of $^{23}\textrm{Na}^{39}\textrm{K}$ Feshbach molecules into rovibronic ground state molecules via STImulated Raman Adiabatic Passage (STIRAP). Using photoassociation spectroscopy from the diatomic scattering threshold in the $a^3Σ^+$ potential, we locate the resonantly mixed electronically excited intermediate states $|B^1Π, v=8\rangle$ and $|c^3Σ^+, v=30\rangle$ which, due to their singlet-triplet admixture, serve as an ideal bridge between predominantly $a^3Σ^+$ Feshbach molecules and pure $X^1Σ^+$ ground state molecules. We inve…
Cooling of molecules via free-space dissipative scattering of photons is thought not to be practicable due to the inherently large number of Raman loss channels available to molecules and the prohibitive expense of building multiple repumping laser systems. The use of an optical cavity to enhance coherent Rayleigh scattering into a decaying cavity mode has been suggested as a potential method to mitigate Raman loss, thereby enabling the laser cooling of molecules to ultracold temperatures. We discuss the possibility of cavity-assisted laser cooling particles without closed transitions, identif…
Ultracold molecules have experienced increasing attention in recent years. Compared to ultracold atoms, they possess several unique properties that make them perfect candidates for the implementation of new quantum-technological applications in several fields, from quantum simulation to quantum sensing and metrology. In particular, ultracold molecules of two-electron atoms (such as strontium or ytterbium) also inherit the peculiar properties of these atomic species, above all the possibility to access metastable electronic states via direct excitation on optical clock transitions with ultimate…
