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arXiv:1405.1674 · Physical Review A

Optical Feshbach resonances through a molecular dark state: Efficient manipulation of p -wave resonances in fermionic Yb171 atoms

Subrata Saha, Arpita Rakshit, Debashree Chakraborty, Arpita Pal, Bimalendu Deb

Published Jul 1, 2014• 8 citations
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Abstract

In a recent experiment by Yamazaki et al. [Phys. Rev. A 87, 010704(R) (2013)], a $p$-wave optical Feshbach resonance in fermionic $^{171}\mathrm{Yb}$ atoms using purely long-range molecular excited states has been demonstrated. We show theoretically that, if two purely long-range excited states of $^{171}\mathrm{Yb}$ are coupled to the ground-state continuum of scattering states with two lasers, then it is possible to significantly suppress photoassociative atom loss by a dark resonance in the excited states. We present a general theoretical framework for creating a dark state in an electronically excited molecular potential for the purpose of increasing the efficiency of the optical Feshbach resonance. This can be accomplished by properly adjusting the relative intensity, phase, polarizations, and frequency detunings of two lasers. We present selected numerical results for atom-loss spectra and $p$-wave elastic- and inelastic-scattering cross sections of $^{171}\mathrm{Yb}$ atoms to illustrate the effects of the molecular dark state on the optical Feshbach resonance.

DOI
10.1103/physreva.90.012701

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In a recent experiment by Yamazaki et al. [Phys. Rev. A 87, 010704(R) (2013)], a ppp-wave optical Feshbach resonance in fermionic 171Yb^{171}\mathrm{Yb}171Yb atoms using purely long-range molecular excited states has been demonstrated. We show theoretically that, if two purely long-range excited states of 171Yb^{171}\mathrm{Yb}171Yb are coupled to the ground-state continuum of scattering states with two lasers, then it is possible to significantly suppress photoassociative atom loss by a dark resonance in the excited states. We present a general theoretical framework for creating a dark state in an electronically excited molecular potential for the purpose of increasing the efficiency of the optical Feshbach resonance. This can be accomplished by properly adjusting the relative intensity, phase, polarizations, and frequency detunings of two lasers. We present selected numerical results for atom-loss spectra and ppp-wave elastic- and inelastic-scattering cross sections of 171Yb^{171}\mathrm{Yb}171Yb atoms to illustrate the effects of the molecular dark state on the optical Feshbach resonance.
We study the use of an optical Feshbach resonance to modify the ppp-wave interaction between ultracold polarized 171Yb^{171}\mathrm{Yb}171Yb spin-1/21/21/2 fermions. A laser exciting two colliding atoms to the 1S0+3P1{}^{1}{S}_{0}+{}^{3}{P}_{1}1S0​+3P1​ channel can be detuned near a purely-long-range excited molecular bound state. Such an exotic molecule has an inner turning point far from the chemical binding region, and thus, three-body recombination in the Feshbach resonance will be highly suppressed in contrast to that typically seen in a ground-state ppp-wave magnetic Feshbach resonance. We calculate the excited molecular bound-state spectrum using a multichannel integration of the Schr\"odinger equation, including an external perturbation by a magnetic field. From the multichannel wave functions, we calculate the Feshbach resonance properties, including the modification of the elastic ppp-wave scattering volume and inelastic spontaneous scattering rate. The use of magnetic fields and selection rules for polarized light yields a highly controllable system. We apply this control to propose a toy model for three-color superfluidity in an optical lattice for spin-polarized 171Yb^{171}\mathrm{Yb}171Yb, where the three colors correspond to the three spatial orbitals of the first excited ppp band. We calculate the conditions under which tunneling and on-site interactions are comparable, at which point quantum critical behavior is possible.
We demonstrate a ppp-wave optical Feshbach resonance (OFR) using purely long-range molecular states of a fermionic isotope of ytterbium 171{}^{171}171Yb, following the proposition made by K. Goyal et al. [Phys. Rev. A 82, 062704 (2010)]. The ppp-wave OFR is clearly observed as a modification of a photoassociation rate for atomic ensembles at about 5 $\ensuremath{\mu}$K. A scattering phase shift variation of $\ensuremath{\delta}\ensuremath{\eta}=0.022$ rad is observed with an atom loss rate coefficient $K=28.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ cm3{}^{3}3/s.
We propose a general method for optical control of magnetic Feshbach resonances in ultracold atomic gases with more than one molecular state in an energetically closed channel. Using two optical frequencies to couple two states in the closed channel, inelastic loss arising from spontaneous emission is greatly suppressed by destructive quantum interference at the two-photon resonance, i.e., dark-state formation, while the scattering length is widely tunable by varying the frequencies and/or intensities of the optical fields. This technique is of particular interest for a two-component atomic Fermi gas, which is stable near a Feshbach resonance.
This thesis reports on the first observation of optical tuning of interactions in an ultracold atomic gas. To control the elastic interactions, we use optically induced scattering resonances. They are realized by coupling pairs of atoms to a bound molecular state with the help of laser light. In comparison to the well-known magnetic Feshbach resonances, these optical Feshbach resonances allow a very fast switching of interactions in a spatially resolved way. This is because laser light can be switched on and off much faster than magnetic fields and because the light intensity can be spatially structured. Our experiments are performed with a Bose-Einstein condensate (BEC) of Rb atoms, which we produce in our new BEC setup. This apparatus has been constructed in the framework of this thesis and offers excellent optical access to the ultracold atom cloud. To investigate the tunability of atomic interactions with single-frequency laser light, we optically couple pairs of free atoms to an electronically excited molecular state. The coupling is achieved with a laser that is tuned close to the respective free-bound photoassociation transition. By changing the laser detuning we are able to adjust the atomic scattering length to any value between 10 a0 and 190 a0, where a0 denotes the Bohr radius. The atomic scattering length is a single quantity that fully describes ultracold elastic interactions. Since laser induced atom losses restrict the duration of our experiments, Bragg spectroscopy is employed as a fast method to measure the scattering length. We also demonstrate that optical Feshbach resonances can be realized with a two-color Raman coupling scheme. To implement this scheme, a second laser is added that couples the excited molecular state to a weakly bound molecular level in a ground state potential. Tuning the second laser results in a modification of the atomic scattering length similar to the one observed with a single laser. In addition, this scheme allows to control the width of the resonance via the detuning of the first laser. For the one-color and two-color coupling schemes, we investigate how the elastic scattering properties depend on the different available parameters, i.e. the laser detunings and intensities. Our experimental findings can successfully be described with an available theoretical model, which treats ultracold collisions in the presence of laser light. Optical Feshbach resonances have the possibility of becoming a valuable tool for fast switching of interactions. This is of special interest in quantum computation schemes with neutral atoms, where interactions between different atomic qubits need to be controlled.
We propose a scheme to efficiently tune the scattering length of two colliding ground-state atoms by off-resonantly coupling the scattering state to an excited Rydberg molecular state using laser light. For the sss-wave scattering of two colliding 87Rb^{87}\mathrm{Rb}87Rb atoms, we demonstrate that the effective optical length and pole strength of this Rydberg optical Feshbach resonance can be tuned over several orders of magnitude, while incoherent processes and losses are minimized. Given the ubiquity of Rydberg molecular states, this technique should be generally applicable to homonuclear atomic pairs as well as to atomic mixtures with sss-wave (or even ppp-wave) scattering.
We present a theory for manipulation of ddd-wave interaction of cold atoms with two lasers strongly driving two photoassociative transitions. The theory predicts the occurrence of a coherence between two excited ro-vibrational bound states due to the photoassociative dipole couplings of ground-state ddd-wave scattering state to the bound states. We show that this excited-state coherence significantly influences atom-atom interaction. In particular, this leads to the enhancement of ddd-wave elastic scattering and to the suppression of inelastic scattering. In the presence of an sss-wave magnetic Feshbach resonance, the two lasers can couple the sss-wave resonance with the ddd-wave scattering state, leading to the further enhancement in ddd-wave scattering at relatively low energy. Our numerical calculations based on realistic parameters show that ddd-wave manipulation would be most effective in case of atoms having excited diatomic states with narrow natural linewidth. We estimate that at an energy of 100 $\ensuremath{\mu}$K the inelastic scattering rate in Yb can be reduced to 20 s${}^{\ensuremath{-}1}$ while the elastic scattering rate can be two orders of magnitude larger.