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arXiv:1306.0395 · Physical Review A

Optical control of a magnetic Feshbach resonance in an ultracold Fermi gas

Zhengkun Fu, Pengjun Wang, Lianghui Huang, Zengming Meng, Hui Hu, Jing Zhang

Published Oct 9, 2013• 36 citations
arXivHTMLPDF

Abstract

We use laser light near resonant with a molecular bound-to-bound transition to control a magnetic Feshbach resonance in ultracold Fermi gases of ${}^{40}$K atoms. The spectrum of excited molecular states is measured by applying a laser field that couples the ground Feshbach molecular state to electronically excited molecular states. Nine strong bound-to-bound resonances are observed below the ${}^{2}{P}_{1/2}+{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}$ threshold. We use radio-frequency spectroscopy to characterize the laser-dressed bound state near a specific bound-to-bound resonance and show clearly the shift of the magnetic Feshbach resonance using light. The demonstrated technology could be used to modify interatomic interactions with high spatial and temporal resolutions in the crossover regime from a Bose-Einstein condensate to a Bardeen-Cooper-Schrieffer superfluid.

DOI
10.1103/physreva.88.041601

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We use laser light near resonant with a molecular bound-to-bound transition to control a magnetic Feshbach resonance in ultracold Fermi gases of 40{}^{40}40K atoms. The spectrum of excited molecular states is measured by applying a laser field that couples the ground Feshbach molecular state to electronically excited molecular states. Nine strong bound-to-bound resonances are observed below the 2P1/2+2S1/2{}^{2}{P}_{1/2}+{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}2P1/2​+2S1/2​ threshold. We use radio-frequency spectroscopy to characterize the laser-dressed bound state near a specific bound-to-bound resonance and show clearly the shift of the magnetic Feshbach resonance using light. The demonstrated technology could be used to modify interatomic interactions with high spatial and temporal resolutions in the crossover regime from a Bose-Einstein condensate to a Bardeen-Cooper-Schrieffer superfluid.
We theoretically investigate the control of a magnetic Feshbach resonance using a bound-to-bound molecular transition driven by spatially modulated laser light. Due to the spatially periodic coupling between the ground and excited molecular states, there exists a band structure of bound states, which can uniquely be characterized by some extra bumps in radio-frequency spectroscopy. With the increasing of coupling strength, the series of bound states will cross zero energy and directly result in a number of scattering resonances, whose position and width can be conveniently tuned by the coupling strength of the laser light and the applied magnetic field (i.e., the detuning of the ground molecular state). In the presence of the modulated laser light, universal two-body bound states near zero-energy threshold still exist. However, compared with the case without modulation, the regime for such universal states is usually small. A unified formula, which embodies the influence of the modulated coupling on the resonance width is given. The spatially modulated coupling also implies a local spatially varying interaction between atoms. Our work proposes a practical way of optically controlling interatomic interactions with high spatial resolution and negligible atomic loss.
Spectroscopic measurements are made and obtained for three molecular levels within 50 MHz of the atomic continuum, along with their variation of magnetic field in ultracold 40 K Fermi gases. We use spectroscopic measurements to modify the scattering properties near magnetic Fano–Feshbach resonances with a radio-frequency (RF) field by measuring the loss profile versus magnetic field. This work provides the high accuracy locations of ground molecular states near the s-wave Fano–Feshbach resonance, which can be used to study the crossover regime from a Bose–Einstein condensate to a Bardeen–Cooper–Schrieffer superfluid in the presence of an RF field.
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.
This thesis reports on a novel scheme for controlling the scattering properties of an ultracold atomic gas with laser light. We show theoretically and experimentally that light near-resonant with a molecular bound-to-bound transition can be used to shift the magnetic field at which a magnetic Feshbach resonance occurs. This makes it possible to tune the interaction strength with laser light and at the same time induce considerably less loss than an optical Feshbach resonance would do. To find an appropriate bound-to-bound transition for this scheme, we perform excited-state spectroscopy. When applying laser light resonant with one such bound-to-bound transition we observe an Autler-Townes splitting of the magnetic Feshbach resonance. We find good agreement between the theoretical model and experimental data. Our scheme can be used for a variety of future applications. For example, light patterns created by holographic masks could serve as a tool for changing the scattering length of ultracold atoms on very short time and length scales.
In this habilitation-thesis a collection of 14 experiments is presented. In these experiments we have developed and used novel techniques to control ultracold gases on the quantum level. In brief, Bose-Einstein condensates (BEC) and degenerate Fermi gases of laser-cooled alkali atoms are subjected to specially designed optical and magnetic fields. The fields influence in a coherent way the behavior, properties and dynamics of the gases. On this basis we developed tools to prepare, coherently manipulate, and analyze pre-defined quantum states. With this high level of control, it was possible to investigate interesting physics phenomena, and we were able to achieve several breakthroughs in the field of ultracold atoms. Highlights of our experimental results include the demonstration of phase engineering of a BEC wavefunction where we created solitons in a BEC and studied their propagation. In another experiment we constructed a spatially resolved matter wave interferometer with which we mapped out the phase distribution on a condensate. Illuminating the BEC with a periodic pattern, i.e. an optical lattice, allowed us to study solid states physics phenomena. In a second set of experiments we demonstrated optically tuning of the interaction between atoms with optical Feshbach resonances. Employing either magnetically tunable Feshbach resonances or laser radiation, we were able to produce ground state molecules in well defined quantum states. This led to the production of the first molecular BEC. By tuning the coupling of atom pairs in a Fermi gas of atoms, we could for the first time investigate the so-called BEC-BCS crossover which describes the continuous change from a BEC superfluid to a Bardeen-Cooper Schrieffer (BCS) superfluid. Due to the excellent control on the quantum level, the developed cold atom techniques open up intriguing prospects for future applications and experiments. They represent a general tool box to investigate fundamental physical phenomena in a pure and undisturbed environment. In the future, cold atoms might serve as a testing ground for fundamental theories or as quantum simulators for complex systems.
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.