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arXiv:1702.02216 · Physical Review A

Dispersive optical detection of magnetic Feshbach resonances in ultracold gases

Bianca J. Sawyer, Milena S. J. Horvath, Eite Tiesinga, Amita B. Deb, Niels Kjærgaard

Published Aug 18, 2017• 14 citations
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Abstract

Magnetically tunable Feshbach resonances in ultracold atomic systems are chiefly identified and characterized through time-consuming atom loss spectroscopy. We describe an off-resonant dispersive optical probing technique to rapidly locate Feshbach resonances and demonstrate the method by locating four resonances of $^{87}\mathrm{Rb}$, between the $|\mathrm{F}=1,{\mathrm{m}}_{\mathrm{F}}=1\ensuremath{\rangle}$ and $|\mathrm{F}=2,{\mathrm{m}}_{\mathrm{F}}=0\ensuremath{\rangle}$ states. Despite the loss features being $\ensuremath{\lesssim}0.1$ G wide, we require only 21 experimental runs to explore a magnetic field range $>18$ G, where $1\phantom{\rule{0.16em}{0ex}}\mathrm{G}={10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The resonances consist of two known $s$-wave features in the vicinity of 9 G and 18 G and two previously unreported $p$-wave features near 5 G and 10 G. We further utilize the dispersive approach to directly characterize the two-body loss dynamics for each Feshbach resonance.

DOI
10.1103/physreva.96.022705

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Magnetically tunable Feshbach resonances in ultracold atomic systems are chiefly identified and characterized through time-consuming atom loss spectroscopy. We describe an off-resonant dispersive optical probing technique to rapidly locate Feshbach resonances and demonstrate the method by locating four resonances of 87Rb^{87}\mathrm{Rb}87Rb, between the ∣F=1,mF=1\ensuremath⟩|\mathrm{F}=1,{\mathrm{m}}_{\mathrm{F}}=1\ensuremath{\rangle}∣F=1,mF​=1\ensuremath⟩ and ∣F=2,mF=0\ensuremath⟩|\mathrm{F}=2,{\mathrm{m}}_{\mathrm{F}}=0\ensuremath{\rangle}∣F=2,mF​=0\ensuremath⟩ states. Despite the loss features being \ensuremath≲0.1\ensuremath{\lesssim}0.1\ensuremath≲0.1 G wide, we require only 21 experimental runs to explore a magnetic field range >18>18>18 G, where 1G=10\ensuremath−4T1\phantom{\rule{0.16em}{0ex}}\mathrm{G}={10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\mathrm{T}1G=10\ensuremath−4T. The resonances consist of two known sss-wave features in the vicinity of 9 G and 18 G and two previously unreported ppp-wave features near 5 G and 10 G. We further utilize the dispersive approach to directly characterize the two-body loss dynamics for each Feshbach resonance.
Trapped ultracold gases are highly controllable experimental systems with unique quantum properties, making them a brilliant medium for research in solid state physics, quantum computation, and precision measurements. The standard technique for observing a cold atomic sample is time-of-flight absorption imaging. While this provides valuable information regarding the distribution and quantum state of atoms, the process destroys the sample via spontaneous emission and recoil heating, allowing for acquisition of just one data point per experimental run. In this thesis we develop and implement a complementary hyperfine state sensitive, off-resonant, optical dispersive interrogation system that uses frequency modulation spectroscopy to monitor the temporal dynamics of atomic population during coherent dynamical processes in cold atom systems. The applicability of this robust and powerful tool is expanded to simultaneously measure both the F = 2 hyperfine state population of 87Rb and the F = 9/2 hyperfine state population of 40K. This dual-species probe is used to monitor the sympathetic cooling process, whereby 40K is cooled indirectly via collision with a sample of 87Rb, which itself is undergoing forced RF evaporative cooling. The dispersive probing technique is then used to investigate a range of magnetic Feshbach resonances in 87Rb. Feshbach resonances allow the atomic interaction strength to be precisely tuned via an external magnetic field. Rapid and effcient location and identification of four such resonances were demonstrated between the |F=1, mF=1> and |F=2, mF=0> states. Despite the resonance-induced loss features being <~0.1 G wide, only a small number of experimental runs was sufficient to explore a magnetic field range >18 G. The resonances consist of two known s-wave features in the vicinity of 9 G and 18 G, and two previously unobserved p-wave features near 5 G and 10 G. The dispersive detection approach is further used to monitor the atomic loss in real-time, and directly characterise the two-body loss dynamics for each Feshbach resonance. We further characterise the four observed Feshbach resonances by dispersively measuring the 2-body loss rate coefficient, K21, as a function of magnetic field. Finally, the dispersive probe method is paired with an FPGA-based feedback algorithm and used to perform reliable transfer of 87Rb atoms to a target quantum state in the presence of an unknown, random magnetic field. The algorithm processes off-resonant optical measurements of state populations during a microwave-induced adiabatic rapid passage in real time, and terminates the process when a certain threshold in population transfer is achieved.
Ultracold atomic gases have realized numerous paradigms of condensed matter physics, where control over interactions has crucially been afforded by tunable Feshbach resonances. So far, the characterization of these Feshbach resonances has almost exclusively relied on experiments in the threshold regime near zero energy. Here, we use a laser-based collider to probe a narrow magnetic Feshbach resonance of rubidium above threshold. By measuring the overall atomic loss from colliding clouds as a function of magnetic field, we track the energy-dependent resonance position. At higher energy, our collider scheme broadens the loss feature, making the identification of the narrow resonance challenging. However, we observe that the collisions give rise to shifts in the center-of-mass positions of outgoing clouds. The shifts cross zero at the resonance and this allows us to accurately determine its location well above threshold. Our inferred resonance positions are in excellent agreement with theory.Studies on energy-dependent scattering of ultracold atoms were previously carried out near zero collision energies. Here, the authors observe a magnetic Feshbach resonance in ultracold Rb collisions for above-threshold energies and their method can also be used to detect higher partial wave resonances.
Abstract Ultracold atomic gases have realized numerous paradigms of condensed matter physics, where control over interactions has crucially been afforded by tunable Feshbach resonances. So far, the characterization of these Feshbach resonances has almost exclusively relied on experiments in the threshold regime near zero energy. Here, we use a laser-based collider to probe a narrow magnetic Feshbach resonance of rubidium above threshold. By measuring the overall atomic loss from colliding clouds as a function of magnetic field, we track the energy-dependent resonance position. At higher energy, our collider scheme broadens the loss feature, making the identification of the narrow resonance challenging. However, we observe that the collisions give rise to shifts in the center-of-mass positions of outgoing clouds. The shifts cross zero at the resonance and this allows us to accurately determine its location well above threshold. Our inferred resonance positions are in excellent agreement with theory.
K) molecules in the rovibrational ground state. We prepare the atoms and molecules in various hyperfine levels of their ground states and observe the loss of molecules as a function of the magnetic field. The atom-molecule Feshbach resonances are identified by observing an enhancement of the loss. We have observed 11 resonances in the magnetic field range of 43 to 120 gauss. The observed atom-molecule Feshbach resonances at ultralow temperatures probe the three-body potential energy surface with exceptional resolution and will help to improve understanding of ultracold collisions.
Magnetically tunable Feshbach resonances are an indispensable tool for experiments with atomic quantum gases. We report on 37 thus far unpublished Feshbach resonances and four further probable Feshbach resonances in spin mixtures of ultracold fermionic 40K^{40}\mathrm{K}40K with temperatures well below 100 nK. In particular, we locate a broad resonance at B=389.7GB=389.7\phantom{\rule{0.16em}{0ex}}\text{G}B=389.7G with a magnetic width of 26.7G26.7\phantom{\rule{0.28em}{0ex}}\text{G}26.7G. Here $1\phantom{\rule{0.28em}{0ex}}\text{G}={10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\text{T}$. Furthermore, by exciting low-energy spin waves, we demonstrate a means to precisely determine the zero crossing of the scattering length for this broad Feshbach resonance. Our findings allow for further tunability in experiments with ultracold 40K^{40}\mathrm{K}40K quantum gases.
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.