We propose and experimentally investigate a scheme for observing Feshbach resonances in atomic quantum gases in situ and with a high temporal resolution of several ten nanoseconds. The method is based on the detection of molecular ions, which are optically generated from atom pairs at small interatomic distances. As test system we use a standard rubidium gas (87Rb) with well known magnetically tunable Feshbach resonances. The fast speed and the high sensitivity of our detection scheme allows to observe a complete Feshbach resonance within one millisecond and without destroying the gas.
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