Atom–molecule coherence for ultralong-range Rydberg dimers

Abstract

An important goal in quantum chemistry is the coherent control of reversible reactions between pure initial and final states of individual constituents. Recent examples of coherent control of such chemical reactions include the photoassociation of ultracold ground-state dimer molecules 1,2 and of Feshbach molecules in highly excited vibrational states 3 . Here, we extend coherent control of such reactions to the photoassociation of dimer states in highly excited electronic states. We demonstrate coherent transfer of initially free pairs of rubidium ground-state atoms to ultralong-range Rydberg molecules consisting of a highly excited Rydberg atom and a ground-state atom 4,5 . The coherent evolution of the molecular system is monitored in echo and Ramsey-pulse sequences by measuring the timescales for the energy-conserving dephasing rate, T 2 , and for non-energy-conserving decay processes, T 1 . These experiments demonstrate an atom-molecule interferometer with Rydberg states.Ultralong-range Rydberg molecules are based on an exotic binding mechanism based on attractive electron-atom scattering. Since their first experimental observation 5 , spectroscopic studies of these unusual molecular states have unveiled another new binding mechanism for ultralong-range Rydberg molecules based on quantum reflection 6 . However, the investigation of the dynamical properties of the ultralong-range Rydberg molecules demands experimental methods going beyond continuous-wave spectroscopy.In the following we demonstrate that our narrow-band laser set-up can coherently drive a system consisting of two rubidium ground-state atoms between the unbound 5s-5s pair state and the bound 3 (5s-35s) molecular ground state by means of a rotary echo-type technique. We model the experimental results with a two-level optical Bloch calculation. The found dephasing and decoherence properties of the system are convincingly reproduced in a Ramsey-type experiment, that is set up to realize an atommolecule interferometer.We create the ultralong-range Rydberg molecules in the 3 (5s-35s)(ν = 0) vibrational ground state from a sample of approximately 2 × 10 6 87 Rb atoms at a temperature of 3 µK in the ground state 5s 1/2 (F = 2; m F = 2) by means of a twophoton transition. The two lasers at 780 nm and 480 nm are detuned by ≈400 MHz from the intermediate 5p 3/2 state (see Methods). A schematic diagram of the levels and the excitation lasers is given in Fig. 1a.Recently, Rabi oscillations of single Rydberg atoms were observed over several microseconds 7,8 . In contrast, previous coherence measurements in many-body ultracold Rydberg systems did not allow observation of coherence times of more than a few hundred nanoseconds owing to laser linewidths of ∼ > 1 MHz (refs 9-11). However, for an atomic sample at a temperature of 3 µK, we expect a Doppler broadening of only 125 kHz full-width at half-maximum as the dominant line-broadening mechanism. Therefore, we have 5. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, German...

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