Atom chips provide a versatile quantum laboratory for experiments with ultracold atomic gases1. They have been used in diverse experiments involving low-dimensional quantum gases2, cavity quantum electrodynamics3, atom–surface interactions4,5, and chip-based atomic clocks6 and interferometers7,8. However, a severe limitation of atom chips is that techniques to control atomic interactions and to generate entanglement have not been experimentally available so far. Such techniques enable chip-based studies of entangled many-body systems and are a key prerequisite for atom chip applications in quantum simulations9, quantum information processing10 and quantum metrology11. Here we report the experimental generation of multi-particle entanglement on an atom chip by controlling elastic collisional interactions with a state-dependent potential12. We use this technique to generate spin-squeezed states of a two-component Bose–Einstein condensate13; such states are a useful resource for quantum metrology. The observed reduction in spin noise of -3.7 ± 0.4 dB, combined with the spin coherence, implies four-partite entanglement between the condensate atoms14; this could be used to improve an interferometric measurement by -2.5 ± 0.6 dB over the standard quantum limit15. Our data show good agreement with a dynamical multi-mode simulation16 and allow us to reconstruct the Wigner function17 of the spin-squeezed condensate. The techniques reported here could be directly applied to chip-based atomic clocks, currently under development18.
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We thank K. Mølmer, J. Reichel, A. Smerzi and A. Sørensen for discussions and J. Halimeh for reading the manuscript. This work was supported by the Nanosystems Initiative Munich and by the European Community's Seventh Framework Programme under grant agreement no. 247687 (Integrating Project AQUTE). T.W.H. acknowledges support by the Max Planck Foundation.
Author Contributions A.S. and P.T. jointly conceived the study. M.F.R., P.B. and P.T. performed the experiment and analysed the data. Y.L. and A.S. carried out the simulations. All authors discussed the results and contributed to the manuscript.
This Supplementary Information file comprises: Experimental Setup; Imaging System; Data evaluation; Phase noise; Depth of Entanglement; Wigner Function reconstruction; Using the squeezed state in an atomic clock. It also contains Supplementary Figures 1-8 with legends and Supplementary References.