Atom-chip-based generation of entanglement for quantum metrology

Atom chips provide a versatile quantum laboratory for experiments with ultracold atomic gases(1). They have been used in diverse experiments involving low-dimensional quantum gases(2), cavity quantum electrodynamics(3), atom-surface interactions(4,5), and chip-based atomic clocks(6) and interferometers(7,8). 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 simulations(9), quantum information processing(10) and quantum metrology(11). Here we report the experimental generation of multi-particle entanglement on an atom chip by controlling elastic collisional interactions with a state-dependent potential(12). We use this technique to generate spin-squeezed states of a two-component Bose-Einstein condensate(13); 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 atoms(14); this could be used to improve an interferometric measurement by -2.5 +/- 0.6 dB over the standard quantum limit(15). Our data show good agreement with a dynamical multi-mode simulation(16) and allow us to reconstruct the Wigner function(17) of the spin-squeezed condensate. The techniques reported here could be directly applied to chip-based atomic clocks, currently under development(18).