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Direct observation of second-order atom tunnelling

Abstract

Tunnelling of material particles through a classically impenetrable barrier constitutes one of the hallmark effects of quantum physics. When interactions between the particles compete with their mobility through a tunnel junction, intriguing dynamical behaviour can arise because the particles do not tunnel independently. In single-electron or Bloch transistors, for example, the tunnelling of an electron or Cooper pair can be enabled or suppressed by the presence of a second charge carrier due to Coulomb blockade1,2. Here we report direct, time-resolved observations of the correlated tunnelling of two interacting ultracold atoms through a barrier in a double-well potential. For the regime in which the interactions between the atoms are weak and tunnel coupling dominates, individual atoms can tunnel independently, similar to the case of a normal Josephson junction. However, when strong repulsive interactions are present, two atoms located on one side of the barrier cannot separate3, but are observed to tunnel together as a pair in a second-order co-tunnelling process. By recording both the atom position and phase coherence over time, we fully characterize the tunnelling process for a single atom as well as the correlated dynamics of a pair of atoms for weak and strong interactions. In addition, we identify a conditional tunnelling regime in which a single atom can only tunnel in the presence of a second particle, acting as a single atom switch. Such second-order tunnelling events, which are the dominating dynamical effect in the strongly interacting regime, have not been previously observed with ultracold atoms. Similar second-order processes form the basis of superexchange interactions between atoms on neighbouring lattice sites of a periodic potential, a central component of proposals for realizing quantum magnetism4,5,6,7.

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Figure 1: Schematics of double-well generation, loading and detection sequences.
Figure 2: Tunnelling dynamics.
Figure 3: Tunnelling frequencies versus short-lattice depth (barrier height).
Figure 4: Conditional tunnelling.

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Acknowledgements

We thank A. M. Rey and B. Paredes for discussions. We acknowledge funding through the DFG and the European Union (MC-EXT QUASICOMBS). R.S. acknowledges support by the EU QUDEDIS programme as well as SJCKMS and the Kempe I and II foundations.

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Correspondence to I. Bloch.

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Fölling, S., Trotzky, S., Cheinet, P. et al. Direct observation of second-order atom tunnelling. Nature 448, 1029–1032 (2007). https://doi.org/10.1038/nature06112

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