Spin resolved microscopy of doped Hubbard chains (T. Hilker)
The doping of an antiferromagnet can lead to the complex physics related to high temperature superconductivity. In one dimension, however, the competition between the spin and density sectors is largely absent due to the separation of the spin and density modes at low energy. Ultracold fermions in optical lattices allow for the simulation of such systems with a unique control over kinetic energy, interactions and doping. A challenge has been to reach the required temperature for spin order and to measure the antiferromagnetic correlations with cold atoms. I will present our direct, single-atom resolved detection of antiferromagnetic correlations over several sites in spin-1=2 Hubbard chains . Upon doping the correlations seem to decrease. With our full access to the spin and density distribution we can directly measure three-point spin-hole-spin correlations and thus confirm that a hole in 1d only acts as a domain boundary of the spin-sector . This is direct consequence of the phenomenon of spin-charge separation and it allows to reveal the full correlations with non-local string operators .
Our technique can be extended to higher dimensions to study the complex interplay between magnetic order and density fluctuations and it demonstrates how topological order could directly be measured in experiments.
1. Boll, M. et al. Science 353, 1257–1260 (2016).
2. Hilker, T. A. et al. arXiv: 1702.00642 (2017).
3. Den Nijs, M. & Rommelse, K. Phys. Rev. B 40, 4709–4734 (1989).
Testing Bell's inequality with atoms entangled over a large distance (Dr. W. Rosenfeld)
Bell's inequality allows testing experimentally whether nature can be described in a local-realistic way by measuring correlations between two separated systems. While being conceptually simple, such tests set very stringent experimental requirements in order to avoid opening “loopholes” for possible local-realistic descriptions. From the discovery of Bell's inequality in 1964 it thus took more than 50 years of experimental development until such tests became possible. In this talk I will present our experiment closing the major experimental loopholes . It is based on heralded entanglement of single trapped Rb atoms over a distance of 400 m combined with a highly reliable and fast readout of their internal spin state. Our results show a clear violation excluding local-realistic theories with a very high level of significance. Beyond their fundamental importance, tests of Bell's inequality also form the basis for novel communication methods. In particular they allow for so-called “device-independent” scenarios, where secure key distribution or generation of random numbers is possible even with untrusted devices.
 arXiv:1611.04604 [quant-ph]