The precision of classical interferometers is limited by the quantum mechanics of single particles. The so-called projection noise limit fundamentally appears in the measurement process when the individual particles, being in a superposition of the two interferometer arms, are projected to one of the two output modes. However, entanglement among the particles can be used as a resource that allows for measurements beyond this limit. In this lecture note we give an introduction to the concept of spin squeezing to achieve quantum enhanced precision. We summarize the Heidelberg experiments that realized spin-squeezed states and interferometric measurements beyond the standard quantum limit using few mode Bose-Einstein condensates. These experimental results showed that direct interatomic interactions in a Bose-Einstein condensate (next to cavity-mediated interactions, quantum non-demolition measurements or state transfer from non-classical light) can be employed to achieve spin squeezing
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