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Paper on Measurement-induced localization is published in Physical Review Letters

The act of observation has profound consequences on a quantum system. In this work, we experimentally demonstrate the famous 'Heisenberg microscope' and show the dramatic effect of continuous position measurements on the evolution of a quantum system. In particular, we confine nanoKelvin temperature atoms within an optical lattice and show that the act of imaging these atoms induces their localization, i.e. suppresses tunneling. In other words, we freeze the quantum motion of these atoms simply by the act of gazing at them. This phenomenon, a manifestation of the quantum Zeno effect, has no classical equivalent. 

The nature of measurement and its influence on a quantum system presents one of the foremost puzzles in our current understanding of quantum physics. Its ramifications include the intrinsic obstacles to 'ideal' measurements, the origin of the so-called standard quantum limit to measurement precision and the incongruity of such concepts when extended to the macroscopic domain (as encapsulated, for instance, in Schrodinger's famous cat paradox). Our experiments distil the essence of a position measurement by observing the effect of single photon scattering events on the spatial evolution of an atom. Furthermore, the tunability and simplicity of our measurement technique potentially allows it to be used as a valuable resource for the control of interacting quantum many-body systems.

Our two-photon imaging technique also allows us to tune the 'strength of the measurement', i.e. the rate at which the atoms scatter photons, over a wide dynamic range. As we gradually increase this measurement strength, we observe the continuous crossover from the regime of weak measurements, where the act of observation has negligible influence on the atoms, to the regime of strong measurements, where the act of observation causes a dramatic suppression of tunneling. This crossover behavior is an instance of emergent classicality in a quantum system due to continuous measurement. 

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Sep. 2015