Abstracts

Quantum control and squeezing of collective spin

Enrique Montano, University of Arizona

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Enrique Montano, Daniel Hemmer, Poul Jessen. Center for Quantum Information and Control (CQuIC). College of Optical Sciences and Department of Physics, University of Arizona. Ben Baragiola, Leigh Norris, Ivan Deutsch. Center for Quantum Information and Control (CQuIC). Department of Physics and Astronomy, University of New Mexico. Quantum control of many body atomic spins is often pursued in the context of an atom-light quantum interface, where a quantized light field acts as a "quantum bus" that can be used to entangle distant atoms. One key challenge is to improve the coherence of the atom-light interface and the amount of atom-light entanglement it can generate, given the constraints of working with multilevel atoms and optical fields in a 3D geometry. We are currently exploring new ways to achieve this, through rigorous optimization of the spatial geometry, and through control and optimization of the internal atomic state. Our basic setup consists of a quantized probe beam passing through an atom cloud held in a dipole trap, first generating spin-probe entanglement through the Faraday interaction, and then using backaction from a measurement of the probe polarization to squeeze the collective atomic spin. In this scenario the relevant figure of merit is the signal-to-noise ratio for a measurement of the collective spin projection noise in the presence of probe shot noise. With an optimized free-space geometry we readily achieve a signal-to-noise ratio of 10dB, and by using a 2-color probe scheme to suppress tensor light shifts we can translate this into as much as 7dB of metrological squeezing. It is possible to further increase atom-light coupling by "amplifying" the initial projection noise per atom through a suitable internal state preparation. For example, by preparing the atom ensemble in a "cat" state, the spin projection noise can be increased by a factor of 2f (8 for Cs) relative to the commonly used spin coherent state. Under the right conditions such an increase in projection noise can lead to stronger measurement backaction and increased atom-atom entanglement. If so, we can in principle use further internal-state control to map this entanglement to a basis where it corresponds to improved squeezing of, e.g., the physical spin-angular momentum or the collective atomic clock pseudospin. In practice, controlling the collective spin of N~10^6 atoms in this fashion is an extraordinarily difficult challenge because errors in the control of individual atoms tend to be highly correlated. We will discuss recent, encouraging progress towards the preparation and detection of projection noise limited "cat" states, and the general prospect of using the internal atomic structure as a resource for ensemble control.