17 April 2015 Congratulations to New ICFO PhD graduate

Dr. Naeimeh Behbood


Thesis Committee

Dr. Naeimeh Behbood graduated with a thesis in ‘Generation of Macroscopic Spin Singlets in Cold Atomic Ensembles’ Dr. Naeimeh Behbood received her Master Degree in Theoretical Physics from the Tarbiat Moallem University of Azarbijan, Iran, before joining the Quantum information with cold atoms and non-classical light, led ICREA Prof. at ICFO Morgan Mitchell. She centered her doctoral work on studying and understanding the quantum properties of cold atomic ensembles through the studying and manipulation of their macroscopic properties. Dr. Behbood’s thesis, entitled ‘Generation of Macroscopic Spin Singlets in Cold Atomic Ensembles’, was supervised by Prof. Morgan Mitchell.

Abstract

This thesis describes the generation of macroscopic spin singlets in a cold atomic ensemble by performing quantum non-demolition measurement. Toward this goal we have implemented a realtime shot-nose limited detection system, incoherent feedback and spin state preparation via optical pumping, an upgraded absorption imaging system and coherent rotation of atomic spin via magnetic field control.

Working with a magnetically sensitive atomic system triggered the development of a vector field magnetometry technique. We demonstrate a fast three-axis optical magnetometer using cold, optically trapped 87Rb gas as a sensor. By near-resonant Faraday rotation we record the free-induction decay following optical pumping to obtain the three field components and one gradient component. A single measurement achieves shot-noise limited sub-nT sensitivity in 1 ms, with transverse spatial resolution of about 20 micrometer. We make a detailed analysis of the shot-noise-limited sensitivity.

We apply entropy removal by measurement and feedback to a cold atomic spin ensemble. Using quantum non-demolition probing by Faraday rotation measurement, and feedback by weak optical pumping, we drive the initially random collective spin variable F toward the origin F = 0. We use inputoutput relations and ensemble quantum noise models to describe this quantum control process and identify an optimal two-round control procedure. We observe 12 dB of spin noise reduction, or a factor of 63 reduction in phase-space volume. The method offers a non-thermal route to generation of exotic entangled states in ultra-cold gases, including macroscopic singlet states and strongly correlated states of quantum lattice gases.

We generate approximate singlet states using the tools of measurement induced spin squeezing: quantum non-demolition measurement and coherent magnetic rotations. By squeezing all three spin components, we approach the zero spin noise. Using a cold Rubidium atomic ensemble and near resonant Faraday rotation probing, we have observed up to 3 dB of squeezing relative to the standard quantum limit, and a violation of the generalized spin squeezing inequality by more than 5 standard deviations.

Thesis Committee:

Prof. Maria Checkhova, Max Planck Institute for the Science and Light
Prof. Hugues de Riedmatten, ICFO
Prof. Geza Toth, Universidad País Vasco

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