27 November 2012 Congratulations to new ICFO PhD Graduate

Dr. Brice Dubost

Thesis Committee

Dr. Brice Dubost has successfully defended his thesis on light-matter interaction with atomic ensembles. Dr. Brice Dubost joined the Quantum information with cold atoms and non-classical light group, led by ICREA Professor at ICFO Morgan Mitchell after receiving his MSc in Physics from École normale supérieure de Cachan, in France. At ICFO, his research centered on the detection of non-Gaussian states in trapped atoms.

Dr. Dubost presented a thesis titled ‘Light-Matter Interaction with Atomic Ensembles’. The thesis was a joint effort between ICFO and the Laboratoire Matériaux et Phénomènes Quantiques, CNRS, France. His thesis work at ICFO was supervised by group leader Prof. Morgan Mitchell. The research in Paris dealt with the storing of light in an ion Coulomb crystal and was supervised by Prof. Samuel Guibal.


The study of quantum light-matter interaction with atomic ensembles is an active research area. This kind of system allows fundamental studies on measurement in the context of continuous variables, in collective entanglement and in quantum simulations. This field of research is also interesting in the context of quantum metrology, quantum networking and quantum computation. In this thesis two complementary aspects of light matter interaction with atomic ensembles have been studied with trapped ions and cold neutral atoms. The trapped ion experiment is intended to evaluate the possibility to use large ion clouds for realizing a quantum memory with long coherence times. Whereas the cold atom experiment focused on the use of quantum non demolition measurements to evaluate non-Gaussian states, this experiment is similar to quantum networking experiment currently planned.

Laser cooled trapped ions can reach a crystalline phase due to the strong Coulomb repulsion between ions. In this phase the relative positions between the ions is fixed avoiding collisions and the ions to explore magnetic field inhomogeneities which can be a source of coherence loss. At low ion number, long coherence times have been demonstrated. With large ion numbers, the trapping mechanism can induce heating of the ion cloud thus making more difficult to obtain the crystallized regime. During this thesis, large Coulomb crystals containing more than 1 X 106 ions have been obtained and signature of electromagnetically induced transparency in such system have been obtained. This study also revealed limitations of this kind of systems which have to be further studied to allow strong light matter interaction probability with cold large ion ensembles in a regime allowing for long coherence times.

Neutral atoms systems allow strong light matter coupling probabilities but usually reduced coherence times. Quantum memories, entanglement between atoms and light, high precision magnetometry have been demonstrated with neutral atomic vapors. The system used during this thesis is designed to allow strong light matter coupling probability with detuned polarized light pulses, allowing to precisely measure the spin state of the atomic system without destruction and low noise. The measurement noise of the system is lower than the atomic noise opening the way for collective entanglement (via measurement induced spin squeezing) and ultra sensitive magnetic _eld measurements. This system is closely related with systems designed for quantum networking and quantum memories. Non Gaussian atomic states are a resource for quantum computation and quantum communication, in the context of atomic physics experiments, their detection can be difficult.

The work presented in this thesis focuses on the detection of non Gaussian states in atomic ensembles using cumulants, and in particular their noise properties.

President: Prof. Carlo Sirtori , Université de Paris VII , FRANCE
Vocal: Dr. Aurelien Romain Dantan, Aarhus University, DENMARK
Secretary: Prof. Hughes de Ridmatten, ICFO- Institute of Photonic Sciences, SPAIN