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This Week at Physics

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Events on Friday, January 27th, 2012

Atomic Physics Seminar
Preparation and stabilisation of a non-classical field in cavity by quantum feedback
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Clément Sayrin, Laboratoire Kastler Brossel, l'Ecole Normale Supérieure
Abstract: Feedback loops are central in most classical control procedures. A controller compares the signal measured by a sensor with the target value. It then adjusts an actuator to bring the signal close to the target value. Generalizing this scheme to the quantum world must overcome a fundamental difficulty: the sensor measurement causes a random back-action on the system. I will present how we have been able to continuously operate a quantum feedback loop stabilizing photon number states in a very high finesse Fabry-Perot cavity. Circular Rydberg atoms repeatedly achieve weak quantum non-demolition measurements of the photon number. A classical computer estimates in real-time the density matrix of the field, based on the outcome of these measurements, and taking into account all known experimental imperfections. It then calculates the amplitude of small classical microwave fields injected into the cavity to bring the field into the target state. We have been able to prepare on demand and stabilize Fock states containing from 1 to 4 photons. This achievement opens interesting perspectives for the production and control of non-classical states for quantum information processing.
Host: Mark Saffman
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Physics Department Colloquium
Atom Trap, Krypton-81, and Global Groundwater
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Zheng-Tian Lu, Argonne National Laboratory & the University of Chicago
Abstract: The long-lived noble-gas isotope 81Kr is the ideal tracer for old water and ice in the age range of 10^5 - 10^6 years, a range beyond the reach of 14C. 81Kr-dating, a concept pursued over the past four decades by numerous laboratories employing a variety of techniques, is now available for the first time to the earth science community at large. This is made possible by the development of an atom counter based on the Atom Trap Trace Analysis (ATTA) method, in which individual atoms of the desired isotope are selectively captured and detected with a laser-based atom trap. ATTA possesses superior selectivity, and is thus far used to analyze the environmental radioactive isotopes 81Kr, 85Kr, and 39Ar, These three isotopes have extremely low isotopic abundances in the range of 10^-16 to 10^-11, and cover a wide range of ages and applications. In collaboration with earth scientists, we are dating groundwater and mapping its flow in major aquifers around the world.
Host: Heeger
Poster: http://www.physics.wisc.edu/twap/posters/2012/2244.pdf
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