Thesis Introduction

This is a HTML version of the introduction to my masters thesis 'Construction of and Experiments with a Linear Paul Trap':

The precision of atomic physics experiments normally depend on how well the system under investigation can be isolated from unwanted perturbations. Such isolation can be achieved to various degrees of success in dilute gasses, plasmas or accelerated beams, but the ideal situation would often be free atomic particles at rest in space. For atomic ions with the same sign of charge, the Coulomb repulsion makes the ideal situation impossible by the very nature of the particles. In the fifties, the development of the particle accelerator technique and mass spectrometers led to the design of various traps for charged particles. The first charged particles were trapped in a hyperbolic radio frequency trap [1], also called a Paul trap after Wolfgang Paul, who proposed the design of the trap. W. Paul was rewarded the Nobel Prize in Physics in 1989 for his contribution to methods for trapping particles [2]. Also in 1989, further improvement of the ion trap technology appeared with the demonstration of the linear Paul trap [3]. Compared to the hyperbolic trap, the linear trap makes it possible to trap larger quantities of ions and also to reduce the perturbing effect of the radio frequency field within the trap.


Laser cooling of trapped ions and atoms is a rapidly growing field in atomic physics and quantum optics. The Nobel Prize in Physics in 1997 equally honored the work on laser cooling and trapping atoms of Steven Chu, William D. Philips and Claude Cohen-Tannoudji. The combination of ion traps and laser cooling techniques are one of the most efficient ways of reducing perturbations in atomic physics experiments. Under the right conditions, laser cooled ions will form complex spatially ordered structures, called ion crystals. In such ion crystals, even single ions can be observed, and the crystals are very useful for precision spectroscopy and quantum optical experiments. For example, trapped ions have been used to make quantum logic gates [4], and proposals suggest they could be used for implementing a quantum computer [5][6]. Another example of the use of trapped ions in quantum optics is the observation of the quantum Zeno effect [7][8], which has also attracted attention in recent years. The quantum Zeno effect is the inhibition of transitions between different quantum states in a system, due to frequent measurements of the systems state.

The motivation for this thesis is an experiment which uses an ion crystal with magnesium ions to show how optical pumping can be achieved without absorption - an effect which can be interpreted as a continuous quantum Zeno effect. The optical pumping without absorption is examined both theoretically and experimentally. However, before the optical pumping experiment with an ion crystal could be performed, a linear Paul trap first had to be constructed and then a series of experiments had to be conducted. These experiments aimed at achieving control over the ion crystals physical properties, such as the content and motion of the ions within the crystal.

The experimental and theoretical work presented in this thesis was done during 1998 and 1999 at the Institute of Physics and Astronomy, University of Aarhus, supervised by assistant professor Michael Drewsen. The first part of the my work was to complete the design and build a linear Paul trap, in a versatile experimental setup which has since been used for a variety of different experiments The experiments presented in this thesis will also show the wide scope of experiments which can be made with an ion trap, covering phenomena from plasma and quantum physics as well as chemistry.


Structure of the Thesis

Because of the different nature of the experimental work, this thesis is divided into three parts. The first part introduces the basic theory for trapping ions in a linear Paul trap, the method of laser cooling, and finally describes the features of the constructed experimental setup. The second part discusses the experiments performed to improve the control over the physical properties of the ion crystals. It starts by reviewing the basic properties of a trapped ion plasma, to provide a basis for understanding the phenomena observed in the experiments. Then the process of loading ions into the trap and observations of the structure of ion crystals are discussed. This leads to a discussion of experimental methods developed to control the content and shape of the crystal. Then ways of observing the ion motion and compensating for imperfections of the fields in the ion trap are presented. Finally, reactions involving the trapped laser cooled magnesium ions and gasses are investigated. The third part of the thesis investigates optical pumping without absorption. An interpretation in terms of a continuous quantum Zeno effect is presented, to help understand the theoretical and experimental results. Theoretical predictions are made using computer simulations and the optical pumping is observed experimentally using an ion crystal consisting of 24Mg+ ions. The observed effect is finally discussed in terms possible future experiments measuring the photon correlation functions. The final conclusion is a brief summary of the conclusions drawn from the presented experiments and an outlook to future experiments with the present trap.


[1] Gosh95

[2] Paul90

[3] Prestage89

[4] Monroe95

[5] Cirac95

[6] Sorensen99

[7] Itano90

[8] Misra77