Riedel, Max (2010): Multiparticle entanglement on an atom chip. Dissertation, LMU München: Faculty of Physics 

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Abstract
The controlled generation of entanglement forms the basis for currently emerging ‘quantum technologies’, such as quantum simulation, computation, and metrology. In the field of quantum metrology, multiparticle entangled states, such as spinsqueezed states, are investigated as a means to improve measurement precision beyond the ‘standard quantum limit’. This limit arises from the quantum noise inherent in measurements on a finite number of uncorrelated particles and limits today’s best atomic clocks. Atom chips combine exquisite coherent control of ultracold atoms with a compact and robust setup, suggesting their use for quantum metrology with portable atomic clocks and interferometers. A severe limitation of atom chips, however, is that techniques to control atomic interactions and to generate entanglement have not been experimentally available so far. In this thesis, I present experiments where we generate for the first time multiparticle entanglement on an atom chip. We achieve this by controlling elastic collisional interactions with a statedependent potential. We employ this novel technique to generate spinsqueezed states of a twocomponent BoseEinstein condensate and show that they are a useful resource for quantum metrology, as they could be used to improve an interferometric measurement by 2.5 dB over the standard quantum limit. The statedependent potential is created with the help of a coplanar microwave guide, which is integrated on our atom chip. In the vicinity of this waveguide a microwave nearfield is formed. When a BoseEinstein condensate of 87Rb is brought into this nearfield, the hyperfine energy levels of the atoms are shifted differentially due to the AC Zeeman effect. The strong gradients in the field can be used to stateselectively shift the minimum of a static magnetic atom trap and thus coherently split an ensemble of atoms which have been prepared in a superposition of two internal states. During this process, nonlinear atomic interactions lead to the formation of a spinsqueezed state. I tomographically analyze the produced state, reconstruct its Wigner function, and deduce that it is at least fourparticle entangled. I compare our results with a dynamical multimode simulation which takes not only the atomic motion and internal state dynamics but also particle losses into account and find good agreement. Moreover, I use this comparison to identify technical noise sources in our experiment, which currently limit the achieved amount of squeezing, and make suggestions on how to eliminate them in future experiments. Our method can in principle create a very large amount of squeezing and entanglement and is applicable to a wide variety of atomic systems, in particular to those for which no convenient Feshbach resonance exists. We envisage the implementation of this technique in portable atomic clocks and interferometers operating beyond the standard quantum limit. Furthermore, it is a valuable tool for experiments on manybody quantum physics and could enable quantum information processing on atom chips.
Item Type:  Thesis (Dissertation, LMU Munich) 

Keywords:  atom chip, entanglement, BoseEinstein condensate, BEC, ultracold atoms, Wigner function, microwave potential, microwave nearfield, microchip, state selective potential, spin squeezing, one axis twisting, atomic clock, standard quantum limit 
Subjects:  600 Natural sciences and mathematics 600 Natural sciences and mathematics > 530 Physics 
Faculties:  Faculty of Physics 
Language:  English 
Date Accepted:  18. November 2010 
1. Referee:  Hänsch, Theodor W. 
Persistent Identifier (URN):  urn:nbn:de:bvb:19126195 
MD5 Checksum of the PDFfile:  af0033916a1aa558afb10975d980d953 
Signature of the printed copy:  0001/UMC 19235 
ID Code:  12619 
Deposited On:  10. Feb 2011 10:36 
Last Modified:  20. Jul 2016 10:27 