Thesis (pdf) - Swinburne University of Technology

lost—making any meaningful interferometry impossible after this point when
the atoms are localised in the left or right well. This collapse into separate
BECs with undefined relative phase was experimentally seen not only for two
wells [Shi05] but also for an array of traps [Gre02a, Gre02b]. These results
seem to indicate that the splitting of the BEC has to stay in the so-called
Josephson regime to be useful for interferometry [Jav86, Sme97, Sak02], or
that the whole process of splitting, phase evolution and recombining for the
measurement has to be done on short timescales to maintain the coherence
between the wells. The coherence between the wells will only survive as long
as the atom numbers are undefined. One way to achieve this is to split only
so far that some tunnelling is still allowed, so that the atom numbers of the
wells fluctuate and are not defined, like in the the above-mentioned Josephson
regime. Another way to keep the coherence is to split the single atoms between
the wells. As long as localisation can be prevented, the atoms in the two wells
then are coherent. However, a BEC consists of interacting particles, and these
interactions can be interpreted as measurements of the location of the atoms.
This limits the coherence time, but there are ways to reduce the interaction
strength by so-called Feshbach resonances [Mar02, Web03, Shi04].
Theoretical work is continuing on this topic, examining the splitting pro-
cess more closely [Bor04] for weakly interacting atoms and ways are being re-
searched on how to increase the sensitivity of such an interferometer [Neg04].
Other proposals use the BEC only as a means to create atoms in the ground
state of the potential well. Atoms that do not interact with each other, because
they have a scattering length of zero, will overcome some of the problems of
the interferometers proposed with interacting BECs. These then in principle
work like multiple single atom interferometers [Dud03], where the single atom
interferometers are the ones proposed for atom chips [Hin01, H¨01c]. Other
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