Two-Level Systems


  What are Two-Level Systems ?
By the term Two-Level Systems (TLS), systems with only two relevant (energy) levels are described. More precisely these levels can be any two different quantum levels where of course at least one of the quantum numbers of the two states must be different from one another. Think e.g. of two different quantum states of the hydrogen-atom.
What do we do with TLS ?
We let them interact with radiation. Then we have a simple model of interaction of radiation with matter. The approximation that matter has only two levels can be fairly good in the case of resonant or near resonant interaction of the radiation with e.g. an atom. No other levels are needed to be taken into account because they can not be excited.
Such models are studied in the context of nuclear magnetic resonance where the magnetic dipole moment of a spin system interacts with a rf electromagnetic field. It provides also for a basic model of laser physics and quantum optics where one observes the electric dipole interaction of an atom with the radiation field.
Last but not least also the isospin dublett of a neutron and a proton (p,n) can be examined under the interaction of a bosonic field (pion-field) (Lee Model by T.D. Lee, Phys. Rev. 95 pg. 1329 (1954)) which means nothing else than a TLS interacting with a radiation field.
Recently TLS became rather popular in the field of quantum computing, where TLS are taken to store and process information in qubits.
  Nice introduction to quantum computing.  
  Download paper on quantum computing using high-Q cavities in which the qubits are represented by single cavity modes restricted in the space spanned by the two lowest Fock states, written by V. Giovannetti, D. Vitali, P. Tombesi, and A. Ekert.  

Christoph Wildfeuer
Last modified: Tue Feb 8 15:35:25 MET 2000
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