Page 174 - Principles and Applications of NanoMEMS Physics

162 Chapter 4

4.3.1.1 The Ion-Trap Qubit
The ion-trap qubit was proposed by Cirac and Zoller [191]. It is embodied by
atomic ions confined by an electrode structure designed in such a way that a 3-
dimensional harmonic potential well (trap) is produced [190]. Cooling the ions
lowers their energy and, were it not because of Coulomb’s force of repulsion, which
maintains them apart, they would descend to, and meet at, the bottom of the well.
Instead, the collective state of the ions is the result of a balance between the
potential well energy profile and the forces of repulsion between ions, which
manifests in their assuming a linear array, see Figure 4-10.

Figure 4-10. Sketch of ion trap qubit. The electrodes create a 3-D harmonic potential well
that confines the ions.
The ion trap simultaneously implements two types of qubit, Fig. 4-11. In one

Figure 4- . Qubit realizations with ion trap.
1 1
instance, the two states of the qubit are embodied by the direction of the
ion’s magnetic moment, which is parallel or antiparallel to an externally
applied magnetic field. In the second instance, the collective motion of an
array of ions forms the qubit. In particular, when expressed in terms of
normal modes, the two states of the motional qubit are the one in which the
ions move simultaneously in the same direction, common mode (CM) and
the one in which adjacent ions move in opposite directions, stretched mode.
In the motional case, the qubit is not associated with any individual ion,