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The specifics of implementing the initialization, measurement, and universal
control required by the DiVincenzo criteria, of course, need to be solved with the
quantum nature of the circuits in mind and can thus not be assumed as a priori
obvious. Nevertheless this flexibility is making it easy to find feasible solutions.
2.2 Idea
2.2.1 Quantum Mechanics in Electrical Circuits
To turn an electrical circuit into a qubit, one needs to find a regime in which it
exhibits quantum behavior. The simplest general system that exhibits quantum
behavior is the harmonic oscillator, provided it is driven at extremely low energies.
Thus, it is natural to start the circuit design with the electrical analog of the
harmonic oscillator, the inductor-capacitor oscillator (or LC oscillator for short).
In this circuit, a capacitor and an inductor are connected in series inside a loop
configuration as shown in Figure 2.1a. If a charge is present on the capacitor, it
tries to force electrons around the loop to remove the charge, while the inductor
in the circuit tries to maintain the current flowing through it at a constant level.
This circuit can be readily analyzed using Kirchhoff’s current law. All current
flowing around the loop is flowing through the inductor and the capacitor, which
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