Page 203 - Principles and Applications of NanoMEMS Physics

4. NANOMEMS APPLICATIONS: CIRCUITS AND SYSTEMS 191

Read out of the qubit state is accomplished by exploiting tunneling
through the barrier separating the potential well from the continuum, and
subsequent self-amplification due to the negative slope potential, see Fig. 4-
21(f). In particular, since the barrier becomes thinner at higher energies, and
those higher energy states have an increasing probability of escape, the one
state is measured by sending a probe signal to induce a particle in the one
state to tunnel out of the well. Upon tunneling out of the well, the downward
acceleration of the potential leads to the appearance of a voltage 2∆ e /
across the junction. This voltage is associated with reading a one state for the
qubit; zero voltage is associated with reading a zero state.
In terms of operating temperature, it is clear that superconducting qubits
must be operated at temperatures such that kT << = ω 01 << ∆ , where ω is
01
the transition frequency between the energy levels representing states 0
and 1 , and ∆ is the energy gap of the superconducting material. This
necessitates cooling to temperatures of the order of 20mK.

4.4 Summary

This chapter has dealt with a number of aspects surrounding the actual
implementation of NanoMEMS circuits and systems. We began discussing
architectural issues, as this is the first step in defining a NanoMEMS system
on chip (SoC). Then, emerging candidate building blocks, intended for
applications ranging from interfaces to signal processing functions, were
described. These included a charge detector, which-path electron
interferometer, torsional MEM resonator for parametric amplification,
Casimir effect oscillator, magnetomechanically actuated beam, functional
arrays, and a quantum entanglement generator. These building blocks
represented nanoelectromechanical quantum circuits and systems (NEMX),
as they exploited the coexistence of electronic and mechanical structures.
The chapter concluded with a presentation of physical implementations of
quantum bits (qubits), such as the ion-trap, the nuclear magnetic resonance,
the semiconductor solid-state, and superconducting qubits, upon which
quantum computing paradigms might be predicated.