Circuit Quantum Electrodynamics - Yale School of Engineering ...

CHAPTER 1. INTRODUCTION 21
|1〉
|0〉
E = h f
Figure 1.1: a. Any two quantum energy levels can be used as a qubit. Relaxation occurs when the
qubit decays from the excited state to the ground state. b. When in a superposition, the phase, φ,
of the qubit precesses at a frequency determined by the energy difference between the two levels. If
this energy varies in time, the clock speed will vary causing it to lose time or dephase.
is the bane of most would-be qubits (see Fig. 1.1b). It is for this reason that there is an intimate
relationship between the study of atomic clocks and quantum bits [Monroe1995].
In order to prevent decoherence, this loss of quantum character, the elements of a quantum
computer must both be isolated from sources of noise, and yet be strongly coupled to each other,
all the while being controlled by the classical experimenter. These nearly antithetical requirements
pose a shared challenge for all quantum computing experiments, and have stimulated the flow of
ideas among disciplines. In this project, we apply analogies of atomic physics to superconducting
circuits, as a means of taming the large decoherence present in solid-state environments. Applying
these analogies in reverse we use superconducting circuits to access areas difficult or impossible to
study in natural atomic systems.
1.2 Cavity Quantum Electrodynamics
The electromagnetic field, though it has a wave-like nature, is composed of discrete packets known
as photons. At first glance, this seems like a rather innocuous postulate, a matter of bookkeeping
rather than a qualitative shift from classical waves. This discretization, however, has subtle and far
reaching effects, explaining many mysteries, including the color of a hot object such as our sun, and
why excited atoms decay, emitting light only at certain frequencies. The complexities of the photon
postulate become more apparent when asking even a simple question like, “What is the shape of
a photon?” The answer to this is subtle. Both the spatial and temporal distributions of energy
are not fixed, but are dependent and controllable by the boundary conditions imposed upon the
photon by matter. If a photon is so malleable how can it be discrete? How does matter interact
B