Circuit Quantum Electrodynamics - Yale School of Engineering ...

CHAPTER 1. INTRODUCTION 33
cesses in the Cooper pair box, with a focus on how the CPB can be made robust against each type
of noise. As a result of this type of analysis a new derivative of the CPB called the “transmon” is
introduced that has very promising characteristics. Next, in chapter 5, the design and fabrication
of the cavities and qubits is discussed. This section is meant to be a design guide, by which one can
realize the desired parameters in a physical circuit. The appendices associated with these chapters,
while not necessary for understanding, present some more involved derivations and perhaps most
importantly provide a convenient formulary for circuit QED. The measurements are performed at
gigahertz frequencies at one-hundredth of a degree above absolute zero, requiring demanding mi-
crowave and cryogenic engineering. The cryogenic and microwave engineering techniques used in
these experiments are explained in chapter 6. The data begins to flow in chapter 7 where the cavities
and circuit QED system are experimentally characterized, with a focus on how to find one’s way in
a rather large parameter space. The rest of the results are divided into two general classes of exper-
iments focusing on spectroscopic and time domain results respectively. In Chapter 8 spectroscopic
experiments observe the vacuum Rabi splitting, emphatically demonstrating that these circuit QED
experiments reach the strong coupling limit. Spectroscopy in the dispersive limit shows the ac Stark
effect, where the transition frequency of the qubit is shifted proportional to the number of photons in
the cavity. Further experiments take this one step further making the dispersive coupling so strong
that the qubit absorption spectrum splits into photon-number peaks, the first demonstration of the
strong dispersive coupling regime. Chapter 9 then studies time resolved measurements performing
detection and manipulation of the qubit state. While spectroscopic measurements are well suited to
study the static energy spectrum of the system, time domain measurements are particularly adept
at observing the dynamics of a single qubit or photon. In this section we realize the first high visi-
bility measurement of a superconducting qubit, and study the fidelity of single-shot readout. Using
the cavity-qubit coupling we are also able to map the qubit state onto a single photon, creating an
on-demand single photon source. This thesis work was not a single experiment, but the creation of
a platform for studying cavity QED and quantum information in a new way. Chapter 10 discusses
a few possible directions for further experiments.