Circuit Quantum Electrodynamics - Yale School of Engineering ...

CHAPTER 1. INTRODUCTION 25
a
Figure 1.4: a. A self assembled quantum dot inside a micro-pillar of alternating dielectrics (GaAs
and AlAs) to create a distributed bragg reflector cavity [Reithmaier2004]. b. A two-dimensional
photonic band gap crystal supported by a micro pillar [Yoshie2004]. c. Top view of the photonic
crystal showing defect in crystal forming the cavity. The quantum dot is located underneath the
surface inside the defect.
Another atomic cavity QED system that heavily influences this work, uses Rydberg atoms and
three-dimensional microwave cavities [Raimond2001]. Rydberg atoms are highly excited alkali atoms,
with much larger electron orbits than ground state atoms (1000 ˚A vs. 1 ˚A). Their large size allows
them to possess much larger dipole moments, and thus to interact more strongly with light. In
addition, radiative decay is much slower for microwave excitations than optical excitations, giving
more time for the atoms to interact with the photons. However, these benefits come at the price of
having less energy available, making direct detection of the photons difficult and requiring cryogenic
cooling of the cavity to ∼ 1 K. Fortunately, the atoms can be detected directly by selective ioniza-
tion. This leads to a different and interesting reversal of focus from the previous implementation,
with atoms as “meters,” probing the cavity photons. In addition to measuring the vacuum Rabi
oscillations described above (see Fig. 1.3b), the Rydberg atoms can be used to count the number
of photons in the cavity [Brune1996] (see Fig. 1.3c). Another experiment measured the presence of
a single photon without destroying it, constituting a quantum non-demolition (QND) measurement
of a single photon [Nogues1999], while a more recent experiment measured quantum jumps as the
number of cavity photons changed [Gleyzes2007]. In section 8.3 we present a new technique for
counting photons without destroying them.
In addition to atomic systems, rapid progress is also being made in semiconducting systems,
using quantum dots as artificial atoms. In one technique, self assembled dots and a distributed
Bragg reflector cavity made from alternating layers of epitaxially grown dielectric materials are
b
c