Maria Ekström, Microtechnology and Nanoscience - MC2

​Title: Quantum acoustics with propagating phonons

​Maria is a PhD student at the Quantum Technology Laboratory
Faculty opponent: Prof. Christopher Bauerle, CNRS and Institut NEEL, Grenoble, France
Examiner and main supervisor: Prof. Per Delsing
Surface acoustic waves (SAWs) are mechanical vibrations that propagate on the surface of
solids while dissipating little power, consequently enabling them to propagate freely over
long distances. The speed and wavelength of SAWs are reduced five orders of magnitude
compared to when light is used as a carrier at gigahertz frequencies. The unique properties
of SAWs combined with the possibility to let them interact with artificial atoms,
discovered and shown for the very first time in the appended Paper I of this thesis, open
up for exploration of new regimes of quantum physics. The appended Paper II is a book
chapter providing an overview of many of the new areas of research, as well as going into
depth of the method and significance of the results of the appended Paper I.

The essential interaction between artificial atoms and SAWs was further investigated
by using Autler-Townes splitting to achieve fast control of the interactions. The appended
Paper IV, shows a transmitted field extinction of 80 %, and provides proof of concept
for a SAW router in the quantum regime. In addition, due to the artificial atom’s highly
frequency dependent coupling to SAWs, electromagnetically induced transparency (EIT)
could be observed in the appended Paper V. Furthermore, the EIT region was distinguished
from the Autler-Townes splitting region by a threshold in the applied power.
The results produce parallel findings to quantum optics, but are perhaps best described
as part of a different field, quantum acoustics.

Among the many possible areas of research emerging as an outcome of this work, a
variety of potential quantum experiments would benefit greatly from a higher conversion
efficiency between electric signals and SAWs. Due to this, focus was put on improving
this conversion efficiency by studying superconducting unidirectional transducers (UDTs),
making use of advances in classical SAW devices. The appended Paper III shows that
99.4 % of the acoustic power can be focused in a desired direction and that the conversion
between electric signals and SAWs is greatly improved by using UDTs, thereby eliminating
the largest source of loss of standard symmetric inter-digital transducers. There is,
however, a trade-off between conversion efficiency and bandwidth. This finding allows
tailoring of quantum experiments based on SAWs that may pave the way towards further
investigating quantum sound.

Keywords: Surface acoustic wave, interdigital transducer, unidirectional transducer,
quantum acoustics, superconducting circuits, artificial atom, qubit, phonon, phonon
router, Electromagnetically Induced Transparency

Category Thesis defence
Location: Kollektorn, lecture room, Kemivägen 9, MC2-huset
Starts: 21 August, 2020, 10:00
Ends: 21 August, 2020, 13:00

Published: Mon 15 Jun 2020.