Robert Jonsson, Tillämpad kvantfysik

Diskussionsledare:
Prof. Tomas McKelvey

Huvudhandledare:
Prof. Göran Johansson

Examinator:
Prof. Janine Splettstösser

Sammanfattning:

With increasingly sensitive measurements being made possible by
technological development, there arises situations where the effects of
quantum mechanics has to be taken into account. While quantum mechanics
tells us that there are fundamental limits of measurement sensitivity,
it also gives us the tools to constructively push the same limits for experimental systems.
The field of quantum metrology investigates how sensitive a measurement can be made,
and how to realize such a setup.

Quantum metrology as a topic is well established for the field of quantum
optics in the visible light frequency range, and quantum enhanced measurement
setups have been experimentally realized. In the last couple of decades,
similar types of setups are starting to be possible at microwave frequencies,
where a thermal background can be significant.

In this thesis and the appended articles, we have studied various
quantum probes applied to radar-like scenarios where the task is to
measure a weak signal in the presence of thermal noise. Our focus has been
two-fold. On the one hand, we have studied the quantum illumination
protocol which uses entanglement to beat classical protocols in the task of
binary discrimination. We have elucidated the scenario
where the advantage is realized and argued that it is difficult to find
useful applications for the protocol. One the other hand, we have studied
the task of estimating the attenuation coefficient in a lossy Bosonic
channel, and established the optimal Gaussian probe states based on
maximization of quantum Fisher information. These results serve to
illustrate situations where a proper understanding of quantum mechanics can
be applied to enhance radar-like tasks, or quantum radars.