Microwave Gaussian quantum metrology

Licentiate thesis, 2023

With increasingly sensitive measurements being made possible by technological development, there arises situations where the effects of quantum mechanics have 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 an advantage is realized and argued that it is difficult to find useful applications for the protocol. On 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.