Generation and Application of Squeezed States of Light in Quantum Sensing

Quantum technology has seen a surge in attention throughout the 21st century in part due to the promise of superior performance in many fields of science and technology compared to classical counterparts. One of the areas that show much potential is the field of quantum metrology, where systems exploiting quantum mechanical principles can overcome the limitations of classical metrology systems and reach extreme levels of precision enabling the exploration of domains previously unavailable.

A subfield of quantum metrology is quantum phase estimation, where a system is probed by an optical field and information about said system is extracted by subsequent estimation of the optical phase. Classical phase estimation systems are limited by the quantum mechanical nature of light, leading to a measurement sensitivity which scales as ∼1/√ N  , where N is the number of photons in the optical field.

Using quantum entangled states like the famous NOON states, allows for theoretical performance beyond this classical limit with sensitivities scaling as  ∼ 1/N . Entangled states are, however, very diffcult to generate and extremely fragile making them quite impractical in a real world implementation.

In this thesis, I will be working with quantum mechanical states of light called squeezed states. The thesis will go into great detail of the theoretical and practical aspects of squeezed light, explaining how optical loss and phase noise in the squeezed quadrature measurement are the two main limitations to generating high degrees of squeezed light.

The thesis will cover the construction of a small footprint squeezed light source and the initial attempt at building an extremely high performing squeezed light source.  The main results of the thesis come in the form of two phase estimation experiments. In the first experiment, squeezed vacuum states of light are shown both theoretically and experimentally to not only break the classical limit, but also give better performance than the before mentioned NOON states.

An important figure of merit for phase estimation experiments is the Fisher Information per photon, and in our experiment we reach 15:8(6) rad-2 , which to the best of our knowledge is higher than has ever been measured in other phase estimation systems. Finally, as an extension of the estimation protocol, the squeezed vacuum states of light are also used to sense a small 3 kHz sine wave phase modulation.

The second experiment implements a variational quantum algorithm to optimize a general squeezed state probe for phase estimation in noisy, practical systems. The preliminary results presented in this thesis seem to suggest that the addition of displacement in the case of thermal noise could lead to better performance, but certain problems in the algorithm together with phase instability of the experiment, means that the performance of the system will have to be improved, before this hypothesis can be fully tested.