Phase stabilization of field-distributed quantum states for fiber-based continuous-variable QKD protocols

Abstract

Squeezed light is a key resource that exhibits quantum correlations and is subject of current research and application. First described in 1970, squeezed light is used today in gravitational wave detectors and is being researched and developed for fields such as quantum sensing, quantum communication, and photonic quantum computers. By overlapping the outputs of two squeezed light sources, entangled states are created that exhibit quantum correlated results of quadrature measurement. Together with the no-cloning theorem, these two properties form a basis for the realization of continuous-variable quantum key distribution. The potential key rate is proportional to the bandwidth of the generated squeezed states. For this reason, monolithic squeezed light resonators with GHz bandwidth were chosen for this work. The focus of this dissertation is on the distribution and phase stabilization of squeezed vacuum states and entangled (two-mode squeezed) states, which were distributed over a noisy fiber channel of one kilometer length to another building. To compensate for environmental disturbances, I developed and implemented control loops based on the principle of a quantum noise lock. In a first application, the phase stabilization of squeezed vacuum states after 1 km of fiber transmission was demonstrated. Noise reduction of 4.7 dB at a sideband frequency of 30 MHz and over 2 dB above 1 GHz were measured, and a long-term measurement of five minutes was completed. In a second step, I extended the scheme to include real-time synchronization and subtraction to stabilize the readout phase of spherically symmetric entangled states. Without distribution, a correlated noise reduction of approximately 4 dB was achieved, and approximately 3 dB with states distributed between two laboratories. The results show that the feedback control scheme is suitable for this application and exhibits no fundamental limitations. An extension to approximately 5 km of fiber length is possible, as is implementation inother applications such as distributed fiber sensing.