To ensure future proof security against quantum computers, quantum key distribution (QKD) is at the forefront of nascent technology. The Quantum Nano Photonics lab at AlbaNova University Center and Ericsson lab in Kista is connected by a telecom fiber, which is used to send entangled photons in different polarization states as part of a QKD setup. In the current setup, two lasers are used as reference when stabilizing the polarization of the light, since linearly polarized light can be measured in two different bases. This project has explored the possibility of reducing the number of lasers from two to one, by using a fast motor that rotates a half-waveplate, which in turn switches the basis of the polarization of the laser.

A stepper motor has been used for this purpose, driven by a stepper motor driver, a CNC shield, as well as an Arduino UNO microcontroller. To be able to place the motor in an optical setup, a mounting bracket and waveplate adapter was designed and manufactured. A half-waveplate was utilized to change the polarization of the incoming light. When the aforementioned parts had been composed, an optical setup including the motor was constructed, where the motor's speed and repeatability was tested. The speed of both a $360^\circ$ and a $45^\circ$ rotation was tested. To test the repeatability, the motor was rotated $45^\circ$ back and forth $1000$ times, and the resulting angles were measured to see if the motor missed any steps.

The speed of $45^\circ$ rotations was on average $24.3$ ms, and for $360^\circ$ rotations it was on average $179$ ms. During the $1000$ rotations of $45^\circ,$ the azimuth angle of the state of polarization of the light had not varied with more than 0.33$^\circ$ for one of the positions, and $0.37^\circ$ for the other, which is within the margin of error of the polarimeter.

It was noticed that the motor sometimes unpredictably takes a few steps when initialized, which is a problem that needs fixing to be able to implement the motor in the QKD setup. Apart from that, the results were satisfactory, with a speed and repeatability that is acceptable for use in the QKD setup. Since the torque of the motor is limiting when it comes to speed, building a waveplate adapter with lower moment of inertia could be for future projects to explore.