Self-aligning Fiber-based Time-bin Qubit Decoder

Sammanfattning

In our connected society, there is a need for secure information channels where sensitive data can be confidentially transferred between two or more parties. Quantum communication is a field of research that use techniques from quantum mechanics to create secure information channels based on the fundamental laws of physics. By encoding bit values to quantum states of a wavefunction, creating a qubit, it is physically impossible for an eavesdropper to listen on that information without affecting the wavefunction and open herself up for detection. One popular technique for encoding qubits is called time-bin encoding, which uses different arrival times of photons as the different quantum states. Time-bin encoding have many attractive properties for quantum communications over fibers, especially since it shows good compatibility with existing telecommunication infrastructure. However, to read and decode a time-bin encoded signal with good clarity the decoder regularly needs to be carefully aligned, which is a time-consuming and tedious process. This thesis explores implementing a time-bin decoder based on an unbalanced Mach-Zehnder interferometer with a self-aligning algorithm that automatically aligns the system with the press of a button, requiring no further calibration from the user. Using this self-aligning algorithm an interferometric visibility of 90-95% have been achieved with no noticeable drift of the alignment over a 12-hour measurement period.