PhD defence by Thjalfe Ulvenberg

Abstract

Light carries the world’s data, but many promising quantum light sources emit at colors that do not travel well in standard telecom fibers. This thesis develops an all-fiber method to translate the color (wavelength) of single photons while keeping their quantum information intact. The core idea is an optical “mixer” called Bragg-scattering four-wave mixing: two laser tones and a single photon interact inside a glass fiber, so the photon emerges at a new color.

Unlike most approaches that keep all light in the same shape, this work mixes light in different physical spatial distributions (different spatial patterns, or modes) inside a multimode fiber, which makes the process broadly tunable and efficient. Bringing quantum signals onto telecom networks matters because it enables communication security guaranteed by the laws of physics (quantum key distribution) and lays groundwork for future distributed sensing and computation.

Real fibers are not perfectly uniform, so the thesis introduces a practical model for tiny diameter variations along the fiber and shows how to choose wavelengths and operating points that make the conversion resilient to these imperfections.

Experiments first verify wide, controllable color shifts with bright light and then demonstrate low-noise conversion of true single photons from a semiconductor quantum-dot source, preserving their single-photon character.

Together, these results point toward more flexible quantum networks: diverse quantum emitters can be linked to low-loss telecom infrastructure, and color-mismatched systems can be connected reliably. The same concepts can also benefit classical sensing and signal-processing tasks that need precise, low-noise wavelength translation.

Supervisors

• Principal supervisor: Professor, Karsten Rottwitt, Department of Electrical and Photonics Engineering, DTU
• Co-supervisor: Senior Researcher Lars Søgaard Rishøj, Department of Electrical and Photonics Engineering, DTU
• Co-supervisor: Associate Professor Michael Galili, Department of Electrical and Photonics Engineering, DTU
  
  

Evaluation Board

• Examiner: Professor Magnus Karlsson, Chalmer University of Technology, SEExaminer: Professor, Christophe Peucheret, University of Rennes, Fr

Chairman

• Associate Professor, Peter Tidemann-Lichtenberg, Department of Electrical and Photonics Engineering, DTU

Master of the Ceremony

• TBA, Department of Electrical and Photonics Engineering, DTU