MSc theses

We always welcome interested Master students to do their thesis in our group. With your thesis project you will usually be part of a larger team effort, working with PhD students and postdocs at the forefront of quantum photonics research with state-of-the-art equipment and methods.

You will also learn about the theory of quantum-optical phenomena and semiconductor physics. The experience gained in these subjects forms an excellent basis for future industrial or academic careers. Please contact us via email and visit us (Gregor WeihsRobert Keil, Stefan Frick, Vikas Remesh) for an introduction to our work and labs. Here are some exemplary potential thesis topics: 

Born's rule and quaternion quantum mechanics

Three slit experiment - IQC

With multipath interference experiments using single photons we are investigating whether higher-order interferences might occur in nature or how accurately we can exclude them. Regular quantum mechanics only shows pairwise interference terms, due to Born's rule. Multipath interference experiments could also reveal if nature were described by quaternion quantum mechanics, because then the phase differences in an interferometer need not sum to zero.

In your MSc thesis you would work on one or more aspects of our experiment, which uses a SiN photonics integrated circuit, a source of heralded single photons and superconducting nanowire single photon detectors. Open questions that could be tackled in a thesis are about the nonlinearity and dead time of the detectors in connection with the single photon source. Is there a way to reduce the repetition rate of the source, so that the dead time will become irrelevant? 

  • T. Kauten, R. Keil, T. Kaufmann, B. Pressl, Č. Brukner, and G. Weihs, Obtaining tight bounds on higher-order interferences with a 5-path interferometer, New J. Phys. 19, 033017 (2017). http://doi.org/10.1088/1367-2630/aa5d982.
  • U. Sinha, C. Couteau, T. Jennewein, R. Laflamme, and G. Weihs, Ruling Out Multi-Order Interference in Quantum Mechanics, Science 329, 418 (2010). http://doi.org/10.1126/science.1190545 

High-efficiency semiconductor waveguides

Wellenleiter LaserIn our research group we spontaneous parametric down-conversion in AlGaAs waveguides to create entangled photon pairs at the telecommunication wavelength of 1550 nm [1]. One of the goals is to make this process as efficient of possible by improving several aspects: the structure design can be improved for better nonlinear interaction and easier fabrication, fabrication steps such as electron-beam or optical lithography and plasma etching should be optimized to produce smoother waveguides, and finally the losses occuring in the coupling in and out of the waveguides should be reduced by antireflective coatings or waveguide tapers or grating couplers. In you MSc thesis project you would work on one or more of these goals in order to eventually create a high-efficiency source of quantum light. All fabrication happens in our clean room, the Quantum Nano Center Tyrol. 

  • H. Chen, S. Auchter, M. Prilmüller, A. Schlager, T. Kauten, K. Laiho, B. Pressl, H. Suchomel, M. Kamp, S. Höfling, C. Schneider, and G. Weihs, Time-bin entangled photon pairs from Bragg-reflection waveguides, APL Photonics 3, 080804 (2018), https://doi.org/10.1063/1.5038186

Boosting quantum dot photon extraction efficiency

Semiconductor quantum dots (QD) are excellent sources for generating single photons and entangled photon pairs. Since they are typically grown in a high refractive index host medium, due to total internal reflection, only a tiny fraction of the emitted light can be collected by an external objective lens that lies in vacuum. Embedding QD in a planar DBR microcavity improves the efficiency a bit. Alternatively, photonic nanostructures that provide Purcell enhancement and directional emission, offer significant improvement in collection efficiency. In this Master's thesis work, the student would work on either (1) fabricating circular Bragg gratings (in collaboration with our partners in Linz) on top of the QD crystal or (2) employ solid immersion lenses on a QD sample with modified cavity designs to investigate the collection efficiency enhancement. The improvement in collection efficiency will be verified by an experiment where a stream of single photons from a QD is routed into four different outputs by fast electro-optic modulators which are then fed into a waveguide interferometer.

  • Barnes, W. L., et al. "Solid-state single photon sources: light collection strategies." The European Physical Journal D-Atomic, Molecular, Optical and Plasma Physics 18.2 (2002): 197-210. https://link.springer.com/article/10.1140/epjd/e20020024
  • Liu, J., Su, R., Wei, Y. et al. A solid-state source of strongly entangled photon pairs with high brightness and indistinguishability. Nat. Nanotechnol. 14, 586–593 (2019). https://doi.org/10.1038/s41565-019-0435-9
  • Kolatschek, Sascha, Cornelius Nawrath, Stephanie Bauer, Jiasheng Huang, Julius Fischer, Robert Sittig, Michael Jetter, Simone Luca Portalupi, and Peter Michler. "Bright Purcell enhanced single-photon source in the telecom O-band based on a quantum dot in a circular Bragg grating." Nano Letters (2021). https://pubs.acs.org/doi/10.1021/acs.nanolett.1c02647
  • Yao, Beimeng, et al. "Design for Hybrid Circular Bragg Gratings for a Highly Efficient Quantum-Dot Single-Photon Source." Journal of the Korean Physical Society 73.10 (2018): 1502-1505. https://link.springer.com/article/10.3938/jkps.73.1502

Multi-photon interference

When two or more identical photons impinge on a beam-splitter from different sides, then they won't leave the beam-splitter independently but always together through one of the two possible outputs. This two-photon interference phenomenon (known as Hong-Ou-Mandel effect [1]) is a consequence of the bosonic properties of the photons and the symmetry of the beam-splitter. Even though no physical forces act between the two photons, one can create an effective interation with this method and apply it in quantum information processing. Recently we were able to prove that this effect can be generalized to arbirary numbers of photons and beam-splitter in- and outputs [2].

Mehrphotoneninterferenz

Your master theses can be in theory or experiment. On the theory side you could look into the question of whether some effects that are not captured by the general description [2] can be reconciled with the same. In the experiment you could investigate certain multiport beam-splitters that are realized as waveguide arrays as shown in the figure (see also [3]) and send entangled photon pairs instead of independend single photons into the network to verify a true four-particle phase through the interference pattern.

  1. K. Hong, Z. Y. Ou, and L. Mandel, "Measurement of Subpicosecond Time Intervals between Two Photons by Interference," Phys. Rev. Lett. 59, 2044 (1987), https://doi.org/10.1103/PhysRevLett.59.2044.
  2. C. Dittel, G. Dufour, M. Walschaers, G. Weihs, A. Buchleitner, and R. Keil, "Totally Destructive Many-Particle Interference," Phys. Rev. Lett. 120, 240404 (2018), https://doi.org/10.1103/PhysRevLett.120.240404.
  3. J. Münzberg, C. Dittel, M. Lebugle, A. Buchleitner, A. Szameit, G. Weihs, R. Keil, "Symmetry allows for distinguishability in totally destructive many-particle interference", PRX Quantum 2, 020326 (2021), 10.1103/PRXQuantum.2.020326

Magnetoluminescence spectroscopy of single quantum dots- measuring the g-factor

Semiconductor quantum dots (QD) are potentially the best candidates to realize entangled photon pairs. In a QD, this is usually achieved by the excitation of the biexciton state, from which a spontaneous decay results in a pair of entangled photons Consequently, a deterministic preparation of the biexciton state is an important requirement, for which several optical excitation schemes exist (above-band gap excitation, two-photon resonant excitation, etc.). In this project, we investigate an alternate scheme, based on the optically inactive dark-exciton state with the help of an external magnetic field. As part of this Master’s thesis project, the student will perform the initial magneto-optical characterization of GaAs/AlGaAs quantum dots and measure the electron-hole g-factor. The student would work with Ph.D. student(s) and a postdoc, to build an optical setup, perform experiments and data analysis, and if necessary, do some numerical simulations.

 

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