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 

 

Quantum rangefinding

Rangefinding has a broad field of applications in the defence and public sectors. However, state of the art rangefinders rely on few photons being reflected from a target which originally stem from a bright laser beam aimed at the target. The brightness and unique spectral signature of lasers make them detectable for the target.

Quantum rangefinding is a quantum protocol using the single photons from a spontaneous parametric down conversion (SPDC) source, which enables efficient camouflaging against background light. Keeping one mode of the two-mode squeezed state locally at the transmitter and illuminating the target with the other only transmits perfect noise, indistinguishable from background light.

The topic of this thesis is to implement a quantum rangefinder using our Bragg reflection waveguides as a source of photon pairs.

Characterisation of fast electronics and optics to pump time-bin entangled spontaneous parametric down-conversion

Spontaneous parametric down-conversion is a process happening in non-linear media where a photon of a pump laser gets destroyed to create a pair of sister photons. If the pump laser is temporally shaped such that it exhibits two subsequent pulses defining an early and late time-bin, photon pairs can either be created early or late. This creates a so called time-bin entangled state. As a result both photons of a pair either exist in the early or late time-bin at the same time.

To generate these fast laser pulses our group has engineered fast electronics capable of directly modulating laser diodes. The scope of this thesis is to characterise the optical response of different laser diodes to the drive current and to analyse the temporal shape of the produced laser light. Subsequently, short pulses should be coupled to non-linear Bragg reflection waveguides and the resulting time-bin entangled state needs characterisation towards its concurrence and fidelity.

Wellenleiter Laser


Hybrid integration of Bragg reflection waveguides

Bragg reflection waveguides in Aluminium Gallium Arsenide produce interesting and useful quantum states of light. Spontaneous parametric down-conversion, occurring in these structures, yields always only pairs of photons. These type of sources have applications in quantum communication and information processing, since both photons are essentially created at the same time and can be created in an entangled state. Bragg reflection waveguides are advantageous in myriad ways when compared to commonly used  down-conversion sources implemented with bulk non-linear crystals. Most prominently they promise integration of a complete photon-pair source on only a few millimeters of waveguide length, whereas bulk-sources can require meters of optical path lengths. This claim, however only holds true if the optical access, necessary to inject pump light into the waveguides and to collect photon pairs from the waveguides, is achieved in a scalable and integrable way.

During the course of this master thesis we want to investigate different ways to achieve such optical connections to and from our waveguides. Firstly, direct fibre butt coupling to the waveguides’ end-facets will be evaluated. Fibre butt coupling involves the fine alignment of fibre modes to waveguide modes and the development of a reliable procedure for packaging. Secondly,
direct coupling of laser diodes to waveguides will be explored. Similar to the fibre approach this again involves advanced optical coupling techniques together with the development of reliable packaging methods.

Difference frequency generation from quantum dots and a long wavelength pump

Single photon sources at the telecom wavelength range are a long standing goal in the science community. The telecom wavelength range is especially attractive for quantum communication since substantial classical communication infrastructure is readily available.

Quantum dots are arguably the most proficient single photon sources today and are extensively used for this purpose in quantum optics laboratories around the world. Unfortunately, quantum dot systems which are easy to use and manufacture, emit single photons at wavelengths in the near-infrared wavelength range. A possible solution to this caveat is difference frequency generation, where a single photon from a quantum dot together with a short-wave-infrared pump laser is frequency converted to the telecom wavelength in a non-linear process.

This projects involves alignment of advanced laser systems to generate the needed short-wave-infrared pump beam. Afterwards, frequency conversion should be characterised in bulk crystals, first with classical laser light and ultimately with single photons from quantum dot sources.

  • A Schlager et al. Difference-frequency generation in an AlGaAs Bragg-reflection waveguide using an on-chip electrically-pumped quantum dot laser. Journal of Optics 23 (2021), 085802. https://doi.org/10.1088/2040-8986/ac13ae

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

Boosting quantum dot photon extraction efficiency (Experimental/ Programming)

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.

This project offers two major directions:

(1) setting up an automated QD positioning system based on widefield imaging to identify locations of bright QDs ,

(2) designing and fabricating circular Bragg gratings and metallic nanorings (in collaboration with our partners in Linz) around QDs 

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

Repetition rate multipliers (Experimental)

In order to realize QD-based multi-qubit photonic quantum information processing, one desires high brightness QDs with higher photon rates, which can then be demultiplexed. The latter can be achieved either via employing clever cavity designs which provide Purcell enhancement, supported by high efficient photon extraction methods (see the thesis topic above) or by employing higher repetition rate lasers: the more you pump the QD source, the more photons you get out. Most common solid-state, short-pulse width lasers have their repetition rates around 80 MHz, which, once the laser is built, cannot be altered without changing the cavity design. An alternate, cost-effective solution for multiplying the repetition rate is to employ an externally coupled cavity. In this project, the student will design and realize a passive repetition rate multiplier to generate laser pulse trains at a higher rate (at first aiming at 3x the repetition rate). This will be then tested by sending the laser pulses to excite quantum dots, achieving a higher photon rate. At a later stage of the project, the student would investigate alternate schemes that employ fiber Bragg gratings and/or compare the performance of the setup with active repetition rate multiplier methods developed earlier in the group.

Magnetoluminescence spectroscopy of single quantum dots- measuring the g-factor (Experimental)

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.

 

Design of quantum dot nanopillar structures to study collective effects (FDTD simulations)

Photosynthesis, the largest energy-collecting, transferring and trapping process in nature, is mediated by an elegant architecture of ring-shaped molecular structures called light harvesting complexes (LHCs). Recent theoretical investigations pointed out that collective subradiant modes in these systems might be playing a key role in mediating the efficient energy trapping. However, experiments on LHCs are extremely challenging due to their low photostability and quantum efficiency. Quantum dots (QD) are bright, photostable point emitters to realize a simulator system to study such effects. In this project, the student will use a commercial Finite Difference Time Domain (FDTD) solver (Lumerical) to design a quantum dot nanopillar structure that mimics a simple LHC molecule, considering parameters like the nanowire and ring diameters, refractive index, radiative rate enhancement/suppression, and Q factor to characterize the subradiant modes in the system. Based on the optimized simulation, the sample will be fabricated by our collaborators and will be used in our experiments. 

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