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: 

Testing Born's rule and quaternion quantum mechanics in a waveguide interferometer

Three path waveguide chip

The Born rule dictates for quantum mechanics that only pairwise interference can arise in multipath interferometers and no higher-order interference from the combination of multiple paths in a single term can exist. We test this prediction with single photons and measure how accurately one can exclude such higher-order interferences. The same experiments could also reveal if nature were described by quaternion quantum mechanics, because then the phase differences in the interferometer need not sum to zero.

In your MSc thesis you would work on one or more aspects of our experiment, which currently uses a SiN photonics integrated circuit (see picture). Open questions that could be tackled in a thesis are about thermal crosstalk between individual phase shifters on the chip as well as other error sources limiting the interference contrast. After mitigation of some errors, experiments with heralded single photons can be performed.

  • S. Gstir, Waveguide Interferometers for Fundamental Investigation of Quantum Mechanics, PhD thesis (2023)
  • 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).
  • U. Sinha, C. Couteau, T. Jennewein, R. Laflamme, and G. Weihs, Ruling Out Multi-Order Interference in Quantum Mechanics, Science 329, 418 (2010). 

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.

Simultaneous generation of multiple entangled photon states from quantum dots

Semiconductor quantum dots (QDs) are bright and efficient sources of high-purity single photons and entangled photon pairs. In QDs, entangled photons states are generated through from a two-photon laser excitation of the so-called biexciton states. If the laser pulses are chirped, it provides a robust method to generate entangled photon states from multiple, spectrally distinct quantum dots.

In our recent works, we demonstrated robust techniques [1,2] to prepare chirped laser pulses and established high-efficiency generation of single-photon states. In this Master's thesis work, we will extend this work, to (a) develop a state-of-the art excitation scheme using chirped pulses [1], characterize them [3], to target vertically stacked quantum dots in a nanowire (see [4,5]), and then (b) establish and quantify the generated entangled photon states.

  1. V. Remesh et al., Compact Chirped Fiber Bragg Gratings for Single Photon Generation from Quantum Dots, arXiv preprint, 2023, 
  2. F. Kappe et al., Collective Excitation of Spatio-Spectrally Distinct Quantum Dots Enabled by Chirped Pulses, Mater. Quantum. Technol. 3, 025006 (2023),
  3. Fan et. al.,  Measurement of the chirp characteristics of linearly chirped pulses by a frequency domain interference method."  Opt. Express 21, 13062 (2013):
  4. Laferriere, et al. Multiplexed single-photon source based on multiple quantum dots embedded within a single nanowireNano Lett. 20, 3688 (2020),
  5. M. Khoshnegar, et al. A solid state source of photon triplets based on quantum dot moleculesNature Commun. 8, 15716 (2017).

Anti-reflection coatings for Bragg reflection waveguides

Bragg reflection waveguides made from AlGaAs are a promising avenue to exploit the high second-order non-linearity of this material. Processes relying on this material parameter include all three-wave mixing phenomena, such as second-harmonic generation, sum-frequency generation, difference-frequency generation and spontaneous parametric down-conversion.

For all of these processes, but especially for spontaneous parametric down-conversion, high transmittivity is favourable and increases the efficiency with which the conversion of light occurs. While losses inside the waveguide are mainly determined by imperfections occurring during the manufacturing process, reflections at the end facets cannot be avoided.

The high refractive index contrast between the waveguide and air causes reflections at this interface which should be mitigated using anti-reflection coatings. This thesis requires the design of anti-reflection coatings for telecom wavelengths using the transfer matrix method, as well as the manufacturing of these coatings on the waveguide facets using sputter deposition in the  university clean room. Subsequently, the quality of these coatings needs to be characterized in the optics lab and evaluated towards the increased efficiency of non-linear processes.

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.

Enhanced multiphoton state generation from quantum dots

To realize a quantum dot-based photonic quantum information processing system in future, the essential building blocks are efficient quantum dot sources which provide high-purity single photons at a high rate, which can then be spatio-temporally demultiplexed [1]. To enhance the multiphoton rate in such a system, we have multiple options as we have demonstrated in our recent works: (a) enhance the quantum dot brightness via photonic cavity structures [2], and (b) develop efficient excitation schemes to generate high-purity photon states in tailored polarization basis with high coherence and indistinguishability [3,4].

In the proposed Master's thesis work, you will extend these works [2,3,4] and develop a high-repetition rate excitation method to demonstrate enhanced multiphoton rate from a photonic cavity-quantum dot structure. 

  1.  J. Münzberg, F. Draxl, S. F. C. da Silva, Y. Karli, S. Manna, A. Rastelli, G. Weihs, and R. Keil, Fast and efficient demultiplexing of single photons from a quantum dot with resonantly enhanced electro-optic modulators, APL Photonics 7, 070802 (2022),
  2. M. Weinreich, Circular Bragg gratings as high-brightness photonic cavities for quantum dots, Master's thesis, 2022-23
  3. Y. Karli, D. A. Vajner, F. Kappe, P. C. A. Hagen, L. M. Hansen, R. Schwarz, T. K. Bracht, C. Schimpf, S. F. C. da Silva, P. Walther, A. Rastelli, V.M. Axt, J. C. Loredo, V. Remesh, T. Heindl, D.E. Reiter, and G. Weihs, Controlling the Photon Number Coherence of Solid-state Quantum Light Sources for Quantum Cryptography, arXiv preprint, 2023,
  4. V. Remesh, R. G. Krämer, R. Schwarz, F. Kappe, Y. Karli, M. P. Siems, T. K. Bracht, S. F. C. da Silva, A. Rastelli, D.E. Reiter, D. Richter, S. Nolte, and G. Weihs, Compact Chirped Fiber Bragg Gratings for Single Photon Generation from Quantum Dots, arXiv preprint, 2023, 

Aluminium Gallium Arsenide on Insulator

Aluminium Gallium Arsenide (AlGaAs) is highly non-linear material which, when combined with guiding light inside nano-fabricated structures, promises to be a powerful platform for the generation of quantum light and applications in quantum  information processing. Typically, AlGaAs is grown on a substrate of Gallium Arsenide (GaAs) which is of higher refractive index than AlGaAs. Bragg-reflection waveguides are one possibility to guide light inside AlGaAs, on a GaAs substrate. A drawback of this approach is the high complexity of the structure which is required to realise the Bragg-reflectors. AlGaAsoI on the other hand is a promising alternative to this approach. Here, AlGaAs is brought onto an insulating substrates, such as silica, with a flip-chip process.

The aim of this project is to characterise newly fabricated AlGaAs on insulator samples in the lab and estimate characteristic parameters such as the loss of the waveguides and the non-linear coefficient.

Multi-photon interference for quantum sensing

When two identical photons impinge on a beam-splitter from different sides, they don'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-sSagnac Sourceplitter. This effect and its extensions to more particles (multi-photon interference) crucially rely on the indistinguishability of the photons, that is, that their internal properties, such as polarisation and spectrum, are identical or at least overlap. We could demonstrate very recently via entangled four-photon states generated in Sagnac interferometers (see image) that not only the magnitude of this overlap has an influence on the resulting interference, but also its phase - amouting to what is known as the collective phase of the particles [2,3].

In your master thesis you would investigate how one could use that collective phase for quantum sensing, that is the measurement of a phase shift with higher precision than possible with classical light of the same wavelength. In order to achieve that you would set up a second Sagnac-based photon-pair source in parallel to an already existing one and then probe an interferometer with variable delay with the so generated four-photon states.

  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),
  2. A. E. Jones , A. J. Menssen , H. M. Chrzanowski, T. A. W. Wolterink , V. S. Shchesnovich, and I. A. Walmsley, "Multiparticle Interference of Pairwise Distinguishable Photons", Phys. Rev. Lett. 125, 123603 (2020),
  3. T. Faleo, E. Brunner, J. Webb, C. Dittel, G. Weihs, G. Dufour, A. Fedrizzi and R. Keil, "Entanglement-induced collective multiparticle interference," (to be published).

Synthesis of square-shaped, picosecond laser pulses (Nanofabrication/ Optics) 

The main goal of this project is to generate square shaped (in time domain) picosecond pulses from a conventional Gaussian/sech2 shaped femtosecond or picosecond laser pulses. Such pulses have important applications in our ongoing work towards the development of advanced excitation methods for quantum dots [1-2]. There are several methods to generate such square shaped pulses, of which in this thesis, we will explore (a) fabricate a phase mask by etching fused silica in various thicknesses to impart optical phase delay (specifically as linear 0-pi-0-pi- phase mask, with etch depth such that there is a pi phase for ~ 800 nm [3], and at a later stage, (b) pulse-shaping approaches employing a spatial light modulator or a grating stretcher [4]. 

In this project, the student would ideally have a basic experience, or willingness to learn nanofabrication tools like e-beam lithography and etching. Later part of the work will also involve experiments in the laser laboratory.  

Please approach for further information. 

 [1] M. Neumann, F. Kappe, T. K. Bracht, M. Cosacchi, T. Seidelmann, V. M. Axt, G. Weihs, and D. E. Reiter, Optical Stark shift to control the dark exciton occupation of a quantum dot in a tilted magnetic field, Phys. Rev. B 104, 075428 (2021),

[2]  Y. Karli, F. Kappe, V. Remesh, T. K. Bracht, J. Münzberg, S. F. C. da Silva, T. Seidelmann, V.M. Axt, A. Rastelli, D.E. Reiter,  and G. Weihs, SUPER Scheme in Action: Experimental Demonstration of Red-detuned Excitation of a Quantum Dot,  Nano Lett. 22 6567 (2022), 

[3] Weiner, A. M., Heritage, J. P., & Thurston, R. N. Synthesis of phase-coherent, picosecond optical square pulsesOptics letters, 11 153 (1986),

[4] Lozovoy, V. V., Rasskazov, G., Ryabtsev, A., & Dantus, M. (2015). Phase-only synthesis of ultrafast stretched square pulses. Optics Express, 23, 27105 (2015),



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