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MSc Theses – University of Innsbruck

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 Weihs, Robert Keil, Stefan Frick, Vikas Remesh) for an introduction to our work and labs. Here are some exemplary potential thesis topics: 

A receiver for space-borne quantum key distribution

Space-Borne QKD

Fiber based quantum key distribution (QKD) is reaching its limits because of the optical loss persistent in even the highest performing optical cables. A long proposed pathway around this issue is space-borne QKD: A satellite linked through the atmosphere to distant optical ground stations can act as trusted node or a quantum repeater to establish quantum links across thousands of kilometers.

These ground stations need to be equipped with receivers capable of measuring single photons and reconstructing the quantum information carried by them. Deployment of these receivers is often at remote locations far away from the light pollution of civilization. Thus the goal of this theses is to design and implement such a receiver for polarization encoded qubits in a rugged and compact form factor, which can be deployed reliably without maintenance.

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.

 

Optimal generation of high-quality photon states from quantum dots

To operate as an on-demand single-photon source, the quantum dot has to be prepared in its exciton state, which recombines to emit a single photon. The most prominent scheme to this end is the Rabi scheme, where a laser pulse tuned to the transition energy inverts the quantum emitter population. The only way to distinguish the required single photon stream from the excitation laser stream is to employ cross-polarisation filtering, or by employing specially fabricated photonic cavities. Given this context, an alternative scheme has gained recent attention, which off-resonantly excites the quantum dot in a so-called Swing-UP of quantum EmitteR method (SUPER). Following the theoretical proposal (1), the first experimental implementation of this method was demonstrated in our research group (2), confirming nearly unity preparation efficiency. Despite promising initial results on the application of SUPER scheme across various platforms, several open questions remain unexplored. In this project, we will explore the measured single-photon indistinguishability at various range of pulse-pair detunings in the SUPER scheme. To this end, the student will modify the existing 4f-pulse shaper equipped with a programmable SLM, create phase-amplitude shaped pulse pairs and excite a quantum dot and finally investigate the indistinguishability of the generated photon states in a wide range of detunings, and compare with other state-of-the-art excitation schemes. 

(1) PRX Quantum 2, 040354 (2021), https://link.aps.org/doi/10.1103/PRXQuantum.2.040354 

(2) Nano Lett. 22, 6567 (2022) https://pubs.acs.org/doi/10.1021/acs.nanolett.2c01783

(3) Adv Quantum Technol. 7, 2300359 (2024) https://onlinelibrary.wiley.com/doi/full/10.1002/qute.202300359 

 

Deterministic generation of time-bin entangled photon pairs via dark states in a quantum dot

Quantum dots are bright and deterministic sources of single photons and entangled photon pairs–key constituents for building a future quantum network. To operate as an entangled photon pair source, quantum dots have to be excited to the so-called biexciton state, for which the well-known two-photon excitation is the most popular method. Various research works including ours have therefore demonstrated polarization entanglement (1), entanglement-based quantum key distribution (2), time-bin entanglement (3) from quantum dots. Although well-suited for decoherence-free transmission through optical fibers, time-bin entanglement remains largely unexplored in quantum dot research. Our goal is to establish a deterministic route to generate bright time-bin entangled photon pairs from a quantum dot via the so-called dark exciton states. The most important step in this process is the direct optical access of the dark state, which couples only weakly to optical fields. We have recently developed a versatile excitation method (see (4) for details) using magnetic field and chirped laser pulses to achieve this. In the proposed Master thesis project, the student will utilize this method to demonstrate a bright, deterministic and robust method of generating time-bin entangled photon pairs from a quantum dot.

(1) Nano Lett.. 14, 7107 (2014), https://doi.org/10.1021/nl503581d 

(2) Sci Adv 7, eabe8905 (2021), https://www.science.org/doi/full/10.1126/sciadv.abe8905

(3) Nat Commun 5, 4251 (2014) https://doi.org/10.1038/ncomms5251 

(4) arXiv preprint, 2024, https://arxiv.org/abs/2404.10708

 

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] to target vertically stacked quantum dots in a nanowire (see [3,4]), and then (b) establish and quantify the generated entangled photon states.

  1. V. Remesh et al.,  APL Photonics 8, 101301 (2023), https://doi.org/10.1063/5.0164222,  F. Kappe et al, Adv Quantum Technol. 2024, 2300352, https://doi.org/10.1002/qute.202300352
  2. F. Kappe et al., Collective Excitation of Spatio-Spectrally Distinct Quantum Dots Enabled by Chirped Pulses, Mater. Quantum. Technol. 3, 025006 (2023), https://iopscience.iop.org/article/10.1088/2633-4356/acd7c1
  3. Laferriere, et al. Multiplexed single-photon source based on multiple quantum dots embedded within a single nanowire, Nano Lett. 20, 3688 (2020), https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.0c00607
  4. M. Khoshnegar, et al. A solid state source of photon triplets based on quantum dot molecules, Nature Commun. 8, 15716 (2017). https://doi.org/10.1038/ncomms15716

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. https://doi.org/10.1088/2040-8986/ac13ae

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 et al., Fast and efficient demultiplexing of single photons from a quantum dot with resonantly enhanced electro-optic modulators, APL Photonics 7, 070802 (2022), https://doi.org/10.1063/5.0091867
  2. M. Weinreich, Circular Bragg gratings as high-brightness photonic cavities for quantum dots, Master's thesis, 2022-23
  3. Y. Karli et al., Controlling the Photon Number Coherence of Solid-state Quantum Light Sources for Quantum Cryptography, arXiv preprint, 2023, https://arxiv.org/abs/2305.20017
  4. V. Remesh et al., Compact Chirped Fiber Bragg Gratings for Single Photon Generation from Quantum Dots, APL Photonics 8, 101301 (2023), https://doi.org/10.1063/5.0164222 

Pulse shaping approaches for quantum control

Rapid advancements in laser technology in the past decades also gave rise to various methods to control the properties of laser pulses. From a simple intensity modulation to complex spatiotemporal waveform generation, these "pulse shaping” techniques have been phenomenally effective in understanding light-matter interactions. In quantum technologies recently, these pulse-shaping techniques have found renewed interest, for instance in spectrotemporal mode shaping, coherent control of quantum dot states [1-4] and so on.

In this project, the student would utilise a spatial light modulator in a 4f pulse shaper to generate tailored spatiotemporal waveforms to excite quantum dot(s) and investigate the properties of the generated single and entangled photon states.

  1. Gamouras et al. "Simultaneous deterministic control of distant qubits in two semiconductor quantum dots." Nano Lett. 13, 4666 (2013)
  2. Praschan et al. "Pulse shaping for on-demand emission of single Raman photons from a quantum-dot biexciton." Phys. Rev. B 105, 045302 (2022)
  3. Karli et al. "SUPER scheme in action: Experimental demonstration of red-detuned excitation of a quantum emitter." Nano Lett. 22, 6567 (2022)
  4. unpublished and ongoing works, Uni Innsbruck

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.

 

 

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