MSc theses

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.

• Stefan Frick, Alex McMillan, and John Rarity. Quantum rangefinding. Optics Express 28 (2020), 37118. https://doi.org/10.1364/OE.399902

Future quantum networks and communication applications require sending single photons from one party to another.  While distributing photons through optical fibre networks is a strategy that works, it is affected by noisy environments and polarization instabilities. Time-bin encoding is an alternative in such cases. Our pioneering works [1] have demonstrated time-bin entanglement generation from quantum dots, yet the quest for near-unity fidelity and a deterministic generation strategy remains to date. In this work, the student will investigate the generation of time-bin entangled photon states from semiconductor quantum dots using advanced excitation schemes such as SUPER [2], chirped pulse excitation via dark states [3], relying on a combined theoretical and experimental effort.

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

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

(4) Sci. Adv. 11, eadu4261 (2025) https://www.science.org/doi/10.1126/sciadv.adu4261 

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

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.

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

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.

• M Placke et al. Engineering AlGaAs-on-insulator toward quantum optical applications. Optics Letters 454 (2020), 6763. https://doi.org/10.1364/ol.406152
• Minhao Pu et al. Efficient frequency comb generation in AlGaAs-on-insulator. Optica 3 (2016), 823. https://doi.org/10.1364/optica.3.000823

This century is evolving into the quantum era, with applications advancing from single-particle to many-particle effects. At the heart of this evolution lies interference, a phenomenon driving cutting-edge quantum technologies and rooted in the cornerstone of quantum theory—wave-particle duality.

Indistinguishable particles routed through a scattering object can follow multiple paths to form the output state. However, any degree of distinguishability leads to incoherent evolution, degrading collective effects. Decoding and quantifying multi-particle phenomena is not straightforward, and it is not generally known which effects stem from genuine interference. Since the amount of wave character limits the amount of particle character, and vice versa, the indistinguishability of the input particles bounds the interference visibility measures. Prior investigations have looked into this complementary nature through a range of different lenses, including coherence, polarization, entanglement, and asymmetric beam interference. Recent theoretical advances were able to define such quantitative measures (1), however, measuring multi-particle systems presents open challenges in the field.

In this Master’s thesis project, we will investigate how the internal and external degrees of freedom of the involved particles influence quantum interference and wave-particle duality beyond the well-understood single-particle systems. The research will combine theoretical modeling with experimental approaches to uncover new insights into multi-particle phenomena.

(1) PHYS REV X 11, 031041 (2021), https://doi.org/10.1103/PhysRevX.11.031041

For more info see UNAGI project

The generation of single and multi-photon states is a crucial feature of the emerging field of quantum technologies [1]. A number of platforms exist for this purpose, such as trapped ions, neutral atoms, and molecules. Solid-state platforms, such as quantum dots (QDs) offer a number of advantages, however, the single-photon emission from each QD has a different spectral profile [2]. In the lab, successive experiments are usually performed using different emitters, and practical applications are made more complex by having to deal with this variation in emission wavelength and quality. The goal of this project is to develop an automated protocol for mapping samples of randomly distributed QDs. Each emitter would be tagged by position, emission wavelength, and relative intensity , such that a desirable emitter can always be recovered after sample exchange cycles. The methods used will involve cryogenic experiments, micro-photoluminescence measurements (optical excitation of single emitters), and wide-field imaging. If this initial goal is achieved, the project may be extended in many exciting directions. Possibilities include studying distributions within the sample, pulse-shaping to access advanced control schemes [4], single-photon characterisation, and more. 

[1] https://iopscience.iop.org/article/10.1088/2058-9565/aa91bb

[2] https://www.nature.com/articles/s41598-022-10451-1 

[3] https://pubs.acs.org/doi/10.1021/acs.nanolett.2c01783