Spin waves (magnons) are the quantized excitations of the magnetic lattice in solid state systems. The field of magnonics is exploring concepts to use these magnons for information transport and processing. Of particular interest is to achieve non-reciprocity for opposite spin wave propagation directions, which can be realized in hybrid structures of a periodic artificial magnetic array on top of a magnonic waveguide. These systems would be potential candidates for compact microwave directional couplers and circulators operational at low temperatures. The goal of this thesis is to develop and optimize such nonreciprocal devices based on periodic magnetic arrays. This implementation is a first step towards compact low temperature microwave circuits relevant for superconducting quantum circuits.You are a resourceful master student willing to contribute with your thesis towards the successful implementation of nonreciprocal microwave devices at cryogenic temperatures. You will use state-of-the-art nanofabrication techniques using electron beam lithography and thin film deposition machines to design your hybrid systems. You will also gain experience in cryogenic microwave spectroscopy utilizing vector network analyzing techniques. Utilizing a combination of numerical and analytical models, you will drive the optimization of such hybrid devices.

Contact: Stephan Geprägs (stephan.gepraegs@wmi.badw.de)

Nano-mechanical strings are archetypical harmonic oscillators and can be straightforwardly integrated with other nanoscale systems. For example, the field of nano-electromechanics studies the coupling of nano-strings to microwave circuits, which resulted in the creation of mechanical quantum states and concepts for microwave to optics conversion. Here, we plan to investigate an alternative hybrid system based on ferromagnetic nanostructures integrated with nano-strings or nano-mechanical platforms. These hybrid devices aim at the efficient conversion between phonons and magnons with the potential to interact with light and are thus ideal candidates for conversion applications. We are looking for a motivated master student for a nano-mechanical master thesis in the context of magnon-phonon interaction. The goal of your project is to investigate the static and dynamic interplay between the mechanical and magnetic properties of a nano-mechanical system sharing an interface with a magnetic layer. In your thesis project, you will fabricate freely suspended nanostructures based on magnetic thin films using state-of-the-art nano-lithography and deposition techniques. Further, you will probe the mechanical response of the nano-structures using optical interferometry while exciting the magnetization dynamics of the magnetic system.

Contact: Hans Huebl (hans.huebl@wmi.badw.de)

The interplay between magnetism and topology makes magnetic topological insulators an interesting platform to investigate controllable topological phase transitions and emerging physical states such as quantum anomalous Hall states and Weyl semimetal phases. In these topological insulators, the long-range magnetic order breaks the time-reversal symmetry and causes an exchange gap in the otherwise gapless surface states, which gives rise to the so-called quantum anomalous Hall effect (QAHE), i.e., a quantized Hall conductance at zero magnetic field. In the framework of this thesis, we will fabricate MnBi_2nTe_3n+1 thin films and investigate their structural, magnetic and electrical transport properties. The material system Mn-Bi-Te is of particular interest, since it exhibits a rich magnetic and topological phase diagram. In your thesis, you will fabricate thin films of magnetic topological insulators using our new molecular beam epitaxy (MBE) setup. You will then investigate their structural and magnetic properties as well as probe the quantum anomalous Hall effect by magneto-transport experiments. Your thesis will contain different physical vapor deposition methods as well as a variety of different techniques to characterize thin film devices. 

Contact: Stephan Geprägs (stephan.gepraegs@wmi.badw.de)

The rare earth spin ensembles are well established by now in the optical domain where the microwave states are used as an intermediate state to extend the storage time [1] and offer a great potential for storage of microwave quantum states. The possibility to store such microwave states in rare earth spin ensembles paves the way towards a secure quantum microwave communication with quantum memory. Number of purely microwave manipulations by spin ensembles is very limited and is bound to coupling of spin ensembles to microwave resonating structures [2], which allows amplifying the microwave signal and enhancing the interaction between the ions and the microwave field. The main disadvantage of using these resonating structures is their fixed frequencies and very small tuning range. Typically fabricated in a coplanar design, the superconducting resonators create strongly inhomogeneous distribution of the field within the spin ensemble, which results into largely detuned Rabi frequencies experienced by the spins.

Aim of this project is to fabricate novel design of microwave transmission line, which would work in a broadband regime and will thus allow to couple to rare-earth spins at a larger bandwidth.  This will allow realizing various spin manipulation schemes, which involve more than two energy levels (beyond Hahn-echo) and thus deploy complex spin-manipulation techniques.

We are looking for a highly motivated master student joining this project. Within the project, you will gain hands-on experience on design and fabrication of superconducting microwave structures. You will design and fabricate superconducting resonating structure, which will then be tested at cryogenic conditions when coupled to rare earth spins ensembles.

[1] Kinos, A. et al. Roadmap for Rare-earth Quantum Computing. arXiv 2103.15743 (2021).
[2] Ranjan, V. et al. Multimode Storage of Quantum Microwave Fields in Electron Spins over 100 ms.

Contact: Nadezhda Kukharchyk

The rare earth spin ensembles are well established by now in the optical domain where the microwave states are used as an intermediate state to extend the storage time [1] and offer a great potential for storage of microwave quantum states. The squeezing and displacement of microwave states also offers a possibility to encode information in a quantum secure way. The possibility to store such microwave states in rare earth spin ensembles paves the way towards a secure quantum microwave communication with quantum memory.

Aim of this project is to study and characterize the interaction of the rare earth spin ensembles with propagating microwave signals via superconducting transmission line. Dependence of the interaction efficiency will be analyzed based with respect to the squeezing and displacement amplitudes of the microwave states.

We are looking for a highly motivated master student joining this project. Within the project, you will gain knowledge in experimental techniques, modern cryogenic setup and data analysis approaches.

[1] Kinos, A. et al. Roadmap for Rare-earth Quantum Computing. arXiv 2103.15743 (2021).
[2] Ranjan, V. et al. Multimode Storage of Quantum Microwave Fields in Electron Spins over 100 ms.

Contact: Nadezhda Kukharchyk

Silicon-based devices are deeply integrated into our todays information and communication technology. Therefore, silicon is one of the most widely used materials and using silicon in quantum information devices is an intriguing task, Of particular interest is deploying it as a host material for spin ensembles, e.g. erbium electronic spins. While erbium ensembles in silicon have already been studied optically, the study in the microwave domain with electron spin resonance techniques is fully missing for these ensembles. Within this thesis, a broadband spin resonance spectroscopy will be carried out on erbium spins implanted in silicon. By the measurement, you will be able to confirm or correct the g-factors for erbium sites, which have been reported in the literature. You will gain hands-on experience with state-of-the-art microwave measurements in the cryogenic environment and data processing steps, including the use of Matlab or Python.

Contact: Nadezhda Kukharchyk, Georg Mair

Silicon-based devices are deeply integrated into our todays information and communication technology. Therefore, silicon is one of the most widely used materials and using silicon in quantum information devices is an intriguing task, Of particular interest is deploying it as a host material for spin ensembles, e.g. erbium electronic spins. While erbium ensembles in silicon have already been studied optically, the study in the microwave domain with electron spin resonance techniques is fully missing for these ensembles. Within this thesis, a broadband spin resonance spectroscopy will be carried out on erbium spins implanted in silicon. By the measurement, you will be able to confirm or correct the g-factors for erbium sites, which have been reported in the literature. You will gain hands-on experience with state-of-the-art microwave measurements in the cryogenic environment and data processing steps, including the use of Matlab or Python.

Contact: Nadezhda Kukharchyk, Ana Strinic

An up-to-the-minute list of bachelor's theses offered at WMI/TUPHE23 can be found on TUM Moodle.