Development of quantum networks is an exciting research topic in quantum science and technology. Microwave quantum communication is set to play an important role in future quantum networks because of its natural frequency compatibility with superconducting quantum processors and modern communication standards. In order to go beyond confines of cryogenic systems, one has to develop means to couple propagating quantum microwave signals generated with Josephson devices to the external open-air environment. This goal can be achieved by exploiting superconducting antennae performing an impedance-matching between the intrinsic cryostat microwave environment and the external world. A successful implementation of the low-loss cryogenic antenna will enable quantum key distribution in the novel microwave regime [1] and can be used in other fundamental experiments, such as entanglement of trapped ions. This master project focuses on the microwave design, electromagnetic simulation, optimization, and experimental testing of a superconducting cryogenic antenna. The project also includes elements of microwave measurements, quantum input-output formalism, and cryogenic techniques.

[1] F. Fesquet et al., arXiv: 2203.05530 (2022)

Contact: Kirill Fedorov (kirill.fedorov@wmi.badw.de)

Entanglement is the phenomenon of composite quantum systems, where respective subsystems cannot be fully described independently of each other. Besides fundamental interest, quantum entanglement also represents a resource for many applications in the fields of quantum computing, quantum communication, and quantum sensing. However, entanglement is known to be a fragile entity, sensitive to unavoidable losses and external noise. Many strategies exist to remedy this problem. Here, the photon subtraction technique with entangled two-mode squeezed states [1] represents a novel experimental approach, relying on the application of single photon detectors. Such detectors can be implemented by parametrically driven superconducting qubits and nowadays are becoming available in state-of-the-art experiments. This master project focuses on the experimental optimization of existing microwave single photon detectors with transmon superconducting qubits and their application to propagating two-mode squeezed states generated with Josephson parametric amplifiers. Respectively, the project includes elements of quantum theory for simulation of dynamics of nonlinear superconducting circuits and cryogenic microwave measurements for benchmarking of the respective devices & entanglement distillation.

[1] H. Takahashi et al., Nat. Phot. 4, 178–181 (2010)

Contact: Kirill Fedorov (kirill.fedorov@wmi.badw.de)

Superconducting resonators are fundamental building blocks in modern quantum computing. Their intrinsic losses play a crucial role in limiting coherence of associated superconducting quantum bits and other types of quantum devices, such as Josephson parametric amplifiers. Various techniques exist to decrease these losses and improve respective coherence times. Encapsulation processes, which passivate the surface of niobium with other superconducting metals and prevent formation of lossy oxides, represent one of the novel approaches which may significantly improve the coherence times [1]. Here, a particular task is to experimentally study various encapsulation techniques & materials with the aim to understand the underlying physical processes of surface dissipation in superconducting resonators and develop optimal solutions for future high-coherence superconducting devices. This master project focuses on fabrication and characterization of encapsulated niobium superconducting microwave resonators and qubits. Particular tasks include electromagnetic simulation of the respective circuits, their fabrication at WMI cleanroom facilities, microwave characterization measurements in dilution cryostats, and the overall fabrication process optimization.

[1] M. Bal et al., arXiv:2304.13257 (2023)

Contact: Kirill Fedorov (kirill.fedorov@wmi.badw.de)

Superconducting spintronics aims to realize novel device concepts utilizing hybrid structures of magnetic materials and superconductors. A unique aspect are Andreev reflections at interfaces between spinpolarized metallic ferromagnets and superconductors. This gives rise to unique conductance variations in electronic transport experiments across the interface. The goal of this thesis is to realize experiments to identify contributions from Andreev reflections in the experiment. This includes the fabrication of thin film hybrid structures and optimization of the electrical characterization setup. You will establish the required experimental tools to carry out Andreev reflection spectroscopy in our labs. You will utilize sophisticated thin film deposition and nanolithography tools to realize superconductor/ferromagnet hybrid structures. You will optimize our low temperature electrical transport setup to carry out the conductance experiments with a special focus on filtering thermal input noise.  

Contact: Matthias Althammer (matthias.althammer@wmi.badw.de)

Utilizing magneto-optical effects enables the investigation of excitations in magnetic systems like magnons or spin waves down to the sub-micrometer scale. In this way, one can probe spin wave propagation in micro-patterned ferromagnetic materials, which is highly relevant for spintronic applications as well the investigation of tailored quantum systems. Especially at low temperatures, novel magnetic phases exist with intriguing magnetization dynamic properties. The goal of this thesis is the optical investigation of spatially resolved magnetization dynamics in spintronic devices as well as hybrid quantum systems at cryogenic temperatures. We are searching for a highly motivated master student to start the experiments on optically detected magnetization dynamics at cryogenic temperatures. You will improve the optical setup used for the detection of magnetization dynamics to increase the sensitivity. In addition, you will work with state-of-the-art microwave equipment to drive the magnetization dynamics in spintronic devices and hybrid systems. After assessing the performance of the setup with state-of-the-art magnetic systems, you will work in the clean room facilities of our institute to carry out the microfabrication steps to define your own spintronic devices or hybrid systems.

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

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)

Dimensionality crucially influences the properties of materials. Two-dimensional (2d) van der Waals materials in the monolayer limit are presently heavily investigated. Within this class of materials systems with magnetic order exist, yet only limited insights have been obtained with respect to their magnetic excitation properties. A major experimental challenge is the small volume and thus low number of spins in these systems. Thus, high sensitivity techniques and large filling factors are key for successful studies of these materials. The goal of this thesis is to use planar superconducting resonators in combination with 2d van der Waals ferromagnets to study magnetic excitations at low temperatures by microwave spectroscopy. You will work on implementing the microwave-based spectroscopy of magnetic excitations in 2d systems. You will use state-of-the-art nanofabrication techniques like electron beam lithography and thin film deposition machines for the superconducting resonators. You will also gain experience in cryogenic microwave spectroscopy utilizing vector network analyzing techniques. Another important aspect will be the development of a quantitative model to illuminate the underlying physics of the magnetic excitations.

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

In a solid-state system, spin angular momentum is mediated by various (quasi-)particles. Among these excitations are phonons, which can carry angular momentum over mm distances. Most importantly, exchange of spin angular momentum from these crystal lattice vibrations to excitations of the magnetic lattice is possible via magneto-elastic coupling effects. This unlocks novel means for coherent and incoherent spin transport concepts without moving charges. Your thesis will be dedicated in assessing the realization of incoherent angular momentum transfer in nanostructured systems. In your thesis you will work on an all-electrical injection and detection scheme to access incoherent angular momentum transfer. You will use state-of-the-art nanofabrication techniques using electron beam lithography and thin film deposition machines for the realization of magnon-phonon hybrid devices. You will also gain experience in cryogenic magnetotransport techniques. You will develop automated evaluation tools and work on modelling the observed phenomena.

Contact: Matthias Althammer (matthias.althammer@wmi.badw.de)

In antiferromagnetic insulators, we obtain two magnon modes with opposite spin chirality due to the two opposing magnetic sublattices. In this way, magnon transport in antiferromagnetic insulators can be considered as the magnonic equivalent of electronic spin transport in semiconductors and the properties can be mapped onto a magnonic pseudospin. At present, most experiments rely on extended epitaxial thin films of antiferromagnetic insulators. Your thesis will be dedicated to confine the lateral dimensions of the magnon transport channel. By conducting all-electrical magnon transport experiments, you will then determine the role of lateral confinement in such measurement schemes. You are interested in providing novel insights into pseudospin properties in antiferromagnetic insulators and provide a spark for theoretical descriptions. In order to answer questions regarding magnon transport in magnetic insulators, your thesis will contain aspects of the fabrication of nano-scale devices using electron beam lithography as well as ultra-sensitive low-noise electronic measurements at high magnetic fields in a cryogenic environment.

Contact: Matthias Althammer (matthias.althammer@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)

Hybrid materials combining nontrivial conducting and magnetic properties are of high current interest, especially in the context of potential spintronic applications. The organic charge-transfer salts (BETS)₂FeX₄ with X = Cl, Br provide perfect natural structures of conducting and magnetic layers alternating on the single-molecule level. Our project is aimed at a quantitative study of the interaction between the two subsystems and of the role of the subtle structural modifications in this family. To this end, high-precision measurements of quantum oscillations in the low-temperature electrical resistance and magnetization will be carried out on single crystals of these compounds under strong magnetic fields. The results will be analyzed in terms of electronic correlation and magnetic interaction effects.

Contact: Mark Kartsovnik (mark.kartsovnik@wmi.badw.de)

Prominent electronic correlations, reduced dimensionality, and high crystal quality make the organic charge-transfer salts κ-(BEDT-TTF)₂X perfect model systems for exploring the correlation-driven Mott metal-insulator transition and associated fascinating quantum phenomena. For understanding the subtle interplay between different electronic instabilities near the Mott transition a thorough knowledge of the parent metallic ground state is indispensable. This Master project is focused on the X = Cu₂(CN)₃ salt, which features a quantum spin-liquid state or unconventional superconductivity, depending on external conditions. You will gain access to fundamental properties of the conduction system via high-field, low-temperature magnetoresistance measurements, particularly via magnetic quantum oscillations. Fine-tuning of the electronic ground state will be achieved by applying precisely controlled pressure. A part of the experiments will be performed at the European Magnetic Field Laboratory (Grenoble site) in fields up to 36 T.

Contact: Mark Kartsovnik (mark.kartsovnik@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.