Research Phase and Master’s Thesis in the M.Sc. Quantum Science & Technology

An essential step for implementation of quantum computing architectures is an efficient readout of qubits. In the field of superconducting quantum circuits, this is typically realized by dispersively coupling a superconducting qubit to a microwave resonator. Then, the frequency of the resonator depends on the state of the qubit. The former can be extracted by probing the resonator with a coherent  tone. However, efficiency of this readout approach is fundamentally limited by quantum  laws. The corresponding threshold is commonly known as the standard quantum limit and bounds quantum efficiency of the readout process by 50%. Nevertheless, recent investigations have shown that it is possible to circumvent this limit and reach quantum efficiency of the qubit readout of 100% by exploiting broadband readout signal combined with Josephson parametric amplifiers.

The goal of this Master project is to build a proof-of-principle experimental setup and perform microwave cryogenic measurements on a superconducting transmon qubit in the broadband regime in order to demonstrate violation of the standard quantum limit in the dispersive readout.

Quantum technologies are on the verge of revolutionizing high-security communication owing to advanced protocols such as teleportation and remote-state preparation. Development of quantum local area networks (QLAN) is an important milestone in this field. Such a microwave QLAN can be realized as  superconducting transmission line cooled down to millikelvin temperatures.  In practice, this system requires careful characterization measurements with quantum signals propagating over the cryogenic link in order to determine the effective losses, noise level, and other imperfections.

In this work, the main goal is to investigate a new prototype of a quantum local area network (QLAN) established between two neighboring laboratories. This includes analysis of experimental results with theoretical and numerical methods. This project will provide you an excellent opportunity to gather expertise in a broad range of topics from quantum physics to microwave engineering and cryogenic techniques.

Quantum communication using single photons provides one route towards physically secure data transmission. However, the total length of today’s quantum key distribution systems is limited to about ~300km due to photon absorption in the “quantum channel” - typically an optical fiber. To overcome this problem, one can build so-called “quantum repeaters” in which the channel is broken down into shorter segments connected by quantum links. In our group we are working towards building a quantum repeater using optically active semiconductor-based quantum dot molecules.  We aim to make use of trapped pairs of charges – singlet-triplet (S-T) spin qubits.

In the first part of this MSc. project you fabricate a quantum photodiode structure containing coupled quantum dots. This will involve clean-room fabrication, as well as electrical characterization of the fabricated diodes. In the second part, your focus will be on optical characterization of the S-T spin qubits. The goal is to measure exceedingly long coherence times (>>1µs) for special electric fields where the energy gap of the qubit is insensitive to electric and magnetic field fluctuations.

Prior knowledge in optics, clean-room fabrication and programming are helpful – but secondary to high motivation and an open and curious mindset to tackle challenging problems. You will get experience in state-of-the-art nanofabrication, optical spectroscopy at cryogenic temperatures, as well as understanding of semiconductors in the context of quantum information and technology.

Quantum experiments often require fast and versatile data processing which allows for a quantum feedback operation. This approach opens the road to many fascinating experiments such as quantum teleportation, entanglement purification,  quantum error correction, among others. Here, we would like to develop a specific measurement and feedback setup, based on a field programmable gate array (FPGA), for experiments with propagating quantum microwaves.

The main goal is to program and experimentally test a specific image for an FPGA  which would allow for acquisition of microwave signals and feedback generation over few hundred of nanoseconds. This timescale is the prerequisite for exploiting quantum correlations effects for quantum communication and cryptography protocols with propagating squeezed microwaves which are conducted in our lab. This project will offer a deep insight into the state‑of‑the‑art FPGA devices, microwave measurements, and cryogenic experiments with superconductors.

The smaller a device is, the higher its resonance frequency becomes. For our future quantum optomechanics experiments we will be working with mechanical resonators that operate at GHz frequencies and simultaneously couple strongly to light. Using mechanical and optical band structure calculations, you will design, make, and measure phononic and photonic cavities. The project involves nanofabrication in the cleanroom, as well as using the extreme sensitivity offered by on-chip optomechanics. We are aiming for devices made from silicon nitride, which has a lot of tensile stress in it. For micromechanical structures this material gives much better mechanical properties compared to, for example, silicon devices. An important aspect of the project is to understand if this is also true for the high frequency devices.

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 enable the design and the investigation of spin-phonon coupling down to the single excitation level and are expected to allow the investigation of the Einstein-de Haas on the nanoscale, where the angular momentum change arising from magnetization reversal is transferred into a mechanical vibration of the beam. In addition, these systems are key for the investigation of strong magnon-phonon coupling.

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 silicon nitride or silicon and deposit on them ferromagnetic multi layers using state-of-the-art nano-lithography and metal 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

In optomechanics, light is used to measure and alter the dynamics of mechanical resonators. It is by far the most sensitive method to observe the tiny vibrations that nanomechanical devices perform: in one second one can determine their position with femtometer precision! Using light to measure the mechanics is not the only aspect of optomechanics. The same light can also be used to change the dynamics of the mechanical device through a process called cavity backaction. The photons exert a force on the resonator, the so-called radiation pressure. In this project we want to explore the ultimate limits to this force. The goal is to measure the force originating from a single photon! For this it is required that the photon interacts with the mechanical resonator as strongly as possible. For this we need to the design and make very low loss optical cavities, such as microring resonators. Also, the mechanical device should have a quality factor as high as possible. You will make both the optical and mechanical components from chips with highly-stressed silicon nitride using state-of-the-art nanofabrication in the cleanroom. Then the devices are placed in a vacuum chamber for their measurement. In our highly-automated setup you can very quickly characterize many of the devices on your chip. Then, with the perfect device parameters you can start to explore the more advanced measurements. Initially we can measure the devices in with pulsed light, but by using single photons we want to explore the ultimate limits to optomechanical forces.

See http://www.groups.ph.tum.de/en/qtech/openings/ for a detailed description of this project.

Quantum microwaves emitted by superconducting circuits can be used for distributed quantum computation or improved quantum illumination/radar. You will join the Quantum Flagship activities on the latter, working towards the demonstration of a quantum advantage in a laboratory setting.

Keywords: Quantum microwaves, quantum communication, quantum illumination, quantum radar

The combination of ferromagnetic and superconducting materials leads to intriguing proximity effects at the interface of the two materials. The goal of this thesis is to investigate the spin transport of superconductor/ferromagnet interfaces and model the obtained results in the framework of proximity effects. To this end, we will use broadband ferromagnetic resonance and magnetoresistance effects to inject a spin current into the superconductor. This requires investigations at low temperatures around the critical temperature of the superconductor in large magnetic fields. For the superconductor/ferromagnet magnetoresistance effects, the magnetic field orientation dependence and its influence on the Andreev reflection contribution is the focus of this study.

We are looking for a talented master student to investigate spin transport in superconductor/ferromagnet heterostructures. The thesis deals with the fabrication of these heterostructures using our new UHV sputtering system and structuring of the blanket films with optical and electron beam lithography. In addition, characterization of theses heterostructures at low temperatures will be conducted in superconducting magnet cryostats. Here, high frequency spin dynamics as well magnetotransport studies will be conducted

Optomechanics provides extremely sensitive methods to measure the displacement of mechanical resonators. However, the forces are much smaller in optomechanics compared to those in nanoelectromechanical systems (NEMS). The goal of the project is to make, and measure opto-electromechanical devices which have strong electrostatic interactions. This includes the electrostatic spring effect where the resonance frequency depends strongly on the applied voltage. The next step is trying to measure the potential by measuring the ringdown of different mechanical modes. The devices will be made using advanced nanofabrication techniques such as electron beam lithography and reactive ion etching.

See http://www.groups.ph.tum.de/en/qtech/openings/ for a detailed description of this project.