Gruppe BluhmSimon Schaal
B.Sc.-Arbeit Long Range
Coupling between Spin Qubits
The Double Quantum Dot system in GaAs can be used for quantum information processing. This thesis benchmarks different ways to couple several qubits to enable good multi-qubit operations.
M.Sc.-Arbeit Nuclear Spin Mediated Landau-Zener Transitions in Double Quantum Dots
The Double Quantum Dot system in GaAs is a promising hardware for quantum information processing. In this thesis, a measurement scheme to probe the dynamics of nuclear spins in GaAs quantum dots is presented. Dokument (PDF)Pascal Cerfontaine
M.Sc.-Arbeit High-Fidelity Qubit Gates for Two-Electron Spin Qubits
High-fidelity gate operations for manipulating qubits in the presence of decoherence are a prerequisite for fault-tolerant quantum information processing. This work theoretically develops control pulses for singlet-triplet qubits in GaAs double quantum dots with fidelities as high as 99.9%.
M.Sc.-Arbeit Probing Classical and Quantum-Mechanical Baths with a Qubit
The investigation of deleterious interactions between a qubit and its environment is important for quantum information research. In this thesis, techniques using qubit evolution and readout to investigate interactions with both classical and quantum-mechanical noise baths are introduced.
Pia Döring: Quantum Cryptography with Microwaves
Jan Alexander Herbort: Strong Coupling between a Spin Qubit and a Resonator
Florian Nolte: Measuring non-commuting quantum operators
Dennis Huben: Geometric Layouts of Qubits for New Lattice Codes
Cedric Sodhi: Hall effect in an eight-shaped conductor
Benedikt Placke: Real-World Quantum Hall Gyrators and Circulators
Martin Wosnitzka: Qbit Layouts for Quantum Computation Using Non-Abelian Anyons
M.Sc.-Project Multiqubit Coupling Dynamics and the Cross-Resonance Gate
The cross-resonance gate is implemented by applying a microwave drive to a system of two coupled qubits. I study the nonlocal properties of the driven and undriven system induced by this coupling. In the undriven case, entanglement is still being generated, but it is periodic and bounded linearly by the ratio of coupling strength and qubit frequency detuning. Specifically I derive an upper bound for the concur- rence as a measure of entanglement. Dokument(PDF)Adrián Parra Rodríguez
M.Sc.-Project Software Tool to Analyse Superconducting Qubits
This thesis is a description of the program “Superconducting Qubit GUI” that following the methods of existing literature, it helps automate ordinary electrical network graph theory in superconducting circuits.The main objective of the Superconducting Qubit GUI is to be as useful as possible to a researcher in the field. It is open to changes for improvement since the written code offers a nice interface for further development. DokumentVeit Langrock
M.Sc.-Project Numerical and theoretical investigation of long-range coherent electron shuttling devices in Silicon/Silicon-Germanium quantum wells for scalable quantum computing
The aim of this thesis is to investigate what kind of problems currently envisioned coherent electron transfer devices may encounter in Silicon/Silicon- Germanium gate-confined nano structures, and to lay out proper methods to de- scribe them in a sufficiently efficient manner. Dokument
M.Sc.-Arbeit Quantum Information Processing with Surface Acoustic Waves
In the first part of the present work, I’m going to investigate the wave propagation in cubic piezoelectric crystals such as GaAs and also wave generation by interdigital transducers. In the second part, I propose and investigate the idea of looking at spiral IDT-like structures, which might permit the simultaneous coupling to two cavities in orthogonal directions. Using COMSOL simulations, I’ve calculated several parameters of the spiral transducer, such as its capacitance, input admittance and response function. The coupling rate of the spiral transducer has also been calculated using semi-classical model for the qubit.Moreover, I have studied the IDT-transmons of Delsing's group. The physics of this is rather different than in conventional "circuit QED", because the transmon capacitor will be much bigger than the wavelength of the bosonic mode, makes it a giant atom. Dokument(PDF)Sander Konijnenberg
M.Sc.-Arbeit Theoretical study of Hall effect gyrators and circulators in the time domain
Circulators and gyrators are non-reciprocal circuit elements that play an important role in microwave systems. It would be desirable to keep such elements small in size, so it has been proposed to create such devices that use the Hall effect (as opposed to e.g. the Faraday effect which makes for relatively large devices). It has been shown that by coupling leads capacitively to a 2D Hall conductor, lossless circulators and gyrators can be made. The response of such devices has been studied in the frequency domain, but these results cannot be trivially transformed to the time domain. In this report we study the behaviour of Hall-effect gyrators and circulators in the time domain. Dokument (PDF)Susanne Richer
M.Sc.-Arbeit Perturbative Analysis of Two-Qubit Gates on Transmon Qubits
This thesis treats different schemes for two-qubit gates on transmon qubits and their per- turbative analysis. The transmon qubit is a superconducting qubit that stands out due to its very low sensitivity to charge noise, leading to high coherence times. However, it is also characterized by its low anharmonicity, making it impossible to neglect the higher transmon levels.
Coupled systems of superconducting qubits and resonators can be treated in cavity QED. In the dispersive regime, where qubit and resonator are far detuned from each other, per- turbative methods can be used for the derivation of effective Hamiltonians. Several such methods are presented here, among which the Schrieffer-Wolff transformation results to be the most adequate. Dokument (PDF)
- Ole Jasper: Stability of the Matrix-Numerov Method for Solving the 1D Schrödinger Equation
- Julio Magdalena: Scattering of electrons off strain fluctuations in graphene
- Timon Vaas: Semiclassical Green’s function of a BCS superconductor in a half space
- Florian Venn: Quantum Oscillations in Graphene Rings
- Uta Meyer: Obstruction of Information Transfer by Locality in Majorana Systems
- Lisa Arndt: Time-Scales of Charge Relaxation in a Superconducting Island via an Inductive Shunt
- Jonas Stapmanns: Universal conductance peak in the transport of electrons through a floating topological superconductor
- Felix Stamm: Modeling of a nanomechanical oscillator with adiabatic correction to scattering theory
- Daniel Otten: Magnetic Impurity Coupled to Helical Edge States
- Mauricio Cattaneo: Non-Abelian exchange statistics of Majorana fermions
- Christoph Baumann: Transport properties of a disordered chain of Majorana fermions
Algebraic Methods for the 1D Schrödinger Equation
We discuss algebraic methods to obtain exact results about the eigenvalue spectrum of the one dimensional Schrödinger equation. We focus on degeneracies in the spectrum and eigenvalues that are exactly solvable. The central object in our discussion is supersymmetry. First, we give an introduction to the description of problems using superpotentials. We present insights that can be obtained from the supersymmetric structure, in particular the degeneracies between bosonic and fermionic states and that the groundstate admits an algebraic solution. We use these insights to study the double sine potential. By tuning the double sine potential away from the point where it can be described using supersymmetry, we find an extension of supersymmetry that is related to a class of problems for which multiple eigenstates admit algebraic solutions. Next, we prove, for a subclass of these problems, that a degenerate partner state exists for almost all eigenstates. Finally, we highlight that the class of periodic problems with more than one exactly solvable eigenvalue is not limited to the double sine potential. Dokument (PDF)Elias Walter
Influence of chiral symmetry on electron scattering at disordered graphene boundaries
In this thesis, we study scattering processes at disordered graphene boundaries. In particular, we investigate how the chiral symmetry of quasiparticles in graphene influences diffusive scattering. We find that a boundary that breaks chiral symmetry behaves like a mirror, in the sense that in the long Fermi wavelength limit diffusive scattering is suppressed and incoming electrons are reflected specularly. However, if the disorder on average preserves chiral symmetry, diffusive scattering increases significantly, leading to a breakdown of the mirror-like behavior. Dokument (PDF)Daniel Otten
Second-Order Coherence of Microwave Photons Emitted by a Quantum Point Contact
In this work we present a diagrammatic approach to calculate current cumulants for the electron transport through a quantum point contact. We provide compact expressions for cumulants up to and including the third order. Furthermore, fluctuations in the electronic current lead to emitted radiation in the microwave regime. In this context the current cumulants are linked to the photon counting statistics of the microwave field. For this setup, we calculate the Fano factor F and the second-order coherence function g (2)(τ). Dokument (PDF)Dominique Dresen
Quantum Transport of Non-Interacting Electrons in 2D Systems of Arbitrary Geometries
The scattering formalism for describing the trans p ort properties of systems is discussed. We apply the formalism to GaAs and graphene with different geometries and types of disorder. For example, we discuss impedance matching of graphene to outside leads, Aharnnov-Bohm effect in a ring geometry, and how strain-fluctuations in graphene manifest themselves in the transport properties. Dokument (PDF)Sebastian Rubbert
Tuneable Long Range Interactions in an Array of coupled Cooper Pair Boxes
The Kitaev chain emulating the transverse Ising model can be implemented in an array of superconducting islands with semiconducting nanowires. In this thesis, we will show that adding additional capacitances to the system implements a long-range interaction between the Ising degrees of freedom. Dokument (PDF)
Gruppe SchuchMadita Nocon
B.Sc.-Arbeit The role of boundaries in gapped one-dimensional phases
Quantum phases in one-dimensional systems can be characterized by the number of ground states. This thesis studies how this classification of phases is affected by the presence of boundaries and the particular way in which the boundary conditions are chosen. Dokument (PDF)Stefan Haßler
M.Sc.-Arbeit Entanglement Spectra and Boundary Theories for Gaussian Fermionic PEPS
Entanglement spectra and boundary theories describe the entanglement structure of a correlated quantum system. This thesis develops a framework for studying boundary theories in the context of non-interacting fermionic Projected Entangled Pair States (PEPS).
M.Sc.-Arbeit Semionic Resonating Valence Bond states on the kagome lattice
Resonating Valence Bond states are a class of states which are well suited to model the physics of Heisenberg antiferromagnets. This thesis shows how Resonating Valence Bond states can be used to describe systems with double semion topological order, as opposed to the toric code order conventionally found in these states, and studies their properties as an ansatz for the Heisenberg antiferromagnet.
Nikolas Breuckmann:Quantum Subsystem Codes: Their Theory and Use
Daniel Weigand: Entanglement Entropy of 1D Noninteracting Fermionic Systems
Timo Simnacher: Non-local Boxes: Theory & Implementation in Minecraft
Dominik Michels: The Sign Problem in Boson Sampling.
Jonathan Conrad: Parity Check Schedules for Hyperbolic Surface Codes
M.Sc. Project From quantum circuits to Hamiltonians: analysis of a multi-time construction for QMA
This thesis lies in the area of quantum complexity theory. It proves the QMA-completeness of a class of 2D interacting fermion Hamiltonians.
M.Sc. Project Improving Transmon Qubit Readout Using Squeezed Radiation
The goal of this project has been to analyze whether the use of squeezing in a Mach-Zehnder interferometer can give rise to a faster, high fidelity, measurement of superconducting transmon qubits.