Quantum transport

The everlasting trend to miniaturize classical electronic devices has guaranteed progress over half a century. This evolution is now reaching its fundamental limits due to the granularity of matter and the quantum nature of the electron dynamics, which modifies the circuit properties on extremely short length and time scales. Therefore, future progress of electronics will be increasingly based on the coherent quantum dynamics of individual states. This development is often denoted as the second quantum revolution.

Transport spectroscopy is applied as a tool to explore the dynamics of nanoscale semiconductor quantum circuits, typically at very low temperatures. The aim of this research is to utilize the coherent superposition of quantum states and to pave the way for future technologies such as quantum simulators or quantum computers. Therefore, the coherent dynamics of individual electronic quantum excitations and of their interactions is investigated on a fundamental level.

The investigated nanostructures are fabricated using state-of-the-art cleanroom facilities. The surface of semiconductor heterostructures or nanowires is patterned by optical and electron beam lithography. In this way, nanoscale electrical contacts and metal electrodes are created to shape the local electrostatic potential within a conducting sheet using the electric-field effect. The circuit components include interacting quantum point contacts, coupled quantum dots, ballistic structures, and hybrid devices such as quantum dots coupled to on-chip phonon resonators.

Information processing is always based on non-equilibrium operations, while quantum control and coherence are closely related to interactions on the microscopic level. Hence, aiming at full control while minimizing decoherence, the main focus of this research covers the dynamics, the interaction, and the coherence of non-equilibrium excitations in quantum circuits.


Selected publications

Title: Optimization of Ohmic Contacts to n-Type GaAs Nanowires
Author: L. Hüttenhofer, R. B. Lewis, S. Rauwerdink, A. Tahraoui, H. Küpers, L. Geelhaar, O. Marquardt, and S. Ludwig
Source: Phys. Rev. Applied 10, 034024, 1 – 11 (2018)
DOI: 10.1103/PhysRevApplied.10.034024

Title: Coherent Electron Optics with Ballistically Coupled Quantum Point Contacts
Author: J. Freudenfeld, M. Geier, V. Umansky, P. W. Brouwer, and S. Ludwig
Source: Phys. Rev. Lett. 125, 107701, 1 – 6 (2020)
DOI: 10.1103/PhysRevLett.125.107701

Title: Electrostatic Potential Shape of Gate Defined Quantum Point Contacts
Author: M. Geier, J. Freudenfeld, J. Pires da Silva, V. Umansky, D. Reuter, A. D. Wieck, P. W. Brouwer, and S. Ludwig
Source: Phys. Rev. B 101, 165429, 1 – 8 (2020)
DOI: 10.1103/PhysRevB.101.165429

Title: Determining Amplitudes of Standing Surface Acoustic Waves via Atomic Force Microscopy
Author: J. Hellemann, F. Müller, M. Msall, P. V. Santos, and S. Ludwig
Source: Phys. Rev. Applied 17, 044024, 1 – 9 (2022)
DOI: 10.1103/PhysRevApplied.17.044024

Title: Scanning X-Ray Diffraction Microscopy of a 6-GHz Surface Acoustic Wave
Author: M. Hanke, N. Ashurbekov, E. Zatterin, M.E. Msall, J. Hellemann, P.V. Santos, T.U. Schulli, and S. Ludwig
Source: Phys. Rev. Applied 19, 024038, 1 – 10 (2023)
DOI: 10.1103/PhysRevApplied.19.024038