ERC FUNDED PROJECTS

Macroscopic control of quantum states is a major theme in much of modern physics because quantum coherence enables study of fundamental physics and has promising applications for quantum information processing. The potential significance of quantum computing is recognized well beyond the physics community. For electron spins in GaAs quantum dots, it has become clear that decoherence caused by interactions with the nuclear spins is a major challenge. We propose to investigate and reduce hyperfine induced decoherence with two complementary approaches: nuclear spin state narrowing and nuclear spin polarization. We propose a new projective state narrowing technique: a large, Coulomb blockaded dot measures the qubit nuclear ensemble, resulting in enhanced spin coherence times. Further, mediated by an interacting 2D electron gas via hyperfine interaction, a low temperature nuclear ferromagnetic spin state was predicted, which we propose to investigate using a quantum point contact as a nuclear polarization detector. Estimates indicate that the nuclear ferromagnetic transition occurs in the sub-Millikelvin range, well below already hard to reach temperatures around 10 mK. However, the exciting combination of interacting electron and nuclear spin physics as well as applications in spin qubits give ample incentive to strive for sub-Millikelvin temperatures in nanostructures. We propose to build a novel type of nuclear demagnetization refrigerator aiming to reach electron temperatures of 0.1 mK in semiconductor nanostructures. This interdisciplinary project combines Microkelvin and nanophysics, going well beyond the status quo. It is a challenging project that could be the beginning of a new era of coherent spin physics with unprecedented quantum control. This project requires a several year commitment and a team of two graduate students plus one postdoctoral fellow.

1 377 000 €

Start date: 2008-06-01, End date: 2013-05-31

The main goal of this theory project is to propose engineered topological phases emerging only in strongly interacting systems and to identify the most feasible systems for experimental implementation. First, we will focus on setups hosting topological states localized at domain walls in one-dimensional channels such as parafermions, which are a new class of non-Abelian anyons and most promising candidates for topological quantum computing schemes. Second, in the framework of weakly coupled wires and planes, we will develop schemes for novel fractional topological phases in two- and three-dimensional interacting systems. To achieve these two goals, my team will identify necessary ingredients such as strong electron-electron interactions, helical magnetic order, or crossed Andreev proximity-induced superconductivity and address each of them separately. Later, we combine them to lead us to the desired topological phases and states. On our way to the main goal, as test cases, we will also study non-interacting analogies of the proposed effects such as Majorana fermions and integer topological insulators and pay close attention to the rapid experimental progress to come up with the most feasible proposals. We will study transport properties, scanning tunneling and atomic force microscopy. Especially for systems driven out of equilibrium, we will develop a Floquet-Luttinger liquid technique. We will explore the stability of engineered topological phases, error rates of topological qubits based on them, and computation schemes allowing for a set of universal qubit gates. We will strive to find a reasonable balance between topological stability and experimental feasibility of setups. Our main theoretical tools are Luttinger liquid techniques (bosonization and renormalization group), Green functions, Floquet formalism, and numerical simulations in non-interacting test models.

1 158 403 €

Start date: 2018-01-01, End date: 2022-12-31

We plan to investigate the strong coherent interaction of light and matter on the level of individual photons and atoms or atom-like systems. In particular, we will explore large dipole moment superconducting artificial atoms and natural Rydberg atoms interacting with radiation fields contained in quasi-one-dimensional on-chip microwave frequency resonators. In these resonators photons generate field strengths that exceed those in conventional mirror based resonators by orders of magnitude and they can also be stored for long times. This allows us to reach the strong coupling limit of cavity quantum electrodynamics (QED) using superconducting circuits, an approach known as circuit QED. In this project we will explore novel approaches to perform quantum optics experiments in circuits. We will develop techniques to generate and detect non-classical radiation fields using nonlinear resonators and chip-based interferometers. We will also further advance the circuit QED approach to quantum information processing. Our main goal is to develop an interface between circuit and atom based realizations of cavity QED. In particular, we will couple Rydberg atoms to on-chip resonators. To achieve this goal we will first investigate the interaction of ensembles of atoms in a beam with the coherent fields in a transmission line or a resonator. We will perform spectroscopy and we will investigate on-chip dispersive detection schemes for Rydberg atoms. We will also explore the interaction of Rydberg atoms with chip surfaces in dependence on materials, temperature and geometry. Experiments will be performed from 300 K down to millikelvin temperatures. We will realize and characterize on-chip traps for Rydberg atoms. Using trapped atoms we will explore their coherent dynamics. Finally, we aim at investigating the single atom and single photon limit. When realized, this system will be used to explore the first quantum coherent interface between atomic and solid state qubits.

1 954 464 €

Start date: 2009-09-01, End date: 2014-08-31

"Coherent extreme ultraviolet (XUV) light sources open up new opportunities for science and technology. Promising examples are attosecond metrology, spectroscopic and structural analysis of matter on a nanometer scale, high resolution XUV-microscopy and lithography. The most promising technique for table-top sources is femtosecond laser-driven high-harmonic generation (HHG) in gases. Unfortunately, their XUV photon flux is not sufficient for most applications. This is caused by the low average power of the kHz repetition rate driving lasers (<10 W) and the poor conversion efficiency (<10-6). Following the traditional path of increasing the power, numerous research teams are engineering larger and more complex femtosecond high-power amplifier systems, which are supposed to provide several kilowatts of average power in the next decade. However, it is questionable if such systems can easily serve as tool for further scientific studies with XUV light. The goal of this proposal is the realization of a simpler and more efficient source of high-flux XUV radiation. Instead of amplifying a laser beam to several kW of power and dumping it after the HHG interaction, the generation of high harmonics is placed directly inside the intra-cavity multi-kilowatt beam of a femtosecond laser. Thus, the unconverted light is “recycled”, and the laser medium only needs to compensate for the low losses of the resonator. Achieving passive femtosecond pulse formation at these record-high power levels will require eliminating any destabilizing effects inside the resonator. This appears to be only feasible with ultrafast thin disk lasers, because all key components are used in reflection. Exploiting the scientific opportunities of the resulting table-top multi-MHz coherent XUV light source in various interdisciplinary applications is the second major part of this project. The developed XUV source will be transportable, which will enable the fast implementation of joint measurements."

1 500 000 €

Start date: 2012-03-01, End date: 2017-02-28