Christoph Adelsberger
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 CH4056 Basel, Switzerland

Short CV
2023present:  Postdoc, University of Basel with Prof. D. Loss and Prof. J. Klinovaja 
20192023:  Ph.D. student in the Quantum Theory of Condensed Matter & Quantum Computing group at the University of Basel, supervisors: Prof. D. Loss and Prof. J. Klinovaja 
20162019:  Master of Science in Physics, University of Konstanz 
Master's thesis: "Cavity Quantum Electrodynamics with spin and valley", supervisor: Prof. Guido Burkhard  
20132016:  Bachelor of Science in Physics, University of Konstanz 
Bachelor's thesis: "New materials' exploration and atomic level control for future nano devices", supervisors: Prof. Elke Scheer and Prof. Toyohiro Chikyow 
Publications
Show all abstracts.1.  ValleyFree Silicon Fins Caused by Shear Strain 
Christoph Adelsberger, Stefano Bosco, Jelena Klinovaja, and Daniel Loss. Phys. Rev. Lett. 133, 037001 (2024); arXiv:2308.13448.
Electron spins confined in silicon quantum dots are promising candidates for largescale quantum computers. However, the degeneracy of the conduction band of bulk silicon introduces additional levels dangerously close to the window of computational energies, where the quantum information can leak. The energy of the valley states—typically 0.1 meV—depends on hardly controllable atomistic disorder and still constitutes a fundamental limit to the scalability of these architectures. In this work, we introduce designs of complementary metaloxidesemiconductor (CMOS)compatible silicon fin fieldeffect transistors that enhance the energy gap to noncomputational states by more than one order of magnitude. Our devices comprise realistic silicongermanium nanostructures with a large shear strain, where troublesome valley degrees of freedom are completely removed. The energy of noncomputational states is therefore not affected by unavoidable atomistic disorder and can further be tuned in situ by applied electric fields. Our design ideas are directly applicable to a variety of setups and will offer a blueprint toward siliconbased largescale quantum processors.
 
2.  Microscopic analysis of proximityinduced superconductivity and metallization effects in superconductorgermanium hole nanowires 
Christoph Adelsberger, Henry F. Legg, Daniel Loss, and Jelena Klinovaja. Phys. Rev. B 108, 155433 (2023); arXiv:2306.06944.
Lowdimensional Ge hole devices are promising systems with many potential applications such as hole spin qubits, Andreev spin qubits, Josephson junctions, and can serve as a basis for the realization of topological superconductivity. This vast array of potential uses for Ge largely stems from the exceptionally strong and controllable spinorbit interaction (SOI), ultralong mean free paths, long coherence times, and CMOS compatibility. However, when brought into proximity with a superconductor (SC), metallization normally diminishes many useful properties of a semiconductor, for instance, typically reducing the g factor and SOI energy, as well as renormalizing the effective mass. In this paper we consider metallization of a Ge nanowire (NW) in proximity to a SC, explicitly taking into account the 3D geometry of the NW. We find that proximitized Ge exhibits a unique phenomenology of metallization effects, where the 3D cross section plays a crucial role. For instance, in contrast to expectations, we find that SOI can be enhanced by strong coupling to the superconductor. We also show that the thickness of the NW plays a critical role in determining both the size of the proximity induced pairing potential and metallization effects, since the coupling between NW and SC strongly depends on the distance of the NW wave function from the interface with the SC. In the absence of electrostatic effects, we find that a sizable gap opens only in thin NWs (d≲3 nm). In thicker NWs, the wave function must be pushed closer to the SC by electrostatic effects in order to achieve a sizable proximity gap such that the required electrostatic field strength can simultaneously induce a strong SOI. The unique and sometimes beneficial nature of metallization effects in SCGe NW devices evinces them as ideal platforms for future applications in quantum information processing.
 
3.  Enhanced orbital magnetic field effects in Ge hole nanowires 
Christoph Adelsberger, Stefano Bosco, Jelena Klinovaja, and Daniel Loss. Phys. Rev. B 106, 235408 (2022); arXiv:2207.12050.  
4.  Hole Spin Qubits in Ge Nanowire Quantum Dots: Interplay of Orbital Magnetic Field, Strain, and Growth Direction 
Christoph Adelsberger, Mónica Benito, Stefano Bosco, Jelena Klinovaja, and Daniel Loss. Phys. Rev. B 105, 075308 (2022); arXiv:2110.15039.
Holespin qubits in quasionedimensional structures are a promising platform for quantum information processing because of the strong spinorbit interaction (SOI). We present analytical results and discuss device designs that optimize the SOI in Ge semiconductors. We show that at the magnetic field values at which qubits are operated, orbital effects of magnetic fields can strongly affect the response of the spin qubit. We study onedimensional hole systems in Ge under the influence of electric and magnetic fields applied perpendicular to the device. In our theoretical description, we include these effects exactly. The orbital effects lead to a strong renormalization of the g factor. We find a sweet spot of the nanowire (NW) g factor where charge noise is strongly suppressed and present an effective lowenergy model that captures the dependence of the SOI on the electromagnetic fields. Moreover, we compare properties of NWs with square and circular cross sections with ones of gatedefined onedimensional channels in twodimensional Ge heterostructures. Interestingly, the effective model predicts a flat band ground state for finetuned electric and magnetic fields. By considering a quantum dot (QD) harmonically confined by gates, we demonstrate that the NW gfactor sweet spot is retained in the QD. Our calculations reveal that this sweet spot can be designed to coincide with the maximum of the SOI, yielding highly coherent qubits with large Rabi frequencies. We also study the effective g factor of NWs grown along different highsymmetry axes and find that our model derived for isotropic semiconductors is valid for the most relevant growth directions of nonisotropic Ge NWs. Moreover, a NW grown along one of the three main crystallographic axes shows the largest SOI. Our results show that the isotropic approximation is not justified in Ge in all cases.
 
5.  Squeezed hole spin qubits in Ge quantum dots with ultrafast gates at low power 
Stefano Bosco, Mónica Benito, Christoph Adelsberger, and Daniel Loss. Phys. Rev. B 104, 115425 (2021); arXiv:2103.16724.
Hole spin qubits in planar Ge heterostructures are one of the frontrunner platforms for scalable quantum computers. In these systems, the spinorbit interactions permit efficient allelectric qubit control. We propose a minimal design modification of planar devices that enhances these interactions by orders of magnitude and enables low power ultrafast qubit operations in the GHz range. Our approach is based on an asymmetric potential that strongly squeezes the quantum dot in one direction. This confinementinduced spinorbit interaction does not rely on microscopic details of the device such as growth direction or strain, and could be turned on and off on demand in stateoftheart qubits.
 
6.  Electricfield control and noise protection of the floppingmode spin qubit 
M. Benito, X. Croot, C. Adelsberger, S. Putz, X. Mi, J. R. Petta, and G. Burkard. Phys. Rev. B 100, 125430 (2019); arXiv:1904.13117.
We propose and analyze a “floppingmode” mechanism for electric dipole spin resonance based on the delocalization of a single electron across a double quantum dot confinement potential. Delocalization of the charge maximizes the electronic dipole moment compared to the conventional singledot spin resonance configuration. We present a theoretical investigation of the floppingmode spin qubit properties through the crossover from the double to the singledot configuration by calculating effective spin Rabi frequencies and singlequbit gate fidelities. The floppingmode regime optimizes the artificial spinorbit effect generated by an external micromagnet and draws on the existence of an externally controllable sweet spot, where the coupling of the qubit to charge noise is highly suppressed. We further analyze the sweet spot behavior in the presence of a longitudinal magnetic field gradient, which gives rise to a secondorder sweet spot with reduced sensitivity to charge fluctuations.
 
7.  Ptype polymerbased Ag2S atomic switch for "tug of war" operation 
C. Lutz, T. Hasegawa, T. Tsuchiya, C. Adelsberger, R. Hayakawa, and T. Chikyow. Jpn. J. Appl. Phys. 56, 06GF03 (2017)
The Ag2S gaptype atomic switch based "tug of war" device is a promising element for building a new type of CMOS free neuromorphic computerhardware. Since Ag+ cations are reduced during operation of the device, it was thought that the gapmaterial should be a ntype polymer. In this study, we revealed that the polymer bithiophene–oligoethyleneoxide (BTOE) doped poly(ethylene oxide) (PEO), which was used as gapmaterial in the first demonstration of the "tug of war", is a ptype polymer. For this we used impedance spectroscopy and transistor measurements. We elaborate on how the electrochemical processes in the "tug of war" devices could be explained in the case of ptype conductive gapmaterials.
