arXiv subjects

Baksa Kolok

Publications and source records attributed to Baksa Kolok.

5 recordsLinked to original sources

arXiv2026

Anisotropic g-tensor and strong spin-orbit interaction of holes in hexagonal germanium nanowires

Hexagonal germanium combines direct-gap optical activity with the low-nuclear-spin environment of a group-IV material, making it a candidate platform for spin qubits and spin-photon interfaces. Using a ten-band $\mathbf{k}\cdot\mathbf{p}$ model, we study the response of confined hole states in hexagonal germanium nanowires to magnetic and electric fields. We show that, by symmetry, the top valence band of the bulk crystal has a vanishing $g$ factor in the plane perpendicular to the $c$ axis, in analogy with the in-plane $g$ factor of heavy holes in two-dimensional hole gases. In nanowires, confinement induces only small transverse $g$-tensor components, so the $g$ tensor remains strongly anisotropic. A transverse electric field strongly reduces the nanowire band gap, providing a gate-tunable optical signature and limiting the usable field range. The same field induces a Rashba spin-orbit interaction that is markedly stronger for fields perpendicular to the $c$ axis and depends nonlinearly on the field strength. In narrow wires at strong fields, the spin-orbit length reaches a few tens of nanometers, comparable to hole devices in cubic silicon. We identify the microscopic origin of the Rashba coupling as a second-order process mediated by a few intermediate subbands with distinct symmetry selection rules. Our results provide guidance for the design of electrically controlled hole-spin qubits in hexagonal germanium nanowires.

cond-mat.mes-hall↗

arXiv2026

Rabi oscillations induced by nonresonant gyrating drive

Coherent control of two-level quantum systems is typically achieved using resonant driving fields, forming the basis for qubit operations. Here, we report a mechanism for inducing complete Rabi oscillations in monochromatically driven two-level quantum systems, when the drive frequency is much larger than the Larmor frequency of the qubit. This effect$\unicode{x2015}$Rabi oscillations induced by nonresonant gyrating drive (RING)$\unicode{x2015}$requires a gyrating control field, i.e., an elliptic drive trajectory that encloses a nonzero area per cycle. We illustrate the effect with numerical simulations, and provide an analytical understanding via a simple effective Hamiltonian obtained from Floquet theory and perturbation theory. We show that RING enables coherent oscillations without relying on resonant energy exchange, allows for high-pass noise filtering, provides access to non-Abelian phases in finite magnetic fields. We detail a realization in electrically driven spin-orbit qubits and argue that the RING mechanism enables amplification of the Rabi frequency using the same gate voltage amplitudes at higher drive frequencies. Our results broaden the landscape of quantum control techniques, by highlighting a pathway to achieving coherent oscillations under off-resonant driving conditions.

cond-mat.mes-hall↗

arXiv2026

Acoustic phonons, spin-phonon coupling and spin relaxation via the lattice reorientation mechanism in hexagonal germanium nanowires

Spin relaxation via electron-phonon interaction is an important decoherence mechanism for spin qubits. In this work, we study spin relaxation in hexagonal (2H) germanium, a novel direct-gap semiconductor showing great potential to combine highly coherent spin qubits with optical functionality. Focusing on electrostatically defined quantum dots in hexagonal germanium nanowires, we (i) identify geometries where spin qubit experiments are feasible, (ii) compute the nanowire phonon modes, and (iii) describe spin relaxation of hole spin qubits due to phonon-induced lattice reorientation, a direct spin-phonon coupling mechanism that is absent in cubic semiconductors typically used for spin qubits (GaAs, cubic Si, cubic Ge). We obtain the spin relaxation time as a function of nanowire cross section, quantum dot confinement length, and magnetic field. For realistic parameters, we find relaxation times above 10 ms, and reveal that the magnetic field direction maximizing the relaxation time depends on the qubit Larmor frequency. Our results facilitate the design of nanowire quantum dot experiments with long qubit relaxation times.

cond-mat.mes-hall↗

arXiv2025

Protocols to measure the non-Abelian Berry phase by pumping a spin qubit through a quantum-dot loop

A quantum system constrained to a degenerate energy eigenspace can undergo a nontrival time evolution upon adiabatic driving, described by a non-Abelian Berry phase. This type of dynamics may provide logical gates in quantum computing that are robust against timing errors. A strong candidate to realize such holonomic quantum gates is an electron or hole spin qubit trapped in a spin-orbit-coupled semiconductor, whose twofold Kramers degeneracy is protected by time-reversal symmetry. Here, we propose and quantitatively analyze protocols to measure the non-Abelian Berry phase by pumping a spin qubit through a loop of quantum dots. One of these protocols allows to characterize the local internal Zeeman field directions in the dots of the loop. We expect a near-term realisation of these protocols, as all key elements have been already demonstrated in spin-qubit experiments. These experiments would be important to assess the potential of holonomic quantum gates for spin-based quantum information processing.

cond-mat.mes-hall↗

arXiv2024

Classification and magic magnetic-field directions for spin-orbit-coupled double quantum dots

The spin of a single electron confined in a semiconductor quantum dot is a natural qubit candidate. Fundamental building blocks of spin-based quantum computing have been demonstrated in double quantum dots with significant spin-orbit coupling. Here, we show that spin-orbit-coupled double quantum dots can be categorised in six classes, according to a partitioning of the multi-dimensional space of their $g$-tensors. The class determines physical characteristics of the double dot, i.e., features in transport, spectroscopy and coherence measurements, as well as qubit control, shuttling, and readout experiments. In particular, we predict that the spin physics is highly simplified due to pseudospin conservation, whenever the external magnetic field is pointing to special directions (`magic directions'), where the number of special directions is determined by the class. We also analyze the existence and relevance of magic loops in the space of magnetic-field directions, corresponding to equal local Zeeman splittings. These results present an important step toward precise interpretation and efficient design of spin-based quantum computing experiments in materials with strong spin-orbit coupling.

cond-mat.mes-hall↗