High-field immiscibility of electrons belonging to adjacent twinned bismuth crystals
npj Quantum Materials Springer Nature 9:1 (2024) 12
Abstract:Bulk bismuth has a complex Landau spectrum. The small effective masses and the large g-factors are anisotropic. The chemical potential drifts at high magnetic fields. Moreover, twin boundaries further complexify the interpretation of the data by producing extra anomalies in the extreme quantum limit. Here, we present a study of angle dependence of magnetoresistance up to 65 T in bismuth complemented with Nernst, ultrasound, and magneto-optic data. All observed anomalies can be explained in a single-particle picture of a sample consisting of two twinned crystals tilted by 108° and with two adjacent crystals keeping their own chemical potentials despite a shift between chemical potentials as large as 68 meV at 65 T. This implies an energy barrier between adjacent twinned crystals reminiscent of a metal- semiconductor Schottky barrier or a p-n junction. We argue that this barrier is built by accumulating charge carriers of opposite signs across a twin boundary.
Fault-tolerant qubit encoding using a spin-7/2 qudit
Physical Review A American Physical Society 108 (2023) 062403
Abstract:The implementation of error correction protocols is a central challenge in the development of practical quantum information technologies. Recently, multi-level quantum resources such as harmonic oscillators and qudits have attracted interest in this context because they offer the possibility of additional Hilbert space dimensions in a spatially compact way. Here we propose a quantum memory, implemented on a spin-7/2 nucleus hyperfine-coupled to an electron spin-1/2 qubit, which provides first order X, Y and Z error correction using significantly fewer quantum resources than the equivalently effective qubit-based protocols. Our encoding may be efficiently implemented in existing experimentally realised molecular electron-nuclear quantum spin systems. The strategy can be extended to higher-order error protection on higher-spin nuclei.
Title: experimental realisation of multi-qubit gates using electron paramagnetic resonance.
Nature communications 14:1 (2023) 7029
Abstract:Quantum information processing promises to revolutionise computing; quantum algorithms have been discovered that address common tasks significantly more efficiently than their classical counterparts. For a physical system to be a viable quantum computer it must be possible to initialise its quantum state, to realise a set of universal quantum logic gates, including at least one multi-qubit gate, and to make measurements of qubit states. Molecular Electron Spin Qubits (MESQs) have been proposed to fulfil these criteria, as their bottom-up synthesis should facilitate tuning properties as desired and the reproducible production of multi-MESQ structures. Here we explore how to perform a two-qubit entangling gate on a multi-MESQ system, and how to readout the state via quantum state tomography. We propose methods of accomplishing both procedures using multifrequency pulse Electron Paramagnetic Resonance (EPR) and apply them to a model MESQ structure consisting of two nitroxide spin centres. Our results confirm the methodological principles and shed light on the experimental hurdles which must be overcome to realise a demonstration of controlled entanglement on this system.
Probing the local electronic structure in metal halide perovskites through cobalt substitution (Small Methods 6/2023)
Small Methods Wiley 7:6 (2023) 2370029
Abstract:Inside Front Cover
In article number 2300095, Hesjedal and co-workers demonstrate that the substitution of Co2+ ions into the halide perovskite imparts magnetic behavior to the material while maintaining photovoltaic performance. We utilize the Co2+ ions (shown as robots) themselves as probes to sense the local electronic environment of lead in the perovskite, thereby opening the substitution gateway for developing novel functional perovskite materials and devices for future technologies.
Universal quantum control of an atomic spin qubit on a surface
npj Quantum Information Springer Nature 9:1 (2023) 48