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We present chemical state information on contamination-free Ge2Sb2Te5 thin film using high-resolution x-ray photoelectron spectroscopy (HRXPS) and the corresponding theoretical understanding of the chemical states, on both amorphous and metastable phases, illuminating the phase-change mechanism of the system. HRXPS data revealed that the Sb 4d shallow core level was split into two components having different binding energies and that the spin-orbit splitting feature of the Ge 3d level was enhanced as the system became metastable. Negligible change was observed in the Te 4d shallow core level, and in contrary to the previous report’s prediction less change in valance band spectra was observed. The results imply that Sb movement is also involved in the phase-change mechanism and that acquisition of shallow core-level spectra can be a useful measure for understanding phase-change mechanism. Hydrogenated SbTe6 octahedral-like cluster model was introduced to schematically interpret the generation of the two components in the Sb 4d level in metastable state, having an isotropic six-bonds configuration, and an anistropic six-bonds (three-short and three-elongated bonds) configuration. The amorphous state was modeled to have three-short bonds configuration. Finally, Stibnite-like building block model was used to show that the existence of the above two configurations for Sb atoms is feasible in the Ge2Sb2Te5 system.

Fast and high-fidelity quantum state detection is essential for building robust spin-based quantum information processing platforms in semiconductors. The Pauli spin blockade (PSB)-based spin-to-charge conversion and its variants are widely used for the spin state discrimination of two-electron singlet-triplet (ST 0) qubits; however, the single-shot measurement fidelity is limited by either the low signal contrast, or the short lifetime of the triplet state at the PSB energy detuning, especially due to strong mixing with singlet states at large magnetic field gradients. Ultimately, the limited single-shot measurement fidelity leads to low visibility of quantum operations. Here, we demonstrate an alternative method to achieve spin-to-charge conversion of ST 0 qubit states using energy-selective tunneling between doubly occupied quantum dots (QDs) and electron reservoirs. We demonstrate a single-shot measurement fidelity of 90% and an S-T 0 oscillation visibility of 81% at a field gradient of 100 mT (~500 MHz h (g*•μ B) −1); this allows single-shot readout with full electron charge signal contrast and, at the same time, long and tunable measurement time with negligible effect of relaxation even at strong magnetic field gradients. Using an rf-sensor positioned opposite to the QD array, we apply this method to two ST 0 qubits and show high-visibility readout of two individual single-qubit gate operations is possible with a single rf single-electron transistor sensor. We expect our measurement scheme for two-electron spin states can be applied to various hosting materials and provides a simplified and complementary route for multiple qubit state detection with high accuracy in QD-based quantum computing platforms.

The complex dielectric constant of Rb~-(NH4) H2As04 mixed crystals is measured for the concentrations x 0.17 and 0.19. It reveals a proton-glass state preceded by the ferroelectric phase transition as the temperature decreases. A complementary laser optical study suggests that the ferroelectric orthorhombic symmetry remains down to the low temperature where the glass state is established. Hence, the proton-glass state coexists with the long-range ferroelectric order of orthorhombic symmetry, and the glass freezing may occur quite independently of the ferroelectric order.