Tunable g factor and phonon-mediated hole spin relaxation in Ge/Si nanowire quantum dots

We theoretically consider g factor and spin lifetimes of holes in a longitudinal Ge/Si core/shell nanowire quantum dot that is exposed to external magnetic and electric fields. For the ground states, we find a large anisotropy of the g factor which is highly tunable by applying electric fields. This tunability depends strongly on the direction of the electric field with respect to the magnetic field. We calculate the single-phonon hole spin relaxation times T1 for zero and small electric fields and propose an optimal setup in which very large T1 of the order of tens of milliseconds can be reached. Increasing the relative shell thickness or the longitudinal confinement length further prolongs T1. In the absence of electric fields, the dephasing vanishes and the decoherence time T2 is determined by T2 = 2 T1.Comment: 7 pages, 4 figure

Acoustic phonons and strain in core/shell nanowires

We study theoretically the low-energy phonons and the static strain in cylindrical core/shell nanowires (NWs). Assuming pseudomorphic growth, isotropic media, and a force-free wire surface, we derive algebraic expressions for the dispersion relations, the displacement fields, and the stress and strain components from linear elasticity theory. Our results apply to NWs with arbitrary radii and arbitrary elastic constants for both core and shell. The expressions for the static strain are consistent with experiments, simulations, and previous analytical investigations; those for phonons are consistent with known results for homogeneous NWs. Among other things, we show that the dispersion relations of the torsional, longitudinal, and flexural modes change differently with the relative shell thickness, and we identify new terms in the corresponding strain tensors that are absent for uncapped NWs. We illustrate our results via the example of Ge/Si core/shell NWs and demonstrate that shell-induced strain has large effects on the hole spectrum of these systems.Comment: 21 pages, 5 figure

Long-Range Interaction between Charge and Spin Qubits in Quantum Dots

We analyze and give estimates for the long-distance coupling via floating metallic gates between different types of spin qubits in quantum dots made of different commonly used materials. In particular, we consider the hybrid, the singlet-triplet, and the spin-$1/2$ qubits, and the pairwise coupling between each type of these qubits with another hybrid qubit in GaAs, InAs, Si, and $\mathrm{Si_{0.9}Ge_{0.1}}$. We show that hybrid qubits can be capacitively coupled strongly enough to implement two-qubit gates, as long as the distance of the dots from the metallic gates is small enough. Thus, hybrid qubits are good candidates for scalable implementations of quantum computing in semiconducting nanostructures

Circuit QED with Hole-Spin Qubits in Ge/Si Nanowire Quantum Dots

We propose a setup for universal and electrically controlled quantum information processing with hole spins in Ge/Si core/shell nanowire quantum dots (NW QDs). Single-qubit gates can be driven through electric-dipole-induced spin resonance, with spin-flip times shorter than 100 ps. Long-distance qubit-qubit coupling can be mediated by the cavity electric field of a superconducting transmission line resonator, where we show that operation times below 20 ns seem feasible for the entangling square-root-of-iSWAP gate. The absence of Dresselhaus spin-orbit interaction (SOI) and the presence of an unusually strong Rashba-type SOI enable precise control over the transverse qubit coupling via an externally applied, perpendicular electric field. The latter serves as an on-off switch for quantum gates and also provides control over the g factor, so single- and two-qubit gates can be operated independently. Remarkably, we find that idle qubits are insensitive to charge noise and phonons, and we discuss strategies for enhancing noise-limited gate fidelities.Comment: 6 pages main article + 12 pages supplement, 4 figure

Exchange interaction of hole-spin qubits in double quantum dots in highly anisotropic semiconductors

We study the exchange interaction between two hole-spin qubits in a double quantum dot setup in a silicon nanowire in the presence of magnetic and electric fields. Based on symmetry arguments we show that there exists an effective spin that is conserved even in highly anisotropic semiconductors, provided that the system has a twofold symmetry with respect to the direction of the applied magnetic field. This finding facilitates the definition of qubit basis states and simplifies the form of exchange interaction for two-qubit gates in coupled quantum dots. If the magnetic field is applied along a generic direction, cubic anisotropy terms act as an effective spin-orbit interaction introducing novel exchange couplings even for an inversion symmetric setup. Considering the example of a silicon nanowire double dot, we present the relative strength of these anisotropic exchange interaction terms and calculate the fidelity of the $\sqrt{\text{SWAP}}$ gate. Furthermore, we show that the anisotropy-induced spin-orbit effects can be comparable to that of the direct Rashba spin-orbit interaction for experimentally feasible electric field strengths.Comment: 18 pages, 7 figure

Prospects for Spin-Based Quantum Computing

Experimental and theoretical progress toward quantum computation with spins in quantum dots (QDs) is reviewed, with particular focus on QDs formed in GaAs heterostructures, on nanowire-based QDs, and on self-assembled QDs. We report on a remarkable evolution of the field where decoherence, one of the main challenges for realizing quantum computers, no longer seems to be the stumbling block it had originally been considered. General concepts, relevant quantities, and basic requirements for spin-based quantum computing are explained; opportunities and challenges of spin-orbit interaction and nuclear spins are reviewed. We discuss recent achievements, present current theoretical proposals, and make several suggestions for further experiments.Comment: 21 pages, 5 figure

Peningkatan Keterampilan Menulis Aksara Jawa melalui Penggunaan Model Pembelajaran Kooperatif Tipe Teams Games Tournament dan Media Ular Tangga

Dini Noviana Saputri. K4214008. Peningkatan Keterampilan Menulis Aksara Jawa Melalui Penggunaan Model Pembelajaran Kooperatif Tipe Teams Games Tournament Dengan Media Ular Tangga Pada Siswa Kelas VII F SMP Negeri 8 Surakarta. Skripsi. Surakarta: Fakultas Keguruan dan Ilmu Pendidikan, Universitas Sebelas Maret Surakarta, Oktober 2018. Bagi siswa yang mempelajari bahasa Jawa sebagai bahasa kedua atau ketiga, membaca dan menulis huruf Jawa merupakan materi yang dianggap sulit. Oleh karena itu, siswa tidak suka dan tidak tertarik mempelajarinya. Tujuan penelitian ini adalah untuk meningkatkan motivasi dan kemampuan menulis huruf Jawa dengan metode kooperatif Teams Games Tournament dan meda permainan Ular Tangga pada siswa kelas VII F SMP Negeri 8 Surakarta tahun ajaran 2017/2018, penelitian ini merupakan penelitian tindakan kelas dengan dua siklus. Setiap siklus terdiri atas kegiatan perencanaan, pelaksanaan, pengamatan dan refleksi. Subjek dari penelitian ini adalah siswa kelas VII F yang berjumlah 32 orang siswa dengan 14 siswa laki-laki dan 18 siswa perempuan. Teknik pengumpulan data yang digunakan adalah observasi, wawancara, analisis dokumen dan tes. Pengujian validitas data menggunakan teknik triangulasi sumber dan triangulasi metode. Kemudian, teknik analisis data yang digunakan adalah analisis deskriptif komparatif dan analisis kritis. Analisis data menunjukkan adanya peningkatan baik dalam segi motivasi belajar maupun segi kualitas hasil belajar. Motivasi belajar siswa saat pratindakan hanya sebesar 49,80%, kemudian meningkat pada siklus I menjadi 60,9%, dan meningkat lagi menjadi 80,66% pada siklus II. Sedangkan, pada segi kualitas hasil belajar siswa juga diketahui hanya sebesar 37,5%, kemudian mengalami peningkatan pada siklus I sebesar 59,4%, dan meningkat lagi pada siklus II menjadi 78,1%. Hasil penelitian ini menunjukkan bahwa melalui permainan Ular Tangga yang dilaksanakan bersama teman memungkinkan siswa lebih termotivasi untuk menguasai penulisan Aksara Jawa. Selain itu, dengan diberlakukannya kompetisi antar kelompok dapat menciptakan suasana belajar yang lebih menyenangkan dan menantang. Simpulan dari penelitian ini adalah penerapan metode TGT dan media Ular Tangga dapat meningkatkan motivasi belajar dan kualitas hasil belajar menulis aksara Jawa pada siswa kelas VII F SMP Negeri 8 Surakarta

Strong Spin-Orbit Interaction and Helical Hole States in Ge/Si Nanowires

We study theoretically the low-energy hole states of Ge/Si core/shell nanowires. The low-energy valence band is quasidegenerate, formed by two doublets of different orbital angular momenta, and can be controlled via the relative shell thickness and via external fields. We find that direct (dipolar) coupling to a moderate electric field leads to an unusually large spin-orbit interaction of Rashba type on the order of meV which gives rise to pronounced helical states enabling electrical spin control. The system allows for quantum dots and spin qubits with energy levels that can vary from nearly zero to several meV, depending on the relative shell thickness.Comment: 8 pages, 6 figure

Heavy hole states in Germanium hut wires

Hole spins have gained considerable interest in the past few years due to their potential for fast electrically controlled qubits. Here, we study holes confined in Ge hut wires, a so far unexplored type of nanostructure. Low temperature magnetotransport measurements reveal a large anisotropy between the in-plane and out-of-plane g-factors of up to 18. Numerical simulations verify that this large anisotropy originates from a confined wave function which is of heavy hole character. A light hole admixture of less than 1% is estimated for the states of lowest energy, leading to a surprisingly large reduction of the out-of-plane g-factors. However, this tiny light hole contribution does not influence the spin lifetimes, which are expected to be very long, even in non isotopically purified samples