43 research outputs found
Orbital Contributions to the Electron g Factor in Semiconductor Nanowires
No full textRecent experiments on Majorana fermions in semiconductor nanowires [S. M. Albrecht, A. P. Higginbotham, M. Madsen, F. Kuemmeth, T. S. Jespersen, J. Nygård, P. Krogstrup, and C. M. Marcus, Nature (London) 531, 206 (2016)NATUAS0028-083610.1038/nature17162] revealed a surprisingly large electronic Landé g factor, several times larger than the bulk value - contrary to the expectation that confinement reduces the g factor. Here we assess the role of orbital contributions to the electron g factor in nanowires and quantum dots. We show that an L·S coupling in higher subbands leads to an enhancement of the g factor of an order of magnitude or more for small effective mass semiconductors. We validate our theoretical finding with simulations of InAs and InSb, showing that the effect persists even if cylindrical symmetry is broken. A huge anisotropy of the enhanced g factors under magnetic field rotation allows for a straightforward experimental test of this theory.</p
Coherence and high fidelity control of two-electron spin qubits in GaAs quantum dots
No full textElectron spin qubits confined in GaAs quantum dots are among the most established and well understood qubit systems. Long coherence times due to their weak interactionwith the environment and the electrical tunability of the semiconductor quantum dot have allowed GaAs-based spin qubits to play a central role in demonstrating the keyoperations of semiconductor spin qubits, such as initialization, read-out, universal control and two-qubit gates. Furthermore, spins confined in semiconductor nanostructuresprovide a solid-state approach to quantum computation which leverages current, well established semiconductor production technology for device fabrication and potential scalability.However, the interaction with nuclear spins in the GaAs host material complicates not only the preservation of qubit coherence, but also the precise control of the electronspins. As both these properties, the coherence time and the fidelity of gate operations, play a crucial role as prerequisites for quantum computing, the focus of this thesis areexperiments addressing these challenges on the basis of two-electron spin qubits. Interesting effects arise from the quadrupolar interaction of nuclear spins with electric field gradients. We show experimentally that quadrupolar broadening of the nuclear Larmor precession reduces electron spin coherence via faster decorrelation of transversenuclear fields. However, this effect disappears for appropriate field directions. Furthermore, we observe an additional modulation of coherence attributed to an anisotropicelectronic g-tensor. These results complete our understanding of dephasing in gated quantum dots and point to mitigation strategies. A key requirement for quantum computation are high-fidelity single qubit operations, which so far have not been demonstrated for encoded qubits in GaAs. Here, we realize such accurate operations by iteratively tuning of the all-electrical control pulses. Using randomized benchmarking, a well established characterization method, we find anaverage gate fidelity of F = (98.5 ± 0.1) % and determine the sum of gate leakage out of and back into the computational subspace to be L = (0.4 ± 0.1) %. These results demonstrate that high fidelity gates can be realized even in the presence of nuclear spins as existent in all III-V semiconductors.The potential of a feedback mechanism based on electric dipole spin resonance for narrowing the nuclear hyperfine field and its effectiveness for extending qubit coherencetime is investigated in a last experiment. Compared to a previously developed feedback mechanism, this polarization scheme promises higher and more stable pump rates andthe ability to set local magnetic fields in each quantum dot individuall
Kondo Effect in Carbon Nanotube Quantum Dots with Spin-Orbit Coupling
No full textMotivated by recent experimental observation of spin-orbit coupling in carbon nanotube quantum dots [F. Kuemmeth , Nature (London) 452, 448 (2008)], we investigate in detail its influence on the Kondo effect. The spin-orbit coupling intrinsically lifts out the fourfold degeneracy of a single electron in the dot, thereby breaking the SU(4) symmetry and splitting the Kondo resonance even at zero magnetic field. When the field is applied, the Kondo resonance further splits and exhibits fine multipeak structures resulting from the interplay of spin-orbit coupling and the Zeeman effect. A microscopic cotunneling process for each peak can be uniquely identified. Finally, a purely orbital Kondo effect in the two-electron regime is also predicted.NSFC 10575119
20433070
Major State Basic Research Developing Program 2007CB815004
NCET of Chin
Quantum transport in carbon nanotubes
No full textCarbon nanotubes are a versatile material in which many aspects of condensed matter physics come together. Recent discoveries have uncovered new phenomena that completely change our understanding of transport in these devices, especially the role of the spin and valley degrees of freedom. This review describes the modern understanding of transport through nanotube devices. Unlike in conventional semiconductors, electrons in nanotubes have two angular momentum quantum numbers, arising from spin and valley freedom. The interplay between the two is the focus of this review. The energy levels associated with each degree of freedom, and the spin-orbit coupling between them, are explained, together with their consequences for transport measurements through nanotube quantum dots. In double quantum dots, the combination of quantum numbers modifies the selection rules of Pauli blockade. This can be exploited to read out spin and valley qubits and to measure the decay of these states through coupling to nuclear spins and phonons. A second unique property of carbon nanotubes is that the combination of valley freedom and electron-electron interactions in one dimension strongly modifies their transport behavior. Interaction between electrons inside and outside a quantum dot is manifested in SU(4) Kondo behavior and level renormalization. Interaction within a dot leads to Wigner molecules and more complex correlated states. This review takes an experimental perspective informed by recent advances in theory. As well as the well-understood overall picture, open questions for the field are also clearly stated. These advances position nanotubes as a leading system for the study of spin and valley physics in one dimension where electronic disorder and hyperfine interaction can both be reduced to a low level.QN/Quantum NanoscienceApplied Science
Patent: Method for forecasting e.g. major volcanic event, involves generating test profile representing physiological status of animal, calculating ratio between test profile and reference profile, and setting alert if ratio reaches threshold value. Patent-Nr.: WO2013167661-A2; WO2013167661-A3; EP2847622-A2; US2015153477-A1, Max Planck Ges zur Foerderung der Wissenschaften
No full textCoupling of spin and orbital motion of electrons in carbon nanotubes
No full textElectrons in atoms possess both spin and orbital degrees of freedom. In non-relativistic quantum mechanics, these are independent, resulting in large degeneracies in atomic spectra. However, relativistic effects couple the spin and orbital motion, leading to the well-known fine structure in their spectra. The electronic states in defect-free carbon nanotubes are widely believed to be four-fold degenerate, owing to independent spin and orbital symmetries, and also to possess electron–hole symmetry. Here we report measurements demonstrating that in clean nanotubes the spin and orbital motion of electrons are coupled, thereby breaking all of these symmetries. This spin–orbit coupling is directly observed as a splitting of the four-fold degeneracy of a single electron in ultra-clean quantum dots. The coupling favours parallel alignment of the orbital and spin magnetic moments for electrons and antiparallel alignment for holes. Our measurements are consistent with recent theories that predict the existence of spin–orbit coupling in curved graphene and describe it as a spin dependent topological phase in nanotubes. Our findings have important implications for spin-based applications in carbon- based systems, entailing new design principles for the realization of quantum bits (qubits) in nanotubes and providing a mechanism for all-electrical control of spins in nanotubes
Strong spin-orbit interaction, helical hole states, and spin qubits in nanowires and quantum dots
No full textSemiconducting nanowires (NWs) and quantum dots (QDs) are promising platforms for spintronics and quantum computation. Great experimental and theoretical efforts have been made to continuously improve their performances, which is evident from the large variety of setups, material combinations, and operation schemes under investigation. With the work summarized in this PhD thesis, we want to contribute to a better understanding of some of these systems.
The main result of our work is the discovery of a novel spin-orbit interaction (SOI) of Rashba type that arises for holes in NWs in the presence of an electric field. In contrast to conventional Rashba and Dresselhaus SOI, this mechanism is not suppressed by the fundamental band gap and therefore unusually strong. As a consequence, we find that Ge/Si core/shell NWs can host helical hole states with remarkably large spin-orbit energies on the order of millielectronvolts. Furthermore, we propose a setup for universal and electrically controlled quantum information processing with hole-spin qubits in Ge/Si NW QDs. Single-qubit gates can be performed on a subnanosecond timescale; two-qubit gates can be controlled independently and over long distances; idle qubits are well protected against electrical noise and lattice vibrations (phonons).
Another key result follows from our analysis of the phonon-mediated decay of singlet-triplet qubits in lateral GaAs double quantum dots (DQDs). We find that two-phonon processes lead to strong dephasing when the DQDs are biased, and the predicted temperature dependence provides a possible explanation for recent experimental data. When the DQDs are unbiased, the dephasing is highly suppressed and the decoherence times of the qubits are by orders of magnitude longer than those for biased DQDs.
In the last part of the thesis, we present a technique for manipulating the emission polarization and the nuclear spins of a single self-assembled QD. Our scheme exploits a natural cycle in which an electron spin is repeatedly created with resonant optical excitation when the QD is tunnel coupled to a Fermi sea. Among other things, we find that the nuclear spin polarization and the effective electron g factor can be changed continuously from negative to positive via the laser wavelength, with a region of bistability near a particular detuning. An analogous behavior is observed for the average polarization of the spontaneously emitted photons. Our experimental results, some of which are counterintuitive, are very well reproduced with a quantitative model.
The thesis is organized as follows. In Chapter 1, we review experimental and theoretical progress toward quantum computation with spins in QDs, with particular focus on NW QDs, lateral QDs, and self-assembled QDs. In Chapter 2, we study the low-energy hole states of Ge/Si NWs in the presence of electric and magnetic fields. We also consider the shell-induced strain, which strongly affects the NW and QD spectra. In Chapter 3, hole-spin qubits in Ge/Si NW QDs are investigated. We find a highly anisotropic and electrically tunable g factor and analyze the qubit lifetimes due to phonon-mediated decay. A setup for quantum information processing with these qubits is proposed in Chapter 4, where we also present surprisingly simple formulas for the effective Hamiltonian of the qubits. A detailed analysis of the static strain and the low-energy phonons in core/shell NWs is provided in Chapter 5, completing the part on NWs and NW QDs. In Chapter 6, we investigate the phonon-mediated decay of singlet-triplet qubits in lateral DQDs. The developed technique for controlling the emission polarization and the nuclear spins of optically active QDs is discussed in Chapter 7. Supplementary information to Chapters 2-7 is appended
Metal-nanoparticle single-electron transistors fabricated using electromigration
No full textWe have fabricated single-electron transistors from individual metal nanoparticles using a geometry that provides improved coupling between the particle and the gate electrode. This is accomplished by incorporating a nanoparticle into a gap created between two electrodes using electromigration, all on top of an oxidized aluminum gate. We achieve sufficient gate coupling to access more than ten charge states of individual gold nanoparticles (5–15 nm in diameter). The devices are sufficiently stable to permit spectroscopic studies of the electron-in-a-box level spectra within the nanoparticle as its charge state is varied
Spin-orbit effects in carbon-nanotube double quantum dots
No full textWe study the energy spectrum of symmetric double quantum dots in narrow-gap carbon nanotubes with one and two electrostatically confined electrons in the presence of spin-orbit and Coulomb interactions. Compared to GaAs quantum dots, the spectrum exhibits a much richer structure because of the spin-orbit interaction that couples the electron's isospin to its real spin through two independent coupling constants. In a single dot, both constants combine to split the spectrum into two Kramers doublets while the antisymmetric constant solely controls the difference in the tunneling rates of the Kramers doublets between the dots. For the two-electron regime, the detailed structure of the spin-orbit split energy spectrum is investigated as a function of detuning between the quantum dots in a 22-dimensional Hilbert space within the framework of a single-longitudinal-mode model. We find a competing effect of the tunneling and Coulomb interaction. The former favors a left-right symmetric two-particle ground state while in the regime where the Coulomb interaction dominates over tunneling, a left-right antisymmetric ground state is found. As a result, ground states on both sides of the (11)-(02) degeneracy point may possess opposite left-right symmetry, and the electron dynamics when tuning the system from one side of the (11)-(02) degeneracy point to the other is controlled by three selection rules (in spin, isospin, and left-right symmetry). We discuss implications for the spin-dephasing and Pauli blockade experiments
