Phonon Decoherence of a Double Quantum Dot Charge Qubit

We study decoherence of a quantum dot charge qubit due to coupling to piezoelectric acoustic phonons in the Born-Markov approximation. After including appropriate form factors, we find that phonon decoherence rates are one to two orders of magnitude weaker than was previously predicted. We calculate the dependence of the Q-factor on lattice temperature, quantum dot size, and interdot coupling. Our results suggest that mechanisms other than phonon decoherence play a more significant role in current experimental setups.Comment: RevTex, 7 pages, 5 figures. v2: appendix added, more details provided. Accepted for publication in PR

Coulomb Blockade Oscillations of Conductance at Finite Energy Level Spacing in a Quantum Dot

We find an analytical expression for the conductance of a single electron transistor in the regime when temperature, level spacing, and charging energy of a grain are all of the same order. We consider the model of equidistant energy levels in a grain in the sequential tunneling approximation. In the case of spinless electrons our theory describes transport through a dot in the quantum Hall regime. In the case of spin-1/2 electrons we analyze the line shape of a peak, shift in the position of the peak's maximum as a function of temperature, and the values of the conductance in the odd and even valleys.Comment: RevTex, 13 pages, 13 figure

Phonon-induced decoherence and dissipation in donor-based charge qubits

We investigate the phonon-induced decoherence and dissipation in a donor-based charge quantum bit realized by the orbital states of an electron shared by two dopant ions which are implanted in a silicon host crystal. The dopant ions are taken from the group-V elements Bi, As, P, Sb. The excess electron is coupled to deformation potential acoustic phonons which dominate in the Si host. The particular geometry tailors a non-monotonous frequency distribution of the phonon modes. We determine the exact qubit dynamics under the influence of the phonons by employing the numerically exact quasi-adiabatic propagator path integral scheme thereby taking into account all bath-induced correlations. In particular, we have improved the scheme by completely eliminating the Trotter discretization error by a Hirsch-Fye extrapolation. By comparing the exact results to those of a Born-Markov approximation we find that the latter yields appropriate estimates for the decoherence and relaxation rates. However, noticeable quantitative corrections due to non-Markovian contributions appear.Comment: 8 pages, 8 figures, published online in Eur.Phys.J.B, article in press; the original publication is avaiable at www.eurphysj.or

Spin Qubits in Multi-Electron Quantum Dots

We study the effect of mesoscopic fluctuations on the magnitude of errors that can occur in exchange operations on quantum dot spin-qubits. Mid-size double quantum dots, with an odd number of electrons in the range of a few tens in each dot, are investigated through the constant interaction model using realistic parameters. It is found that the constraint of having short pulses and small errors implies keeping accurate control, at the few percent level, of several electrode voltages. In practice, the number of independent parameters per dot that one should tune depends on the configuration and ranges from one to four.Comment: RevTex, 6 pages, 5 figures. v3: two figures added, more details provided. Accepted for publication in PR

Coulomb Blockade Peak Spacings: Interplay of Spin and Dot-Lead Coupling

For Coulomb blockade peaks in the linear conductance of a quantum dot, we study the correction to the spacing between the peaks due to dot-lead coupling. This coupling can affect measurements in which Coulomb blockade phenomena are used as a tool to probe the energy level structure of quantum dots. The electron-electron interactions in the quantum dot are described by the constant exchange and interaction (CEI) model while the single-particle properties are described by random matrix theory. We find analytic expressions for both the average and rms mesoscopic fluctuation of the correction. For a realistic value of the exchange interaction constant J_s, the ensemble average correction to the peak spacing is two to three times smaller than that at J_s = 0. As a function of J_s, the average correction to the peak spacing for an even valley decreases monotonically, nonetheless staying positive. The rms fluctuation is of the same order as the average and weakly depends on J_s. For a small fraction of quantum dots in the ensemble, therefore, the correction to the peak spacing for the even valley is negative. The correction to the spacing in the odd valleys is opposite in sign to that in the even valleys and equal in magnitude. These results are robust with respect to the choice of the random matrix ensemble or change in parameters such as charging energy, mean level spacing, or temperature.Comment: RevTex, 11 pages, 9 figures. v2: Conclusions section expanded. Accepted for publication in PR

Phonon-induced decoherence of the two-level quantum subsystem due to relaxation and dephasing processes

Phonon-related decoherence effects in a quantum double-well two-level subsystem coupled to a solid are studied theoretically by the example of deformation phonons. Expressions for the reduced density matrix at T=0 are derived beyond the Markovian approximation by means of explicit solution of the non-stationary Schrodinger equation for the interacting electron-phonon system at the initial stage of its evolution. It is shown that as long as the difference between the energies of the electron in the left and the right well greatly exceeds the energy of the electron tunneling between the minima of the double-well potential, decoherence is primarily due to dephasing processes. This case corresponds to a strongly asymmetric potential and spatially separated eigenfunctions localized in the vicinity of one or another potential minimum. In the opposite case of the symmetric potential, the decoherence stems from the relaxation processes, which may be either "resonant" (at relatively long times) or "nonresonant" (at short times), giving rise to qualitatively different temporal evolution of the electron state. The results obtained are discussed in the context of quantum information processing based on the quantum bits encoded in electron charge degrees of freedom.Comment: 20 pages, no figure

Semiconductor Spintronics

Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role. In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism. This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics. The most fundamental spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling. Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians. The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions. Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations. A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors. In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field.Comment: tutorial review; 342 pages, 132 figure