Decoherence and dissipation during a quantum XOR gate operation

The dynamics of a quantum XOR gate operation in a two-qubit system being coupled to a bath of quantum harmonic oscillators is investigated. Upon applying the numerical quasiadiabatic propagator path integral method, we obtain the numerically precise time-resolved evolution of this interacting two-qubit system in presence of time-dependent external fields without further approximations. We simulate the dissipative gate operation for characteristic experimental realizations of condensed matter qubits; namely, the flux and charge qubits realized in superconducting Josephson systems and qubits formed with semiconductor quantum dots. Moreover, we study systematically the quality of the XOR gate by determining the four characteristic gate quantifiers: fidelity, purity, the quantum degree, and the entanglement capability of the gate. Two different types of errors in the qubits have been modelled, i.e., bit-flip errors and phase errors. The dependence of the quality of the gate operation on the environmental temperature, on the friction strength stemming from the system-bath interaction, and on the strength of the interqubit coupling is systematically explored: Our main finding is that the four gate quantifiers depend only weakly on temperature, but are rather sensitive to the friction strength.Comment: 16 pages including 1 table and 5 figure

Landau Zener transitions in a dissipative environment: Numerically exact results

We study Landau-Zener transitions in a dissipative environment by means of the numerically exact quasiadiabatic propagator path-integral. It allows to cover the full range of the involved parameters. We discover a nonmonotonic dependence of the transition probability on the sweep velocity which is explained in terms of a simple phenomenological model. This feature, not captured by perturbative approaches, results from a nontrivial competition between relaxation and the external sweep.Comment: 4 pages, 5 figures; published version (minor changes

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

Exciton dynamics and Quantumness of energy transfer in the Fenna-Matthews-Olson complex

We present numerically exact results for the quantum coherent energy transfer in the Fenna-Matthews-Olson molecular aggregate under realistic physiological conditions, including vibrational fluctuations of the protein and the pigments for an experimentally determined fluctuation spectrum. We find coherence times shorter than observed experimentally. Furthermore we determine the energy transfer current and quantify its "quantumness" as the distance of the density matrix to the classical pointer states for the energy current operator. Most importantly, we find that the energy transfer happens through a "Schr\"odinger-cat" like superposition of energy current pointer states.Comment: enlarged and final version, 7 pages, 5 figure

Rashba induced chirality switching of domain walls and suppression of the Walker breakdown

In conventional domain wall systems the aim of a high domain wall velocity may be hindered by the occurrence of a Walker breakdown at comparably low current density. We show how a Rashba interaction can stabilize the domain wall dynamics and thereby shift the Walker breakdown to higher current densities. The Rashba interaction creates a field like spin torque, which breaks the symmetry of the system and modifies the internal structure of the domain wall. Besides a shift of the Walker breakdown it can additionally induce a chirality switch of the domain wall at sufficient Rashba fields. The preferred chirality may then be chosen by the direction of the current flow. Both, the suppression of the Walker breakdown and the chirality switching, affect the domain wall velocity. This is even more pronounced for short current pulses, where an additional domain wall movement after the pulse in either positive or negative direction can determine the final position of the domain wall.Comment: 10 pages, 9 figure

Dynamics of the spin-boson model with a structured environment

We investigate the dynamics of the spin-boson model when the spectral density of the boson bath shows a resonance at a characteristic frequency $\Omega$ but behaves Ohmically at small frequencies. The time evolution of an initial state is determined by making use of the mapping onto a system composed of a quantum mechanical two-state system (TSS) which is coupled to a harmonic oscillator (HO) with frequency $\Omega$. The HO itself is coupled to an Ohmic environment. The dynamics is calculated by employing the numerically exact quasiadiabatic path-integral propagator technique. We find significant new properties compared to the Ohmic spin-boson model. By reducing the TSS-HO system in the dressed states picture to a three-level system for the special case at resonance, we calculate the dephasing rates for the TSS analytically. Finally, we apply our model to experimentally realized superconducting flux qubits coupled to an underdamped dc-SQUID detector.Comment: 26 pages, 11 figures, Chemical Physics Special Issue on the Spin-Boson Problem, ed. by H. Grabert and A. Nitzan, in pres