Optimal laser-control of double quantum dots

Coherent single-electron control in a realistic semiconductor double quantum dot is studied theoretically. Using optimal-control theory we show that the energy spectrum of a two-dimensional double quantum dot has a fully controllable transition line. We find that optimized picosecond laser pulses generate population transfer at significantly higher fidelities (>0.99) than conventional sinusoidal pulses. Finally we design a robust and fast charge switch driven by optimal pulses that are within reach of terahertz laser technology.Comment: 5 pages, 4 figure

Optimal Control of charge transfer

In this work, we investigate how and to which extent a quantum system can be driven along a prescribed path in space by a suitably tailored laser pulse. The laser field is calculated with the help of quantum optimal control theory employing a time-dependent formulation for the control target. Within a two-dimensional (2D) model system we have successfully optimized laser fields for two distinct charge transfer processes. The resulting laser fields can be understood as a complicated interplay of different excitation and de-excitation processes in the quantum system

Optimal Control of Quantum Rings by Terahertz Laser Pulses

Complete control of single-electron states in a two-dimensional semiconductor quantum-ring model is established, opening a path into coherent laser-driven single-gate qubits. The control scheme is developed in the framework of optimal control theory for laser pulses of two-component polarization. In terms of pulse lengths and target-state occupations, the scheme is shown to be superior to conventional control methods that exploit Rabi oscillations generated by uniform circularly polarized pulses. Current-carrying states in a quantum ring can be used to manipulate a two-level subsystem at the ring center. Combining our results, we propose a realistic approach to construct a laser-driven single-gate qubit that has switching times in the terahertz regime.Comment: Phys. Rev. Lett. (in print) (2007

Tailoring laser pulses with spectral and fluence constraints using optimal control theory

Within the framework of optimal control theory we develop a simple iterative scheme to determine optimal laser pulses with spectral and fluence constraints. The algorithm is applied to a one-dimensional asymmetric double well where the control target is to transfer a particle from the ground state, located in the left well, to the first excited state, located in the right well. Extremely high occupations of the first excited state are obtained for a variety of spectral and/or energetic constraints. Even for the extreme case where no resonance frequency is allowed in the pulse the algorithm achieves an occupation of almost 100%

Optimal control of time-dependent targets

In this work, we investigate how and to which extent a quantum system can be driven along a prescribed path in Hilbert space by a suitably shaped laser pulse. To calculate the optimal, i.e., the variationally best pulse, a properly defined functional is maximized. This leads to a monotonically convergent algorithm which is computationally not more expensive than the standard optimal-control techniques to push a system, without specifying the path, from a given initial to a given final state. The method is successfully applied to drive the time-dependent density along a given trajectory in real space and to control the time-dependent occupation numbers of a two-level system and of a one-dimensional model for the hydrogen atom.Comment: less typo

Quantum control with spectral constraints

Various constraints concerning control fields can be imposed in the realistic implementations of quantum control systems. One of the most important is the restriction on the frequency spectrum of acceptable control parameters. It is important to consider the limitations of experimental equipment when trying to find appropriate control parameters. Therefore, in this paper we present a general method of obtaining a piecewise-constant controls, which are robust with respect to spectral constraints. We consider here a Heisenberg spin chain, however the method can be applied to a system with more general interactions. To model experimental restrictions we apply an ideal low-pass filter to numerically obtained control pulses. The usage of the proposed method has negligible impact on the control quality as opposed to the standard approach, which does not take into account spectral limitations.Comment: 6 pages, 4 figure

Aerodynamic impact of swirling Combustor Inflow on endwall heat transfer and the robustness of the film cooling design in an axial turbine

The development of new gas turbines and aero engines is dedicated to reduce pollutant emissions in addition to the continuous strive to improve component efficiency and the consumption of fossil fuels. To foster this trend, new combustion concepts have come into play such as lean combustion. Whereas the emission of carbon dioxide can be reduced by lower fuel consumption, the formation of thermal nitrogen oxide can only be hindered by a leaner fuel-to-air mixture: Lower peak temperatures and avoiding a stochiometric concentration in the combustion chamber slow the thermal reaction process responsible for the formation of nitrogen oxides. Swirl and a recirculation zone are used to stabilize the combustion process and a redistribution of mass flow towards the endwalls occurs. Additionally, a changed temperature profile with reduced peak temperature, but increased temperature near both endwalls due to the reduced injection of dilution air in the combustor approaches the subsequent turbine stage. Associated, positive and negative incidence, high turbulence intensities and increased thermal load to the endwalls challenge the turbine design. To improve the understanding of the complex aerodynamic and aerothermal interaction, the aerodynamic impact of combustor swirl on the first vane row of a turbine, the nozzle guide vane (NGV), is investigated. The experiments are conducted at the Large Scale Turbine Rig (LSTR) in Darmstadt, which consists of a 1.5-stage axial turbine that is subject to an engine-representative swirl. A combustor simulator is used to vary the inflow to the turbine. Further goals of the investigation are to evaluate the robustness of its endwall film cooling design and to investigate endwall heat transfer and film cooling effectiveness experimentally by using infrared thermography and the auxiliary wall method. As a reference, axial and low-turbulent inflow to the turbine is investigated. A variation of the coolant mass flow rate highlights the influence on Nusselt numbers and film cooling effectiveness as well as the aerodynamic flow field. An increase of Nusselt numbers by up to 80% is observed with a concurrent increase of the film cooling effectiveness by up to 25%. In a combined analysis a significant heat flux reduction due to film cooling by 30% is achieved. A coolant mass flow rate (MFR) of one yields the greatest benefit. For higher MFR the further improvement of the film cooling effectiveness is counteracted by the even greater increase in heat transfer. With applied swirl, the flow field changes significantly. Averaged whirl angles of 15 ? to 20 ? and a mass flow redistribution to the endwalls are detected. The NGV exit flow exhibits a dominating influence of swirl on pressure losses instead of the coolant flows as it had been observed for the baseline. For similar settings of the stage parameters, an increase in Nusselt numbers by up to 40% is observed. The film cooling effectiveness is reduced because of the mass flow redistribution. For MFR greater than 1.5, the increase in Nusselt numbers is less decisive and remains at a similar level to the reference case. To achieve the same level of film cooling, the double amount of coolant air is necessary. These general trends are resolved for two clocking positions between swirler and vanes, whereby local differences are observed. The combined analysis of the thermal parameters shows a local increase of endwall heat flux and a local influence on the coolant injection. The coolant injection is still beneficial in reducing the heat flux for low injection rates, but the local extent varies much more. For higher injection rates above 1.5, only sections of the endwall face an improvement and there is a growing area, where increased heat flux and in consequence higher thermal load is applied in comparison to the reference

On the thermalization of a Luttinger liquid after a sequence of sudden interaction quenches

We present a comprehensive analysis of the relaxation dynamics of a Luttinger liquid subject to a sequence of sudden interaction quenches. We express the critical exponent $\beta$ governing the decay of the steady-state propagator as an explicit functional of the switching protocol. At long distances $\beta$ depends only on the initial state while at short distances it is also history dependent. Continuous protocols of arbitrary complexity can be realized with infinitely long sequences. For quenches of finite duration we prove that there exist no protocol to bring the initial non-interacting system in the ground state of the Luttinger liquid. Nevertheless memory effects are washed out at short-distances. The adiabatic theorem is then investigated with ramp-switchings of increasing duration, and several analytic results for both the propagator and the excitation energy are derived.Comment: 7 pages, 4 figure

Optimal Control of Superconducting N-level quantum systems

We consider a current-biased dc SQUID in the presence of an applied time-dependent bias current or magnetic flux. The phase dynamics of such a Josephson device is equivalent to that of a quantum particle trapped in a $1-$D anharmonic potential, subject to external time-dependent control fields, {\it i.e.} a driven multilevel quantum system. The problem of finding the required time-dependent control field that will steer the system from a given initial state to a desired final state at a specified final time is formulated in the framework of optimal control theory. Using the spectral filter technique, we show that the selected optimal field which induces a coherent population transfer between quantum states is represented by a carrier signal having a constant frequency but which is time-varied both in amplitude and phase. The sensitivity of the optimal solution to parameter perturbations is also addressed

Optimization Schemes for Selective Molecular Cleavage with Tailored Ultrashort Laser Pulses

We present some approaches to the computation of ultra-fast laser pulses capable of selectively breaking molecular bonds. The calculations are based on a mixed quantum-classical description: The electrons are treated quantum mechanically (making use of time-dependent density-functional theory), whereas the nuclei are treated classically. The temporal shape of the pulses is tailored to maximise a control target functional which is designed to produce the desired molecular cleavage. The precise definition of this functional is a crucial ingredient: we explore expressions based on the forces, on the momenta and on the velocities of the nuclei. The algorithm used to find the optimum pulse is also relevant; we test both direct gradient-free algorithms, as well as schemes based on formal optimal control theory. The tests are performed both on one dimensional models of atomic chains, and on first-principles descriptions of molecules.Comment: 51 page