D. Liese

Strong field quantum control by selective population of dressed states

We study the dynamics of potassium atoms in intense laser fields using femtosecond phase-locked pulse pairs in order to extract physical mechanisms of strong field quantum control. The structure of the Autler–Townes (AT) doublet in the photoelectron spectra is measured to analyse transient processes. The analysis shows that the physical mechanism is based on the selective population of dressed states (SPODS). Experimental results of closed loop optimization of SPODS are presented in addition. Applications to decoherence measurements with implications for quantum information are also proposed.

Quantum control and quantum control landscapes using intense shaped femtosecond pulses

A hitherto not considered physical mechanism of quantum control with intense shaped femtosecond laser pulses is investigated. Phase modulated pulses are used to exert control on the strong-field ionization of potassium atoms. We use a sinusoidal phase modulation function to manipulate the intensity of the Autler–Townes (AT) components in the photoelectron spectrum. The effect of all sine parameters is studied systematically. In addition, controllability is investigated using parameterized pulse shapes to generate a two-dimensional quantum control landscape. Our results show that the selective population of dressed states is the underlying strong-field physical mechanism. Due to its robustness with respect to the laser parameters, the selective dressed state population is an important general control mechanism.

Chapter 28 - Quantum control beyond spectral interference and population control: Can resonant intense laser pulses freeze the population?

The ability to measure and to control the quantum mechanical phase is the key step towards a deeper understanding of quantum control. Since the energy resolved photoelectron spectra from simultaneous excitation and ionization are directly related to the temporal evolution of the excited state (population and phase), this technique is most suited to elucidate details of the quantum control dynamics. In particular, the use of pulse sequences has proven a strong tool to study interference effects in atomic and molecular systems in detail. This scheme was extended to the continuum in order to demonstrate the coherence transfer from femtosecond laser pulses to ultrashort free electron wave packets. A variety of important control mechanisms is only accessible when strong laser fields are employed. Examples of coherent control by intense sequential laser pulses are coherent transients such as the photon echo and Ramsey fringes as well as the STIRAP. In this contribution, recent results on the control of the quantum mechanical phase of an atomic state in strong laser fields studied using the Autler-Townes (AT) effect in the photoionization of the K (4p) state are discussed. This chapter demonstrates quantum control beyond population control and spectral interference.

FEMTOSECOND PUMP-PROBE PHOTOELECTRON-SPECTROSCOPY ON ELECTRONIC STATES OF NA2: A TOOL TO STUDY ULTRAFAST CONTROL OF CHEMICAL REACTIONS

Microscopic control of the outcome of a chemical reaction is a long-standing dream in physical chemistry. Recently femtosecond lasers have emerged as a particularly suitable tool for quantum control of reaction dynamics. Assion et al. [1] were the first to demonstrate the use of tailored femtosecond laser pulses from a computer-controlled pulse shaper to control the branching ratios of various photodissociation channels in CpFe(CO)2Cl. Moreover, quite recently Levis et al. [2] have shown that high intensity laser pulses permit to control a dissociative rearrangement reaction in which chemical bonds are not only selectively broken, but also newly formed. Quantum control over chemical reactions may be obtained employing various so-called one parameter schemes [3-6]. Currently much attention has been focused on the utilization of adaptive feedback controlled femtosecond pulse shaping making available a most versatile instrument for multiparameter control schemes [1, 7-10]. These techniques have proven to be universal in the sense that they enable to optimize virtually any conceivable quantity and are capable to deliver the optimal electric field without the knowledge of the underlying potential energy surfaces (PES). However, the individual control mechanisms may be inferred, if at all, only for very simple systems. In order to get a better physical insight into the multi-parameter control driving such an experiment it is essential to investigate one parameter control schemes in detail on pertinent model systems.