Quantum control is concerned with active manipulation of physical and chemical processes on the atomic and molecular scale. This work presents a perspective of progress in the field of control over quantum phenomena, tracing the evolution of theoretical concepts and experimental methods from early developments to the most recent advances. Among numerous theoretical insights and technological improvements that produced the present state-of- the-art in quantum control, there have been several breakthroughs of foremost importance. On the technology side, the current experimental successes would be impossible without the development of intense femtosecond laser sources and pulse shapers. On the theory side, the two most critical insights were (i) realizing that ultrafast atomic and molecular dynamics can be controlled via manipulation of quantum interferences and (ii) understanding that optimally shaped ultrafast laser pulses are the most effective means for producing the desired quantum interference patterns in the controlled system. Finally, these theoretical and experimental advances were brought together by the crucial concept of adaptive feedback control (AFC), which is a laboratory procedure employing measurement-driven, closed-loop optimization to identify the best shapes of femtosecond laser control pulses for steering quantum dynamics towards the desired objective. Optimization in AFC experiments is guided by a learning algorithm, with stochastic methods proving to be especially effective. AFC of quantum phenomena has found numerous applications in many areas of the physical and chemical sciences, and this paper reviews the extensive experiments. Other subjects discussed include quantum optimal control theory, quantum control landscapes, the role of theoretical control

https://iopscience.iop.org/article/10.1088/1367-2630/12/7/075008/pdf

Control of quantum phenomena: past, present and future

It's challenging to measure non-repetitive events in real time in the field of instrumentation and measurement. Dispersive Fourier transformation is an emerging method that permits capture of rare events, such as optical rogue waves and rare cancer cells in blood. This Review article covers the principle of dispersive Fourier transformation and its implementation in diverse applications.

Dispersive Fourier transformation for fast continuous single-shot measurements

The development of femtosecond time-resolved methods for the study of gas-phase molecular dynamics is founded upon the seminal studies of Zewail and co-workers, as recognized in 1999 by the Nobel Prize in Chemistry.1 This methodology has been applied to chemical reactions ranging in complexity from bond-breaking in diatomic molecules to dynamics in larger organic and biological molecules, and has led to breakthroughs in our understanding of fundamental chemical processes. Photoexcited polyatomic molecules and anions often exhibit quite complex dynamics involving the redistribution of both charge and energy.2-6 These processes are the primary steps in the photochemistry of many polyatomic systems,7 are important in photobiological processes such as vision and photosynthesis,8 and underlie many concepts in molecular electronics.9 Femtosecond time-resolved methods involve a pump-probe configuration in which an ultrafast pump pulse initiates a reaction or, more generally, creates a nonstationary state or wave packet, the evolution of which is monitored as a function of time by means of a suitable probe pulse. Time-resolved or wave packet methods offer a view complementary to the usual spectroscopic approach and often yield a physically intuitive picture. Wave packets can behave as zeroth-order or even classical-like states and are therefore very helpful in discerning underlying dynamics. The information obtained from these experiments is very much dependent on the nature of the final state chosen in a given probe scheme. Transient absorption and nonlinear wave mixing are often the methods of choice in condensed-phase experiments because of their generality. In studies of molecules and clusters in the gas phase, the most popular methods, laser-induced fluorescence and resonant multiphoton ionization, usually require the probe laser to be resonant with an electronic transition in the species being monitored. However, as a chemical reaction initiated by the pump pulse evolves toward products, one expects that both the electronic and vibrational structures of the species under observation will change. Hence, these probe methods can be * To whom corresondence should be addressed. A.S.: telephone (613) 993-7388, fax (613) 991-3437, E-mail albert.stolow@nrc.ca. D.M.N.: telephone (510) 642-3505, fax (510) 642-3635, E-mail dan@radon.cchem.berkeley.edu. 1719 Chem. Rev. 2004, 104, 1719−1757

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Femtosecond time-resolved photoelectron spectroscopy.

The ultrafast motion of electrons and holes after light-matter interaction is fundamental to a broad range of chemical and biophysical processes. We advanced high-harmonic spectroscopy to resolve spatially and temporally the migration of an electron hole immediately after ionization of iodoacetylene while simultaneously demonstrating extensive control over the process. A multidimensional approach, based on the measurement and accurate theoretical description of both even and odd harmonic orders, enabled us to reconstruct both quantum amplitudes and phases of the electronic states with a resolution of ~100 attoseconds. We separately reconstructed quasi-field-free and laser-controlled charge migration as a function of the spatial orientation of the molecule and determined the shape of the hole created by ionization. Our technique opens the prospect of laser control over electronic primary processes.

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Measurement and laser control of attosecond charge migration in ionized iodoacetylene

A comprehensive effort in photodynamical ab initio simulations of the ultrafast deactivation pathways for all five nucleobases adenine, guanine, cytosine, thymine, and uracil is reported These simulations are based on a complete nonadiabatic surface-hopping approach using extended multiconfigurational wave functions Even though all five nucleobases share the basic internal conversion mechanisms, the calculations show a distinct grouping into purine and pyrimidine bases as concerns the complexity of the photodynamics The purine bases adenine and guanine represent the most simple photodeactivation mechanism with the dynamics leading along a diabatic ππ* path directly and without barrier to the conical intersection seam with the ground state In the case of the pyrimidine bases, the dynamics starts off in much flatter regions of the ππ* energy surface due to coupling of several states This fact prohibits a clear formation of a single reaction path Thus, the photodynamics of the pyrimidine bases is much richer and includes also nπ* states with varying importance, depending on the actual nucleobase considered Trapping in local minima may occur and, therefore, the deactivation time to the ground state is also much longer in these cases Implications of these findings are discussed (i) for identifying structural possibilities where singlet/triplet transitions can occur because of sufficient retention time during the singlet dynamics and (ii) concerning the flexibility of finding other deactivation pathways in substituted pyrimidines serving as candidates for alternative nucleobases

https://www.pnas.org/content/pnas/107/50/21453.full.pdf

Relaxation mechanisms of UV-photoexcited DNA and RNA nucleobases

Random orientation of molecules within a sample leads to blurred observations of chemical reactions studied from the laboratory perspective. Methods developed for the dynamic imaging of molecular structures and processes struggle with this, as measurements are optimally made in the molecular frame. We used laser alignment to transiently fix carbon disulfide molecules in space long enough to elucidate, in the molecular reference frame, details of ultrafast electronic-vibrational dynamics during a photochemical reaction. These three-dimensional photoelectron imaging results, combined with ongoing efforts in molecular alignment and orientation, presage a wide range of insights obtainable from time-resolved studies in the molecular frame.

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Time-resolved molecular frame dynamics of fixed-in-space CS2 molecules.

The study of many fundamental processes in chemistry relies on the understanding of the dynamics of the valence electrons, which make and break chemical bonds. A laser method now provides direct information on the dynamics of the valence electrons—separate from any vibrational motion—during a polyatomic chemical reaction, without the need for strong laser fields that unavoidably influence the motions of these electrons.

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Time-resolved imaging of purely valence-electron dynamics during a chemical reaction

We show experimentally that field-free alignment of iodobenzene molecules, induced by a single, intense, linearly polarized 1.4-ps-long laser pulse, can be strongly enhanced by dividing the pulse into two optimally synchronized pulses of the same duration. For a given total energy of the two-pulse sequence the degree of alignment is maximized with an intensity ratio of 1:3 and by sending the second pulse near the time where the alignment created by the first pulse peaks.

Observation of enhanced field-free molecular alignment by two laser pulses.

We present a summary of recent advances in the understanding of the UV photophysics of the isolated DNA base adenine, emphasizing a discussion of the mechanisms behind the ultrafast relaxation following excitation to the pipi* band. Drawing on our femtosecond time-resolved photoelectron spectroscopy experiments, we discuss differences in the ultrafast relaxation of adenine and 9-methyladenine and consider the relative merits of the various proposed mechanisms.

Excited-state dynamics of isolated DNA bases: a case study of adenine.

Nonadiabatic laser alignment of an asymmetric top molecule is studied using the combination of a quantum dynamical theory and time-resolved photofragment imaging experiments. In particular, the degree of alignment of iodobenzene, induced by an intense, linearly polarized picosecond laser pulse, is calculated and measured. Pronounced alignment is obtained under field-free conditions.

Christer Z. Bisgaard

Papers

Time-resolved molecular frame dynamics of fixed-in-space CS2 molecules.

Time-resolved imaging of purely valence-electron dynamics during a chemical reaction

Observation of enhanced field-free molecular alignment by two laser pulses.

Excited-state dynamics of isolated DNA bases: a case study of adenine.

Nonadiabatic Alignment of Asymmetric Top Molecules: Field-Free Alignment of Iodobenzene