Philip Bucksbaum Stanford PULSE Center Chapter 2 Control of Electrons and Nuclei in Atoms, Molecules, and Materials

Fundamental science challenges: Coherence and Control Chapter 2 describes the challenge of understanding and controlling coherence in new ways. Main concepts: Coherence and Control –Quantum coherence in materials control new phenomena Quantum degeneracy and quantum coherence Quantum coherence in photochemistry Quantum coherence and information science –Coherence properties of novel light sources to control new materials Chemical composition and chemical bond control Laser-driven materials properties Imaging materials in important new ways

We know there can be strong connections between materials and quantum coherence Superconductivity is just one of a number of phases related to quantum coherence of electrons at low temperatures in certain materials. Sophisticated magnetic materials are used widely (information storage, nanoscale sensors, and in the future for spintronics

The quantum state of matter at low temperatures: Quantum simulators Challenges: Quantum spin liquid at T  0: A triangular antiferromagnetic spin lattice (Physics Today, February 2007) Vortex arrays in superfluids made of atoms, molecules, and BCS pairs (Ketterle, MIT) The future: simulating Quantum Chromodynamics? ((F. Wilczek, Nat. Phys 3, 375 (2007).)

Excited state chemistry requires a new description Avoided crossings of “spaghetti” of states of diatomics becomes a puff pastry of conical intersections in polyatomic molecules. H 2 O anion - D. Haxton H C. H. Greene NH 3 “The Born-Oppenheimer approximation may be irrelevant. We don’t yet have a language to describe the physics these experiments can probe” -- W. Kohn

Why we can’t just calculate this stuff… Moore’s original graph predicting Moore’s Law in Chip capacity will double every two years. This must fail soon (2007). Too bad for us, because we need much more computing power: Kohn’s law: “Traditional multiparticle wave-function methods when applied to systems of many particles encounter an exponential wall when the number of atoms N exceeds a critical value which currently is in the neighborhood of N~10 (to within a factor of about 2)” (W. Kohn, Nobel Prize Address, 1999)

Quantum simulators, or some other new computing paradigm, is required Quantum computing? (Quantum entanglement as a resource.) Analog logic? 17nm features on a crossbar circuit, showing atomic-scale bumpiness. Analog circuit elements like memristors may be able to use such circuits more effectively.

Intense coherent sub-picosecond x-ray light sources will be able to track matter at extremes MD simulation of FCC copper X-ray diffraction image using LCLS probe of the (002) shows in situ stacking fault information 0 0 Diffuse scattering from stacking fault Peak diffraction moves from 0,0 due to relaxation of lattice under pressure Periodic features  average distance between faults S. K. Saxena & L. S. Dubrovinsky, American Mineralogist 85, 372 (2000). J. C. Boettger & D. C. Wallace, Physical Review B 55, 2840 (1997). C. S. Yoo et al., Physical Review Letters 70, 3931 (1993).

Coherent Control The control of quantum phenomena takes engineering control principles into the realm of quantum mechanics Time scales are picoseconds to attoseconds, and the size of objects under direct control are Angstroms to nanometers. The intellectual pay-off of this field is vast: essentially all dynamics events start with the atomic and molecular scale, including all of chemistry, and much of materials science.

COHERENT CONTROL IN MOLECULAR SYSTEMS Pulse Shaping: the optimal field discovered by OCT often has a broad bandwidth, with its phases adjusted to give a highly structured pulse. Learning Control: The learning loop brings the same feedback used in optimal control algorithms into the laboratory.

Connections to nature: photosynthesis Electron motion drives nuclear motion The retinal molecule (light blue) in the center of rhodopsin bends after absorbin light, to help move a proton across a membrane. Coherence enhancement? Some molecules appears to utilize quantum coherence in the process of photosynthesis. Challenges: 1. Discover the general principles for control 2. Real-time feedback for quantum control

New Experiments are showing us attosecond electron dynamics for the first time

QUANTUM ELECTRON SCATTERING Figure: Collecting an image of a nitrogen molecule as it undergoes strong-field ionization and recollision (cartoon at left). The image collected from the radiation produced in the recollision is shown at the top right, and a calculation of the most loosely bound ground state electron in nitrogen is shown on the bottom right. (from AMO2010: Controlling the Quantum World. Original artwork from: D. Villeneuve, NRC, Ottawa.

X-ray laser science Figure 13: The LCLS, Linac Coherent Light Source, when it is completed in 2009 at Stanford University, will be one-billion times brighter than currently existing synchrotrons.

Imaging with XFEL’s XFEL light will be a billion times more brilliant than current sources, in bursts shorter than the movement of the atoms in a molecule. Fundamental mechanisms of damage at such high intensities are not well understood. Can the coherence of the x-ray laser change the character of the damage?

Questions that frame the challenges in this chapter: A. Materials and coherence 1. How does electronic quantum coherence affect the properties of materials? 2. What is the role of quantum coherence in dynamics, especially photo-chemistry? B. Coherence and control 1. How can we control the quantum states of matter by applying coherent fields? (Coherent control) 2. How does matter behave on the timescale of electron motion? (Attoscience) 3. How can we utilize new generations of coherent sources for materials science and chemical science?