At Last, Alchemy Arrives in a Burst of Light—From Lasers

The idea sounds like magic, pure and simple. You create a light beam that can make substances vanish, give them properties they shouldn’t possess, or turn them into a perfect mimic of another substance entirely. It’s 21st-century alchemy, in principle capable not just of making lead resemble gold, but of turning ordinary materials into superconductors.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research develop­ments and trends in mathe­matics and the physical and life sciences.The general approach, developed over the course of decades, is to use tailored optical pulses to reshape the electron clouds of atoms and molecules. Earlier this summer, a team of researchers at Tulane University in New Orleans and their collaborators extended the idea. They figured out how to apply the pulse strategy to solids and bulk materials, rewriting the usual laws governing how their properties are dictated by their chemical composition and structure. Using quantum control, said Gerard McCaul at Tulane, “you can almost make anything look like anything.”

Meanwhile, other researchers have already used light pulses to conjure up superconductivity—the ability to conduct electricity without resistance—in materials that would not otherwise behave this way.

But perhaps the real potential of the technique doesn’t lie in enabling marvels of mimicry but in inducing other kinds of transformation. Light beams might be used to create optical computers powerful enough to solve difficult problems such as factorization. Chemical substances could become temporarily and selectively invisible, which would assist the analysis of complex mixtures. The theoretical possibilities seem limited only by our imagination. In practice, the limitations may stem from how well we can understand and control the interactions of light and matter.
A Plan for a Pulse

After the invention of the laser in the early 1960s, many researchers quickly realized that these devices could be used to manipulate molecules, since the molecules’ electron clouds feel and respond to the laser light’s electromagnetic fields, in which all the waves oscillate in step (that is, coherently). But to truly control something, you need to be able to prod or guide it on the timescale on which its trajectory changes—which is very fast for molecules and even faster for electrons. At first, laser pulses simply couldn’t be made short enough to deliver a sufficiently rapid sequence of nudges.

During the late 1980s and early 1990s, however, the pulse durations were brought down to as little as a few femtoseconds (a femtosecond is equal to 10–15 second), approaching the time frame of atomic motions. This enabled lasers to stimulate and probe those motions selectively. However, to actually control such movements, in the early 1990s Herschel Rabitz, a chemist at Princeton University, and his co-workers pointed out that one would need shaped pulses: complex waveforms that might guide molecular behavior along particular paths. That technology for pulse-shaping was, by good fortune, being developed at the time for optical telecommunications.

Herschel Rabitz, a chemist at Princeton University, pioneered the use of laser pulses to alter a substance’s quantum properties.Photograph: C. Todd Reichart/Princeton University
But the challenge is immense. To control the path taken by a macroscopic object—a glider, say—you need to know the trajectory that you’re seeking to modify. For a quantum mechanical system, the equivalent is to know how its quantum wave function evolves in time, which is determined by a mathematical function called the Hamiltonian. And there’s the rub—in all but the simplest systems, such as a hydrogen atom, the Hamiltonian becomes too complicated for researchers to calculate the dynamics of the wave function exactly.

In the absence of that knowledge—needed to calculate in advance what control pulse you need—the only alternative seemed to be trial and error: trying out some initial control pulse and then iterating it by running the same experiment again and again. It’s like a glider pilot learning to land by trying out random motions of the control stick and then gradually refining those movements after seeing what works.

That’s a lot more complicated (if less hazardous) for quantum systems than gliders. Shaping the pulse means adding more frequencies. The challenge is to figure out which combination of frequencies is needed. “It’s like a piano, but worse, because it had about 128 keys,” said Rabitz. (Today, pulse-shaping might involve a thousand or so frequency components.)
Now McCaul, working with Denys Bondar at Tulane and his colleagues, has described a theoretical scheme for calculating the required pulse in advance.