Chemical reactions take place when molecules held together by atoms meet and reorganize into new compounds. It is one of nature’s most fundamental processes. Unfortunately, this transition happens too quickly for us to easily see – in a matter of femtoseconds which is to a second as a second is to 32 million years. Of course, this inability to see atoms move slowly hindered the detailed scientific study of chemical reactions.
All this changed In the late 1980s when the Egyptian scientist and later, Nobel Prize-winning chemist, Ahmed Zewail developed methods for studying chemical reactions at high speeds. By using ultrafast laser technology to produce flashes of light just a few femtoseconds long, he was able to visualize these reactions in slow motion, allowing him to see what actually happens when chemical bonds break up and new ones are formed. In doing so, he brought about a revolution in chemistry and related sciences.
Up until the advent of Zewail’s work, all chemists and physicists were convinced that it was impossible to track the movements involved in the transition states related to super-fast reactions as such efforts would run against the roadblock imposed by the Heisenberg Uncertainty Principle. 
Zewail changed all that. As a young Assistant Professor without tenure at Caltech, he delved deeply into concepts of coherence previously clarified by the Hungarian-American physicist, Eugene Wigner . From the late 1970s onwards, his work led him inexorably to his discovery of ultrafast laser spectroscopy and to his reward in Stockholm. At the ceremony in Stockholm in December 1999, when Zewail was the sole winner of the Nobel Prize for Chemistry that year, Professor Bengt Norden, a member of the Nobel Committee introduced the recipient with the following words:
“Zewail’s use of the fast laser technique can be likened to Galileo’s use of his telescope, which he directed towards everything that lit up the vault of heaven. Zewail tried his femtosecond laser on literally everything that moved in the world of molecules. He turned his telescope towards the frontiers of science.”
As though his discovery of the rapid laser technique was not enough, in 1991 Zewail embarked on another major scientific venture. This time, he designed a new type of electron microscope that, through dexterous use of ultrafast laser pulses and the photoelectric effect, created a stream of ultrafast electrons repeated on a femtosecond scale. This enabled him to chart movement of atoms in solids and their surfaces and biological material a thousand million times as fast as had been done before. This 4D electron microscope — three dimensions of space and one of time—is now transforming the whole landscape of physical, biological, medical and engineering science.
Roger Kornberg, a Nobel Laureate for his work in structural biology, in commenting on Zewail’s most recent book, “The 4D Visualization of Matter”, describes it as “a chronicle of an extraordinary journey of invention and discovery”. This work itself could have garnered Zewail a second Nobel Prize but for the fact that Zewail passed on in 2016, aged 70.
 Quantum coherence deals with the idea that all objects have wave-like properties. If an object’s wave-like nature is split in two, then the two waves may coherently interfere with each other in such a way as to form a single state that is a superposition of the two states.
 Zewail exploited the concept of coherence among molecular wave functions to achieve atomic-scale resolution of dynamics. Molecular wave functions are spatially diffuse and exhibit no motion. Superposing a number of separate wave functions of appropriately chosen phases, however, can produce a spatially localized and moving coherent superposition state, referred to as a wave packet The packet has a well-defined velocity and position which now makes it analogous to a moving classical object but at atomic resolution. The femtosecond light induces the coherence and makes it possible to reach atomic-scale spatial and temporal resolution, without violating the Uncertainty Principle.