Every atom or molecule has a characteristic spectrum that it emits when excited by light. But this “chemical fingerprint”—it turns out—can be forged. Theoretical work demonstrates that tailoring an incoming light pulse can make a single atom of hydrogen, for example, emit a signal that mimics that of argon. The work demonstrates a new degree of flexibility available for controlling the quantum states of atoms. The theoretical approach could eventually lead to solutions to a related, reverse problem: how to discriminate between two molecules that have very similar spectral features.
Researchers often use light to make atoms and other systems behave in an unusual way. This so-called quantum control can, for example, steer a set of chemical reactions to yield more of a desired molecule than would be produced without the light. Another application is in high harmonic generation (HHG), where a strong light pulse excites a gas of atoms and causes them to emit at a series of frequencies that are much higher than normal [1]. Calculating the complicated, time-dependent input light pulse that optimizes HHG emission is computationally intense, so theorists have tried calculating the pulse shape analytically using approximations. But so far there is no general solution.
A team of theorists from Princeton University led by Herschel Rabitz tackled the problem of controlling the light emission from an atom using quantum control. Unlike previous attempts to calculate the required excitation pulse, where theorists dealt with a finite number of atomic states, the Princeton team's model assumes that the incoming light pulse could excite the atom into one of an infinite number of states. Some of these states are ionized states, in which an electron is freed from the atom but only temporarily. By incorporating this broader range of states, the team realized that they could control the quantum state of the atom in practically unlimited ways.
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