In the quantum world, identity is a hazy idea

The colours emitted by an atom are a dead giveaway for its identity, but now American physicists have worked
out how to make any atom impersonate any other—by hitting it with a specially crafted pulse of light.

The technique realises “an aspect of the alchemist dream to make different elements or materials look alike,” study author Andre Campos and colleagues from Princeton University describe in the journal Physical Review Letters,“albeit for the duration of a control laser pulse.”

For 200 years scientists have identified chemicals by the light they
emit. You can recognise a sodium lamp from its orange glow. It’s the same
orange you get by holding salt to a flame, a standard trick of chemistry. Astronomers can deduce the components of stars or dust clouds from their particular light fingerprint.

Now, the Princeton team show that, in theory at least, these fingerprints can be forged.

As the Danish physicist Niels Bohr and others worked out in the early 20^th century, atoms and molecules emit light when the electrons around them shuffle about between different levels of energy. An electron falling from a high energy level to a lower one gives out the colour of light that exactly matches the energy difference.

It’s a bit like a dog’s squeaky toy tumbling down the stairs, making a different sound depending on the height of the steps.

What Campos’s team realised was that they could use lasers to excite an atom into a state of any energy, at least temporarily, by hitting it with a specially designed light pulse. When the electron fell back down, it would then emit whatever light colour the physicists chose.

To illustrate their idea, the researchers worked out how to make hydrogen “look like” argon

This means that in principle astronomers detecting what they assume to be a cloud of argon gas might be mistaken.

[I]t could be a little green man with a fancy laser trying to mislead us by making hydrogen atoms look like argon atoms,” study co-author Denys Bondar told the American Physical Society.

In fact, two atoms or molecules can be made to look alike, by applying specific pulse shapes. In general, this means that simply measuring the light given out by a material is not enough to know what it’s composition. You need to know the input light, too.

Revealing a surprising flexibility in the behaviour of light and matter, the study might also be put to use for designing special materials with weird properties—perhaps molecules that fluoresce at will at different colours.

It might also be useful helping in biology, where it is a major challenge to distinguish very similar molecules in certain mixtures. Specific laser pulses will cause different molecules to emit different colours, and so provide a means to tell between them.

Though the team haven’t done the actual experiment yet, they point to recent work in Nature which show the technology to do it should be available. On to the experimenters!