A guitar string plucked exactly at its middle produces only odd overtones. The same prohibition holds in symmetric crystals: when light knocks out electrons, their re-emitted radiation contains only odd harmonics. But physicists have found a way to bypass this rule — without shifting a single atom.

The secret is electric current. When current flows through the same crystal, the electron cloud distorts and symmetry breaks. To analyze this process, the scientists used Stochastic modeling — it tracks the random hops of electrons between atoms driven by the contacts.

The experiment was done on a chain of atoms with alternating bonds (the SSH model), connected to two leads with different electron densities. A steady current arose, pushing the system into a non-equilibrium state. Although the crystal itself remained symmetric, the electron distribution became asymmetric. As a result, even harmonics appeared in the emission.

The discovery provides a new tool for ultrafast spectroscopy — a method that determines the properties of a material from its emission spectrum. Now, by measuring the photometry (brightness) of even harmonics, one can monitor currents in quantum chips with picosecond accuracy — and without interfering with their operation. (A picosecond is to a second what one day is to thirty thousand years.)