Current sparks forbidden harmonics express
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.)
Original abstract
as written by the authors arXiv:2607.27847v1
High-harmonic generation (HHG) in inversion-symmetric systems is typically restricted to odd harmonics by symmetry. Here, we show that this selection rule can be broken without modifying the underlying Hamiltonian. We investigate a boundary-driven Su-Schrieffer-Heeger (SSH) chain coupled to source and sink reservoirs and demonstrate that dissipative dynamics generates a nonequilibrium steady state carrying a finite DC current. While the SSH Hamiltonian retains inversion symmetry, the current-carrying steady-state density matrix does not, leading to the emergence of even harmonics in the emitted spectrum. Using a correlation-matrix approach based on the Lindblad master equation, we obtain the steady state and calculate the resulting HHG response. We find that the intensity of the even harmonics is directly controlled by the transport current, establishing a link between nonequilibrium charge transport and HHG selection rules. Our results uncover a mechanism for even-harmonic generation that relies solely on nonequilibrium steady-state symmetry breaking and provide a route to probing transport currents through ultrafast nonlinear spectroscopy in centrosymmetric quantum systems.
The prohibition on even harmonics in symmetric systems stood for nearly a century — since the early days of quantum mechanics. This result shows that a non-equilibrium current can break symmetry without a single moving atom.