Quantum cascade lasers are compact, electrically pumped light sources in the technologically important mid-infrared and terahertz region of the electromagnetic spectrum^1,2. Recently, the concept of topology³ has been expanded from condensed matter physics into photonics⁴, giving rise to a new type of lasing^5,6,7,8 using topologically protected photonic modes that can efficiently bypass corners and defects⁴. Previous demonstrations of topological lasers have required an external laser source for optical pumping and have operated in the conventional optical frequency regime^5,6,7,8. Here we demonstrate an electrically pumped terahertz quantum cascade laser based on topologically protected valley edge states^9,10,11. Unlike topological lasers that rely on large-scale features to impart topological protection, our compact design makes use of the valley degree of freedom in photonic crystals^10,11, analogous to two-dimensional gapped valleytronic materials¹². Lasing with regularly spaced emission peaks occurs in a sharp-cornered triangular cavity, even if perturbations are introduced into the underlying structure, owing to the existence of topologically protected valley edge states that circulate around the cavity without experiencing localization. We probe the properties of the topological lasing modes by adding different outcouplers to the topological cavity. The laser based on valley edge states may open routes to the practical use of topological protection in electrically driven laser sources.

But the entanglement takes longer than the memory holds its state.

Brillouin scattering has applications ranging from signal processing^1,2, sensing³ and microscopy⁴ to quantum information⁵ and fundamental science^6,7. Most of these applications rely on the electrostrictive interaction between light and phonons^3,7,8. Here we show that in liquids optically induced surface deformations can provide an alternative and far stronger interaction. This allows the demonstration of ultralow-threshold Brillouin lasing and strong phonon-mediated optical coupling. This form of strong coupling is a key capability for Brillouin-reconfigurable optical switches and circuits^9,10, for photonic quantum interfaces¹¹ and to generate synthetic electromagnetic fields^12,13. While applicable to liquids quite generally, our demonstration uses superfluid helium. Configured as a Brillouin gyroscope¹⁴ this provides the prospect of measuring superfluid circulation with unprecedented precision, and exploring the rich physics of quantum fluid dynamics, from quantized vorticity to quantum turbulence^15,16.

A basic rule of quantum physics is that knowing too much about an experiment will break quantum interference, but now physicists have discovered a way to bend that rule.