A New Frontier in X-Rays and Quantum Sensing

When certain atoms are irradiated with laser light, they can produce laser pulses with extremely high frequencies in the X-ray range. Until now, the theoretical model of this effect predicted an upper limit to the energy, known as the energy cutoff. Past this point, hardly any X-rays are produced.

New research from the University of California San Diego, TU Wien (Austria) and the University of Salamanca (Spain) succeeds in overcoming this cutoff. Using helium atoms, the researchers reached a much higher energy range than standard theory predicts, because the atom's two electrons can release their energy together as a single X-ray photon.

For this experiment, UC San Diego Assistant Professor of Physics Tenio Popmintchev's team used intense UV lasers and helium atoms. The first electron is released and accelerated, followed by the second. The two electrons are not independent of one another, but are quantum-mechanically correlated and entangled from the moment they are freed until the moment they return.

Using UV driving pulses, the team could arrange for both electrons to recombine with the same ion at exactly the same instant, releasing their combined energy as one higher-energy X-ray photon. This double-electron recombination is the reverse of a process in which a single photon ejects two electrons at once — something that can happen only because the electrons are correlated. Here it has been observed for the first time.