An atom is hit by a laser pulse. One electron is ripped out of the atom, another electron is shifted into a state with higher energy. Image Credit: TU Wien Quantum theory describes events that occur ...

So, after the laser pulse, one electron flies away and one remains with the atom with unknown energy. "We can show that these two electrons are now quantum entangled," says Burgdörfer.

Now, each of these paired electrons can have one of two spin states: up or down. If one electron spins upward, the other spins downward. Since it is not possible to tell which electron has which spin ...