In a silicon crystal, a phosphorus atom holds an extra electron on two very closely spaced orbits. These orbits are like two neighboring floors in a building where the staircase is almost invisible. A short laser pulse in the mid-infrared range pushes the electron from the lower floor to the upper one, and it starts swinging between them like a pendulum.
To observe this swinging, the scientists sent a second, weak pulse right after the first one, just enough to slightly rotate the plane of light oscillation. By measuring how much this second pulse rotated, they could tell where the electron was at any given moment. It turned out that the amplitude of the oscillation and the time it takes to decay depend on the number of free charges already excited in the crystal. The more such charges, the faster the electron loses its rhythm.
Sometimes, under certain conditions, the laser pulse does not start the oscillation but sharply shifts the very "floor"—that is, changes the shape of the orbits. Then the electron jumps to the third floor, which is usually closed. This shift is like not swinging a swing but giving the whole structure a sudden push. Thanks to this, they managed to excite a transition that was forbidden by symmetry rules for ordinary oscillation.
This experiment shows that quantum states in silicon can not only be created but also tuned by changing the environment. In the future, such controlled oscillations could become the basis for qubits — the cells of a quantum computer built on ordinary semiconductors.
Silicon is the second most abundant element in Earth's crust after oxygen. Almost all the sand on beaches consists of its oxide.