Peng-Bo Li

A tuning fork sounds when you strike it. Its prongs vibrate, and you hear the sound until the vibration dies out. The quantum world has its own "tuning forks"—for example, a defect in diamond that responds to magnetic fields, and a carbon tube that can tremble at frequencies billions of times higher than the note A.

A quantum computer is like an orchestra where strings and brass play in different languages. Superconducting processors operate with microwave photons in centimeter-scale circuits, while spin qubits, such as nitrogen-vacancy centers, store…

A typical photon source can be likened to a club entrance with a single bouncer. He tries to let people in one at a time, but occasionally couples sneak past. Physicists call this the purity problem: photons must arrive strictly solo, otherwise quantum computing breaks down.