Two quantum dots are like two tiny pockets in a piece of silicon where an electron can sit, like a ball in a cup. When the pockets are far apart, each electron lives on its own, and its energy resembles the levels of a solitary atom. But when the pockets are brought closer, the electron starts to feel its neighbor, and the levels split — creating a molecule-like structure with shared, distinct states.
Physicists studied this picture by varying three things: how much the pockets are shifted in energy, how easily an electron can hop between them, and the applied magnetic field. They watched the atomic levels smoothly transition into molecular ones — bonding and antibonding. In a magnetic field, even the electron's spin (its tiny magnetic moment) splits the levels, like a compass needle that can point up or down.
When two electrons are in the trap, they form a pair that can be either in a 'singlet' state (spins opposite) or a 'triplet' state (spins aligned). The energy difference between these states is crucial for quantum computing: it can serve as the logical zero and one. The authors learned to measure this difference directly, simply by adjusting the levels in the dots.
This method is like spectroscopy, but for artificial atoms. Instead of light absorbed by real atoms, electrical signals are used to peer inside the quantum dot. This opens the door to studying more complex materials where electron spins are more strongly coupled to their motion, and where new quantum effects may be hiding.
Quantum dots are so tiny that several thousand of them could fit side by side across a human hair.
In the series 'The Expanse,' quantum computers on silicon dots control ship navigation, and precise tuning of energy levels is key to their reliability.