What Kinds of Correlations are Allowed by Quantum Mechanics? A Link Between Quantum Chemistry and Quantum Data Storage

Quantum systems, unlike classical(non-quantum) systems, can be in a “superposition” of all possible states at the same time. Being in a superposition allows quantum states to have stronger correlations between pairs of particles than what would be possible through classical probability. Quantum systems, such as atoms and molecules, want to minimize their energy which depends on correlations between pairs of particles; the lowest energy states of a system will have “extreme” correlations, the lowest energy correlations allowable.

Despite their importance, very little about these extreme correlations is known. It was believed that in principle, if a state’s extreme two-particle correlations were known, one could determine the entire state. We find counterexamples using quantum error correction codes, which essential to the eventual realization of a large-scale quantum computer.