The Quantum-House Effect: Filling the Gap Between Classicality and Quantum Discord

We introduce the quantum-house effect, a non-local phenomenon which apparently does not require quantum discord to be present. It suffices for the effect if neither subsystem of a bipartite system is in a pure state. This way, the quantum-house effect completely fills the gap between trivial correlations and quantum discord. However, we discuss why the situation is more subtle than that, by showing that in a concrete cryptographic setting called "the quantum-house game", the ability to produce quantum discord is in fact necessary for the quantum-house effect to work. Then, we suggest a principle called "quantum detachment" to characterize where quantumness in general departs from classicality, based on the information a physical system contains about itself. The quantum-house effect is demonstrated on SpinQ Gemini, a 2-qubit liquid-state NMR desktop quantum computer.