Research questions in quantum science

Unitary dynamics predicts an entangled system and apparatus. What connects that description to a definite recorded result?

Bell inequalities exclude local hidden-variable models, but no-signalling alone allows correlations stronger than quantum theory.

Loss of local interference is only part of the story: stable, independently accessible records must also be explained.

A branch-dependent gravitational phase offers a tractable starting point, but interpreting an entanglement signal requires explicit assumptions.

Interference gives efficient algorithms for some problems, but the boundary of efficient quantum computation is not settled.

Efficiently producing a quantum evolution is different from efficiently extracting a scientifically useful prediction.

Error correction changes the scaling of failure, but practical overhead depends on codes, noise, gates, and architecture together.

Pure-state entanglement has a clean asymptotic currency. Mixed states reveal irreversibility and limits to that description.

Quantum channel capacity requires both a coding strategy and a converse bound; optimizing one use is not generally enough.

Recovery depends on what the environment learned, not simply on whether a noisy transformation has an algebraic inverse.

A third particle removes the Schmidt decomposition, so tripartite entanglement comes in kinds rather than amounts. For three qubits the genuinely tripartite pure states fall into two classes that no local protocol interconverts.

A scalable device must preserve useful control and verifiable performance as calibration, connectivity, and correlated errors grow.

Repeaters promise a different rate–distance trade-off, but loss, memory, and scheduling must be accounted for together.

The ideal Heisenberg scaling is a starting point; useful sensing advantage must survive noise and equal-resource comparisons.