Quantum reference frames and indefinite causal order

6 problems

• Can causal inequalities be violated when spacetime itself is superposed?

  Partially resolved

  Some theoretical processes give correlations that no fixed, or randomly chosen, order of events could explain; these violate causal inequalities. They cannot appear for events localized in an ordinary fixed spacetime; whether a spacetime in superposition, as quantum gravity may allow, can produce them is unknown.

• Quantum reference frame changes for general and imperfect frames

  Partially resolved

  Describing physics from the viewpoint of a quantum particle needs rules for switching viewpoints. Several frameworks give such rules for idealized frames; for realistic frames such as small quantum clocks they disagree or are undefined.

• Measure quantum time dilation of a clock in a momentum superposition

  Open

  A moving clock ticks slower by an amount fixed by its speed. If the clock is in a superposition of two speeds, theory predicts an extra effect that no ordinary random mixture of speeds can produce, and no measurement of it has been reported.

• Which process matrices are physically realizable?

  Partially resolved

  The mathematics allows many exotic arrangements of operations, but only some can be built with real devices. The task is the exact list, for four or more parties and for processes that cannot be completed into a reversible one.

• Do laboratory quantum switches realize genuinely indefinite causal order?

  Contested

  A quantum switch sends a photon through devices A and B in a superposition of the orders A-then-B and B-then-A. Critics say each device actually acts at two different times, so the order is definite in spacetime and only appears indefinite.

• Bell test for temporal order using a mass in superposition

  Open

  If a heavy mass is in a superposition of two positions, nearby clocks tick at superposed rates, which can put the order of two events in superposition. A proposed Bell-type test would show this order is not fixed in advance by hidden variables.