Quantum Reference Frames

Abstract illustration of relational descriptions in quantum theory. The blurred figure represents a delocalized quantum reference frame A, the intermediate object B denotes a general quantum system, and the sharply defined observer C represents a classical reference frame. The visual contrast emphasizes how localization, definiteness, and "classicality" emerge only relative to the chosen quantum reference frame.

Reference frames are the systems or structures relative to which we describe positions, times, and other observable properties. In standard physics they are usually treated as classical. In our work, however, we study both quantum reference frames understood as physical quantum systems—such as quantum rulers and clocks—and more abstract quantum coordinate systems, which generalise the classical notion of coordinates to situations in which spacetime relations themselves exhibit quantum features.

This requires extending familiar ideas such as changes of frame, rest frames, and covariance into the quantum regime. Covariance is the principle that physical laws preserve their form under changes of reference frame or coordinates. Our research asks how this principle must be reformulated when such changes are themselves quantum, namely when transformations relate quantum reference frames or quantum coordinates that may be in superposition or entangled with other systems. In this setting, the description of localisation, motion, superposition, entanglement, and even temporal order can become frame-dependent. These questions lie at the heart of our research at the interface of quantum theory, relativity, and gravity, and connect naturally to indefinite causal structures.