August 31, 2026 -- Quantum theory has long treated space and time differently. Quantum states describe systems at a given moment, while changes that unfold over time are represented separately as quantum processes. Finding a consistent way to describe quantum information across both space and time has remained a fundamental challenge.

A research team, led by Professor Seok Hyung Lie of the Department of Physics at UNIST and Professor Hyukjoon Kwon of the School of Computational Sciences at the Korea Institute for Advanced Study (KIAS) has developed a theoretical framework for measuring quantum states that extend across spacetime. Their work shows that these states can be reconstructed using interferometry, a technique widely used in quantum experiments.

The researchers asked whether such measurements could be made without knowing the causal order between different regions in advance. They defined a causally agnostic measurement as one that remains valid regardless of that order, and proved that any measurement meeting this condition can be carried out through interferometry.

Interferometry uses the wave-like behavior of quantum systems to extract information from the interference between different paths. Applying the technique across space and time provides an experimental basis for describing both within the same framework.

The theory also reveals a limit to what interferometry alone can distinguish. Different quantum dynamics can sometimes produce the same quantum state over time, making them indistinguishable through these measurements.

To address this ambiguity, the team considered an additional quantum system that could serve as a reference. This led them to identify a new form of spatiotemporal correlation, which they call synchronization. When two quantum dynamics evolve in sync, one can provide a temporal reference for the other, allowing certain processes to be distinguished even under time-reversal symmetry.

A simple analogy illustrates the idea. A video of a ball moving through the air may not reveal whether it is playing forward or backward from the trajectory alone. A clock visible in the same frame provides a reference for the direction of time. In the researchers' framework, the additional quantum system serves a similar purpose.

“How to define a quantum state that encompasses both space and time, and how such a state can be measured, have remained important questions in quantum information science and the foundations of quantum theory,” said Professor Lie. “This study approaches the problem through measurement, asking what can actually be learned about a spatiotemporal quantum state from experimental observations.”

Professor Lie added that the framework could help describe and analyze quantum information processes in optical, condensed-matter, and superconducting systems. It may also provide a basis for studying situations in which the timing and location of events are not known in advance, including potential applications in quantum radar and particle-collision experiments.