Abstract
Quantum mechanics and relativity are the two pillars of modern physics. However, for over a century, their treatment of space and time has remained fundamentally disconnected. Relativity unifies space and time into a single fabric called spacetime, describing it seamlessly. In contrast, traditional quantum theory employs different languages: Quantum States (Density Matrix) for spatial systems and Quantum Channels for temporal evolution.
A recent breakthrough by Assistant Professor Seok Hyung Lie from the Department of Physics at UNIST offers a way to describe quantum correlations across both space and time within a single, unified framework.
In this study, the team developed a new theoretical approach that treats the entire timeline as one quantum state. This concept introduces what they call the Multipartite Quantum States over Time. In essence, it allows us to describe quantum processes at different points in time as parts of a single, larger quantum state. This means that both spatially separated systems and systems separated in time can be analyzed using the same mathematical language.
The researchers showed that, starting from just two simple assumptions-namely, that the initial state behaves linearly and that a quantum version of classical conditional probability, called the Quantum Conditionability holds-the mathematical structure of these multipartite states over time is uniquely determined. This result provides a solid foundation for describing quantum systems consistently across space and time.
Figure 1. (a) The Markovian extension of a QSOT uniquely characterized in Theorem 1 allows for sampling observables at time through temporally localized interventions, i.e., quantum snapshotting. (b) Contrarily, non-Markovian QSOTs do not have such a simple decomposition and sampling an observable at each time requires global access to multiple time steps.
Interestingly, the team also found a direct link between these multipartite quantum states over time and Kirkwood-Dirac quasiprobability distributions, a concept already well-known in quantum physics. This connection suggests new possibilities for experimentally probing quantum correlations over time, especially using recent techniques like Quantum Snapshotting, which can reconstruct these correlations in the lab with high precision.
This new framework bridges the gap between the traditional ways of describing spatial quantum states and temporal quantum processes, offering a more integrated way to understand how quantum systems behave in spacetime. It opens up exciting avenues for research in quantum information, measurement, and even the quest for a unified theory that combines quantum mechanics and gravity.
The work was led by Assistant Professor Seok Hyung Lie as the first author, with Professor James Fullwood from Hainan University serving as the corresponding author. Their collaboration creates new tools that could significantly impact future studies in quantum science and beyond.
Supported by the Institute for Information & Communications Technology Planning & Evaluation (IITP), this study has been published in Physical Review Letters, one of the most respected journals in physics. According to the Nature Index, more than a quarter of Nobel laureate research from 1997 to 2017 appeared in this journal.
In 2023 alone, the Department of Physics of UNIST published six papers in Physical Review Letters and was recognized as one of Korea's leading institutions for natural sciences and physics by the JoongAng Ilbo university rankings.
Journal Reference
Seok Hyung Lie and and James Fullwood, "Multipartite Quantum States over Time from Two Fundamental Assumptions," Phys. Rev. Lett., (2025).
