In classical physics, the idea of memory is straightforward. A system is considered memoryless if its future behavior depends only on its current state. If past states continue to influence what happens next, the system is said to have memory.

Quantum physics is far less clear-cut. Quantum systems can store and transfer information in ways that have no classical counterpart, and measurements themselves play a key role in how these systems evolve. Because of this, defining memory in quantum mechanics has remained a challenge.

Rethinking Memory in Quantum Systems

In a study published in PRX Quantum, researchers from the University of Turku in Finland, the University of Milan in Italy, and Nicolaus Copernicus University in Toruń in Poland revisited the concept of memory in quantum systems to better understand its meaning.

"Our work shows that memory is not a single concept but can manifest in different ways depending on how the evolution of a system is described," says first author, Doctoral Researcher Federico Settimo from the University of Turku.

Two Perspectives in Quantum Mechanics

Scientists have long studied memory by tracking how quantum states change over time, an approach rooted in the work of Erwin Schrödinger. However, quantum theory also offers another equally important framework developed by Werner Heisenberg. Instead of focusing on states, this perspective looks at how observable quantities evolve, meaning the measurable properties seen in experiments.

Although both approaches produce the same experimental results, the new research shows they are not interchangeable when it comes to describing memory.

Hidden Memory Effects Revealed

The team found that these two perspectives can reveal different types of memory. Some memory effects only appear when analyzing the evolution of quantum states, while others become visible only when focusing on observables.

This means a quantum system can appear memoryless in one description but show clear signs of memory in another. The finding suggests that quantum memory is more complex than previously believed and cannot be fully understood by looking at quantum states alone.

Implications for Quantum Technology

"Our findings open up new research avenues into the dynamics of quantum systems. Moreover, our work has implications beyond its foundational significance for quantum technologies, where the external environment induces noise and memory effects. Knowing how memory can be witnessed is essential for developing strategies to mitigate noise or exploit environmental effects in realistic quantum devices," says Professor of Theoretical Physics Jyrki Piilo from the University of Turku.

By clarifying how memory works in quantum systems, the study sheds new light on a fundamental aspect of quantum dynamics. It also highlights how the unique nature of quantum time evolution reshapes even basic concepts like memory, with potential consequences for future technologies.