Quantum systems can remember and forget at the same time

A quantum system can seem to remember its past and forget it at the same time. It all depends on how you look at it, a study suggests.

In physics, memory means a system's past still affects how it behaves.

Some systems are memoryless. Once information leaks into their surroundings, it is gone for good. Others have memory, because some of that lost information can flow back.

This matters for quantum computers, whose fragile qubits constantly leak information, causing errors.

Physicists have two main ways to describe how quantum systems change over time.

One, from Erwin Schrödinger, follows how the state of a system changes. The other, from Werner Heisenberg, follows how the things we can measure change.

Both give exactly the same predictions in experiments. For about a century, they have been treated as equal.

But researchers from the University of Turku in Finland, the University of Milan in Italy and Nicolaus Copernicus University in Poland found a surprising difference when it comes to memory.

Using mathematical analysis, they showed that the same quantum system can look memoryless in one picture and still show memory in the other.

Some memory effects appear only when you track quantum states. Others appear only when you track measurable quantities.

"Memory is not a single concept but can manifest in different ways," said first author Federico Settimo of the University of Turku.

Why does this matter? Understanding where memory hides could help engineers reduce noise in quantum computers, or even put the environment to use.

"Knowing how memory can be witnessed is essential for developing strategies to mitigate noise," said coauthor Jyrki Piilo.

There are caveats. This is a theoretical study, based on mathematics rather than experiments.

The practical uses for quantum technology are still potential, not proven.

RESEARCH PAPER 

Federico Settimo et al., "Divisibility of Dynamical Maps: Schrödinger Versus Heisenberg Picture," PRX Quantum (2026).