Researchers from the University of Turku in Finland, the University of Milan in Italy, and Nicolaus Copernicus University in Poland have examined the idea of memory in quantum systems. In ordinary physics, a process is called memoryless when the future depends only on the current state of the system. If earlier states keep affecting later results, the process has memory. Quantum systems follow quantum mechanics, which is different from ordinary physics.
The new work, published in PRX Quantum, returns to a basic question: what exactly counts as memory when quantum rules apply? Quantum mechanics offers two equally valid but different ways to describe change over time. One approach tracks quantum states, which contain all possible information about a system. The other tracks observables, which are the physical quantities that experiments actually measure, such as energy levels or positions.
Different pictures of quantum change
These two descriptions always agree on experimental outcomes, yet they disagree on memory. Certain memory effects become visible only when following quantum states, while others appear only when following observables. As a result, the same quantum process can seem completely memoryless from one viewpoint and full of memory from the other. This means the concept of memory in quantum physics is richer and more flexible than earlier studies assumed.
“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,” say the researchers in a press release.
The discovery matters for practical applications. Quantum technologies, such as quantum computers and sensors, must deal with noise from their surroundings, and this noise often carries memory effects. Knowing exactly where and how memory shows up helps engineers design ways to reduce harmful noise or even harness useful environmental influences. The study therefore clarifies a core feature of quantum dynamics, the way quantum systems evolve, and points toward fresh lines of research in both theory and device development.