Quantum systems never quite forget where they came from
Even the most chaotic quantum systems keep a permanent mark of their own past – a "quantum birthmark" – that never fades. Researchers from Tampere University, Harvard University, and TU Dresden discovered the feature in their recent study. Their findings shed new light on the elusive relationship between classical and quantum mechanics. Looking ahead, these quantum birthmarks, along with related phenomenon known as "scars," could eventually be harnessed to power next-generation nanoelectronics.
Stir a drop of milk into a cup of tea and it is gone for good. The swirl blurs, spreads and evens out, and no amount of staring at the cup will tell you where the drop first landed. Physicists refer to this kind of unpredictable, memory-erasing behavior simply as chaos, and it underpins much of how we explain the everyday world: heat spreading through a room, smoke filling the air, a pinball rattling away from wherever it was launched.
A new study shows that the quantum realm – the world of atoms and electrons – does not play by that rule. There, the earliest moments leave a signature that never washes out.
"In the everyday world, chaos wipes the slate clean. What we found is that quantum systems can't hide their origin, even in the middle of chaos," says Dr. Joonas Keski-Rahkonen, a researcher in the Quantum Control and Dynamics (QCAD) group at Tampere University's Computational Physics Laboratory, and one of the study's lead authors.
The team worked with one of physics' venerable testing grounds: a ball bouncing endlessly around a table with curved walls, ricocheting off in a new direction every time — picture a billiard table with rounded ends, what physicists call a "stadium". If you track the ball long enough, it will have visited every part of the table, with no hint left of where it began. But quantum objects do not behave like billiard balls. They behave more like ripples spreading across water, and what physicists determine is the chance of finding them here rather than there.
When the researchers set such a quantum ripple (a "wave packet" in the physics lexicon) loose on the table, it scrambled within moments into what appears at any instant like a completely random pattern. Yet averaging that featureless-looking motion over very long stretches of time revealed a clear bias towards the system's own early history: it remained at least twice as likely to be found back in its original condition as in any other comparable one. And this partiality does not vanish. It persists for as long as the system stays quantum.
The result generalizes a phenomenon that has intrigued physicists for four decades. In 1984, Eric Heller, a Harvard professor and co-author of the new study, discovered that quantum systems sometimes retain surprisingly sharp imprints of repeating paths, an anomaly known as "quantum scarring" that has long been regarded as a rare exception. The new work establishes that scarring is just a special case of something universal: every quantum system carries a birthmark from its own beginnings.
Keski-Rahkonen says that the team aims at turning this into a general way of asking how much of its own history a quantum system can ever truly forget.
"That matters beyond textbook physics. It speaks to how quantum systems settle into equilibrium, and how our familiar classical world emerges from the quantum-mechanical rules. Moreover, quantum simulators and nanoscale electronics are nowadays small enough that these effects count. For instance, it matters that a system which quietly remembers its starting point behaves differently from one that forgets," he says.
Reference Quantum Birthmarks: Ergodicity Breaking Beyond Scarring
Anton M. Graf, Saul Atwood, Mingxuan Xiao, Roland Ketzmerick, Eric J. Heller, and Joonas Keski-Rahkonen
