A quantum birthmark keeps a chaotic particle at least twice as likely to return to its starting state, even after a very long time.<\/p>\n
Stir a drop of milk into a cup of tea, and within seconds there\u2019s no telling where the drop first landed. <\/p>\n
Chaotic mixing like that erases any record of a starting point. In the quantum world of atoms and electrons, new research suggests, the erasing never finishes.<\/p>\n
Physicists at Tampere University<\/a> in Finland, Harvard University<\/a> and TU Dresden<\/a> in Germany call what\u2019s left behind a quantum birthmark.<\/p>\n Even a particle bouncing around chaotically stays at least twice as likely to be found back in its starting condition as in any comparable one. <\/p>\n That holds when its motion is averaged over a very long time. The work is theory, built on math and computer simulations rather than a lab experiment.<\/p>\n Joonas Keski-Rahkonen, a co-author, works in the Quantum Control and Dynamics<\/a> group at Tampere\u2019s Computational Physics Laboratory.<\/p>\n \u201cIn the everyday world, chaos wipes the slate clean. What we found is that quantum systems can\u2019t hide their origin, even in the middle of chaos,\u201d he said.<\/p>\n Testing a quantum billiard table<\/p>\n The team worked with one of physics\u2019 oldest test cases: a ball bouncing forever around a billiard table with rounded ends. <\/p>\n Physicists call that shape a stadium, and waves in oddly shaped spaces<\/a> like it tend to turn chaotic.<\/p>\n A real ball on that table bounces off the curved walls in a new direction every time. Track it long enough and it visits every spot equally, with no hint left of where it began.<\/p>\n Particles as small as electrons don\u2019t behave like billiard balls. <\/p>\n They spread out like ripples on water, and the most a physicist can say is how likely a particle is to be found in one spot rather than another. <\/p>\n A small, bunched-up ripple of that kind is called a wave packet.<\/p>\n The researchers simulated wave packets inside a stadium with soft walls the particle could seep into slightly, closer to what electrons meet in nanoscale devices<\/a>. <\/p>\n Within a few bounces, each wave packet looked like random noise spread across the whole stadium.<\/p>\n The starting point never fully fades<\/p>\n At any single moment, that noise looked featureless. The memory showed up only when the team averaged the motion over a very long stretch of time.<\/p>\n Averaged that way, a wave packet stayed at least twice as likely to be found back in its starting state as in a typical state with the same energy. <\/p>\n For some kinds of starting states, the minimum rises to three times. The advantage didn\u2019t belong to the first moment alone. <\/p>\n Every shape the wave packet passed through on its way kept it too, and the result held no matter when the team began averaging.<\/p>\n Classical physics predicts no advantage at all. By the team\u2019s math, every sealed-off quantum system that doesn\u2019t begin in a settled state carries a birthmark of its own.<\/p>\n Early bounces leave deeper marks<\/p>\n To see what makes a birthmark stronger, the team launched four wave packets. <\/p>\n Three left the center of the stadium along paths that repeat: straight up and down, straight side to side, and a bow-tie shape. The fourth started off-center on an ordinary chaotic path.<\/p>\n The ordinary launch came out close to the minimum, at about 1.8 times. The team noted that a single wave packet can land slightly below two, since two is an average. <\/p>\n The up-and-down launch, which kept bouncing back to where it began, reached about 6.6 times, and about 8 in a stadium with hard walls.<\/p>\n Early bounces mattered most. Ripple patterns from the wave packet\u2019s second hit on the wall still showed up in the long-time average.<\/p>\n Even the most ordinary wave packet reached only about half of the combinations of position and direction open to it. A classical ball would eventually have covered them all.<\/p>\n Quantum scars become a special case<\/p>\n In 1984, Eric Heller, a Harvard physicist and co-author of the new study, found that some quantum states keep sharp imprints along repeating paths, a pattern called quantum scarring<\/a>.<\/p>\n Scars were long treated as a rare exception to chaos. The team now treats them as one case of something broader: a scar is a birthmark made stronger by a path the particle keeps returning to.<\/p>\n Keski-Rahkonen tied the result to a larger question. <\/p>\n \u201cThat 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,\u201d he said.<\/p>\n He also pointed to a practical side. \u201cFor instance, it matters that a system which quietly remembers its starting point behaves differently from one that forgets,\u201d Keski-Rahkonen said.<\/p>\n In tiny electronic devices holding even thousands of electrons<\/a>, several statistical measures commonly run three or more times higher than random behavior would predict. <\/p>\n The authors read that as a sign of a birthmark. The American Physical Society\u2019s magazine Physics highlighted the study in a feature article<\/a>.<\/p>\n Quantum birthmarks in the real world<\/p>\n The math assumes a quantum system sealed off from its surroundings. Once such a system leaks information into the world around it, the universal part of the birthmark fades. <\/p>\n The particle then starts behaving more like a classical ball, though marks left by early bounces may still remain.<\/p>\n So far, the team hasn\u2019t tested the idea in a lab. How strongly a birthmark shows up also depends on what physicists choose to measure, and the researchers call that an important question for future work.<\/p>\n They suggest looking with quantum simulators, machines related to quantum computers<\/a> that are built from carefully controlled atoms<\/a>. <\/p>\n In those machines, physicists can set how many states a system has room for, so they could test whether a birthmark holds as the system grows.<\/p>\n For now, the birthmark exists on paper and in simulations. The next step is measuring one in a real device kept isolated well enough to hold on to it.<\/p>\n The full study was published in the journal Physical Review X<\/a>.<\/p>\n \u2014\u2013<\/p>\n Like what you read? Subscribe to our newsletter<\/a> for engaging articles, exclusive content, and the latest updates.<\/p>\n Check us out on EarthSnap<\/a>, a free app brought to you by Eric Ralls<\/a> and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"A quantum birthmark keeps a chaotic particle at least twice as likely to return to its starting state,…\n","protected":false},"author":2,"featured_media":896852,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[49],"tags":[199,79],"class_list":["post-896851","post","type-post","status-publish","format-standard","has-post-thumbnail","category-physics","tag-physics","tag-science"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts\/896851","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/comments?post=896851"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts\/896851\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/media\/896852"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/media?parent=896851"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/categories?post=896851"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/tags?post=896851"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}