The double-slit experiment<\/a>, first performed in 1801 by British physicist Thomas Young, illustrates the dual nature of light in the quantum world. When you shine a beam of light\u2014photon \u201cparticles\u201d following a direct path\u2014through two parallel slits on a screen, what appears on the other side is an interference pattern resembling the union of two ripples in a pond, like a \u201cwave.\u201d But if you try to catch this mysterious transition in action by peering into the slit, you lose the interference pattern.<\/p>\n Niels Bohr, Einstein\u2019s main opponent in this debate, referred to this result as\u00a0complementarity<\/a>, the idea that it\u2019s impossible to simultaneously measure complementary properties of a quantum system. But Einstein surmised that, if a paper-thin slit held in place by a spring was struck with light, the individual photons would shake the spring in a particle-like manner. That way, we could catch the duality of light in action.<\/p>\n To test this hypothesis, the MIT team stripped down their experimental setup to the scale of single atoms, which they cooled down to microkelvin temperatures (for context, one kelvin is equivalent to -460 degrees Fahrenheit or -272 degrees Celsius). They used lasers to arrange more than 10,000 atoms into a neat, crystal-like configuration. This highly controlled environment allowed the researchers to adjust each atom\u2019s \u201cfuzziness,\u201d or the certainty of its location. Simply, a fuzzier atom increases the probability that a photon passing through will exhibit particle-like behavior.<\/p>\n \u201cThese single atoms are like the smallest slits you could possibly build,\u201d explained Wolfgang Ketterle, the study\u2019s senior author, to MIT News<\/a>. By repeatedly bombarding the atomic \u201cslits\u201d with photons, Ketterle, a 2001 Nobel laureate, and his team were able to record the diffraction pattern from the photons scattering off the atomic slits.<\/p>\n What they found, unsurprisingly, was that Bohr was correct. The more they zoomed in on the path of an individual photon, the weaker the diffraction pattern became, confirming we can\u2019t observe light as both a wave and a particle simultaneously. They also tried shutting off the lasers holding the atoms in place\u2014the \u201cspring\u201d for their setup. Even then, it was impossible to track a photon\u2019s path without disrupting the wave-like interference pattern.<\/p>\n \u201cIn many descriptions, the springs play a major role. But we show, no, the springs do not matter here; what matters is only the fuzziness of the atoms,\u201d explained Vitaly Fedoseev, study lead author, also to MIT News. \u201cTherefore, one has to use a more profound description [like Bohr\u2019s complementarity], which uses quantum correlations between photons and atoms.\u201d<\/p>\n![\"Niels](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/08\/niels-bohr-albert-einstein-quantum-debate-1280x1036.jpg\")
In the early 20th century, renowned physicists Niels Bohr (left) and Albert Einstein (right) engaged in a public debate on some key concepts in quantum mechanics. Credit: Science Museum\/Science & Society Picture Library <\/p>\n