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MIT scientists have conducted a refined interpretation of one of the most renowned experiments in quantum physics. Their results illustrate, with atomic-level accuracy, the dual yet elusive characteristic of light. They also happen to validate that Albert Einstein was incorrect regarding this specific quantum scenario.

The experiment under discussion is the double-slit experiment, initially carried out in 1801 by British researcher Thomas Young to demonstrate the wave behavior of light. Nowadays, with the advent of quantum mechanics, this experiment is acknowledged for its surprisingly straightforward portrayal of a perplexing reality: that light exists as both particle and wave. Even more intriguingly, this duality cannot be observed simultaneously. Detecting light as particles immediately obscures its wave-like characteristics, and vice versa.

The initial experiment involved directing a beam of light through two parallel slits in a barrier and observing the resulting pattern on a second distant screen. One might anticipate observing two overlapping spots of light, suggesting that light exists as particles, or photons, akin to paintballs following a direct trajectory. Yet, instead, the light forms alternating bright and dark bands on the screen, resembling the interference pattern created when two ripples in a pond intersect. This indicates light behaves as a wave. Surprisingly, when attempting to measure which slit the light traverses, the light abruptly acts as particles, and the interference pattern vanishes.

Today, the double-slit experiment is introduced in most high school physics courses as a concise means of illustrating the fundamental concept of quantum mechanics: that all physical entities, including light, simultaneously exhibit particle and wave characteristics.

Nearly a century ago, the experiment was central to a friendly discourse between physicists Albert Einstein and Niels Bohr. In 1927, Einstein contended that a photon should traverse only one of the two slits, generating a slight force on that slit, akin to a bird disturbing a leaf in its flight. He posited that one could detect such a force while simultaneously observing an interference pattern, thereby capturing light’s particle and wave nature concurrently. In response, Bohr employed the quantum mechanical uncertainty principle and demonstrated that detecting the photon’s trajectory would erase the interference pattern.

Since then, scientists have conducted various adaptations of the double-slit experiment, all of which have, to varying extents, affirmed the validity of the quantum theory articulated by Bohr. Now, MIT physicists have executed the most “idealized” iteration of the double-slit experiment to date. Their version distills the experiment to its quantum essentials. They utilized individual atoms as slits and employed weak beams of light to ensure that each atom scattered at most one photon. By preparing the atoms in various quantum states, they were able to alter the information the atoms received about the path of the photons. The researchers thus validated the predictions of quantum theory: The more information gathered about the path (i.e., the particle aspect) of light, the lower the visibility of the interference pattern became.

They revealed where Einstein erred. Whenever an atom is “disturbed” by a passing photon, the wave interference diminishes.

“Einstein and Bohr would have never imagined that it is possible to conduct such an experiment with single atoms and single photons,” states Wolfgang Ketterle, the John D. MacArthur Professor of Physics and head of the MIT team. “What we have accomplished is an idealized Gedanken experiment.”

Their findings are published in the journal Physical Review Letters. Ketterle’s co-authors from MIT include primary author Vitaly Fedoseev, Hanzhen Lin, Yu-Kun Lu, Yoo Kyung Lee, and Jiahao Lyu, all affiliated with MIT’s Department of Physics, the Research Laboratory of Electronics, and the MIT-Harvard Center for Ultracold Atoms.

Cold confinement

Ketterle’s group at MIT engages with atoms and molecules they supercool to temperatures just above absolute zero and arrange in configurations confined by laser light. Within these ultracold, meticulously tuned clouds, exotic phenomena that only arise at the quantum, single-atom scale can emerge.

In a recent study, the team explored a seemingly unrelated query, examining how light scattering could unveil the properties of materials composed of ultracold atoms.

“We realized we can measure the extent to which this scattering process resembles a particle or a wave, and we quickly recognized that we could implement this new technique to realize this famous experiment in a very idealized manner,” Fedoseev explains.

In their new research, the team worked with over 10,000 atoms, which they cooled to microkelvin temperatures. They employed an array of laser beams to arrange the frozen atoms into a regularly spaced, crystal-like lattice structure. In this configuration, each atom is sufficiently distanced from the others that each can effectively be viewed as a single, isolated, identical atom. And 10,000 such atoms can create a signal that is easier to detect compared to a solitary atom or two.

The researchers reasoned that with this setup, they might direct a weak beam of light through the atoms and observe how a single photon scatters off two neighboring atoms, behaving as a wave or a particle. This would mirror how, in the original double-slit experiment, light transitions through two slits.

“What we have accomplished can be seen as a new variation of the double-slit experiment,” Ketterle notes. “These individual atoms are akin to the smallest slits one could possibly create.”

Tuning fuzz

Operating at the single photon level required repeating the experiment many times and using an ultrasensitive detector to record the pattern of light scattered off the atoms. From the intensity of the recorded light, the researchers could directly determine whether the light acted as a particle or a wave.

They were particularly fascinated by the scenario in which half the photons sent in behaved as waves, while half exhibited particle characteristics. They accomplished this by employing a method to adjust the likelihood that a photon would manifest as a wave versus a particle, by modifying an atom’s “fuzziness,” or the certainty of its position. In their experiment, each of the 10,000 atoms is secured by laser light that can be fine-tuned to tighten or relax the grip of the light. The more loosely an atom is confined, the fuzzier, or more “spatially extensive” it appears. The fuzzier atom responds more readily and records the photon’s path. Hence, by tuning an atom’s fuzziness, the researchers can boost the likelihood that a photon will exhibit particle-like behavior. Their observations completely aligned with the theoretical predictions.

Springs away

In their investigation, the group assessed Einstein’s hypothesis regarding how to detect the photon’s path. Conceptually, if each slit were carved into a very thin sheet of paper suspended in midair by a spring, a photon traversing through one slit should cause the corresponding spring to vibrate by a certain degree, signaling the photon’s particle nature. In earlier implementations of the double-slit experiment, physicists included such a spring-like element, which significantly contributed to describing the photon’s dual nature.

However, Ketterle and his colleagues managed to conduct the experiment without the proverbial springs. The team’s cloud of atoms is initially retained in position by laser light, akin to Einstein’s image of a slit suspended by a spring. The researchers reasoned that if they were to remove their “spring,” and observe precisely the same phenomenon, then it would indicate that the spring has no impact on a photon’s wave/particle duality.

This, too, was what they discovered. Over several trials, they disabled the spring-like laser holding the atoms steady and quickly took a measurement in a millionth of a second before the atoms became fuzzier and ultimately fell due to gravity. In this brief time frame, the atoms were essentially floating in free space. In this spring-free condition, the team noticed the same phenomenon: A photon’s wave and particle nature could not be perceived concurrently.

“In many explanations, springs play a major role. But we demonstrate that, no, the springs are not relevant here; what is significant is solely the fuzziness of the atoms,” Fedoseev states. “Thus, a more profound description is required, one that employs quantum correlations between photons and atoms.”

The researchers note that the year 2025 has been declared by the United Nations as the International Year of Quantum Science and Technology, marking the centenary of the formulation of quantum mechanics. The debate between Bohr and Einstein regarding the double-slit experiment transpired only two years later.

“It’s an extraordinary coincidence that we could assist in clarifying this historic disagreement in the same year we celebrate quantum physics,” remarks co-author Lee.

This research was partially supported by the National Science Foundation, the U.S. Department of Defense, and the Gordon and Betty Moore Foundation.

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