MIT Scientists Conduct Cleanest Test of Double-Slit Experiment

Scientists at the Massachusetts Institute of Technology (MIT) have successfully conducted the most precise demonstration of the well-known double-slit experiment, a cornerstone of quantum mechanics. By utilizing two single atoms as slits, the research team was able to infer the path of a photon through subtle measurements of changes in the atoms’ properties after photon scattering. Their findings align with predictions made by quantum theory, showcasing interference patterns when no path information was observed and distinct bright spots when the path was detected.

First carried out in the early 1800s by Thomas Young, the double-slit experiment is renowned for its seemingly paradoxical outcomes. The experiment’s design is straightforward: light is directed toward two slits in a barrier, allowing researchers to observe the resulting pattern. When light passes through both slits unobserved, it produces an interference pattern resembling ripples in water. Conversely, if the light’s path is monitored, the interference pattern disappears, yielding two distinct bright spots. This phenomenon raises profound questions about the nature of reality, suggesting that the act of observation itself influences physical outcomes.

The implications of these findings were hotly debated by physicists, notably Albert Einstein and Niels Bohr. Einstein argued that observation introduces noise, which disrupts the interference pattern, while Bohr maintained that the fundamental nature of quantum systems dictates that they can exhibit either wave-like or particle-like behavior, but not both simultaneously. Despite numerous replications of the experiment supporting Bohr’s perspective, real-world noise has left lingering doubts about the robustness of this principle.

To commemorate the International Year of Quantum Science and Technology, physicists led by Wolfgang Ketterle at MIT undertook a direct implementation of Einstein’s thought experiment. The team began by cooling over 10,000 rubidium atoms to near absolute zero, trapping them in a laser-created lattice so that each atom functioned as an individual scatterer of light. A faint beam of light was directed through this lattice, allowing a single photon to scatter off an atom.

The experiment presented significant challenges, as Hanzhen Lin, a PhD student involved in the study, explained: “This was the most difficult part. We had to repeat the experiment thousands of times to collect enough data.” The critical aspect of their methodology was controlling how much path information the atoms provided. By adjusting the laser traps, the research team manipulated the “fuzziness” of the atoms’ positions. Tightly trapped atoms had well-defined locations, which according to Heisenberg’s uncertainty principle, restricted their ability to reveal the photon’s path, resulting in an observable interference pattern.

In contrast, loosely trapped atoms possessed greater uncertainty in their positions, enabling them to move freely. This movement allowed an atom that interacted with a photon to carry a trace of that interaction, which was sufficient to eliminate the interference fringes and yield only distinct spots. Once again, Bohr’s predictions were confirmed.

While Lin acknowledged that other experiments have measured scattered light from trapped atoms, this study is notable for its ability to repeat measurements after the traps were removed, which allowed the atoms to float freely. This approach surpassed Einstein’s initial concept of spring-mounted slits, demonstrating that the results remained unchanged, thus ruling out interference from the traps.

The significance of this experiment has not gone unnoticed. Thomas Hird, a physicist at the University of Birmingham who was not part of the research, remarked, “I think this is a beautiful experiment and a testament to how far our experimental control has come. This probably far surpasses what Einstein could have imagined possible.”

Looking ahead, the MIT team aims to investigate the effects of having two atoms per site in the lattice, which could yield intriguing results from the interactions between the atoms. Their findings are documented in the journal Physical Review Letters. This groundbreaking work not only reinforces the principles of quantum mechanics but also showcases the advancements in experimental physics, offering new avenues for exploration in the quantum realm.