“Einstein and Bohr would have never thought that this is possible, to perform such an experiment with single atoms and single photons,” said Wolfgang Ketterle, head of the MIT group whose new experiment has reconfigured the double-slit experiment in its purest quantum form. This achievement comes at a timely moment: the centenary of quantum mechanics, a discipline that has both mystified and inspired generations of physicists.
The double-slit experiment, originally invented by Thomas Young in 1801, has been the icon of quantum strangeness for a long time. In its traditional form, a light beam is passed through two parallel slits and an interference pattern is projected onto a remote screen a telltale sign of wave behavior. But as quantum mechanics developed, the experiment uncovered a more profound mystery: light, and consequently all matter, has both wave-like and particle-like properties, but never simultaneously. Measurement collapses the duality and requires light to “decide” on a single nature.
Just under a century ago, this mystery prompted a famous debate between Albert Einstein and Niels Bohr. Einstein argued that a photon, when acting like a particle, ought to leave a small force on the slit it passes through, a gentle tug like a bird stirring a leaf. If this push were measurable, he said, one could also see the path of the photon and the interference pattern, both sides of quantum reality. Bohr responded with the uncertainty principle, maintaining that any way of trying to get path information would destroy the interference.
The MIT group’s solution bare-bones the experiment to quantum basics. Instead of macroscopic slits, they used more than 10,000 ultracold atoms, laser-cooled to microkelvin temperatures and organized in a crystal-like optical lattice. Each isolated atom, separated by precisely controlled laser fields, serves as an atomic-sized “slit”. By shining this lattice with a faint beam of light, they made sure that every atom scattered no more than one photon, allowing the final test of quantum theory’s predictions with atomic-scale accuracy.
A key innovation was in the capability of tuning each atom’s spatial localization its “fuzziness” to suit by varying the trapping lasers’ intensity. An atom tightly trapped is precisely localized, cutting down on its capacity to register a photon’s passage. A loosely bounded, fuzzier atom has a greater chance of “rustling” when a photon travels through, storing which-path information. This tunable fuzziness enabled the scientists to determine the likelihood that a photon would exist as a wave or a particle. With increasing fuzziness of the atom, the probability of particle-like behavior increased, and the interference pattern disappeared accordingly. The experimental results matched exactly with quantum theory: the less information gained about the route of the photon, the more visible the interference pattern.
The result gives a direct blow to Einstein’s spring analogy. In previous versions of the experiment, slits were thought of as suspended on springs, and therefore a passing photon would give a measurable displacement. Ketterle’s team experimented by briefly switching off the “spring” the laser trap so that the atoms floated freely in a vacuum for a millionth of a second before gravity took over. Even without this spring, the inherent quantum behavior held up: The simultaneous observation of wave and particle traits remained out of reach. As the lead author on this study, Vitaly Fedoseev, explained, In 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. Therefore, one has to use a more profound description, which uses quantum correlations between photons and atoms.
This realization is in line with current quantum measurement theory, where decoherence, the loss of quantum coherence, arises not through classical noise such as spring displacements but from entanglement between the system to be measured and its surroundings. Entanglement-induced decoherence is now known to be the mechanism through which quantum probabilities effectively become classical, as investigated in recent theory. The MIT experiment shows that decoherence in the double-slit experiment occurs due to quantum correlations and not mechanical back-action.
The technical feat of imaging and arranging thousands of ultracold atoms with such accuracy builds on developments in optical lattice trapping and quantum gas microscopy. High-fidelity, fast imaging made possible through methods like accordion lattices and binarization has emerged as a critical requirement for quantum simulation and computation, enabling scientists to image individual atoms and their quantum states with fidelities above 99.4% within microseconds. These technologies not only facilitate such basic tests but also open the door to more advanced quantum technologies.
As the International Year of Quantum Science and Technology progresses, the MIT group’s achievement serves as a reminder of the continuing power of simple idealized experiments to shed light on the deepest enigmas of the quantum universe.
