The double-slit experiment has often been described as the most beautiful demonstration in physics. In one striking image, it shows the strange dual character of matter and light. When particles such as electrons or photons are sent through two narrow slits, the resulting pattern on a screen behind them is not the simple outline of the slits, but a series of alternating bright and dark bands. This pattern looks exactly like the ripples produced by waves on the surface of water when two stones are thrown in together. But when detectors are placed to see which slit each particle passes through, the pattern changes: the wave-like interference disappears and the particles line up as if they had travelled like microscopic bullets.
This puzzling switch between wave and particle behaviour became the stage for one of the deepest disputes of the 20th century. The two central figures were Albert Einstein and Niels Bohr, each with a different vision of what the double-slit experiment really meant. Their disagreement was not about the results themselves but about how these results should be interpreted, and what they revealed about the nature of reality.
Einstein believed strongly that the purpose of physics was to describe an external reality that exists independently of us. For him, the universe must have clear properties whether or not anyone is looking. In a double-slit experiment, this meant an electron or photon must in fact have taken a definite path, through one slit or the other, before striking the screen. The interference pattern might suggest some deeper process that we don’t yet understand but, to Einstein, it couldn’t mean that the particle lacked a path altogether.
Based on this idea, Einstein argued that quantum mechanics (as formulated in the 1920s) couldn’t be the full story. The strange idea that a particle had no definite position until measured, or that its path depended on the presence of a detector, was unacceptable to him. He felt that there must be hidden details that explained the apparently random outcomes. These details would restore determinism and make physics once again a science that described what happens, not just what is observed.
Bohr, however, argued that Einstein’s demand for definite paths misunderstood what quantum mechanics was telling us. Bohr’s central idea was called complementarity. According to this principle, particles like electrons or photons can show both wave-like and particle-like behaviour, but never both at the same time. Which behaviour appears depends entirely on how an experiment is arranged.
In the double-slit experiment, if the apparatus is set up to measure which slit the particle passes through, the outcome will display particle-like behaviour and the interference pattern will vanish. If the apparatus is set up without path detectors, the outcome will display wave-like interference. For Bohr, the two descriptions are not contradictions but complementary views of the same reality, each valid only within its experimental context.
Specifically, Bohr insisted that physics doesn’t reveal a world of objects with definite properties existing independently of measurement. Instead, physics provides a framework for predicting the outcomes of experiments. The act of measurement is inseparable from the phenomenon itself. Asking what “really happened” to the particle when no one was watching was, for Bohr, a meaningless question.
Thus, while Einstein demanded hidden details to restore certainty, Bohr argued that uncertainty was built into nature itself. The double-slit experiment, for Bohr, showed that the universe at its smallest scales does not conform to classical ideas of definite paths and objective reality.
The disagreement between Einstein and Bohr was not simply about technical details but a clash of philosophies. Einstein’s view was rooted in the classical tradition: the world exists in a definite state and science should describe that state. Quantum mechanics, he thought, was useful but incomplete, like a map missing a part of the territory.
Bohr’s view was more radical. He believed that the limits revealed by the double-slit experiment were not shortcomings of the theory but truths about the universe. For him, the experiment demonstrated that the old categories of waves and particles, causes and paths, couldn’t be applied without qualification. Science had to adapt its concepts to match what experiments revealed, even if that meant abandoning the idea of an observer-independent reality.
Though the two men never reached agreement, their debate has continued to inspire generations of physicists and philosophers. The double-slit experiment remains the clearest demonstration of the puzzle they argued over. Do particles truly have no definite properties until measured, as Bohr claimed? Or are we simply missing hidden elements that would complete the picture, as Einstein insisted?
A new study in Physical Review Letters has taken the double-slit spirit into the realm of single atoms and scattered photons. And rather than ask whether an electron goes through one slit or another, it has asked whether scattered light carries “which-way” information about an atom. By focusing on the coherence or incoherence of scattered light, the researchers — from the Massachusetts Institute of Technology — have effectively reopened the old debate in a modern setting.
The researchers trapped rubidium atoms held in an optical lattice, a regular grid of light that traps atoms in well-defined positions, like pieces on a chessboard. By carefully preparing these atoms in a particular state, each lattice site contained exactly one atom in its lowest energy state. The lattice could then be suddenly switched off, letting the atoms expand as localised wavepackets (i.e. wave-like packets of energy). A short pulse of laser light was directed at these atoms. The photons it emitted were scattered off the atoms and collected by a detector.
By checking whether the scattered light was coherent (with a steady, predictable phase) or incoherent (with a random phase), the scientists could tell if the photons carried hints of the motion of the atom that scattered them.
The main finding was that even a single atom scattered light that was only partly coherent. In other words, the scattered light wasn’t completely wave-like: one part of it showed a clear phase pattern, another part looked random. The randomness came from the fact that the scattering process linked, or entangled, the photon with the atom’s movement. This was because each time a photon was scattered off, the atom recoiled just a little, and that recoil left behind a faint clue about which atom had scattered the photon. This in turn meant that if the scientists looked close enough, they could work out where the photon came from in theory.
To study this effect, the team compared three cases. First, they observed atoms still held tightly in the optical lattice. In this case, scattering could create sidebands — frequency shifts in the scattered light — that reflected changes in the atom’s motion. These sidebands represented incoherent scattering. Second, they looked at atoms immediately after switching off the lattice, before the expanding wavepackets had spread out. Third, they examined atoms after a longer expansion in free space, when the wavepackets had grown even wider.
In all three cases, the ratio of coherent to incoherent light could be described by a simple mathematical term called the Debye-Waller factor. This factor depends only on the spatial spread of the wavepacket. As the atoms expanded in space, the Debye-Waller factor decreased, meaning more and more of the scattered light became incoherent. Eventually, after long enough expansion, essentially all the scattered light was incoherent.
Experiments with two different atomic species supported this picture. With lithium-7 atoms, which are very light, the wavepackets expanded quickly, so the transition from partial coherence to full incoherence was rapid. With the much heavier dysprosium-162 atoms, the expansion was slower, allowing the researchers to track the change in more detail. In both cases, the results agreed with theoretical predictions.
An especially striking observation was that the presence or absence of the trap made no difference to the basic coherence properties. The same mix of coherent and incoherent scattering appeared whether the atoms were confined in the lattice or expanding in free space. This showed that sidebands and trapping states were not the fundamental source of incoherence. Instead, what mattered was the partial entanglement between the light and the atoms.
The team also compared long and short laser pulses. Long pulses could in principle resolve the sidebands while short pulses could not. Yet the fraction of coherent versus incoherent scattering was the same in both cases. This further reinforced the conclusion that coherence was lost not because of frequency shifts but because of entanglement itself.
In 2024, another group in China also realised the recoiling-slit thought experiment in practice. Researchers from the University of Science and Technology of China trapped a single rubidium atom in an optical tweezer and cooled it to its quantum ground state, thus making the atom act like a movable slit whose recoil could be directly entangled with scattered photons.
By tightening or loosening the trap, the scientists could pin the atom more firmly in place. When it was held tightly, the atom’s recoil left almost no mark on the photons, which went on to form a clear interference pattern (like the ripples in water). When the atom was loosely held, however, its recoil was easier to notice and the interference pattern faded. This gave the researchers a controllable way to show how a recoiling slit could erase the wave pattern — which is also the issue at the heart of Bohr-Einstein debate.
Importantly, the researchers also distinguished true quantum effects from classical noise, such as heating of the atom during repeated scattering. Their data showed that the sharpness of the interference pattern wasn’t an artifact of an imperfect apparatus but a direct result of the atom-photon entanglement itself. In this way, they were able to demonstrate the transition from quantum uncertainty to classical disturbance within a single, controllable system. And even at this scale, the Bohr-Einstein debate couldn’t be settled.
The results pointed to a physical mechanism for how information becomes embedded in light scattered from atoms. In the conventional double-slit experiment, the question was whether a photon’s path could ever be known without destroying the interference pattern. In the new, modern version, the question was whether a scattered photon carried any ‘imprint’ of the atom’s motion. The MIT team’s measurements showed that it did.
The Debye-Waller factor — the measure of how much of the scattered light is still coherent — played an important role in this analysis. When atoms are confined tightly in a lattice, their spatial spread is small and the factor is relatively large, meaning a smaller fraction of the light is incoherent and thus reveals which-way information. But as the atoms are released and their wavepackets spread, the factor drops and with it the coherent fraction of scattered light. Eventually, after free expansion for long enough, essentially all of the scattered light becomes incoherent.
Further, while the lighter lithium atoms expanded so quickly that the coherence decayed almost at once, the heavier dysprosium atoms expanded more slowly, allowing the researchers to track them in detail. Yet both atomic species followed a common rule: the Debye-Waller factor depended solely on how much the atom became delocalised as a wave, and not by the technical details of the traps or the sidebands. The conclusion here was that the light lost its coherence because the atom’s recoil became entangled with the scattered photon.
This finding adds substance to the Bohr-Einstein debate. In one sense, Einstein’s intuition has been vindicated: every scattering event leaves behind faint traces of which atom interacted with the light. This recoil information is physically real and, at least in principle, accessible. But Bohr’s point also emerges clearly: that no amount of experimental cleverness can undo the trade-off set by quantum mechanics. The ratio of coherent to incoherent light is dictated not by human knowledge or ignorance but by implicit uncertainties in the spread of the atomic wavepacket itself.
Together with the MIT results, the second experiment showed that both Einstein’s and Bohr’s insights remain relevant: every scattering leaves behind a real, measurable recoil — yet the amount of interference lost is dictated by the unavoidable quantum uncertainties of the system. When a photon scatters off an atom, the atom must recoil a little bit to conserve momentum. That recoil in principle carries which-way information because it marks the atom as the source of the scattered photon. But whether that information is accessible depends on how sharply the atom’s momentum (and position) can be defined.
According to the Heisenberg uncertainty principle, the atom can’t simultaneously have both a precisely known position and momentum. In these experiments, the key measure was how delocalised the atom’s wavepacket was in space. If the atom was tightly trapped, its position uncertainty would be small, so its momentum uncertainty would be large. The recoil from a photon is then ‘blurred’ by that momentum spread, meaning the photon doesn’t clearly encode which-way information. Ultimately, interference is preserved.
By recasting the debate in the language of scattered photons and expanding wavepackets, the MIT experiment has thus moved the double-slit spirit into new terrain. It shows that quantum mechanics doesn’t simply suggest fuzziness in the abstract but enforces it in how matter and light are allowed to share information. The loss of coherence isn’t a flaw in the experimental technique or a sign of missing details, as Einstein might’ve claimed, but the very mechanism by which the microscopic world keeps both Einstein’s and Bohr’s insights in tension. The double-slit experiment, even in a highly sophisticated avatar, continues to reinforce the notion that the universe resists any single-sided description.
(The researchers leading the two studies are Wolfgang Ketterle and Pan Jianwei, respectively a Nobel laureate and a rockstar in the field of quantum information likely to win a Nobel Prize soon.)
Featured image created with ChatGPT.