Two centuries ago, the double-slit experiment revealed the strange nature of light – somewhat wave and somewhat particle. This quietly launched modern quantum theory.
Now, a team at the Massachusetts Institute of Technology (MIT) has stripped that classroom favorite to its bare bones and proved, once again, that you can never observe light behaving as both a particle and a wave at the same time.
Wolfgang Ketterle of MIT’s Research Laboratory of Electronics (RLE) led the work with graduate student Vitaly Fedoseev and colleagues. They made use of ultracold atoms spaced just a few ten-thousandths of an inch apart in a vacuum lattice.
Their results reset the century-old argument between Albert Einstein and Niels Bohr – this time, in favor of Bohr’s quantum view.
Updating a quantum certainty classic
The original demonstration, performed by Thomas Young in 1801, let sunlight pass through two narrow slits and produced an interference pattern, proving that light can travel as a wave.
More than a century later, Einstein proposed adding a delicate balance to each slit so that a passing photon would nudge it.
This setup, he argued, would let observers detect which path the particle took while still preserving the interference stripes.
Bohr responded that the Heisenberg uncertainty principle would make any such measurement blur the pattern beyond recognition, preserving the mystery.
Physicists have since tried countless versions of the experiment, using electrons, neutrons, and even full-sized molecules – each time confirming Bohr’s stance.
A recent study even formalized a conservation rule: the more information one gains about a particle’s path, the less visible its wave behavior becomes.
“Einstein and Bohr would have never thought that this is possible – to perform such an experiment with single atoms and single photons,” said Ketterle. His group went further by removing every classical component except the light and the scatterers.
Atoms stand in for slits
The researchers cooled more than 10,000 rubidium atoms to about 1 microkelvin – just above absolute zero – so that the atoms barely moved.
Laser beams arranged them into a crystal-like grid, with each site roughly 0.00004 inches apart. This spacing allowed any two neighboring atoms to act as the tiniest conceivable double slit.
A faint laser sent photons in, one by one; each photon scattered off the two adjacent atoms before reaching a camera that recorded interference fringes.
Because every atom was identical, the team could repeat the trial millions of times and build up crisp statistics without the noise that plagued earlier setups that used moving slits.
The heart of the design was controllable “fuzziness.” By loosening the trapping laser for a selected pair of atoms, the physicists enlarged each atom’s quantum position spread.
This increased the chance that an incoming photon would leave a telltale recoil – or which-way information.
When the atoms were sharply localized, the camera recorded bright, evenly spaced stripes – hallmarks of wave interference. Making the atoms fuzzier dissolved those stripes into a speckled blob, revealing particle-like hits instead.
Chasing the limits of certainty
Half-wave, half-particle operation came when the lattice depth was tuned so that only about fifty percent of the photons left detectable recoil.
That mix matched the trade-off predicted by complementarity, linking interference visibility to path knowledge.
To be sure the lattice itself was not acting like Einstein’s spring, the team briefly shut off the trapping light after each shot. This left the atoms freely floating for a millionth of a second before they fell under gravity.
Even without the “spring,” probing the path still erased the stripes, proving that it is the entanglement between photon and atom, not any macroscopic support, that decides the outcome.
“In many descriptions, the springs play a major role,” said Fedoseev, the study’s first author. “But we show, no, the springs do not matter here; what matters is only the fuzziness of the atoms.”
The finding dovetails with a recent analysis that simulated a tunable recoiling-slit scenario and reached an identical verdict. The measuring device can be virtual as long as it steals enough momentum information.
Einstein’s spring meets modern lasers
Einstein imagined a real mechanical balance that would move by about one ten-millionth of an inch, a heroic engineering task for 1927.
Today’s optical lattices create forces a thousand times smaller yet still track them, thanks to single-photon detectors cooled to near 0 °F.
Because the MIT arrangement uses atoms that are “Heisenberg-uncertainty limited,” every recoil event instantly entangles the photon with the atomic state.
As a result, the scattered light carries a fringe pattern only when the atom remains unperturbed. This mirrors Richard Feynman’s famous remark that the double-slit “contains the only mystery” of quantum mechanics.
The team’s control also let them test intermediate fuzziness values and verify that interference visibility falls off in strict proportion to path knowledge.
That linear relation is a long-sought benchmark for quantum resource theories that treat information as a conserved quantity.
The experiment closes a conceptual gap left by molecular and neutron versions, which always relied on extended slits or diffraction gratings. Here, the “slit” is a single particle, so nothing classical can be blamed for the trade-off.
Fuzziness still matters
Light-based computers, precision sensors, and secure communication channels all hinge on balancing wave-like coherence against particle-like detection signals.
Engineers use precise knowledge of how entanglement reduces interference to decide how much information they can extract before a quantum state decoheres.
The MIT results arrive in the United Nations-declared International Year of Quantum Science and Technology (IYQ), a timely reminder that foundational questions still guide applied research.
Future work will try the same protocol with molecules and superconducting qubits to test whether the visibility-information law is truly universal.
If it holds, textbooks may soon replace drawings of slits in screens with sketches of floating atoms. This would give students a more faithful picture of how nature hides her clues.
The study is published in Physical Review Letters.
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