A thought experiment that sparked a famous debate between physicists Albert Einstein and Niels Bohr in 1927 has now been realized. This breakthrough addresses one of quantum physics’ fundamental mysteries: is light truly a wave, a particle, or an intricate mix of both?

The debate centers on the double-slit experiment, tracing back another century to 1801, when Thomas Young used it to argue for the wave nature of light, while Einstein contended it is a particle. Bohr’s contributions to quantum physics suggested that both perspectives could hold true. Einstein, critical of this notion, designed a modified version of Young’s experiment to counter it.

<p>Recently, <a href="https://quantum.ustc.edu.cn/web/en/node/137">Chaoyan Lu</a> and his team at the University of Science and Technology of China utilized cutting-edge technology in experimental physics to verify Einstein's theories, demonstrating the unique dual wave-particle character of quantum objects, as theorized in the 1920s. "Witnessing quantum mechanics 'in action' at such a foundational level is awe-inspiring," remarks Lu.</p>
<p>In the classic double-slit experiment, light is directed at two narrow parallel slits in front of a screen. If light were entirely particles, the screen would display a distinct light blob behind each slit. However, researchers observed an "interference pattern" of alternating dark and bright bands instead. This demonstrates that light behaves like waves passing through a slit, creating ripples that collide on the screen. Notably, this interference pattern remains evident even when the light intensity is reduced to a single photon. Does this imply that photons, which exhibit particle-like behavior, also interfere like waves?</p>
<p>Bohr proposed the idea of "complementarity," stating that one cannot simultaneously observe the particle nature of a photon showing wave-like behavior, and vice versa. Amid discussions on this matter, Einstein envisioned an additional spring-loaded slit that would compress when a photon entered. By analyzing the movement of the spring, physicists could determine which slit a photon passed through. Einstein believed this approach allowed for a simultaneous description of both particle and wave behavior, creating an interference pattern that contradicts complementarity.</p>
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<p>Lu's team aimed to create a setup at the "ultimate quantum limit," firing a single photon rather than using a slit, but rather an atom that could recoil similarly. Upon impacting the atom, the photon entered a quantum state that allowed it to propagate left and right, which also produced an interference pattern upon reaching the detector. To achieve this, researchers utilized lasers and electromagnetic forces to significantly cool the atoms, enabling precise control over their quantum properties. This was vital for testing Bohr's claims against Einstein's. Bohr argued that Heisenberg's uncertainty principle could disrupt the interference pattern when momentum fluctuations of the slit due to recoil are well known, rendering the photon’s position highly ambiguous, and vice versa.</p>
<p>"Bohr's response was brilliant, but such thought experiments remained theoretical for almost a century," notes Lu.</p>

<p>By adjusting the laser, Lu's team could control the momentum uncertainty of the atoms as they slitted. They found that Bohr was indeed correct; finely tuning these momentum ambiguities could eliminate interference patterns. Remarkably, the team could access intermediate regions to measure recoil information, observing blurred versions of interference patterns. Essentially, the photon displayed both wave and particle characteristics simultaneously, according to Lu.</p>
<p>``The real intrigue lies in [this] intermediate realm," states <a href="https://physics.mit.edu/faculty/wolfgang-ketterle/">Wolfgang Ketterle</a> from the Massachusetts Institute of Technology. Early this year, he and his team conducted a variation of Einstein's experiment, using ultracold atoms controlled by lasers that could pass through two slits. Lu's group utilized a single atom to scatter light in two directions; both atoms scattered light in the same direction, and changes in its quantum state indicated the influence of the photons colliding with each atom. Ketterle emphasizes that this approach provides a distinct means to explore wave-particle duality, offering clearer insights into photon behavior since this "which direction" information is recorded in one of the two separate atoms, albeit deviating slightly from Einstein's premise.</p>
<p>Furthermore, he and his colleagues performed experiments where they abruptly switched off the laser (similar to removing a spring from a moving slit) and subsequently directed photons at the atoms. Bohr's conclusions held, as the uncertainty principle impacted the momentum exchange between atoms and photons, potentially "washing out" the interference fringes. This spring-free iteration of Einstein's concept had remained untested until now, according to Ketterle. "Nuclear physics presents an excellent opportunity to apply cold atoms and lasers for a clearer illustration of quantum mechanics, a possibility not achievable before."</p>

<p><a href="https://physik.unibas.ch/en/persons/philipp-treutlein/">Philip Treutlein</a> and his colleagues at the University of Basel in Switzerland assert that both experiments strongly reinforce fundamental aspects of quantum mechanics. "From our modern perspective, we understand how quantum mechanics operates on a microscopic level. Yet witnessing the empirical realization of these principles is always impactful." The experiments led by Lu align conceptually with historical records of the debates between Bohr and Einstein, affirming that quantum mechanics behaves as predicted.</p>
<p>For Lu, there remains more work on categorizing the quantum state of the slit and increasing its mass. However, the experiment carries significant educational importance. "Above all, I hope to illustrate the sheer beauty of quantum mechanics," he shares. "If more young individuals witness the real-time emergence and disappearance of interference patterns and think, 'Wow, this is how nature functions,' then the experiment will already be a success."</p>

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