“God does not play dice with the universe,” Albert Einstein famously declared in 1927, sparking one of the most enduring debates in physics. Nearly a century later, a team of Chinese physicists has recreated his proposed counterexample to Niels Bohr’s complementarity principle only to find that Bohr’s argument was correct in every measurable way.
Complementarity A principle proposed by Bohr in the mid-1920s, which argues that some pairs of physical properties, e.g. position and momentum of a particle, cannot be measured simultaneously precisely, is known as complementarity. This concept is the foundation of the wave-particle duality: a quantum object may exhibit either wave-like interaction or particle-like information about “which way” information, but never both with any clarity in one experiment. Einstein, who was doubtful of the randomness inherent in quantum mechanics, tried to come up with a test which would demonstrate both at the same time, thus discrediting the argument of Bohr.
His experiment of thought was based on the work of Thomas Young, who in 1801, using a double-slit apparatus, observed the wave aspect of light, observing fringes of interference. Einstein envisioned the insertion of one slit ahead of the two-slit and had it attached on springs sensitive to momentum. Each photon which passes through would give a recoil, which would enable the experimenters to measure which slit would be proceeded to next, nevertheless creating an interference pattern at the bottom. Bohr refuted that a measurement of the recoil with great accuracy would, according to the uncertainty principle, introduce much positional uncertainty and smear the pattern of interference, without interference affecting complementarity.
In the modern implementation, the single rubidium atom in an optical tweezer was used by Jian-Wei Pan and others at the University of Science and Technology of China to replace the spring-mounted slit. Neutral atoms can be trapped and manipulated with high precision using optical tweezers; these are high-focus laser beams that are able to control the quantum states of neutral atoms. The rubidium atom was an ultralight beam splitter, whose momentum was quantum-entangled with the momentum of the photon. The team measured the sharpness of the interference fringes by adjusting the depth of the tinier of the two traps that adjusts the inherent momentum uncertainty of the atom, which affects the sharpness of the interference fringes directly.
Fringe visibility decreased when the momentum of the atom was well-defined as had been predicted by Bohr. Adding the uncertainty reinstated the interference pattern. The researchers arrived at the conclusion, “Using modern language, the Einstein-Bohr interference visibility is determined by the degree of quantum entanglement in the momentum degree of freedom between the photon and the slit.” This finding not only validated the principle of Bohr but also revealed the close connection between the concepts of complementarity and entanglement, a connection that was codified in recent theoretical studies of quantum complementarity relations.
The experiment required a thorough control of the environmental factors. Laser frequency drifts that led to heating of the atom changed the depth of the tweezer and scattered photons, which could lead to results being obscured. To measure the residual temperature of the atom in real time the team paid by scanning Raman spectroscopy, which measures vibrational modes through light scattering, which is inelastic. This provided that any changes in interference observed were not caused by thermal noise but were as a result of quantum effects.
This ability to trap and manipulate single atoms is one of the frontier capabilities of quantum optics. Optical tweezer array instrumentation, including observable quantum network assembly experiments, enables precise positioning of atoms and efficient entanglement of atoms and photons, enabling scalable quantum communication networks. The optical tweezer was not only a device to trap atoms in Pan experiment, but also a quantum control knob, which allowed to explore the transition regimes between pure wave and pure particle behavior.
The double-slit experiment has been a paradigm since at least the early days when Young used it to demonstrate the principles of quantum theory, and more recently in electron interference in 1927 and current “weak measurement” methods that measure some which-way information without destroying all the fringes. This lineage is furthered by the work of the Chinese team which directly applies the counterexample of Einstein at the quantum limit using the scattering of single photons by one atom to directly observe the details of the interaction of measurement and uncertainty and interference.
To physicists and undergraduate students, the importance is not simply the resolution of a famous controversy but the demonstration of how in the 21 st century tools, namely ultracold atoms, laser trapping, and the ability to control entanglement, can answer some of the biggest questions of modernity with clarity never before achieved. The atomic physics has now provided “real opportunities to showcase quantum mechanics with clarity which was not possible before.” The group of Pan is going to go even further and use quantum state tomography so that the quantum state of the slit can be mapped and then moved to a higher mass so that decoherence can be examined on entanglement.
The result is the shocking reminder; despite having brilliant counterarguments, Einstein could not invert the probabilistic core of quantum theory. The principle of complementarity that Bohr proposed is even stronger today not only empirically demonstrated by theory alone, but literally by observation of the quantum dice of nature at work.
