Double-slit experiment in a hall of mirrors

A purely quantum physical variation of the classic experiment with two atoms reveals surprising interference phenomena

The double-slit experiment is regarded among physicists as one of the most elegant experiments of all time. According to Nobel Prize laureate Richard Feynman, it encapsulates the entire mystery of quantum physics. It impressively demonstrates the wave nature of light and the phenomenon of interference. Scientists from the Max Planck Institute of Quantum Optics in Garching have now rearranged this classic experiment: instead of two slits, they use two individual light-scattering atoms. As in the original experiment, interference phenomena occur. However, in this case, additional quantum phenomena also occur, which the physicists are studying to learn how light interacts with matter. The success of this experiment depended on the physicists capturing and observing atoms in a resonator with unprecedented precision. In the long term, they want to study physicists capturing and observing atoms in a resonator with unprecedented precision. In the long term, they want to study how these observations can be used to discover new techniques for quantum information processing.

Resonant laser light (red arrow) is scattered by two single atoms. The resulting interference pattern (shown here artistically) is determined by the exact position of the atoms and the interaction with the light stored in the optical resonator (mirrors in grey). © Andreas Neuzner / MPI of Quantum Optics Resonant laser light (red arrow) is scattered by two single atoms. The resulting interference pattern (shown here artistically) is determined by the exact position of the atoms and the interaction with the light stored in the optical resonator (mirrors in grey). © Andreas Neuzner / MPI of Quantum Optics

The experiment, which the English physicist Thomas Young first conducted in 1800, is essentially quite simple: light passes through two thin parallel slits and is projected onto a screen. Because the slits are so thin, the light passing through them spreads out and illuminates large parts of the screen. However, the areas in which the light from both slits combines are not twice as bright as the light from a single slit. The result is rather a striped pattern in which dark stripes alternate with stripes that are four times as bright. Physicists refer to this phenomenon as interference and explain the pattern in terms of the wave nature of light. Where two wave crests or troughs meet, the area is light; where crests and troughs are superimposed, the area is dark. For a fixed wavelength, the shape of the striped interference pattern depends on the distance between the two slits.

Instead of two slits, the team at the Max Planck Institute of Quantum Optics used two rubidium atoms which they were able to excite to fluoresce with a laser. They used a sophisticated method to position the atoms precisely. “At the heart of the experimental set-up is an optical resonator consisting of two highly reflective mirrors separated by a distance of half a millimetre,” explains the lead author of the paper, Andreas Neuzner, who also wrote his doctoral dissertation on this experiment. Laser beams generate a standing-wave light field between the two reflectors. Physicists speak of an optical lattice that can be roughly likened to an egg carton. Cooled rubidium atoms are loaded into the lattice and settle into the cups of the carton.

The distance between atoms can be determined to an accuracy of less than 70 nanometres

When another, resonant laser beam impinges on these atoms, they emit fluorescent light. Using a sophisticated method and a high-resolution microscope which they developed specifically for this purpose, the Garching-based physicists are able to locate a single atom with a precision of up to 70 nanometres, i.e. a fraction of the wavelength of light. “The potential mink a of our light lattice are very deep, so the atoms are tightly confined in them,” Neuzner explains. And since the physicists know the dimensions of their light lattice, i.e. the distance between two troughs, to an accuracy of less than one nanometre, the actual distance between the atoms can be determined to a significantly greater accuracy than 70 nanometres. The high precision also allows the researchers to dislodge unwanted atoms from the lattice with another laser beam. “We do this until exactly two atoms remain at the desired distance,” Neuzner says.

The purely quantum mechanical double-slit set-up is then ready for experimentation. When the physicists expose the two atoms to laser light of a suitable wavelength, the atoms emit fluorescent light, which forms an interference pattern by superimposition, as in the classic experiment. This is registered by a detector behind one of the two partially transparent mirrors which captures light from the resonator. The quantum researchers in Garching adjust the distance between atoms from one experiment to the next, thus setting up various interference conditions.

The atoms bask in a flood of light – with interesting consequences

However, on closer inspection, the researchers found deviations from the classic case in their two-atom experiments. At the minimum of the interference pattern, where destructive interference occurs, it is not completely dark, that is, the intensity does not fall to zero. This is because the atoms are not slits. “In the case of destructive interference, the two atoms can be excited simultaneously by the laser and emit light into the resonator,” Neuzner explains. This light is then registered by the detector.

However, some of the light emitted by the two atoms strikes the mirrors and is reflected back and forth between them. As a result, a light field builds up in the resonator, which interacts with the atoms. The atoms bask in a flood of light as it were − with interesting consequences.

For example, in the case of two atoms in free space, the light intensity at the maximum of the interference pattern, where constructive interference occurs, should be four times that of a single atom. In the Garching-based experiment, however, the intensity is only augmented by a factor of 1.3. This is due to the light field in the optical resonator, which is superimposed on the excitation light. “The space around the atoms is dark due to destructive interference,” Neuzner explains. The atoms are therefore less strongly excited from outside and emit less light. Consequently, the intensity decreases.

Atoms in a trap for quantum information processing

“Because we’re able to control and observe the atoms so precisely, we’re able to analyze quantum physics cleanly at the textbook level,” says Gerhard Rempe, Director at the Max Planck Institute in Garching and Leader of the Quantum Dynamics Group. “It’s incredibly exciting to find out exactly what happens when light interacts with atoms,” he adds.

The physicists in Garching are thinking beyond questions of basic research. Trapped atoms are believed to offer a means of realizing a quantum computer. They can also serve as nodes for exchanging information in quantum networks. “They offer the opportunity to implement novel protocols for quantum information processing with multiple quantum bits,” Rempe says. Rempe and his colleagues plan to extend the experiment in this direction using their ultra-precision equipment.

TB