Interference at a double slit made of two atoms

March 1, 2016
Resonant laser light (red arrow) is being scattered from two single atoms. The effects that arise due to interference (here shown in an artist’s view) are determined by the relative spatial phase of the atoms and the interaction with the light mode of the optical resonator (mirrors in grey). Lower left: fluorescence image of two rubidium atoms. Credit: Andreas Neuzner, MPQ

The investigation and exploitation of light-matter-interaction in optical resonators is one of the central research topics in the Quantum Dynamics Division of Professor Gerhard Rempe, Director at the Max Planck Institute of Quantum Optics in Garching. A couple of years ago, the team succeeded in creating single-photon emitters using single atoms stored in optical resonators. The stationary atoms can, for example, serve as nodes for the exchange of quantum information in a long-distance quantum network.

Now, the scientists went one step further. They trapped a pair of with well-defined relative positions in such a resonator and scattered light from this "double slit". They observed interference phenomena that contradict well-established intuition. These results were enabled by the development of a technique that allows for position control of the atoms with an accuracy well below the wavelength of the scattered light. One motivation for this experiment is to better understand fundamental aspects of cavity quantum electrodynamics. Furthermore, the technique paves the way for studying new concepts of entanglement generation between quantum bits and thus opens up new perspectives for quantum information processing (Nature Photonics, AOP, 29 February 2016, DOI: 10.1038/nphoton.2016.19).

Key element of the experimental set-up is an optical resonator consisting of two highly-reflecting mirrors spaced by 0.5 mm. Inside the cavity, a so-called optical lattice is generated by crossing two retro-reflected laser beams, one oriented orthogonal to and the other along the resonator axis. The resulting light pattern of bright and dark spots resembles a checker board with a period of about half a micrometre. These spots define lattice sites at which the atoms can be trapped and where they are localized to about 25 nanometres.

At first, a couple of rubidium atoms, precooled to very low temperatures, are loaded into the optical lattice. By detecting their fluorescence light via a high-resolution microscope objective, the atoms can be identified as individual light spots. Excess atoms are subsequently removed by individually heating them up with a resonant laser beam, until only a pair of atoms with the desired spacing remains. "This is the "double-slit" from which the resonant laser light, propagating transversally through the resonator, is scattered", explains Andreas Neuzner, who performed this experiment as part of his doctoral studies.

"Interference can only be observed if the phase relation between the two light sources is fixed", explains Dr. Stephan Ritter, another scientist on the experiment. "In order to investigate the interference as a function of the phase, we have to know the position of the atoms with a precision well below the wavelength of 780 nanometres." Although the resolution of the imaging system limits the size of the atom images to 1.3 micrometres, the scientists can localize the emitting atoms with an accuracy of 70 nanometres and can thereby assign their position to a particular lattice site. Therefore, the distance between two atoms, typically about 10 micrometres, is precisely known.

The resonator favours emission along its axis and enhances the interaction between the atoms and the scattered light, which is reflected multiple times between the mirrors. The light power leaking through one of the mirrors – i.e. the photon rate – is recorded as a function of the relative phase of the two atoms.

The observed interference pattern displays several intriguing features that are not expected in the simpler picture of two classical dipoles in free space. First, in the case of in-phase (constructive) interference, the intensity is only a factor of 1.3 larger than the rate observed for a single atom, whereas a fourfold larger signal is expected for the simpler picture. This phenomenon goes back to the various light fields inside the resonator that have to be taken into account. In contrast to the classical double-slit experiment, not only the phase relation between the scattered light waves matters. It is rather the superposition of the scattered light with the light field of the resonator that in the end leads to an intensity reduction in the field maxima.

The second feature occurs for out-of-phase (destructive) interference. Here, the photon rate drops below the value measured for a single atom, but does not go to zero as one would expect intuitively. Strikingly, extremely strong intensity fluctuations are observed, so-called photon bunching. "This phenomenon arises, because in the case of destructive interference, the atoms can emit photons only pairwise and at the same time into the resonator", explains Andreas Neuzner.

"In this experiment we have combined three key techniques for the first time: Using an optical lattice, we position the atoms with high accuracy and then localize them with a high-resolution microscope. The interaction with the resonator enables directed detection of the scattered light", says Stephan Ritter. "The newly developed techniques are essential for future experiments aiming to explore collective radiation effects predicted for multi-atom systems", resumes Prof. Gerhard Rempe. "On the other hand, they offer the possibility to implement novel protocols for quantum information processing with several quantum bits."

Explore further: Seeing a photon without absorbing it

More information: A. Neuzner et al. Interference and dynamics of light from a distance-controlled atom pair in an optical cavity, Nature Photonics (2016). DOI: 10.1038/nphoton.2016.19

Related Stories

Seeing a photon without absorbing it

November 14, 2013

Light is of fundamental importance. It allows us to see the world around us and record pictures of our environment. It enables communication over long distances through optical fibers. All current methods of detecting light ...

Squeezed light from single atoms

June 30, 2011

( -- Max Planck Institute of Quantum Optics scientists generate amplitude-squeezed light fields using single atoms trapped inside optical cavities.

Rubidium atoms used as a refrigerant for ytterbium atoms

November 12, 2015

For many years rubidium has been a workhorse in the investigation of ultracold atoms.  Now JQI scientists are using Rb to cool another species, ytterbium, an element prized for its possible use in advanced optical clocks ...

Cold fermions keep distance from each other

January 4, 2016

Today, quantum optical experiments provide methods to prove the rules that have been thought of and pressed into elegant mathematical equations in those days. In this regard, scientists in the Quantum Many-Body Division of ...

Two or one splashing? It's different

January 15, 2015

If two children splash in the sea high water waves will emerge due to constructive superposition. Different observations are made for the microscopic world in an experiment at the University of Bonn, where physicists used ...

A little light interaction leaves quantum physicists beaming

August 24, 2015

A team of physicists at the University of Toronto (U of T) have taken a step toward making the essential building block of quantum computers out of pure light. Their advance, described in a paper published this week in Nature ...

Recommended for you

Researchers discover new rules for quasicrystals

October 25, 2016

Crystals are defined by their repeating, symmetrical patterns and long-range order. Unlike amorphous materials, in which atoms are randomly packed together, the atoms in a crystal are arranged in a predictable way. Quasicrystals ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.