'Double-slit' and multiple-path Interference studies from Rb excited and ionized by high-resolution laser radiation.

The double-slit experiment using electrons to produce interference at a detector was voted as one of the 5 'most beautiful experiments in physics' by Physics World readers in 2002. Recent experiments in 2013 demonstrated that SINGLE electrons that were detected before the next electron was emitted also produce an interference pattern when the signal builds up over time. This convincingly shows individual electrons have both wave-like & particle-like character, as predicted by Richard Feynman in the early 1960's. Feynman thought such experiments would never be done, however advances in technology since then have now made this possible.

The interference pattern arises since we do not know which slit the electron passes through. We assign a wavefunction to the electron, & the slits then define 2 possible pathways for the wave to travel from source to detector. The wavefronts beyond the slits then recombine at the detector, & the square of their sum gives the probability an electron is detected. If the peak of one wave meets the trough of another, the waves cancel & there is zero probability an electron will be detected at that position. By contrast, if two peaks or two troughs arrive at a point, there is then maximum probability an electron will be detected. An interference pattern is hence produced across the detector, which depends on how the waves recombine at any given point.

In Manchester we recently invented a new type of 'double-slit' experiment in a single atom, where the 'slits' are replaced by atomic states 1 & 2 excited by lasers. The laser beam that excites state 1 also ionizes state 2, whereas the laser exciting state 2 ionizes state 1. There are then 2 pathways to ionization, & we do not know which was taken to produce the detected photoelectron. We again have to add the wavefunctions from each path to determine the outcome, leading to interference. The states (slits) can be turned on or off (effectively opening or closing individual slits) by selectively tuning & detuning the lasers & this allows us to determine the interference pattern.

In the new experiments to be carried out in this proposal we will explore this process in much greater detail, by selecting different excited states & by using different laser polarizations. Our collaborators in Germany theoretically predict this will produce large changes to the ensuing pattern. A further prediction we will explore is that injection of a third laser beam can selectively control the interference. This new idea has no analogy in a conventional double-slit experiment & may find application in other areas where wavefunctions must be manipulated (e.g. quantum computing).

There is no reason why these processes must be confined to single atoms & the second facet of this work will explore how laser excitation & ionization can be applied to arrays of atoms. We will first cool the atoms to close to absolute zero in a magneto-optical trap, before creating a periodic array of excited atoms using a standing-wave laser. The atoms will then be ionized by a second laser, set so that the de Broglie wavelength of the emerging photoelectrons is comparable in size to the dimensions of the array. Interference will once again occur, however now the summation is for waves from ALL sites from which the photoelectrons are born. The resulting yield then depends on both the individual atoms, as well as their position in the array. This is expected to be similar to the effect a diffraction grating has on light, however now the waves are for electrons rather than photons. By altering the properties of the lasers we can 'shape' the grating in different ways, which will change the electron distribution that is produced. Initial models from our collaborators support these ideas & experiments are needed to test & refine the models. This work could find application in electron diffraction studies of surfaces & for controlling the injection of electrons into particle accelerators.