October 11, 2005
A century ago, we took the first steps towards recognising, at the level of fundamental physical events, the dual character of nature that had been previously postulated. Albert Einstein was the first to see the implication of this dual character in Max Planck’s quantum hypothesis. Einstein suggested that photons would have the character of particles,
although photons had previously been viewed as electromagnetic waves.
That was the quintessence of his work on the photoelectric effect. Later in 1926, it was deBroglie who postulated that all building blocks of nature known to us as particles - electrons, protons, etc. - would behave like waves under certain conditions.
In its totality, nature is therefore dual. None of its constituents can be considered as only a particle or a wave. To reconcile this duality, in 1923 Niels Bohr proposed his Complementarity Principle: simply put, every component in nature has particle-like, as well as wave-like character, and which character is observed at a given time depends only on the observer. In other words, the experiment determines which characteristic one is measuring - particle or wave.
His whole life long, Einstein viewed with suspicion that natural characteristics would actually depend on the observer and believed that there must be a reality independent of the observer. Indeed, over time quantum physics has simply come to accept as a fact that there does not seem to be an independent reality. Physics has ceased questioning this, because experiments have confirmed it repeatedly and with a growing accuracy.
The best example is Young’s double-slit experiment. Coherent light is passed through a barrier with two slits. On an observation screen behind it, a pattern of light and dark stripes emerges. The experiment can be carried out not only with light, but also with particles - for example, electrons. If single electrons are sent in one after the other through the open Young double slit, a stripe-shaped interference pattern appears on the photo plate behind it. The pattern contains no information about the route the electron took. But if one of the two slits is closed, an image appears of the other open slit from which one can directly determine the path of the electron. What this experiment does not provide, however, is a stripe pattern and a "which way" report at the same time. That requires a molecular double-slit experiment based not upon position-momentum uncertainty, but on mirror, more precisely, inversion symmetry.
The double-slit was voted the most beautiful experiment of all time in a 2002 poll by Physics World, published by the UK's Institute of Physics. Although each electron seems to go only through one of the two slits, at the end a wave-like interference pattern is created, as if the electron is split while going through the slit, but is subsequently re-unified. However, if one of the slits is closed, or an observer sees which slit the electron went through, it behaves like a perfectly normal particle. The particle is only at one position at one time, and not at both at the same time. Hence, depending on how the experiment is carried out, the electron is either at position A, position B, or at both at the same time.
But Bohr’s Complementarity Principle, which explains this ambiguity, requires that one can only observe one of the two electron's manifestations at any given time - either as a wave or a particle, but not both simultaneously. This remains a certainty in every experiment, despite all the ambiguity in quantum physics. Either a system is in a state of "both/and" like a wave, or "either/or" like a particle regarding localisation. This is, in principle, a consequence of Heisenberg’s uncertainty principle, which states that for a complementary pair of measurements - for example, position and momentum - only one can be determined exactly at the same time. Information about the other measurement is proportionally lost.
Recently there has been a set of experiments suggesting that these various manifestations of matter can be "carried over" into each other - in other words, there can be switching from one form to the other and, under some conditions, back again. This class of experiment is called quantum markers and quantum erasers. Researchers have shown in the last few years that for atoms and photons - and now, electrons - "both/and" and "either/or" exist side-by-side. In other words, there is a grey zone of complementarity. There are hence experimentally demonstrable conditions where matter appears to be both a wave and a particle.
These situations can be described by a so -called duality relation. It can be viewed as an extended Complementarity Principle for quantum physics and may be more precisely called a Co-existence Principle. It states that manifestations of matter which would normally be mutually exclusive - e.g., local and non-local, coherent and non-coherent - are indeed measurable and make themselves evident, in a particular "transition regime". One can speak of partial localisation and partial coherence, or partial visibility and partial distinguishability. These are measurements that are connected to each other via the duality relation.
In this transition regime the Complementarity Principle, and the complementary dualism should be extended to the more general Co-existence Principle, describing the parallel dualism of nature. Nature thus has a more ambivalent character than previously recognized. Atomic interferometry provides us with examples of this ambivalence. This was first observed in 1998 in atoms, which consist of an assembly of particles.
In a recent issue of Nature Max Planck researchers in Berlin, together with researchers from the California Institute of Technology in Pasadena, California, report on a molecular double-slit experiment with electrons which are elementary particles, not assemblies of particles, like atoms. Molecules with identical atoms, and thus with inversion symmetry, behave like nature's own microscopically small double-slit. Nitrogen is one such molecule. In it, each electron, including the highly localised inner electrons, is simultaneously at both atoms. If we ionise such a molecule with soft x-rays, we end up with a coherent - that is, wavelike - strongly coupled electron emission from both atomic sites. This is exactly like a double slit experiment with single electrons.
For the first time, the researchers were able to demonstrate the coherent character of electron emission from such a molecule in analogy to the double slit experiment. They used soft x-rays to destabilise the innermost, and thus most strongly localised, electrons of nitrogen from the molecule, and then followed their movement in the molecular frame of reference using electron-ion coincidence detection. In addition, the researchers were able to prove something that has long been doubted: a disruption of the inversion symmetry of this molecule leads to a partial loss of coherence through the introduction of two different heavy isotopes, in this case N14 and N15. The electrons begin to localise partially on one of the two, now distinguishable, atoms. This is equivalent to partially marking one of the two slits in Young’s double slit experiment. It provides partial "which way" information, because the marking gives information about which path the electron took.
The experiments were carried out by members of the "Atomic Physics" working group of the FHI at the synchrotron radiation laboratories HASYLAB at DESY in Hamburg and BESSY in Berlin. The measurements employed a multi-detector array for combined, coincident electron and ion detection behind undulator beam lines, which deliver soft x-rays with high intensity and spectral resolution.