The researchers provided evidence that disrupting the
reflective symmetry of these molecules by introducing two different heavy
isotopes, in this case N14 and N15, leads to a partial loss of coherence.
The electrons partially begin to localise on one of the two, now
distinguishable, atoms. The results could have implications for the building
and control of "artificial molecules", which are made of
semiconductor quantum dots, and are a possible component of quantum computers.
(Nature, September 29, 2005).
A hundred years ago, we took the first steps in recognising, at the level of
elementary physical events, the dual character of nature that had been
postulated in natural philosophy. Albert Einstein was the first who saw Max
Planck’s quantum hypothesis leading to this dual character. Einstein
suggested the photon have an electromagnetic wave character, although
photons had previously been considered as particles. That was the
quintessence of his work on the photoelectric effect. Later in 1926, it was
deBroglie that recognised that all the building blocks of nature known to us
as particles - electrons, protons, etc. - behave like waves under certain
conditions.
In its totality, therefore, nature is dual. None of its components can only
be considered as a particle or as a wave. To understand this fact, Niels
Bohr introduced in 1923 the Complementarity Principle: simply put, every
component in nature has a particle, as well as a wavelike character, and it
depends only on the observer which character he sees at any given time. In
other words, the experiment determines which characteristic one is measuring
- particle or wave.
His whole life long, Einstein suspected that natural characteristics
actually depend on the observer. He believed that there must be a reality
independent of the observer. Indeed, quantum physics has simply come to
accept as a given over the years 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,
there is a pattern made of light and dark stripes. The experiment can be
carried out not only with light, but also particles - for example,
electrons. If single electrons are sent, one after the other, through the
open Young double slit, then a stripe-shaped interference pattern appears on
the photo plate behind it. The pattern contains no information about the
route that 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 read the
path of the electron. What this experiment does not produce, however, is a
stripe pattern and situation report. For that, a molecular double slit
experiment is required that is based not upon position-momentum uncertainty,
but on reflective symmetry.
The double-slit was voted the most beautiful experiment of all time in a
2002 poll by Physics World, published by England’s Institute of Physics.
Although each electron seems to go alone through one of the two slits, at
the end a wavelike interference pattern is created, as if the electron split
while it went through the slit, but then was subsequently re-unified. But if
one of the slits is closed, or an observer sees which slit the electron went
through, then it behaves like a perfectly normal particle. That particle is
only at one position at one time, but not at the same time. So, 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 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,
relating to its localisation. This is, in principle, a consequence of
Heisenberg’s uncertainty principle, which says that given 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 lost, proportionally.
Recently there has been a set of experiments suggesting that these various
manifestations of material can be "carried over into" each other -
in other words, they can switch from one form to the other and, under
certain conditions, back again. This set of experiments 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 therefore experimentally demonstrable
conditions in which the material appears to be both a wave and a particle.
These situations can be described with a duality relation. It can be seen as
an extended Complementarity Principle for quantum physics; it can also be
labelled a co-existence principle. It says that manifestations of material
which would normally be mutually exclusive - e.g., local and not local,
coherent and not coherent - are indeed measurable and make themselves
evident, in a particular "transition area". One can speak of
partial localisation and partial coherence, or partial visibility and
partial differentiability. These are measurements that are connected to each
other via the duality relation.
In this transition area the Complementarity Principle, and the complementary
dualism of nature, can be extended to be a co-existence principle, a
parallel dualism. Nature has thus an ambivalent character previously
unassumed. Atomic interferometry provides us with examples of this
ambivalence. It was first found in 1997 in atoms, which are made from 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 about a molecular double-slit experiment with electrons -
not assemblies of particles, like atoms. Molecules with identical, and thus
reflectively symmetrical, atoms, behave like a microscopically small
double-slit built by nature. Nitrogen is one such molecule. In it, each
electron - also the highly localised inner electrons - stays simultaneously
in both atoms. If we ionise such a molecule with a weak x-ray, we end up
with a coherent - that is, wavelike - strongly coupled electron emission
from both atomic sides. This is just like a double slit experiment with
single electrons.
For the first time, the researchers were able to show the coherent character
of electron emissions from such a molecule, in this analogue to the double
slit experiment. They used a weak x-ray 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 ion
coincidence measurements. In addition, the researchers succeeded in proving
something long doubted: that a disruption of the reflective 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. This is partial "which way" information, because
the marking gives information about which path the electron took.
The experiments were carried out by members of the working group
"atomic physics" of the FHI at the synchrotron radiation
laboratories BESSY in Berlin and HASYLAB at DESY in Hamburg. The
measurements took place using a multi-detector array for combined electron
and ion proof behind what are called undulator beam pipes, which deliver
weak x-rays with a high intensity and spectral resolution.
Original work:
Daniel Rolles, Markus Braune, Slobodan Cvejanović, Oliver Ge ner,
Rainer Hentges, Sanja Korica, Burkhard Langer, Toralf Lischke, Georg Prümper,
Axel Reinköster, Jens Viefhaus, Björn Zimmermann, Vincent McKoy and Uwe
Becker
Isotope-induced partial localization of core electrons in the homonuclear
molecule N2
Nature 437, 711-715, September 29, 2005
Source: Max Planck Society
