July 19, 2016
How would you look for something that can be in two 'places' at
The answer, according to Oxford University research into a
quantum phenomenon called
superposition, seems to be to ask
where it isn't rather than where it is.
'Superposition allows an atom to be
simultaneously 'here' and 'there'.
Electrons behave like tiny magnets
which can point both North and South at the same time,' explains
Professor Andrew Briggs of Oxford University's Department of
'This is a distinctive quantum
effect; it is quite different from anything in our intuitive
every day experience of the world.'
According to Professor Andrew Briggs
you can imagine an electron as being rather like a spinning top,
as it spins it generates a magnetic effect.
'Just as a magnetic compass aligns
itself with the Earth's magnetic field, because its energy is
lower when it points that way, so a single electron in a
magnetic field has a different energy depending on which way its
spin points,' he says.
But in the quantum world nothing is
easy: try to look directly at which way this 'quantum compass' is
pointing and the very superposition you wanted to catch in the act -
of it pointing north and south at the same time - is destroyed.
Instead the superposition state will be
replaced with one where the magnet is pointing either north or
south at random.
To get around this problem Dr Richard George and others from
Oxford worked with colleagues at
TU Delft in the Netherlands to
prepare a series of experiments.
The researchers used the magnetism of a
single atom of nitrogen trapped in a high-purity diamond as their
'quantum mechanical compass'. Under laser light, the nitrogen atom
fluoresced according to how it was magnetized.
Rather than asking,
'Is the magnet pointing north or
south?' the team asked, 'Is it pointing not east?'
Measurements that confirm 'not east'
were still compatible with the quantum superposition of pointing
both north and south at the same time.
The researchers studied three successive
rounds of measurement on the nitrogen quantum compass, and used
correlations between different rounds to prove the presence of
quantum superposition in their system.
'We had previously performed
experiments in which the nuclei of our atoms had two states
available to them. Now we have extended this to a superposition
of three states, if you like North, South, and East,' Professor
'The investigation involved an intermediate measurement, which
was equivalent to opening one of three boxes and seeing if a
ball was not in it. We showed that not only could you not tell
which box had been opened; you could not even tell whether a box
had been opened.
This in turn, thorough some rather
detailed reasoning, allowed us to prove experimentally some
fundamental conjectures about the
nature of reality.'
According to Professor Briggs this work
is pushing the boundaries of 'quantumness' and developing techniques
that will help to investigate whether quantum superposition applies
to larger and more complex objects.
Dr George adds:
'Our confirmation of these subtle
quantum predictions is an important step on the road to
transplanting quantum mechanics from a theoretical and
laboratory curiosity and into the devices which we use in
commerce and everyday life.
Our vision is to scale up and build
computers in which every 'bit' is replaced with a 'quantum bit'
that uses superposition as an integral part of their operation.'