PW_mar13_sample_issue

Volker Steger/Science Photo Library
Quantum frontiers: The lowdown
Beam me up
Entangled photons
can be used for
quantum
teleportation,
quantum computing
and possibly even
secure satellite
communications.
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world according to quantum physics, we can still use
quantum systems to develop new technologies. One
of the key advantages of quantum systems is their
universality – in other words, every quantum system
of sufficient complexity should be able to simulate
efficiently every other quantum system. One of the
most developed quantum simulators at present uses
atoms cooled down to near absolute zero. The interactions
between these cold atoms can be tuned so
well in the laboratory that they can be made to interact
in many different ways, allowing them to simulate
the behaviour of other systems. This is useful first
and foremost because some physical systems, such
as high­temperature superconductors, are so complicated
that it is difficult to determine their exact
quantum description by measuring them directly.
Simulating their behaviour with atoms lets us extract
what we believe to be the essence. More over, we can
also simulate the behaviour of systems we are not
even sure exist in nature.
For example, Majorana fermions are meant to be
fermionic particles (like electrons), but at the same
time they are their own antiparticles (unlike electrons).
At present only bosons, such as photons, are
known to also be their own antiparticles so we do
not really know if Majorana fermions exist. Interestingly,
this does not prevent us from simulating them
with cold atoms. But is our ability to simulate things
that nature does not create itself a deep and fundamental
property of the universe? Or is nature’s way
of making Majorana fermions first to make humans
who then figure out how to artificially make (i.e.
simulate) Majorana fermions? To paraphrase Bohr,
a physicist is just a Majorana fermion’s way of creating
itself. Immanuel Bloch explores this fascinating
topic for us on pages 47–50.
Beyond the physics lab
Quantum physics may often sound like science fiction,
but surely Bohr, Einstein or any of its founders
would never have believed that it might one day be
possible to do quantum experiments in space. The
main motive for taking quantum experiments to this
new frontier is that if our future communication technology
is to be fully quantum, it will involve quantum
physicsworld.com
communication between quantum computers based
on Earth and on satellites. The current world record
in distant quantum­information processing is held
by Anton Zeilinger and colleagues, who teleported
a quantum bit across a distance of 143 km between
the Canary Islands of La Palma and Tenerife. This
terrestrial record will be smashed once we move into
space – the fact that there are very few atoms around
eliminates a great deal of the noise we have to face
when doing experiments on Earth.
Performing quantum experiments in space will
also allow us to test fundamental physics theories in
regimes we have never before been able to access.
That is because it will let us send signals over large
distances, between platforms moving at large relative
speeds, all in near­vacuum conditions. Indeed, certain
tests of alternative theories to quantum physics,
such as Penrose’s gravitationally induced collapse,
are only realistically possible in space. The problem
is the measurements required are very sensitive since
the effects postulated at the boundary of quantum
physics and gravity are rather meagre and any other
potential noise needs to be eliminated. For more on
this fascinating topic of space­based quantum physics,
check out the article by Brendon Higgins and
Thomas Jennewein (pp52–56).
Quantum physics is also proving useful in our
understanding of biology, which has traditionally
been based on classical physics given that biological
mole cules are so large, warm and wet. After
all, systems that interact strongly with their environment
(and so are warm and wet) cannot behave
coherently because the environment itself impedes
quantum coherence. Moreover, large molecules
have many more ways in which quantum coherence
can be destroyed. However, we are now discovering
that biology may use some of the more sophisticated
quantum tricks to improve its processing.
One interesting case, which Jim Al­Khalili explains,
is the possibility that DNA mutations are the result
of the quantum tunnelling of hydrogen (pp42–45).
Other fascinating examples are bacteria implementing
a quantum random walk to optimize photosynthesis,
birds using entangled electrons to determine
the inclination of the Earth’s magnetic field and the
amazing possibility that we humans and other animals
can smell quantum superpositions.
Anybody’s guess
So what of the future? I predict that in 2013 we will
see the first implementation of quantum teleportation
between satellites. We will also see the first
cold­atom simulation of non­Abelian anyons (particles
with a behaviour that lies somewhere between
fermions and bosons). We will have more evidence
that biological energy transport is fundamentally
quantum mechanical. We will almost certainly still
puzzle over the meaning of quantum measurement.
And finally, I am convinced that another Nobel prize
will be given for testing quantum­mechanical effects,
most likely the existence of the Higgs boson.
My predictions will almost certainly be wrong, but
even if they are, quantum physics is guaranteed to
keep us tantalized for many years to come. n
Physics World March 2013