Researchers have recreated the universe's primordial soup in miniature format by
colliding lead atoms with extremely high energy in the 27 km long particle accelerator, the
LHC at CERN in Geneva. The primordial soup is a so-called quark-gluon plasma and
researchers from the Niels Bohr Institute, among others, have measured its liquid
properties with great accuracy at the LHC's top energy. The results have been submitted
to Physical Review Letters.
A few billionths of a second after the Big Bang, the universe was made up of a kind of extremely hot
and dense primordial soup of the most fundamental particles, especially quarks and gluons. This
state is called quark-gluon plasma. By colliding lead nuclei at a record-high energy of 5.02 TeV in
the world's most powerful particle accelerator, the 27 km long Large Hadron Collider, LHC at CERN
in Geneva, it has been possible to recreate this state in the ALICE experiment's detector and
measure its properties.
"The analyses of the collisions make it possible, for the first time, to measure the precise
characteristics of a quark-gluon plasma at the highest energy ever and to determine how it flows,"
explains You Zhou, who is a postdoc in the ALICE research group at the Niels Bohr Institute. You
Zhou, together with a small, fast-working team of international collaboration partners, led the
analysis of the new data and measured how the quark-gluon plasma flows and fluctuates after it is
formed by the collisions between lead ions.
Advanced methods of measurement
The focus has been on the quark-gluon plasma's collective properties, which show that this state of
matter behaves more like a liquid than a gas, even at the very highest energy densities. The new
measurements, which uses new methods to study the correlation between many particles, make it
possible to determine the viscosity of this exotic fluid with great precision.
You Zhou explains that the experimental method is very advanced and is based on the fact that
when two spherical atomic nuclei are shot at each other and hit each other a bit off center, a quark-
gluon plasma is formed with a slightly elongated shape somewhat like an American football. This
means that the pressure difference between the centre of this extremely hot 'droplet' and the
surface varies along the different axes. The pressure differential drives the expansion and flow and
consequently one can measure a characteristic variation in the number of particles produced in the
collisions as a function of the angle.
Mapping the primordial soup
"It is remarkable that we are able to carry out such detailed measurements on a drop of 'early
universe', that only has a radius of about one millionth of a billionth of a meter. The results are fully
consistent with the physical laws of hydrodynamics, i.e. the theory of flowing liquids and it shows
that the quark-gluon plasma behaves like a fluid. It is however a very special liquid, as it does not
consist of molecules like water, but of the fundamental particles quarks and gluons," explains Jens
Jørgen Gaardhøje, professor and head of the ALICE group at the Niels Bohr Institute at the
University of Copenhagen.
Jens Jørgen Gaardhøje adds that they are now in the process of mapping this state with ever
increasing precision -- and even further back in time.
Institute. Note: Materials may be edited for content and length.
Atoms placed precisely in silicon can act as quantum simulator
Date: April 22nd. 2016
Source: University of New South Wales
"Previously this kind of exact quantum simulation could not be performed without interference from
the environment, which typically destroys the quantum state," says senior author Professor Sven
Rogge, Head of the UNSW School of Physics and program manager with the ARC Centre of
Excellence for Quantum Computation and Communication Technology (CQC2T).
"Our success provides a route to developing new ways to test fundamental aspects of quantum
physics and to design new, exotic materials -- problems that would be impossible to solve even
using today's fastest supercomputers."
The study is published in the journal Nature Communications. The lead author was UNSW's Dr Joe
Salfi and the team included CQC2T director Professor Michelle Simmons, other CQC2T
researchers from UNSW and the University of Melbourne, as well as researchers from Purdue
University in the US.
Two dopant atoms of boron only a few nanometres from each other in a silicon crystal were studied.
They behaved like valence bonds, the "glue" that holds matter together when atoms with unpaired
electrons in their outer orbitals overlap and bond.
The team's major advance was in being able to directly measure the electron "clouds" around the
atoms and the energy of the interactions of the spin, or tiny magnetic orientation, of these electrons.
They were also able to correlate the interference patterns from the electrons, due to their wave-like
nature, with their entanglement, or mutual dependence on each other for their properties.
"The behaviour of the electrons in the silicon chip matched the behaviour of electrons described in
one of the most important theoretical models of materials that scientists rely on, called the Hubbard
model," says Dr Salfi.
"This model describes the unusual interactions of electrons due to their wave-like properties and
spins. And on of its main applications is to understand how electrons in a grid flow without
resistance, even though they repel each other," he says.
The team also made a counterintuitive find -- that the entanglement of the electrons in the silicon
chip increased the further they were apart.
"This demonstrates a weird behaviour that is typical of quantum systems," says Professor Rogge.
"Our normal expectation is that increasing the distance between two objects will make them less,
not more, dependent on each other.
"By making a larger set of dopant atoms in a grid in a silicon chip we could realise a vision first
proposed in the 1980s by the physicist Richard Feynman of a quantum system that can simulate
nature and help us understand it better," he says.
The publication of this latest advance towards the development of a silicon-based quantum
computer at UNSW coincided with the opening of the university's new quantum computing
laboratories by Australian Prime Minister Malcolm Turnbull.
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The atom without properties
Date: April 21st. 2016
Source: University of Basel
Everyday objects possess properties independently of each other and regardless of whether we
observe them or not. Einstein famously asked whether the moon still exists if no one is there to look
at it; we answer with a resounding yes. This apparent certainty does not exist in the realm of small
particles. The location, speed or magnetic moment of an atom can be entirely indeterminate and yet
still depend greatly on the measurements of other distant atoms.
Experimental test of Bell correlations
With the (false) assumption that atoms possess their properties independently of measurements
and independently of each other, a so-called Bell inequality can be derived. If it is violated by the
results of an experiment, it follows that the properties of the atoms must be interdependent. This is
described as Bell correlations between atoms, which also imply that each atom takes on its
properties only at the moment of the measurement. Before the measurement, these properties are
not only unknown -- they do not even exist.
A team of researchers led by professors Nicolas Sangouard and Philipp Treutlein from the
University of Basel, along with colleagues from Singapore, have now observed these Bell
correlations for the first time in a relatively large system, specifically among 480 atoms in a Bose-
Einstein condensate. Earlier experiments showed Bell correlations with a maximum of four light
particles or 14 atoms. The results mean that these peculiar quantum effects may also play a role in
Large number of interacting particles
In order to observe Bell correlations in systems consisting of many particles, the researchers first
had to develop a new method that does not require measuring each particle individually -- which
would require a level of control beyond what is currently possible. The team succeeded in this task
with the help of a Bell inequality that was only recently discovered. The Basel researchers tested
their method in the lab with small clouds of ultracold atoms cooled with laser light down to a few
billionths of a degree above absolute zero. The atoms in the cloud constantly collide, causing their
magnetic moments to become slowly entangled. When this entanglement reaches a certain
magnitude, Bell correlations can be detected. Author Roman Schmied explains: "One would expect
that random collisions simply cause disorder. Instead, the quantum-mechanical properties become
entangled so strongly that they violate classical statistics."
More specifically, each atom is first brought into a quantum superposition of two states. After the
atoms have become entangled through collisions, researchers count how many of the atoms are
actually in each of the two states. This division varies randomly between trials. If these variations
fall below a certain threshold, it appears as if the atoms have 'agreed' on their measurement results;
this agreement describes precisely the Bell correlations.
New scientific territory
The work presented, which was funded by the National Centre of Competence in Research
Quantum Science and Technology (NCCR QSIT), may open up new possibilities in quantum
technology; for example, for generating random numbers or for quantum-secure data transmission.
New prospects in basic research open up as well: "Bell correlations in many-particle systems are a
largely unexplored field with many open questions -- we are entering uncharted territory with our
experiments," says Philipp Treutlein.
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