New research from the University of Bristol
may disprove a long-standing conjecture made by one of the founders of quantum
information science: That quantum states featuring ‘positive partial transpose,’
a particular symmetry under time-reversal, can never lead to nonlocality.
When it comes to
space and time, modern physics defies our intuition in the most dramatic way.
Einstein’s relativity theory tells us that time and space are intimately
related and that absolute time is an illusion. Quantum mechanics, however, is
at rest, and its predictions are perhaps even more astonishing than those of
In a nutshell,
quantum theory tells us that two entangled particles behave as a single
physical object, no matter how far apart they are. If a measurement is
performed on one of these particles, the state of its distant twin is
This effect leads
to quantum nonlocality, the fact that the correlation between results of local
measurements performed on these particles are so strong, that they could not
have been obtained from any pair of classical systems, such as two computers.
To cut a long story short, it is as if quantum particles live outside
space-time—and experiments confirm this.
this phenomenon of quantum inseparability, arguably the most counter-intuitive
feature of the theory, represents a major challenge of modern physics. A key
point is that inseparability appears under various forms in quantum mechanics.
Understanding precisely the relation between these various forms is a
Writing in Physical Review Letters, Tamas Vertesi,
PhD, from the Hungarian Academy of Sciences and Nicolas Brunner, PhD, from
the University of
Bristol make a
significant step forward in this direction. They show that the weakest form of
entanglement—so-called undistillable entanglement—can lead to quantum nonlocal
correlations, the strongest form of inseparability in quantum mechanics.
According to Professor Pawel Horodecki, a quantum theorist at the University of Gdansk, “entanglement is almost
‘invisible’ in such systems, which makes it very surprising that they can
The work of
Vertesi and Brunner also goes a long way towards disproving a long-standing
conjecture made in 1999 by Professor Asher Peres, one of the founders of
quantum information science.
Peres argued that
quantum states featuring a particular symmetry under time-reversal—known as
partial transpose—can never lead to nonlocality. All research in this area
supported Peres’ conjecture—until now. Vertesi and Brunner’s work proves, via a
simple example, that the conjecture is false when three (or more) observers are
present. It remains to be seen whether the conjecture could nevertheless hold
true in the case of two observers.
contribution to our understanding of the foundations of quantum theory, this
work raises novel questions in quantum information science. In particular, it
will spark a debate on the role that entanglement and nonlocality play in
quantum information processing tasks, such as in quantum cryptography and