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Peter Anthony Ralph Jones
[email protected]
STAG Research Centre
What is Entanglement?
Entanglement is an essential feature of quantum
mechanics, our best theory for describing the
microscopic world. It is best understood by considering
a pair of particles produced together but travelling away
from each other, as in the picture below.
Quantum mechanics predicts that the two particles
cannot really be considered as separate objects, but
are intrinsically connected or “entangled” with each
other. For example, performing a measurement on
one, such as measuring the particles spin as in the
diagram above, instantaneously affects the result of
measuring the other. This phenomena occurs even if
the particles are very far apart in space, so far that
even light could not travel between them in the time
it takes for the other particle to be affected. This is at
the heart of what Einstein called the “spooky nonlocality” of quantum mechanics:
Quantum particles are intrinsically entangled!
Entangled Fields
In physics, a field is an
object that extends
throughout space. To
describe the system we
therefore have to
specify how it is
behaving at every
single point in space.
Examples of fields
include magnetic fields
as in the picture on the
right, and the height of
the surface of a body of
water as in the picture
below. In both cases a
number is associated
to every point in
space, describing the
value of the field there.
Photo of water:
Picture :
We can study the
entanglement of fields
just as for particles.
Here, the entanglement
is occurring between the
field at different
positions in space. The
way that entanglement
is usually studied in field
theory is by splitting
the space into two
regions A and B as in the
picture on the left, and
studying the so-called
entropy” between the
field contained in A with
the field contained in B.
Looking at Entangled Fields through the Hologram
My research involves using holography to study the entanglement of quantum fields. Quantum field theories provide
our most successful and fundamental descriptions of nature; it is a natural and important question to ask how quantum
entanglement applies to field theories, since entanglement is a fundamental property of all quantum systems.
Holography states that a quantum field theory in d-dimensions (known as the “boundary theory”) is equivalent to a
theory of gravity in d+1-dimensions (known as the “bulk theory”); for example, a quantum field theory in 2D is
equivalent to gravity in 3D. It is often mathematically simpler to perform calculations in gravity than it is to perform
calculations in quantum field theory, and thus holography admits a new way of calculating the entanglement of fields.
As an example, consider the problem mentioned above
of calculating the field entanglement between regions
A and B. Holography claims that this is given by the
area of a particular surface S that ends on the curve
that separates the regions, as in the picture on the
right. This surface extends in the full 3-dimensions of
the bulk theory since the presence of gravity “drags it”
into the bulk. We also see this is closely related to the
area law for the entropy of black holes that motivated
holography in the first place!
Picture of Space: