Quantum entanglement appears to defy basic physics rules, yet it also supports a number of crucial technologies.

There's a
fair probability you've heard about quantum entanglement if you know anything
about quantum mechanics. This property of quantum mechanics is one of the most
remarkable discoveries of the twentieth century, as well as one of the most
promising study areas for new technologies in communications, computers, and
other fields.

But what
exactly is quantum entanglement, and why is it so crucial? What was it about it
that frightened Albert Einstein? And why does it appear to defy one of physics'
most fundamental laws?

## What Is Quantum Entanglement in Simple Terms?

Things get
confusing whenever you talk about quantum mechanics, and quantum entanglement
is no exception.

The first
thing to realise is that until they are noticed, particles exist in a condition
of "superposition." The quantum particles employed as qubits in a
quantum computer are both 0 and 1 at the same time in a common demonstration,
until they are detected, at which point they appear to randomly become a 0 or
1.

To put it
another way, quantum entanglement occurs when two particles are formed or
interact in such a way that their key properties cannot be explained
separately.

If two
photons are created and entangled, for example, one particle may have a
clockwise spin on one axis, whereas the other must have a counterclockwise spin
on the same axis.

This isn't
particularly revolutionary in and of itself. However, because quantum physics
describes particles as wave functions, measuring a particle's spin is said to
"collapse" its wave function to generate that observable feature
(like going from both 0 and 1 to only 0 or only 1).

However,
when we apply this to entangled particles, we get the most fascinating portion
of quantum entanglement. Even if you did not observe the second particle, when
you measure an entangled particle to determine its spin along some axis and
collapse its wave function, the other particle collapses as well, producing the
measurable property of spin.

If you
measure one of two entangled particles as 0, the other entangled particle
collapses to generate a 1 on its own, without any interaction from the
observer.

This
appears to happen instantly and regardless of their distance from one another,
leading to the bizarre conclusion that information about the measured
particle's spin is somehow communicated to its entangled partner faster than
the speed of light.

## Are All Particles Entangled?

Yes, to
some extent.

When most
people talk about quantum entanglement, they use an example of two entangled
particles behaving in a certain way to illustrate the phenomena, although this
is a highly simplified version of a very complex quantum system.

In
actuality, a single particle can be entangled with a variety of other particles
to differing degrees, rather than only the "maximally entangled"
state, in which two particles are one to one correlated and only to each other.

This is
why, in real-world applications, measuring one part of an entangled pair does
not guarantee that you will know the state of the other particle, because the
other particle has additional entanglements to maintain as well. It does,
however, give you a higher probability than chance of knowing the state of the
other particle.

## Who Discovered Quantum Entanglement?

In a 1935
publication in the journal Physical Review titled "Can Quantum-Mechanical
Description of Physical Reality Be Considered Complete," Einstein and his
colleagues Boris Podolsky and Nathan Rosen postulated quantum entanglement, or
at least the principles that characterise the phenomena. Einstein, Podolsky,
and Rosen explained how a very strong connection of quantum states between
particles can lead to a single unified quantum state between them.

They also
discovered that due to this unified state, measurements of one strongly
correlated particle can have a direct effect on the measurement of the other
strongly correlated particle, regardless of the distance between the two
particles.

The goal
of the Einstein-Podolsky-Rosen paper wasn't so much to proclaim the
"finding" of quantum entanglement as it was to describe a phenomenon
that had already been observed and discussed, and to argue that there must be a
missing component of quantum mechanics that hasn't been discovered yet.

Because
the strong correlation phenomenon they described looked to be counterintuitive
and defied Einstein's relativity equations, the paper contended that physicists
were missing something else that would correctly fit the quantum realm under
the cover of relativity. Almost a century later, that "something
else" has yet to be discovered.

Erwin
Schrödinger was the first to adopt the term "entanglement" to
characterise this occurrence, recognising it as one of quantum mechanics' most
fundamental properties and arguing that it wasn't a riddle that would be solved
shortly under relativity, but rather a complete break from classical physics.

## What Did Einstein Say about Quantum Entanglement?

Quantum
entanglement is famously defined as "spooky action at a distance,"
but Einstein described it as more than simply a strange quirk of ghostly
particles with instantaneous knowledge of each other.

Quantum
entanglement, according to Einstein, is a mathematical paradox, or an inherent
contradiction in mathematical logic that indicates that something about the
arguments being made is incorrect.

The
arguments in the so-called Einstein-Podolsky-Rosen conundrum are that the
fundamental rules of quantum mechanics are entirely understood and that general
relativity is correct. Nothing in the cosmos can go faster than the speed of
light, which is 186,000 miles per second if general relativity is correct.

If quantum
mechanics is fully understood, the rules regulating strong particle
correlations are complete, and our observations provide all the information we
require.

Because
quantum particles are "of the universe," they should, like everything
else, be governed by the speed of light. However, quantum entanglement appears
to transfer information instantly between particles that may conceivably be on
opposite ends of the cosmos. Even worse, this data may be able to go back and
forth in time.

Quantum
entanglement in time would have a lot of ramifications for the nature of
causality, which is about as fundamental a physical law as you can get. It
doesn't work the other way around; consequences can't come before their causes,
but some scientists believe that those constraints don't apply to quantum
mechanics any more than they do to the speed of light.

This last
argument is still somewhat speculative, but it does have some experimental
support, and it only adds to the paradox provided by Einstein, Podolsky, and
Rosen in their 1935 study.

## Why Is Quantum Entanglement Important?

Entanglement
in quantum mechanics is significant for two reasons.

To begin
with, quantum entanglement is a fundamental mechanism of the quantum universe
that we can directly interact with and control. It could be a route to
unlocking some of the universe's most fundamental features and pushing our
technology to new heights.

We
understand how to entangle particles and do so on a daily basis in both
laboratory and real-world applications such as quantum computers. Quantum
computers, in particular, show how quantum mechanics may be used in modern
technology, and quantum entanglement is the best instrument we have for
utilising quantum mechanics in this way.

Another
crucial argument for quantum entanglement is because it is a signpost pointing
to something very basic about our reality. It's as evident as it gets that the
quantum world is a purer version of the cosmos than the one we can see, and
that it follows principles that we can understand.

If the
entire universe is a stage, and matter is the actors, quantum entanglement—and
quantum mechanics in general—could be the line riggings that raise the
curtains, the switches that turn on and off the lights, or even the actors'
clothing.

When we
see a play, we can appreciate it in two ways. You can appreciate the tale told
by the play by looking past the theatre and set pieces, or you can appreciate
the quality of the performance, staging, and execution.

By seeing
the same performance again, you can see two quite different things, and quantum
mechanics appears to provide us a new way of experiencing the same universe
we've always seen, and quantum entanglement may be the key to getting us
backstage.

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