Abstract after by Erwin Schrödinger in a paper titled

Abstract

            Quantum
entanglement is when two or more particles interact with each other in such a
way that you cannot describe one particle without mentioning the other particle
or particles. When these particles become entangled, we are able to observe and
measure certain traits of one, and know, with certainty, the traits of the
others across negligible distances. However, there are drawbacks. For example,
only one trait can be measured at a given time. If the velocity of a photon was
measured, it is impossible to simultaneously measure the spin of the entangled
photon.1,2 The theory of quantum entanglement has been prevalent
since it was first discovered by Albert Einstein, published in a 1935 paper
written with Boris Podolsky and Nathan Rosen titled “EPR Paradox”, and expanded
upon soon after by Erwin Schrödinger in a paper titled “Entanglement”.3
Today, quantum entanglement, along with quantum theory, is one of the most
researched topics in physics. There are currently applications in of quantum
entanglement in quantum cryptography, quantum computing and superdense coding,
as well as even teleportation.

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            Keywords: quantum entanglement, quantum
theory, photon, causality, quantum cryptography, uncertainty principle,
superposition, quantum computing, decohere, quantum teleportation

 

 

 

Entanglement

            When
particles become entangled, they lose all independence. They become connected
in such a way that it is impossible to describe one particle without mentioning
the other. Particles can only become entangled through physical interaction,
such as a laser fired through both particles. Once entangled, though, they are
bonded permanently. Even if a particle from an entangled pair becomes entangled
with another particle, they form a system.1

            The
most common example of entangled particles is having two entangled photons
placed an arbitrary distance apart. Photon A has an up-spin state. Because the
photons are entangled, Photon B takes up a state relative to Photon A, in this
case a down-spin state. This travel of information between the two photons,
assuming it’s not literally instantaneous, is many times faster than the speed
of light.2

History

            Quantum
mechanics, the study of physics at the smallest possible scale, was first
discovered and studied by Max Planck. Through a series of experiments, Planck
noticed that energy has certain characteristics of matter, not just waves.
Throughout the next 40 years, notable scientist such as Niels Bohr, Louis de
Broglie, Paul Dirac, Albert Einstein, and Erwin Schrödinger took Planck’s
theory and evolved quantum mechanics into what it is today.4

            In
Einstein’s paper “EPR Paradox” written with Boris Podolsky and Nathan Rosen,
the three scientist argued that quantum mechanics was not, in fact, a complete
physical theory. One of the most basic concepts regarding quantum theory is the
uncertainty principle. Using quantum entanglement, only one property of a
particle or system can be measured at any given time. Einstein detested both
these notions. The scientists involved in the paper hypothesized the existence
of hidden variables that could account for the uncertainties involved in
quantum theory. Einstein also deemed the idea of quantum entanglement “spooky
action at a distance”, saying that it broke one of the most basic ideas of
relativity: information cannot, under any circumstances, be sent faster than
the speed of light, because this would defy causality.

            However,
the point of the “EPR Paradox” paper was not to attempt to disprove quantum
mechanics as a whole. The point of the paper was simply a way for them to state
that, because of the EPR paradox, quantum mechanics was not a complete theory
of physics. Today, scientists generally do not view the EPR paradox as a reason
why quantum mechanics is not a physical theory as Einstein, Podolsky, and Rosen
originally intended, but more as an example as to why quantum mechanics defies
classical physics.3

Quantum
Cryptography

The first widespread use of
quantum entanglement is quantum cryptography. Information sent between two
systems whether it be on the internet or a television provider is encrypted
using conventional math. This conventional math is what protects things like
social security numbers, credit card numbers, and now even electronic currency
like Bitcoin. Coders make this math considerably hard to hack, but it can still
be accomplished with enough computer power or time. In an attempt to produce
systems with greater security, coders have started to turn away from
conventional math, and turn to quantum mechanics. Quantum cryptography is coding
that employs the use of quantum mechanics that is effectively unhackable.

The most common type of
quantum cryptography is called quantum key distribution, or QKD. There are many
different types of QKDs and systems that employ these QKDs. However, all of these
have one thing in common. The “key” to decode a message or to get into a system
is sent by a single or group of entangled particles, usually photons of light.
If an unwanted party were to try and access that sent information, it would
affect the entangled particles and would, without fail, leave a sign that that
QKD was tampered with. The system controlling this key would be notified and
would terminate and change the key instantaneously.5,6
            These systems that employ QKDs
are becoming more and more prevalent. The first recorded use of quantum
cryptography was at bank in Vienna, Austria when the researchers working on
this project transferred 3,000 euros into their bank account. And while QKD is
the most common type of quantum cryptography, it is only one area reseach in a
field of science and technology that is constantly growing. In the 1960s,
Columbia University professor came up with the idea of quantum money. This
money would contain quantum particles that could be verified at banks, making
it impossible to counterfeit.5

 

Quantum
Computing

            In
the last decade, quantum computing has been one of the most researched fields
in science. A quantum computer is the same as a classical computer, except with
one key difference: quantum bits, or qubits. Our traditional, everyday
computers run off of pieces of information in the form of zeros and ones called
bits. Every zero represents a “no” and every one represents a “yes”. At the
very basic level, classical computers use this binary coding to answer
questions and carry out commands. A computer’s processing speed is limited to
how many bits it has. A qubit is some particle or photon that is entangled with
another or group of qubits. Qubits still represent ones and zeros like
classical bits, but because of superposition, they can be both a one and a zero
simultaneously, allowing individual bits to work on multiple commands at one
time.7 All the qubits in a quantum computer are entangled with
another or a system of qubits allowing the system to contain substantially more
information than classical computers. This is called superdense coding. Quantum
computers employ the theories of both superposition and entanglement to be
faster and more efficient than today’s computers.8

            Now,
by definition, we have quantum computers right now; but they aren’t powerful
enough to outperform classical computers yet. The biggest quantum computer so
far contains 12 qubits, made by a team from the Institute of Quantum Computing
and the Massachusetts Institute of Technology. The major problem researchers
are facing is that when they work with and manipulate these qubits, they fall
out of their quantum state, or decohere. According to the Institute of Quantum
Computing: “Decoherence is the Achilles heel of quantum computing, but it is
not insurmountable.” Even though quantum computers exist today, a system that
has more computing power than a classical computer is still years of research
away.7,9

Quantum
Teleportation

            Quantum
teleportation is a process that involves entangled particles “teleporting”
information between each other. Experiments testing this theory have been
happening since the 1990s. So far, though, teleportation has only been achieved
for single particles at a time. At first, the teleportation experiments were
just short distances: across a lab table, for example. Scientist would entangle
two particles, usually using a laser shined through a crystal, and then set the
particles at opposite sides of the table. The basis of the experiment is to
essentially “download” all the information of the first particle, and then send
that information through the quantum link. The second particle then adopts
every aspect of the first one, achieving teleportation.10,11

            Today,
scientists are fundamentally doing the same experiments with the only
differences being the distance teleported, the medium in which particles are
being teleported, and the speed at which they can test sets of particles. The
longest quantum teleportation to date is more than 500 kilometers between a lab
in Ngari in Tibet to a satellite orbiting the earth. They created entangled
photons at the rate of 4,000 per second and sent them one at a time to the
satellite. Even though they were only able to teleport one photon at a time,
they were still able to test millions of photons in the 32 days of research
with positive results in 911 cases.10 Another notable quantum
teleportation experiment was performed in seawater. A team of Chinese
researchers filled 3.3 meter long tanks with water from the Yellow Sea. Like
the other experiments, they would first entangle sets of photons and place them
on opposite sides. What makes this experiment notable, however, is the fact
that water is notorious for distorting light. This team of researchers had a 98
percent success right teleporting the photons to the other end of the tank.12

The applications for quantum
teleportation are endless. While it’s only one particle at at time, scientists
are constantly researching this field of science and making more headway every
day. The instantaneous teleportation of matter and information across open
space will change the way we interact with society in a fundamental way. Still,
scientists are years away from commercial use of quantum teleportation.

Quantum
Revolution

            A
quantum revolution is coming. Because of entanglement, there is and will be
technologies that will change the world forever. Like quantum cryptography,
there is already commercially available quantum technology. Still, technologies
like quantum computers and quantum teleporters won’t be ready for a number of
years. Once they are available the whole world will benefit from the advances
of sciences. Quantum computing, for example, means faster and more complex
simulations for potential jumps in all branches of science. The more we
research quantum entanglement, the more major changes the world will undergo.13