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.

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