# Quantum Entanglement

**- Overview**

Quantum entanglement is a phenomenon that describes how two light particles, or photons, can become inextricably bound to each other. This means that a change to one photon causes a change in the other, no matter how far apart they are.

Albert Einstein called this connection "spooky action at a distance". However, modern cosmology suggests that most of the particles in the visible universe exhibit a high degree of entanglement.

Entangled photons exhibit non-classical light–matter interactions that create new opportunities in materials and molecular science. For example, in entangled two-photon absorption, the intensity-dependence scales linearly as if only one photon was present.

A single entangled system will remain unchanged as long as it doesn't interact with anything else. For example, if you emit the photons along directions that head off into outer space without hitting anything, then in principle the photons will remain a single entangled system forever.

### **- The 2022 Nobel Prize in Physics**

Quantum entanglement is thought to be one of the trickiest concepts in science, but the core issues are simple. And once understood, entanglement opens up a richer understanding of concepts such as the “many worlds” of quantum theory.

Three quantum physicists have won the 2022 Nobel Prize in Physics for their experiments on entangled photons, in which particles of light become inseparable. These experiments have laid the foundation for a host of quantum technologies, including quantum computers and communications.

John Clauser, Anton Zeilinger and Alain Aspect were awarded the 2022 Nobel Prize in Physics for their work on quantum entanglement. The trio's experiments demonstrate that the connections between quantum particles don't boil down to local "hidden variables," unknowns that invisibly link the two outcomes together. Rather, the phenomenon arises from a true correlation, in which manipulating one quantum object affects another distant object. German physicist Albert Einstein called this phenomenon "spooky action at a distance" -- it's now known as quantum entanglement.

The modern version of the experiment pioneered by the three laureates could be at the heart of a major open question in physics today - how to reconcile quantum mechanics with Albert Einstein's general theory of relativity.

**- Entangled Particle Pairs**

Quantum entanglement is a phenomenon that occurs when two particles interact or share spatial proximity in such a way that the quantum state of each particle cannot be described independently of the state of the other. This means that aspects of one particle of an entangled pair depend on aspects of the other particle, no matter how far apart they are.

For example, a laser beam fired through a certain type of crystal can cause individual photons to be split into pairs of entangled photons. The photons can be separated by a large distance, hundreds of miles or even more.

In theory, there's no upper limit on how many particles can share an entangled state. Theoretical physicists have imagined three-way, four-way, even 100-way quantum connections.

Due to the effects of quantum entanglement, measuring the properties of one particle in an entangled pair immediately affects the measurement of the other particle. It's what enables quantum computers - machines that attempt to exploit the ability of quantum particles to exist in multiple states at the same time, to perform calculations that would be impossible on conventional computers.

Today, physicists are using entanglement to develop quantum encryption and the quantum internet for ultra-secure communications and new types of sensors and telescopes.

**- Spooky Action at a Distance**

Spooky action at a distance" is a term used to describe quantum entanglement. It's a phenomenon where one particle can "know" something about another particle, even if they are separated by a great distance.

Albert Einstein coined the term "spooky action at a distance" in a letter to Max Born on March 3, 1947. He used it to describe the strange effects of quantum mechanics, where two particles can interact instantaneously over a distance.

Over the past few decades, physicists have demonstrated the reality of spooky action over ever greater distances.

Some types of quantum entanglement include:

- Entanglement in position and momentum
- Entanglement in time and energy
- Entanglement in angular position
- Orbital angular momentum
- Entanglement in polarization

**The First-ever Image of "Spooky Action at Distance"**

Scientists have captured the first ever image of a phenomenon which Albert Einstein once described as "spooky action at a distance". The photo above shows a strong form of quantum entanglement, where two particles interact and share their physical states for an instant.

It occurs no matter how great the distance between the particles is. It might not look like much, but just stop and think about it for a second: this fuzzy grey image is the first time we've seen the particle interaction that underpins the strange science of quantum mechanics and forms the basis of quantum computing.

Quantum entanglement occurs when two particles become inextricably linked, and whatever happens to one immediately affects the other, regardless of how far apart they are. Hence the 'spooky action at a distance' description.

This particular photo above shows entanglement between two photons - two light particles. They're interacting and - for a brief moment - sharing physical states.

**- Bell Entanglement**

Bell states are four specific quantum states of two qubits that are maximally entangled. They are in a superposition of 0 and 1, which is a linear combination of the two states.

In 1964, Bell proved that quantum entanglement is incompatible with EPR's notion of locality and causality.

Bell's theorem showed a mathematical conflict between Einstein's view and quantum theory. It also outlined a powerful way for experimentally testing the two.

The theorem states that the predictions of quantum mechanics for the Bell's polarization states of two entangled particles cannot be reproduced by any statistical model of hidden variables.

Bell Inequalitiesare mathematical expressions involving correlations between measurements on entangled particles.

In 2015, a loophole-free Bell violation was reported using entangled diamond spins over a distance of 1.3 kilometers. This was corroborated by two experiments using entangled photon pairs.

**- Scientists Unveil Images of Quantum Entanglement**

For the first time ever, physicists have successfully photographed a strong form of quantum entanglement known as Bell entanglement, capturing visual evidence of an elusive phenomenon that a bewildered Albert Einstein once called "spooky action at a distance."

Two particles interacting with each other (such as two photons passing through a beam splitter) can sometimes remain connected, sharing their physical state instantaneously, regardless of the distance between them. This connection is called quantum entanglement, and it underpins the field of quantum mechanics.

Einstein considered quantum mechanics to be "spooky" because the apparent long-range interaction between two entangled particles is instantaneous, which seemed incompatible with elements of his special theory of relativity.

Sir John Bell later formalized the idea of this non-local interaction, describing a form of intense entanglement that exhibited this spooky quality. Today, while Bell entanglement is exploited in practical applications such as quantum computing and cryptography, it has never been captured in a single image.

Einstein's special theory of relativity is about the relationship between space and time. It states that the speed of light is a limit and cannot be reached by any material object.

However, experiments have shown that entanglement does not allow for the transmission of information faster than the speed of light. Some say that the interaction occurs outside of spacetime, in another dimension.

**[More to come ...]**