###### By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)

## Quantum entanglement is a concept that forms when quantum rules are applied to the world outside atoms. Using the Copenhagen Interpretation, it can describe the probabilities of finding atoms or photons in a different state. It might seem confusing, but it is not once you learn about it. So, read on to see what they mean.

Quantum entanglement happens when two particles are entangled, and their information becomes interdependent. To understand this, one must first know how the Copenhagen Interpretation explains quantum mechanics against classical physics.

### The Copenhagen Interpretation

The Copenhagen interpretation distinguishes the classical and quantum worlds with observation. However, observation in the classical terms needs an observer, while observation in the quantum world happens when a particle interacts with anything around it.

There is another concept at work here called the wave function. It is an actual mathematical function that determines the probability of finding something is a specific state. In the quantum realm, nothing is deterministic, but in the classical world, it is. For example, if a ball is thrown at a specific direction, its future movement can be predicted precisely using mathematics and physics. That is not the case in quantum physics.

The wave function is also a mathematical tool used in quantum mechanics, but it can never predict what happens next inside a quantum system, whether it is a single atom or bigger.

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Understanding the Misconceptions of Science. Watch it now, on Wondrium.

### Quantum Entanglement

The classical system consists of many atoms, all interacting with one another. In the quantum realm, there are small and big systems. The wave function works in the small systems. When observation occurs, and the state becomes clear, the wave function collapses.

When a quantum system interacts with another quantum system, the connections can be more profound. Quantum entanglement is when two objects both have wave functions, but the two wave functions are connected in some way. Photons are also quantum particles that can interact.

Learn more about untangling how quantum mechanics works.

### Quantum Entanglement in Photons

In the famous experiment that proved light was a wave, one photon was shot through a slit to the wall behind it. Each photon moved according to the relevant wave function, which determined the probability of the location where the photons were found. Every photon landed on a specific spot, but by taking many of them into consideration, one could measure the probabilities.

The single photon shot from each slit could actually be a pair of photons, entangled. They are shot in a hypothetically zero-spin quantum system. Thus, the spin before and after emission is both zero. The two entangled photons each have a spin that cancels out the other’s, and they are interdependent.

Each photo has a wave function, but because of quantum entanglement, their wave functions are actually one. When one is in the plus state, the other is definitely in the minus state. Before one takes a state, it is simultaneously in the plus and minus state, according to the wave function. When they interact with something, an observation occurs, and the wave function collapses.

Learn more about what’s inside atoms.

### Faster than Light

If the two photons are emitted with a distance, for example one light second, then the situation can get complicated. Upon emission, there is a 50% probability for each state – minus and plus. As they are entangled, knowing the state of one must determine the state of the other with a 100% probability.

As soon as one photon is measured and its state is determined, its wave function collapses. As it is entangled with the other photon, that photon’s wave function also collapses. However, the information must have traveled from the first photon to the second faster than light, which is physically impossible. Otherwise, the information must have traveled instantly, and all the wave functions must have collapsed at the same time.

Einstein, Boris Podolsky, and Nathan Rosen used this to invalidate the Copenhagen interpretation in their famous EPR paper. Thus, they argued that quantum mechanics was an incomplete and approximate theory and that a better theory—which involved other unknown factors, what they called hidden variables—would eventually be invented.

Learn more about whether you can go faster than light.

### Hidden Variables

In 1964, Irish physicist John Bell put the theory to the test and realized that hidden variables and traditional quantum mechanics made different predictions for a specific set of measurements. He found out that plus and minus were vague terms.

The spin direction of photons 1 and 2 can be vertical, horizontal, and a variety of other directions. If only vertical and horizontal directions are considered, the vertical state of photon 1 will determine the vertical state of photon 2. However, if we decide to measure photon 2’s other dimension different from the horizontal, photon 1 cannot provide the information we look for.

Different measurements will lead to different predictions, but in all of them, quantum mechanics predict that you have some ability to predict how often you will detect photon 2 in the plus or minus state of this new direction.

Such experiments have taken place since the 1970s and have confirmed the Copenhagen Interpretation, quantum entanglement, and the instant collapse of the wave function. However, they do not go against Einstein’s theory of special relativity and modern science.

### Common Questions about Quantum Entanglement

**Q: What is quantum entanglement?**

Quantum entanglement focuses on the consequences of imposing quantum thinking on groups of objects larger than a single atom. If two atoms are entangled, the information from one depends on the information of the other.

**Q: What is observation in quantum mechanics?**

An observation in the quantum world does not need an observer. When an entity interacts with another, an observation is made. The quantum nature of observation is essential to understanding quantum entanglement.

**Q: How does quantum entanglement apply to two photons?**

To observe quantum entanglement in two isolated photons, we must imagine that the photons in a quantum system with zero angular momentum. When the photons are released, they spin in opposite directions to keep the sum of the spin zero. The photons are entangled in this situation.

**Q: Do two spinning photons have two wave functions?**

Under the terms of quantum entanglement, when two photons are entangled, they spin in the opposite directions. Although both obey a wave function, it is basically the same function and not two separate ones.