Connections that span space

What is quantum entanglement?

The electrons, photons, and other particles that make up our universe can become inextricably linked and "closely connected" to each other, and in this state, no matter how far apart two particles are, the state observed on one particle accurately reflects the state of another "connected" particle. This connection is called quantum entanglement.

Quantum entanglement occurs when two systems (such as two particles) interact, and their properties are also related. Entanglement often complements another quantum phenomenon called superposition. Superposition means that particles exist in two different states at the same time, for example, photons can display both horizontal and vertical polarization states.

To simplify the understanding of this problem, we can first think of two "entangled" coins. Each coin is hidden under the cup, and as long as the cup is not lifted, the coin is always "spinning", in a state of superposition of the front and back. Once the cup is lifted, the coin will randomly change to a front or a tail.

Now, Bob and Alice each came to different rooms with a cup. If Alice opens her cup first and finds her coin facing upwards (a completely random result), then when Bob opens the cup, she will also find a coin facing upwards.

If they close the cup again to repeat the experiment, the coins will return to the stacked state. Alice lifted the cup again, this time probably to find that her coin was positive. Well, we can expect Bob to also find his coin on the positive side.

Real entanglement

In 1935, Einstein, Podolsky, and Rosen published a paper on the theoretical concept of quantum entanglement. Several physicists have described the idea, but argue that it creates problems for quantum mechanics and makes the theory incomplete.

At the time, many questioned whether entanglement was correct, and even Einstein himself did not believe that two particles could be connected at great distances, calling it "ghostly hyperclide." He thinks this requires them to communicate at faster than the speed of light, which he has shown before.

But over the years, in various experiments, researchers have produced entangled particles that support this theory, and entanglement has undoubtedly become one of the most representative and bizarre phenomena in the quantum world.

Similar to Alice and Bob's experiment, when researchers entangled two photons and then sent each photon in a different direction under precisely controlled conditions, they would continue to be superimposed while polarizing horizontally and vertically. Only when one of the photons is measured will the two photons randomly get one of the two possible polarization states.

Physicists have now shown that entanglement can span hundreds of kilometers. In 2017, a Chinese satellite called Mozi sent entangled photons to different ground stations, separated by more than 1,200 kilometers, breaking the distance record for entangled particles.

Experiments to understand the entanglement of more particles

Quantum correlations are very different from ordinary correlations. Randomness is key. This ghostly intrinsic randomness is actually exactly the problem that plagued Einstein, but it is crucial to the way the quantum world works.

The concept of two entangled particles is already very puzzling, and the situation becomes more complicated when more particles are involved.

For example, in a natural environment such as the human body, not two, but hundreds or more molecules are entangled together to form an intertwined group. In these multibody entanglement systems, the whole is greater than the sum of its parts. To use a figurative metaphor, entanglement is like a thread that passes through each individual particle and instructs them how to connect to each other. The real challenge for scientists is to understand how hundreds or more particles can relate to each other in a similar way.

The first step in understanding multibody entanglement is to create and control it in the lab. Fundamentally, this is very difficult to do. Even if you scale it down a little more, it's not easy.

For example, if researchers create a system that produces 20 entangled particles, and then they send 10 in different directions, they have to measure whether each of the first group of 10 particles is entangled with each particle in the other group.

There are many different ways to test these associations, and the description of these systems is particularly complex. To solve this problem, many researchers are trying to think about computational representations of entangled materials that are simpler and more concise than existing models.

Another difficulty in creating and controlling quantum systems has to do with their fragile nature. Like mimosa, known as a "sensitive plant," entanglement can easily disappear when the environment changes slightly.

In experiments, entangled particles quickly become entangled with their surroundings, disrupting the original entangled state that researchers might have tried to study or use. Even a single stray photon that flies past an experiment can destroy an entire experiment. In the field of quantum computing, this vulnerability also brings many problems, leading to computational errors.

The root of all things

While entanglement is key to advances in quantum information science, it is also a concept of particular interest to theoretical physicists.

Some theoretical physicists argue that space and time are themselves the result of potential networks of quantum connections. Any two points in space-time, no matter how far apart, are actually entangled. The points of time and space that we think are closer to each other may just be more "entangled" than those that feel farther away.

More recent speculation suggests that entanglement can be seen as a line in quantum gravity that stitches together different regions of space-time. This entanglement's connection to space-time may even help solve one of the biggest challenges in fundamental physics, which is to build a unified theory that links the macroscopic laws of general relativity (gravity) with the microscopic laws of quantum physics (the way subatomic particles behave).

#创作团队:

Written by: M Ka

Typography: Wenwen

#参考来源:

https://magazine.caltech.edu/post/untangling-entanglement

https://www.symmetrymagazine.org/article/january-2014/quantum-entanglement

https://theconversation.com/quantum-entanglement-what-it-is-and-why-physicists-want-to-harness-it-171608

#图片来源:

Cover/Header: Principle