Considering some students' curiosity,

here is a layman's explanation of the concept of photons—or rather, quantum entanglement.

As long as you can read, you should be able to understand.

First, let's give an example.

Suppose two identical stationary disks in space, placed side by side, are blown apart by a person named Hao using a bomb.

So, they begin to rotate.

After they have flown quite far apart, we capture one of the disks and measure it.

We find that its angular velocity is w.

Then we can immediately know that the other disk's angular velocity must be -w.

Because according to the conservation of angular momentum, the sum of the angular momentum of the two disks must be zero, so their angular velocities must be opposite.

That is, w and -w cancel each other out.

Quantum entanglement is somewhat similar.

After a pair of entangled photons has flown far away in opposite directions, we capture one of these photons.

The measured result shows its polarization direction is counterclockwise.

At that moment, we can instantly know that the far-off other photon has a clockwise polarization direction.

At this point, some may feel.

Quantum entanglement doesn't seem so special, so why is it discussed so much?

What's the actual difference between the quantum entanglement experiment and the one in the classical world mentioned earlier?

The main difference is that in the classical world, the states of the two disks are already determined at the moment of the explosion.

No matter when and where we measure, we'll get the same results.

But in the quantum entanglement experiment,

as the two photons fly in opposite directions, each photon's polarization direction is not definite.

Instead, it's a quantum state superimposing 50% clockwise polarization and 50% counterclockwise polarization.

The measurement result has a 50% chance of being clockwise polarization and a 50% chance of being counterclockwise polarization.

This photon's state is only determined when you measure it, and it's entirely a probabilistic event.

What does this mean?

Here's the crucial part.

It means when you measure one of the photons, that one photon's state collapses to, say, clockwise polarization.

The state of the other distant photon simultaneously collapses into a definite counterclockwise polarization.

It's as if there's an instantaneous connection between the two photons that transcends the speed of light, allowing them to reach an agreement instantly.

The specific experimental process uses entangled photon pairs generated by spontaneous parametric down-conversion through a type-II BBO crystal to produce photon pairs with orthogonal polarization states.

It's completed using polarizers and single-photon counters.

There are quite a few related papers, so I won't elaborate here; there's no need to delve deeper.

Of course.

Some classmates may ask a deeper question:

How do you know the quantum state is not determined before measurement?

Couldn't it be that it's already determined objectively?

In other words, this photon here was always clockwise polarized, and the other photon was counterclockwise polarized.

We just didn't know their states before observing them?

This involves the issue of superposition.

Looking at Bell's inequality combined with experimental results, it is proven that quantum states are in superposition before being observed.

What does this mean?

It means the same photon, which might measure as clockwise polarized the first time.

But if you switch the basis, it could be counterclockwise polarized the second time.

For instance, you have two fridges in front of you, A contains an egg, and B contains a piece of beef.

When you open A the first time and find an egg, you know without looking that B must have beef.

But when you close A and open it again, it turns out to be beef, even though you didn't do anything but close it.

The third time it turns back to an egg.

After going back and forth, the probability of finding beef and an egg is both 50%, the only certainty is that after A is found to contain something, B must contain the other item.

Of course.

The so-called popular explanation means it's not rigorous enough, so theoretically there must be some difference from reality.

But in terms of nature, the given examples are basically consistent with the experiment, and that's enough.

After all, no one's doing experiments or taking exams.

Additionally, Academician Pan's research on quantum teleportation is based on this rule.

It means if I say a character '0' here, you can instantly get a character '1' at superluminal speed over there.

Even if they're millions of light-years apart, entanglement happens instantly.

Only, the transmission of information requires the carrier of the classical channel, so it can at most approach light speed without violating relativity.

Now let's return our attention to the helicopter cabin.

Once everything was ready,

Academician Pan made a gesture of invitation to Li Bai'an:

"Elder Li, please operate the equipment."

As a student who was once briefly educated by Li Bai'an, Academician Pan knows that this nearly seventy-year-old man's lifelong wish is to observe space once?

Or rather, it's the dream of every physicist.

The experiment process is almost risk-free, but even if it let them die immediately after seeing the result, countless people would still be willing to give their lives.

Li Bai'an nodded to Academician Pan and walked over to the equipment platform.

This elder, who has scarcely contended for fame or fortune in his life, did not choose to demur this time because he is one of countless physicists too.

Then he took a deep breath and pressed the button.

As mentioned before, the speed of photons in a classical channel does not exceed the speed of light.

But in reality, that speed is indistinguishable from the speed of light for ordinary people.

It's a matter of the blink of an eye.

So when Li Bai'an pressed the start key, the feedback result appeared on the screen almost instantly.

A photon was placed on the spatial light modulator at the image plane of the crystal, reflected and displayed in phase, then collected into a single-mode fiber, and finally detected by a single-photon avalanche diode.

The ICCD camera on the micro-robot directly added the threshold frames, soon displaying four separate quantum entanglement images on the screen.

They correspond to four directions θ2 = {0°, 45°, 90°, 135°}.

Meanwhile.

The computer swiftly defined an annular region of interest along the edge of the phase circle object in each image.

Academician Pan quickly glanced over the images on the main control screen:

"Elder Li, the four parallel images of the phase circles at different parts of the photosensitive array have appeared. Shall we proceed with the addition?"

Li Bai'an hesitated for a moment, shook his head, and said:

"No rush, let's wait a little longer."

Just as long-unused kettles often discard the first boiled water, many times the first test results in research are ignored.

After thirty seconds, the beam results in arm2 were updated.

This time Li Bai'an decisively spoke up:

"Xiaopan, start the cumulative calculation."

Academician Pan nodded, quickly typing a section of encryption keys on the keyboard.

Soon, the cumulative phase appeared.

This was a cumulative phase with four colors.

The four colors are red, yellow, purple, and green.

Among them, red, purple, and green represent experimental photons, negative charge, and electrons, respectively.

But that yellow...

The moment he saw it, Academician Pan's pupils constricted sharply:

"How... how is this possible?"

Then he abruptly raised his head and said to Li Bai'an:

"Elder Li, at the edge of space there are lots of...

positrons!"

....

Note:

I spent an afternoon deriving the experimental process myself, it should be fine, though the 45° phase calculation is a bit tricky, I won't list the formula.

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