Another world conquest manual

Considering the curiosity of some unknown classmates,

Here we explain the concept of photons—or quantum entanglement—in as simple a way as possible.

Basically, as long as you can read the text, you should be able to understand it.

First give an example.

Suppose that two identical discs standing next to each other in space were blown up by a bomb by a bomber named Hao.

The two of them started to spin.

After they flew a long way, we captured one of the disks and measured it.

And find that its rotational angular speed is w.

Then we can immediately know that the angular velocity of the other disk 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 rotational angular velocities must be opposite.

That is, w and -w cancel out.

Quantum entanglement is somewhat similar.

When a pair of quantumly entangled photons fly very far in opposite directions, we capture one of the photons.

Its polarization direction is measured to be counterclockwise.

Then at this moment, we can know that the polarization direction of another photon far away is clockwise polarized.

Seeing this, some people may think so.

So quantum entanglement doesn't seem to be anything special, so why is it discussed so much?

What is the difference between quantum entanglement experiments and the previous experiments in the classical world?

The most important difference is that in the classic world, the status of the two disks is already determined at the moment after the explosion.

No matter where and when we measure, we will get the same result.

But in the experiment of quantum entanglement.

When two photons fly in opposite directions, the polarization direction of each photon is not determined.

Instead, it is in a quantum state with a superposition of 50% probability of clockwise polarization and 50% probability of counterclockwise polarization.

There is a 50% probability that your measurement result is clockwise polarization, and a 50% probability that it is counterclockwise polarization.

The state of this photon can only be determined when you measure it, and it is a completely probabilistic event.

What does this mean?

Here comes the most critical part.

That is to say, you measure one of the photons, and the state of this photon collapses into, for example, clockwise polarization.

The state of another photon in a distant place collapses into a definite counterclockwise polarization at the same time.

It is as if there is a connection between the two photons that can exceed the speed of light, allowing them to reach a consensus instantly.

The specific experimental process is to use the spontaneous parametric down-conversion of the second-type BBO crystal to generate entangled photon pairs with two orthogonal polarization states.

Then use the polarizer and single photon counter to complete the measurement.

There are quite a lot of related papers, so I won’t go into details here, and there is no need to understand them too deeply.

Of course.

Maybe some students will ask a deeper question:

How do you know that the state of a quantum is uncertain until it is measured?

Can't it be objectively determined?

That is to say, the photon here is already polarized clockwise, while the other photon is polarized counterclockwise.

Is it just that we don’t know their status before observing them?

This involves a superposition state problem.

Combining Bell's inequality with experimental results, it proves that quantum is in a superposition state before being observed.

What does this mean?

In other words, the same photon may be clockwise polarized when you measure it for the first time.

But if you change the basis vector for the second time, it will become counterclockwise polarized.

For example, there are two refrigerators in front of you. There is an egg in A and a piece of beef in B.

When you open A for the first time, you find it is an egg. At the same time, you know that B must be beef without looking at B.

But when you close A and open it again, it turns into beef the second time, and you did nothing except close the door.

The third time it turned back into an egg.

Over and over again, the probability of beef and eggs appearing is 50%. The only thing that remains unchanged is that after determining that there is a certain object in A, there must be another object coming out of B.

Of course.

The so-called popular statement also means that it is not rigorous enough, so the theory must be different from the reality.

But in terms of nature, the examples given are basically consistent with the experimental situation, and this is enough.

After all, we don’t need to do experiments or exams.

In addition, the quantum hidden state teleportation studied by Academician Pan and his colleagues is based on this rule.

That is to say, if I say a 0 character here, you can immediately get a 1 character at super-light speed.

Even if they are millions of light years apart, entanglement will occur instantaneously.

It’s just that the transmission of information requires the carrier of a classical channel, so it can only be close to the speed of light at most, and does not violate the theory of relativity.

The sight returns to the helicopter cabin.

After everything is ready.

Academician Pan made an inviting gesture towards Li Baian:

"Mr. Li, it's up to you to start the equipment."

As a student who Li Baian once taught for a short period of time, academician Pan didn't know that the lifelong wish of this nearly 70-year-old man was to observe space?

In other words, this is every physics person’s dream.

There are almost no risks in the experimental process, but even if they die immediately after seeing the results, countless people will still be willing to sacrifice their lives.

Li Baian nodded to Academician Pan and walked to the equipment table.

This old academician who had hardly competed for fame and fortune in his life did not choose to give in this time because he was also one of the countless physics people.

Then he took a deep breath and pressed the button.

As mentioned before, the propagation speed of photons in classical channels does not exceed the speed of light.

But in reality, that speed is actually not much different from the speed of light to ordinary people.

It's all in the blink of an eye.

Therefore, when Li Baian pressed the start button, feedback results appeared on the screen almost instantly.

A single photon is reflected by a spatial light modulator placed on the image plane of the crystal and displays a phase object, which is then collected into a single-mode fiber and ultimately detected by a single-photon avalanche diode.

The threshold frames obtained by the ICCD camera on the microrobot are directly added, and four independent quantum entangled images soon appear on the screen.

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

at the same time.

The computer quickly defines a circular region of interest along the edge of the phase circle object in each image.

Academician Pan quickly came to the main control screen and took a few glances:

"Mr. Li, four parallel images of the phase circles at different parts of the photosensitive array have come out. Do you want to add them up now?"

Li Baian hesitated for a while, shook his head and said:

"No rush, just wait."

Just like a hot water kettle that has not been used for a long time is often discarded when boiling water, the first test results are often ignored in scientific research.

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

This time Li Baian spoke decisively:

"Xiao Pan, let's start adding up."

Academician Pan nodded and entered a certain key on the keyboard.

Soon, the cumulative phase appeared.

This is 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 charges and electrons respectively.

But that yellow.

The moment he saw it, Academician Pan's pupils suddenly tightened:

"How is this possible?"

Then he suddenly raised his head and said to Li Baian:

"Mr. Li, there are a lot of things at the edge of space.

Positron! "

Note:

I deduced the experimental process myself for an afternoon, and it should be no problem. However, the calculation of the 45° phase is a bit troublesome, so the formula will not be listed.