Chapter 316

Considering the curiosity of some rare students,

Let's explain the concept of photon - or quantum entanglement in as popular a way as possible.

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

Let's start with an example.

Let's say that in space, two stationary disks next to each other are forced to explode by a bomb surnamed Hao.

Both of them therefore began to have a rotation.

When they flew far, we captured one of the disks and measured it.

It is found that its rotational angular velocity is w.

So we immediately know that the angular velocity of the other disk must be - W.

According to the conservation of angular momentum, the sum of the angular momentum of the two disks must be zero, so the rotational angular velocities of the two disks must be opposite.

That is, W and - W cancel out.

Quantum entanglement is somewhat similar.

When a pair of quantum entangled photons fly far in the opposite direction, we capture one of them.

It is measured that its polarization direction is counterclockwise.

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

See here, maybe someone will feel like saying.

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

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

The main difference is that in the classical world, the state of the two discs is determined at the moment after the explosion.

No matter when and where we measure it, 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.

It is a quantum state with 50% probability of clockwise polarization and 50% counterclockwise polarization.

The result of your measurement has a 50% probability of clockwise polarization and a 50% probability of counterclockwise polarization.

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

What does this mean?

Here comes the most crucial place.

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

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

It seems that there is a connection between the two photons that can surpass the speed of light, so that they can reach a consensus in an instant.

The specific experimental process is entangled photon pairs. Using the spontaneous parametric down conversion of class II BBO crystals, two entangled photon pairs with orthogonal polarization states can be generated.

The measurement can be completed by using polarizer and single photon counter.

There are still a lot of relevant papers. I won't repeat them here, and there's no need to understand them too deeply.

Yes, of course.

Maybe some students will ask a deeper question:

How do you know that the quantum state is uncertain before measurement?

Can't it be objectively determined?

That is, the photon here has long been clockwise polarized, while the other photon is counterclockwise polarized.

Is it just that we don't know their state before we observe them?

This involves a superposition state problem.

Bell inequality, combined with the experimental results, proves that the quantum is in the superposition state before being observed.

What does that mean?

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

You can change the basis vector to counterclockwise polarization the second time.

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

The first time you opened a, you found it was an egg. At the same time, you don't have to look at B to know that B must be beef.

But when you close a and open it again, the second time it turns into beef, and you don't do anything except close the door.

The third time it turned back into an egg.

Over and over again, the probability of beef and eggs is 50%. The only constant is that after determining that there is an object in a, there must be another object in B.

Yes, of course.

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

But in terms of nature, the example basically does not run away from the experimental situation, which is enough.

After all, we don't want to do experiments or exams.

In addition, the quantum teleportation studied by academician pan is based on this rule.

That is, I said a 0 character here, and you can immediately get a 1 character at the speed of superluminal speed.

Even if they are millions of light-years apart, entanglement will occur in an instant.

Only the transmission of information needs the carrier of classical channel, so it can only be close to the speed of light at most, which does not violate the theory of relativity.

Return to the helicopter cabin.

When everything is ready.

Academician pan made an invitation gesture to Li Bai'an:

"Mr. Li, you can start the equipment."

As a student of Li Bai'an who had been teaching for a short time, academician pan did not know that the lifelong wish of this 70 year old man is to observe space once?

Or this is the dream of every physical person.

There is little risk in the experiment, but even if they die immediately after seeing the results, countless people will still be willing to pay their lives.

Li Bai'an nodded to academician pan and walked to the equipment platform.

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

Then he took a deep breath and pressed the button.

As mentioned earlier, the propagation speed of photons in the classical channel will not exceed the speed of light.

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

It's all in the blink of an eye.

Therefore, when Li Baian just pressed the start button, there was feedback on the screen almost instantaneously.

Only one photon is reflected by the spatial light modulator placed on the image plane of the crystal and displays the phase object. Then it is collected into the single-mode fiber and finally detected by the single-photon avalanche diode.

The threshold frames obtained by the ICCD camera on the micro robot were added directly, and four independent quantum entanglement images soon appeared on the screen.

They correspond to θ 2 = {0 °, 45 °, 90 °, 135 °} in four directions.

meanwhile.

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 looked at it:

"Mr. Li, four parallel images of phase circles at different parts of the photosensitive array have come out. Now can we accumulate them?"

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

"No hurry, wait."

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

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

This time, Li Baian spoke decisively:

"Xiao pan, start accumulating."

Academician pan nodded and input 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

Red, purple and green represent experimental photons, negative charges and electrons respectively.

But the yellow

At the moment of seeing it, academician Pan's pupils suddenly contracted:

"This... How is this possible?"

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

"Li Lao, there are a lot of at the edge of space

Positron! "

.......

Note:

During the experiment, I deduced it myself for an afternoon. It should be no problem, but the phase calculation of 45 ° is a little troublesome, so the formula is not listed.