When you come to the West 12th Teaching Building, there are classrooms in the front, on the left, and in the back. The classrooms are connected by intricate corridors and stairs, and Mengshu, a veteran road idiot, immediately loses his sense of direction.

"Dabai, how did you find a classroom here?"

Su Bai smiled and turned to the right, patiently explaining to her: "Stretch out your right hand facing the direction of the rising sun, and point your fingers to the south, which is the S area." In the same way, the direction of the fingers of the left hand is the N-zone. Large 9-12 on the west side, small 1-4 on the east side, and M 5-8 on the half floor. ”

Seeing Su Bai introduce Meng Shu with interest, Ning Chuan nodded with satisfaction, it was not in vain that he had bothered to teach her so much in the first place.

Meng Shu looked to the right and asked cautiously, "Then, the first classroom here is N109?" ”

"Hmm." Su Bai nodded, as if he had thought of something, and said with a smile, "Ah Meng, I remember that your sense of direction is not very good. In the first few days of college, did you find the direction? ”

Meng Shu smiled and shook his head, "In the first few days of school, I was indeed often lost. Especially when there are two different classes in a row, I walk out of a classroom and change the origin of the coordinate system, and I can't find the north. You can only go around to the outside of the school building and then walk into another classroom. ”

Yi Zhan opened her arms and hugged Meng Shu when she was caught off guard.

Meng Shu struggled and protested in a low voice: "What about the students?" ”

"This is the university." Yi Zhan greedily sniffed the faint fragrance of Meng Shu Qingsi, since he hugged her, he didn't want to let go.

Su Bai on the side said angrily: "There are still us here!" ”

Breaking free from Yi Zhan's embrace, Meng Shu looked at the man in front of her who was smirking at her, and immediately lost his temper, and could only laugh along.

"In the first semester of my freshman and sophomore years, we had to take a major course called mathematical analysis, three times a week, and I quickly memorized the classroom of this course. Using this classroom as a reference point, it is not difficult to find other classrooms. ”

"Three semesters?" Yi Zhan was a little surprised, he only needed to study one semester of courses in college.

Meng Shu nodded and said with a smile: "In the past, the mathematics major was also a five-year system, and it took four semesters to study the number of points. Now it has been changed to a four-year system, and it only takes three semesters to study for points. ”

Looking at Meng Shu, who is now smiling lightly, Yi Zhan felt a little distressed in his heart. With his sentence back then, "The greatest talent for learning mathematics is to be serious", how many helpless times will she encounter when she breaks into the land of math tigers and wolves alone.

The pestle silently looked at the three chattering people on the side, and finally couldn't help but remind: "Comrades, we've been standing here for a few minutes, where are we going?" ”

Yi Zhan laughed, "Teacher Xia, where are we going to listen to the class?" ”

"Then this way!"

I looked at four classrooms in a row, all of them were self-study classrooms, and the four of them had to go up, but fortunately, N209 was a classroom for exercises.

The blackboard is the façade of a classroom. Walking into a classroom, Mengshu, as a people's teacher, always focuses on the blackboard first.

Half of the huge blackboard is obscured by a white curtain that displays the topics to be covered this afternoon. The remaining half is a space for passionate young men to write books.

Meng Shu, who was sitting in the last row, soon felt out of place.

The questions on the screen were all in English, and she would always get stuck on a few key words when reading, making it difficult to understand the topic. The students were basically sitting in the front row, and there were only a few of them in the last three rows.

Yi Zhan also felt the same way, and couldn't help but smile and said, "A few of us, it's obvious that we're not here to learn." ”

The only thing to be thankful for is that several of them are far away from the front row, as long as the volume is controlled, it will not affect the students in front of them.

"What are these students thinking about, Ma Yuan, are Mao Xuan so serious?"

"In the case of professional classes, everyone will almost take the front row position ten minutes in advance. And in the general education class, to be honest, I feel embarrassed for the teacher. The students who came sat in the back row, writing essays and doing homework in other subjects, and few people were listening. If it weren't for answering a question to add a little bit of regular points, the whole class would be a one-man show for the teacher. ”

Yi Zhan touched Ning Chuan with his shoulder, "Old Ning, are you like this too?" ”

Although there is no expression on the face, the nodding motion is enough to explain the answer.

"You husband and wife, you are delicate enough!"

"I studied in the Guanke, and here I learned only in professional courses."

Meng Shu looked at Yi Zhan, "Ah Zhan, what about you studying economics?" ”

Yi Zhan said with a smile: "I try to listen to it, I feel that those teachers are quite interesting in their lectures." ”

Meng Shu nodded, it seems that he has learned the theory of the "invisible hand", but he still has not become a refined egoist.

"What are they talking about?" Seeing that there was no formula on the blackboard that could be understood, Su Bai, who was a doctor, couldn't help but doubt life a little.

"It's not quantum mechanics, it's higher quantum mechanics!" From the word "wave function", Mengshu has already gotten the answer.

Yi Zhan smiled, "What a coincidence! ”

"I don't understand anything." Su Bai tried very hard to understand the first question, but the result was still unsatisfactory.

"You look at this question, not only the knowledge of calculus and probability theory is used, but also some simple knowledge of complex variable functions, and to understand quantum mechanics, you need a good foundation in linear algebra."

"I see, I've only learned a little bit of advanced algebra." Su Bai adjusted his mentality, "Insiders look at the doorway, laymen look at the excitement, then you should take a look at the excitement this afternoon." ”

Because the area of the blackboard is really limited, and the formula derivation of each question is extremely lengthy, the young man on the podium has to keep writing and erasing, erasing and writing. Between the wipes, the sweat on his face couldn't stop seeping out.

"In the class, Mr. Ma has already introduced the working principle of the Mach-Zehnder interferometer, so I will not repeat it here. The last question of the first week's homework was a question made by Mr. Ma based on the Elitzur-Vaidman bomb testing problem, I changed everyone's homework, about three-quarters of the students thought that this bomb might explode, and the rest of the students thought that the bomb would not explode, so would this bomb explode at all? ”

Yi Zhan, who was more curious about new things, immediately became interested, "Hey, this topic is interesting!" The truth is often in the hands of a few, and I will not blow it up if I stand in line. If the bomb blows up, what is the point of this quantum detection? Just set the bomb on fire. ”

"Seconded." Su Bai and Yi Zhan agreed.

Ning Chuan was silent, quietly looking at the blackboard, and because he didn't know the idea of solving the problem at all, he refused to express his opinion.

Meng Shu smiled, "It's not enough to learn quantum mechanics just by thinking, in order to convince people, experiments and mathematical expressions are essential. Lens AB has a 50% probability of transmitting or reflecting photons, and I feel that if this bomb is explosive, there should be a certain probability that it will explode. ”

Looking at the students' expectant eyes, the teaching assistant on the podium pushed his eyes upward, "The background to this question is this: Suppose there is a batch of bombs in an arsenal, some of which are explosive and some of which are duds. Each explosive bomb is equipped with a "photon trigger sensor" that detonates when a photon is sensed. Since the dud is not equipped with a photon-triggered sensor, it does not interact with photons, so it cannot sense photons and will not explode. Elitzur and Vaidman questioned in 1993: Is there a way to detect some explosive shells in these bombs without causing all of them to explode? In classical mechanics, this is an impossible task; But in quantum mechanics, the use of zero-action measurements can provide the answer. ”

Hearing "some explosive bombs detected", Yi Zhan immediately understood that he was on the wrong line, and he couldn't help but be a little frustrated.

Back then, he didn't know how many times he took the chalk from the teacher's hand and wrote his unique opinions on the blackboard, just to enjoy Mengshu's adoring eyes during the dozen or so steps back to his seat. But now, he expressed his opinion with high spirits, which was quickly denied. What made him feel even more that the time had changed was that the little girl who didn't know anything about mathematics, physics and chemistry back then put forward the right opinion!

Seeing Yi Zhan's slight pouting movements, Meng Shu knew that the man next to him began to be more serious again. She had to hold back a smile and said softly, "It's okay, this question is very colloquial, similar to the example I gave in class." This question is indeed very interesting, and if the question is a little more polished and expressed more clearly, then our thinking will be clearer. ”

Yi Zhan scratched his head embarrassedly and said with a smile: "It seems that it is not possible to express opinions based on experience only before understanding the situation." ”

"Let's hear the answer to this question."

Several people turned their attention to the blackboard in unison.

The correct answer is: If the bomb is a dud, then the bomb can be removed, and only detector A will go off. If the bomb is explosive, there is a 50% chance that the bomb will be detonated, and neither detector will go off. If the bomb is exploding and not detonated, there is a 50% chance that both detectors will go off. To sum up, as long as detector B sounds, then the bomb must be detonable. According to the quantum Zeno effect, the experiment can also be improved to achieve a yield rate of 100 percent.

After listening to the teaching assistant's explanation, Yi Zhan sighed from the bottom of his heart: "Wow, what a magical quantum mechanics!" ”

The three people around him nodded in unison.

"It's just a little bit hard to understand."

The four people sitting in the back row laughed and nodded again in agreement.

One of the teaching assistants put down the chalk in his hand, and the other quickly walked up to the podium with a few pieces of paper.

"In the second week's homework, there were a few questions selected from Landau's base, and many students left messages in the group to make you cry. But everyone is also very good, despite tears, they still gritted their teeth and made it. ”

The classroom burst into laughter.

Landau, known as an all-round physicist, proposed a syllabus for theoretical physics, known as Landau's barrier. Before Landau's death, only 43 people passed the barrier, and many of those 43 were elected academicians of the Soviet Union. ”

Mengshu silently blessed these children in his heart, hoping that they could truly become successful and contribute to the scientific cause of the motherland.