Quantum communication: a "magical change" telepathy
Abandon all "common sense" in the macro world, break free from the shackles of traditional experience, and maintain a state of "open brains". Liberal arts students must also "grit their teeth and persevere." Believe me, this time you will discover quantum communication. ,I see!
On January 8, the first prize of the 2015 National Natural Science Award was awarded to such a project: "Multiphoton Entanglement and Interference Measurement".
The project was led by Academician Pan Jianwei of the University of Science and Technology of China and jointly completed by Peng Chengzhi, Chen Yuao, Lu Chaoyang, and Chen Zengbing.
After the awards were awarded, various popular science articles were published.
To put it simply, multi-photon entanglement and interference measurement is a new way based on the principles of quantum physics to encode, store, transmit and logically operate information, and to conduct microscopic particles such as photons and atoms. Precise manipulation to ensure communication security and increase computing speed can break through the bottlenecks of classic information technology.
But it is not easy to truly understand this passage.
The first question is, what are the principles of quantum physics?
Before opening the door to the magical quantum world, you need to travel lightly: throw away all the "common sense" you have acquired in the macroscopic world, and get rid of those The shackles built by traditional experience keep the mind open and ready to accept all kinds of "How is this possible?" ”
Then, let’s get started.
The first stop is wave-particle duality - it is important to see or not to see
Quantum is the smallest unit of energy.
Chen Yuao, a professor of modern physics at the University of Science and Technology of China, said that microscopic particles are all quanta. The atoms, electrons and photons we have come across in junior high school chemistry books, They all belong to the quantum family.
It can be said that the entire world is composed of quanta.p>
However, because the quantum is too small, for most people, it is the "most familiar stranger".
Microscopic particles have a magical property called wave-particle duality.
The double-slit interference experiment confirmed this.
Imagine that you have an electron emitter in your hand, a partition with two slits in front of you, and a screen placed behind the partition.
When you hit enough electrons, what should appear on the screen?
If electrons were particles, then there should be two stripes on the screen - the electron randomly chooses which of the two slits to pass through, leaving a trace on the screen.
However, the reality is that multiple "interference fringes" of alternating light and dark are left on the screen.
After studying the distribution of these stripes, people were surprised to find that photons seemed to have some kind of "wave" characteristics when passing through the gaps.
In other words, it did not choose one of the two gaps to pass through, but passed through both gaps at the same time in the form of a "wave" and interfered with itself—— If the crest of one wave happens to meet the trough of another wave, the brightness just cancels out, forming a "dark spot" on the screen.
However, electrons clearly exhibit the characteristics of particles.
When we emit electrons one by one, you will find that after the electrons pass through the gap in the partition, they will appear at a certain point on the sensing screen. A bright spot is struck at a position.
But its distribution conforms to the distribution pattern of interference fringes: the probability of falling in the bright area is high, and the probability of falling in the dark area is low.
Why? Scientists gave a bold explanation: before hitting the sensor screen, no one interfered, and the electron did pass through the two slits in the form of waves; but once it hit the sensor screen, The wave function immediately collapses into a point
The sensor screen plays the role of "observer" here.
In other words - electronic What state is displayed depends on "observation"
Is observation important? Look left, right, look up and down, that girl is not simple? But in the microscopic particle world, any kind of intervention will cause fatal interference to the measurement object.
< p>You can never get all the information about a particle - when you know its position, its speed also depends on your "knowledge""Tao" has changed - this is also the famous "uncertainty" principle.Second stop quantum entanglement - incredible telepathy
With a little hazy realization, we continue on the road.
Superposition state, just like Sun Wukong's clone technique.
Because he has 72 clones, Sun Wukong can be here and there at the same time.
However, if Tang Monk wants to see clearly Where is Sun Wukong? All the clones of this naughty Monkey King will randomly disappear, leaving only one. 0px; margin-bottom: 0px; border: 0px; line-height: 24px; font-size: 14px; font-family: simsun, Arial; white-space: normal;">At this point, the concept of quantum entanglement should come into play: independent particles can be completely "entangled" together, and observing one particle can instantly affect other particles, no matter how far apart they are. Far.
The famous scientist Einstein This is unacceptable and is called "spooky action at a distance"
Let's continue to imagine: Now there is a large particle decaying into two small particles. The two are not in harmony and fly away in opposite directions.
Suppose this kind of particle There are two possible spins - "left-handed" and "right-handed".
According to global conservation, if particle a is left-handed, then particle b must be right-handed; and vice versa.
However, before we observe a and b, their states are uncertain, and each particle is in a superposition state of left/right possibilities.
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What happens next is a scene that even Einstein couldn't understand - once we observe particle a, its wave function collapses instantly and randomly selects At this time, even though it is far away from a, the state of particle b is instantly determined-it is "right rotation".
Even if these two particles are at opposite ends of the universe, they can still maintain such a terrible "tacit understanding" - once you randomly choose the left, then I will definitely choose the right.
Any so-called Telepathy is not as profound as "quantum entanglement"
The third stop of quantum teleportation - a "magical change"
Finally, we are going to enter the core attraction - if, I want to entangle the two What about communication between particles?
At this time, we prepared photon α and photon β in a quantum entangled state, and gave α to A in Beijing and β to B in Shanghai.
What we actually want to pass is photon γ
First, we let photon α and photon γ interfere and record the interference results; then, A needs to use classic communication methods, such as calling, sending text messages, sending emails, etc., to tell B the result.
After getting the results, we can look forward to a "photon transformation show".
B will operate something called a wave plate to turn β into γ.
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What? What does this mean? Don’t panic, you can understand it this way: α and β are in an entangled state; so when α and γ interfere, γ and β automatically have a certain relationship; A tells B α The result of the interference with γ actually tells B what relationship β and γ should have.
So, B can change β into γ through “backwards”.
Please note that quantum teleportation does not really transmit anything, but a "magical change".
In In this "communication", both α and β are "cannon fodder" sacrificed for the protagonist γ
The ultimate goal is to let β become γ so that B can obtain some information about γ. p>
However, the things that can be transmitted using quantum teleportation are currently quite limited.
The top ten major breakthroughs in the field of international physics in 2015 were Pan Jianwei, Lu Chaoyang and others. The scientific research project "Multi-freedom Quantum Teleportation"
The breakthrough of this work is that it transmits two properties of photons - "spin" and "orbital angular momentum" for the first time.
True application indeed has a long way to go.
Fourth Website quantum key distribution - cannot be eavesdropped, absolutely safe
However, the door to the practical attractions of quantum communication technology has been opened, and the first one to walk in is called "quantum secure communication".
It is most closely related to classical communication. The difference is that the key is transmitted using quantum methods
Pan Jianwei gave a speech at the 2015 China Computer Conference.
He said that there is a bottleneck in information security in information science: using chips may have backdoors, and using optical fibers may be susceptible to insensible eavesdropping. ; Even if we encrypt information, with the development of information technology, things that are traditionally considered difficult to crack may become window paper, which can be broken with a poke.
However, if we Using quantum methods to transmit passwords does not have this problem.
Zhao Yifu, a Ph.D. from the Laboratory of Quantum Information of the University of Science and Technology of China, introduced in an interview with the media that quantum key distribution is to encode a piece of information on a photon. A photon has different quantum states, representing 0 and 1; the photon passes through the optical fiber. After transmitting, the receiver decodes the key after receiving it.
We have said before that the state of a quantum is unknown. According to the "uncertainty principle", we cannot obtain all the state information of a quantum. Therefore, the quantum cannot be accurately measured and copied accurately; and Quantum cannot continue to be divided, It is impossible for an eavesdropper to split it in half, take one half away, and pass the other half to the transporter; what's even more incredible is that during this transmission process, once someone eavesdrops, the tiny photons can respond immediately - because On the quantum scale, the presence of eavesdroppers is too strong!
Therefore, the security of quantum secure communications can be greatly guaranteed.
Today, quantum secure communications are even called the "next trillion blue ocean" by the capital market.
< p>Now, we have visited four attractions in the world of quantum communication. In view of the numerous deaths and injuries of brain cells, this journey has come to an end.
In the macro world, my country's long-distance quantum communication backbone network "Beijing-Shanghai Trunk Line" project is under construction and is expected to be completed in 2016; also in 2016, my country will also launch the world's first quantum satellite, which is expected to be completed in 2016. Complete three major tasks: absolutely secure quantum key distribution between satellites and ground, verifying Bell's inequality in space, and realizing communication between ground and satellitesTeleportation.
Next time, when you enter the world of quantum communication, there will be more and more interesting things to see.
