Quantum Technology Series Report ③
The establishment of quantum mechanics brought about the first quantum revolution and gave birth to the third industrial revolution represented by modern information technology, which fundamentally changed the way of life and social outlook of human beings.
With the continuous deepening of human knowledge, understanding and research on quantum mechanics, quantum information technology that uses microscopic particle systems as the object of manipulation and uses its unique physical phenomena for information acquisition, processing and transmission has emerged, and is expected to promote the second quantum Revolution will have an essential impact on the future society.
Quantum information technology mainly includes three major areas: quantum computing, quantum communication, and quantum measurement. Among them, quantum communication has become one of the focuses of information and communication technology evolution and industrial upgrading.
The entanglement feature allows a pair of quantum "to communicate with each other"
Even if a pair of entangled quanta is far apart, the behavior of one particle will instantly affect the state of the other. This spatial influence speed can exceed the speed of light, breaking the locality principle proposed by Einstein.
Quantum communication uses quantum superposition states or quantum entanglement effects for information or key transmission, and guarantees transmission security based on the principles of quantum mechanics. The main quantum teleportation and quantum key distribution are two types.
In this process, the superposition state properties of quantum play an important role, and even quantum entanglement is a superposition state of multiple particles.
Quantum entanglement refers to the phenomenon that particles affect each other in a system composed of two or more particles. Even if they are far apart, the behavior of one particle will affect the state of the other.
When one of them is manipulated (for example, quantum measurement) and the state changes, the other one will also have a corresponding state change immediately.
This kind of quantum entanglement that spans space and affects both parties instantaneously was once called "ghost-like over-distance action." Einstein once questioned the completeness of quantum mechanics on this basis, because this over-distance action violated his proposal. The principle of locality, that is, the speed of mutual influence in any space cannot exceed the speed of light.
On the basis of Einstein's locality, the physicist Bohm proposed the hidden variable theory to explain this super-distance interaction. He believed that microscopic particles have no objective reality, and they have certain properties only when people measure them. .
The physicist Bell demonstrated the existence of quantum nonlocality through experiments, and proved to the world that quantum entanglement is nonlocal, and the hidden variable theory is wrong.
In 1984, Bennett of IBM and Brasard of the University of Montreal proposed the first practical quantum key distribution system, called the BB84 scheme, which officially marked the birth of quantum secure communication.
The basic principle of the BB84 scheme is that the information of the sender and receiver can be represented by the photon polarization state. If Zhang San sends the information using random polarization, Li Si discovers and records the information.
Then, Zhang San informed Li Si of the polarization frequency on the public frequency, and the two sides compared the selected information part according to the correct polarization comparison.
In fact, although the BB84 scheme uses quantum channels, it still transmits classical information. The real quantum communication is to encode information on qubits, and transmit the qubits from Party A to Party B on the quantum channel to directly realize the information. Of delivery.
For example, in the classic communication, Zhang San sends the information obtained after scanning the documents to be transmitted to Li Si through the classic channel, who can print the documents.
However, it is impossible for Zhang San to transmit a quantum state to Li Si in this way.
Because it must be measured to be transmitted, the quantum state collapses once measured and is no longer the original quantum state.
So, how to transfer a quantum state without causing collapse?
In 1993, Bennett and others proposed a teleportation protocol based on EPR pairs (pairs of particles with zero total momentum and total spin), using two classical bit channels and a winding bit to achieve the transmission of a qubit.
This transfer process first prepares two entangled quantum (particles) A and B, if Zhang San and Li Si each hold one.
Then, Zhang San made a "Bell measurement" on the quantum state X to be transmitted and A in his hand to confirm that the two particles were entangled.
After the measurement, the quantum state of X collapses, but its state information is hidden in A, which makes A also change, but not collapse.
Because A and B are entangled with each other, the change of A immediately affects B, making B also change.
But at this time, Li Si could not observe B until he got the information from Zhang San from the classic channel.
Zhang San tells Li Si of the measurement result (that is, the change in A), and then Li Si performs corresponding transformation processing on B to make B become the same quantum state as the original X.
After this transmission process is completed, although X collapses, all information of X is transmitted to B. This process is called teleportation.
The quantum information transmitted in quantum teleportation is a quantum state. When the B particle obtains the original state of the A particle, the state of the A particle will inevitably change.
Only one particle can be in the target state at any time, so it is just a "movement" of the state, not a "copy".
Strictly proven unconditional safety
According to the unclonable nature of quantum, any eavesdropper who tries to intercept quantum communication will cause damage to the quantum state.
Both parties who send and receive information can determine whether the information has been intercepted by comparing partial keys.
Before copying (ie cloning) the state of any particle, the state must be measured first.
But the quantum state is very fragile. Any measurement will change the quantum state itself, that is, make the quantum state collapse, so the quantum state cannot be arbitrarily cloned.
This quantum non-cloning property has been rigorously proven mathematically.
When an eavesdropper tries to intercept classic information, it will copy this classic information.
In the process of quantum state transmission, because it is impossible to clone any quantum state, when an eavesdropper intercepts quantum communication, it will destroy the quantum state he has intercepted.
Because of this, when we directly transmit qubits, there is no need to establish quantum cryptography.
The generation process of the quantum key is also a distribution process. The two parties in the communication obtain the key at the same time, without the need for a third party messenger to transmit in the middle.
Quantum key distribution is to establish a completely secure key transmission channel.
Because photons have two polarization directions and they are perpendicular to each other, both the sender and receiver of the information can simply select the 90-degree measurement method or the 45-degree measurement method to measure the photon.
After all the measurement results are obtained, they establish contact through classic channels, such as telephone, QQ, etc., and share the measurement methods they have used with each other. The binary bits corresponding to the same measurement method are the passwords they finally generate.
It should be noted that only when the measurement method selected by the sender and receiver are the same, the transmitted bits can be reserved and used as a key.
Want to know whether there is an interceptor, the sender and receiver only need to take out a small part of the key to compare.
If they are found to be 25% different from each other, then it can be concluded that the information has been intercepted.
Similarly, if the information is not intercepted, the same rate of the two passwords is 100%.
If an eavesdropping is found, immediately close the communication or redistribute the key until no one is eavesdropping.
It is precisely because of the quantum unclonability theorem that the receiver can detect the error of the key and stop the key communication, thus ensuring the absolute security of the quantum encrypted information.
Ultra-light communication is still impossible
Even if the speed of quantum entanglement can exceed the speed of light, this change in quantum state is not changed by human will.
The specific information consists of a series of ordered symbols, and the transmission of this information cannot exceed the speed of light.
Chinese scientist Pan Jianwei led his team to test the lower limit of the quantum entanglement speed in Qinghai. Experiments show that the speed of quantum entanglement is at least tens of thousands of times the speed of light.
So, can the use of quantum entanglement be used to achieve superluminal communication?
the answer is negative.
If the two entangled particles of A and B are placed in the two places of AB, and the particles of A are changed, the particles of B will also change accordingly.
Due to the non-locality of quantum mechanics, this kind of change can only be random, and will not be made completely according to people's wishes.
Because valid information must be a series of most basic ordered symbols. If 01011000 represents the information of an apple, then we must manipulate a quantum entangled system to force it to send symbols like 0101100 in order.
It is a pity that the results obtained by manipulating quantum are only random.
This is the most puzzling point of quantum physics: when you know the complete state of the system and measure the rest of the system, you can obtain information about a certain part of the system through entanglement, but you cannot create a combination from a certain part of the entangled system. Send information to another part.
Although this idea is very clever, it is still impossible to achieve ultra-light communication.
The change of state in quantum mechanics is instantaneous. In science fiction movies, there are often shots that transfer people from one place to another instantaneously. It is even believed that quantum teleportation achieved by entanglement can overturn the theory of relativity and achieve superluminal transmission. .
In fact, this is also a misunderstanding.
The quantum teleportation scheme includes several steps, one of which is Zhang San above tells Li Si of the measurement result (that is, the change in A), so that the state of the B particle can be turned into the target state.
This information needs to use classic communication methods, such as phone calls, emails, etc., and the speed cannot exceed the speed of light, so the speed of quantum teleportation based on quantum entanglement cannot exceed the speed of light.
Our reporter Wu Changfeng