This month, three scientists won the Nobel Prize in Physics for proving one of the most counterintuitive but far-reaching reality in the world of quantum . Their research shows that two entangled quantum particles must be treated as a single system, even if the particles are far apart. In practice, this "nonlocality ()" phenomenon means that the quantum in front of you can be immediately affected by quantum thousands of kilometers away.
Entanglement and non-locality enable computer scientists to create unbreakable passwords. In a technology called "device-independent quantum key distribution ", a pair of particles are entangled and then distributed to two people. The common properties of particles can be used as a kind of code, which is almost impossible to crack even with quantum computer .
But why (entanglement) is only on two particles? In theory, there is no upper limit for the number of particles that share entangled state . For decades, theoretical physicist has been scheming 3-way (three-way) , 4-way, and even 100-way quantum connections (quantum connections), which will develop a fully distributed, quantum-protected Internet.
Recently, a laboratory in our country seems to have achieved non-domain entanglement between three particles, which may increase the intensity of quantum cryptography and the possibility of generalized quantum networks.
Physicists have entangled more than two particles before. But the distance between them is very short, at most only a few centimeters. In order for multi-party entanglement to be useful for cryptography , scientists need to prove non-locality.
's key to proof of non-locality is to test whether the properties of one particle match the properties of another (a sign of entanglement) and nothing else can cause this effect once they are far enough. For example, if a particle is physically close to its entangled object, it may affect the entangled object by emitting radiation. But if they are one kilometer apart, then they are likely to be connected only by entanglement. Experiments used a set of equations called Bell inequality to exclude all other explanations of particle association properties.
For three particles, the process of proving non-locality is similar, but there are more possibilities to be ruled out. This increases the complexity of the measurement and also increases the difficulty of mathematicality.
A team in Hefei made a key leap in the research results published in August this year. First, by emitting laser light to a special type of crystal, they entangled three photons and placed them in different regions, hundreds of meters apart. They then measured the random properties of each photon at the same time. The researchers analyzed the measurement results and found that the relationship between the three particles can be best explained by 3-way quantum non-locality . This is the most comprehensive 3-way non-localistic argument to date.
Researchers built their experiments on a new, more stringent 3-way non-localistic definition, and his three devices were unable to communicate with each other, this limitation was very useful in encrypted scenarios. Now, researchers have successfully entangled particles closer together, and they want to further expand the distance.
This technology can support the wider distribution of quantum key . If entangled particles are used as encryption keys, testing the non-localistic Bell inequality can ensure that the secret is completely secure. Even if the device used to send or receive information is maliciously manipulated by hackers, hackers cannot determine the quantum key. The key is only known to you and the owner of another entangled particle.
- Researchers entangled three particles and placed them in a device marked Alice, Bob and Charlie, hundreds of meters apart.
This is a completely new phenomenon that can extend device-independent cryptography from basic two-way communication to the entire secret shared network.
In addition to cryptography, multiple entanglement also opens up possibilities for other types of quantum networks. Researchers are studying quantum internet , which can connect to quantum computers like ordinary Internet connected to ordinary devices. The system will bring together the computing power of many quantum devices by connecting millions of particles spanning different distances and with varying degrees of entanglement. Dutch scientists have successfully entangled three particles in a network of spanning two independent laboratories.
This study on three-way entanglement was just an "interesting phenomenon" at the beginning. However, when there are things that quantum mechanics can do and other methods cannot do, this will open up all kinds of new technical possibilities.
At present, some laboratories have demonstrated the four-way non-locality between particles. Nobel Prize winners took half a century to eliminate some loopholes in their two-way experiments, and eventually achieved success in 2017. We have made great progress in technology since then.
