Last month, dad scientists won the Nobel Prize in Physics for their work demonstrating one of the most counterintuitive yet consequential realities of the quantum world. They showed that two entangled quantum particles must be treated as a single system—their states are inexorably intertwined—even when the particles are very far apart. In practice, this “non-local” phenomenon means that the system you have in front of you can be instantly affected by something thousands of miles away.
Entanglement and non-localization allow computer scientists to create codes that cannot be traced. In a technique known as device-independent quantum key distribution, a pair of particles is entangled and then distributed to two people. The shared properties of particles can now act as a code, a code that will keep communications secure even with quantum computers — machines capable of disrupting engineering Classic coding.
But why stop at two counties? In theory, there is no upper limit on the number of particles that can share the entangled state. For decades, theoretical physicists have imagined three-way, four-way, even 100-dimensional quantum connections — the kind that would enable a fully distributed quantum-secured internet. Now, a lab in China has achieved what appears to be nonlocal entanglement between three particles at once, potentially advancing the power of quantum cryptography and the possibilities for quantum networks. Generally speaking.
“The settlement of the two sides is crazy enough as it is,” said Peter Bierhorst, a quantum information theorist at the University of New Orleans. “But it turns out that quantum mechanics can do things even further than that when you have triplets.”
Physicists have entangled more than two particles before. The record is somewhere in between 14 seeds and 15 trillion won, depending on who you ask. But they are only at short distances, only a few inches apart at most. To make multiparty entanglement useful for cryptography, scientists need to go beyond simple entanglement and demonstrate nonlocality—“a high threshold to reach,” says Elie Wolfea quantum theorist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada.
The key to proving nonlocality is to test whether the properties of one particle match the properties of the other—a hallmark of entanglement—when they are far enough apart that nothing else can. cause effects. For example, a particle that is physically still close to its entangled twin can emit radiation that affects the other. But if they’re a mile apart and are practically instantaneously measured, it’s likely that they’re just linked together by entanglement. Experimenters use a set of equations called Bell inequality to the exclusion of all other explanations for the bonding properties of the particles.
For three particles, the process of proving nonlocality is similar, but more likely to be ruled out. This explains the complexity of both the measurement and the mathematical rounds that scientists have to go through to prove the non-local relationships of the three particles. “You have to come up with a creative way to approach it,” says Bierhorst — and have the technology to create the right conditions in the lab.
In results published in August, a research team in Hefei, China, took an important step forward. First, by firing a laser through a special type of crystal, they cumbersome three photons and place them in different areas of the research facility, hundreds of meters apart. They then simultaneously measured a random property of each photon. The researchers analyzed the measurements and found that the relationship between the three particles is best explained by three-dimensional quantum nonlocality. It is the most comprehensive demonstration of three-dimensional nonlocality to date.
