The spooky world of quantum mechanics might someday make for a faster and more secure internet. Now a study makes strides towards that future with a new distance record for quantum entanglement between particles of light and matter. Researchers have managed to send a photon, entangled with an ion, down a 50-km-long (31-mi) optical fiber.
Quantum entanglement describes a strange bond that can form between two particles, even when separated by huge distances. Information about their quantum states can become so inextricably linked that looking at one particle can tell you about the other, and even influence changes in it instantly, no matter how far away it is.
Since that essentially amounts to teleporting information, this eerie phenomenon could form the basis of a new type of communication. A quantum internet could allow data to be transferred instantaneously around the world, and far more securely than it is now – after all, if a hacker tries to intercept a message, the act of observing it garbles the data.
The new study is a step towards that kind of quantum internet. Researchers from the University of Innsbruck and the Austrian Academy of Sciences have entangled a calcium ion and a photon, before sending the latter down a long optical fiber.
The team started with a calcium ion, suspended in an ion trap. Using a laser, they wrote a quantum state onto the ion, which also excites it to emit a photon. When this photon is created, it has the same quantum state as the ion, meaning the two particles are entangled.
The next step is to send the photon on its merry way, and make sure that this quantum link is maintained. The problem is, because the photon has a wavelength of 854 nanometers (nm), it will be quickly absorbed into the fiber. It needs to be converted to 1,550 nm, which is the current standard wavelength for telecommunications.
To do so, the researchers first have to send the photon through a nonlinear crystal, which is lit up by a powerful laser. Then it’s ready for its long journey.
In tests, the team showed that not only did the photon manage to survive a 50-km trip through a fiber optic cable, but it kept its entanglement with the ion, even after the wavelength conversion.
That makes this a new distance record for entangling light and matter. Entangled pairs of photons have been seen to travel further apart – up to 1,200 km (745 mi) through the Micius satellite – but this new study is the furthest using the kind of infrastructure that might make up a quantum internet.
The team could effectively double that distance too. Calcium ions placed 100 km (62 mi) apart could beam their entangled photons towards each other, meeting at an intersection in the middle. There, the two photons could become entangled with each other and lose connection with their ions. A quantum internet therefore could be made up of nodes placed 100 km apart, connecting cities to one another.
The research was published in the journal Quantum Information.
Source: University of Innsbruck
