Physicists in China have forged a mysterious quantum connection between particles, called entanglement, over dozens of kilometers of standard optical fiber, setting a new record. The advance marks a long step toward a fully quantum mechanical internetalthough such a network is still years away. The achievement springs not from one particular breakthrough, but from the careful implementation of multiple techniques, says David Awschalom, a physicist at the University of Chicago. Im very impressed that theyve integrated these various technologies into a full system, he says. Its a beautiful piece of work. Entanglement links the strange states of tiny quantum mechanical objects. For example, a top can spin either clockwise or counterclockwise, but an atom can spin both ways at onceat least until it is measured and that two-way state collapses one way or the other. Two atoms can be entangled so that each is in an uncertain two-way state, but their spins are definitely correlated, say, in opposite directions. So if physicists measure the first atom and find it spinning clockwise, they know instantly the other one must be spinning counterclockwise, no matter how far away it is. Entanglement would be key to a fully quantum internet that would let quantum computers of the future communicate with one another and be immune to hacking. If hackers messed with communication, they would spoil the entanglement, revealing their presence. Various companies already sell systems that send messages in quantum states of light that are largely unhackable. But to use such links, the information must still be decoded at each network node, which is potentially vulnerable. In a quantum internet, any node could be entangled with any other, so messages between them couldnt be decoded at intermediate nodes. But developers must first stretch entanglement over greater distances. Previously, researchers had demonstrated entanglement of two bits of matter over 1.3 kilometers of optical fiber. Now, Xiao-Hui Bao, Jian-Wei Pan, and colleagues at the University of Science and Technology of China, Beijing, have demonstrated entanglement over fiber optic links of up to 50 kilometers, as they report this week in Nature. The details are dizzying, but the basic idea of the experiment is relatively simple. Researchers start with two identical stations in a single lab, each containing a cloud of rubidium atoms. Prodding each cloud with a laser, they generate a photon whose polarization, which can corkscrew clockwise or counterclockwise, is entangled with the clouds internal state. They then send the photons down two parallel optical fibers to a third station in another lab 11 kilometers away, where the photons interact in a way that instantly passes the original entanglement connection to the two faraway atom clouds. To do that, physicists take advantage of the fact that, according to quantum mechanics, a measurement can affect the state of the measured object. At the destination lab, the physicists set up a measurement of the photons polarizations that, even as it consumes the photons, it also projects them into a specific entangled state with 25% probability. For those trials, the measurement instantly passes the entanglement back to the atom clouds. The researchers performed a variant of the experiment that extended the link from 22 kilometers to 50 kilometers, albeit with fibers wound on spools. To make the experiment work, the team had to get several elements just right, Pan says. A major hurdle was avoiding absorption of the photons in the optical fiber. To do that, Pan and colleagues used another laser pulse and a device called a waveguide to stretch the photons wavelength 60% to the sweet spot for transmission down a standard optical fiber. At the same, the researchers made life easier for themselves because the atoms clouds were actually less than 1 meter apart and merely connected by a long optical fiber. That closeness made synchronizing the experiment significantly simpler. So, strictly speaking, the record of entangling atomic-scale particles separated by 1.3 kilometers still stands, says, Ronald Hanson, a physicist at Delft University of Technology, who led that earlier effort. Still, Hanson says, the experiment is significant because, for a network, the setup link is about half of the basic element called a quantum repeater. A repeater would consist of two systems like the one in the experiment placed end to end. Once physicists had entangled the atom clouds at the ends of each system, they could perform additional measurements on clouds in the middle that would swap the entanglement to the clouds on the ends, stretching the entanglement twice as far. This experiment is a big step toward a quantum repeater, Hanson says. But several aspects of the work need to be improved before it can be used to make a quantum repeater, Hanson says. In particular, the atom clouds do not yet hold their delicate quantum states long enough to allow the multiple linking needed in a quantum repeater. Pan agrees, but says his group is working on that and urges patience. I think a true quantum network is at least 10 years away. *Correction, 13 February, 4:30 p.m.: The story has been updated to clarify that the experiment sets a new standard for the longest optical fiber used to entangle two quantum memories and not for the actual distance between the memories.
~ssciencemag.org, 13 February 2020