Expanding Horizons of Long-Haul Quantum Communications

Since quantum teleportation first became a reality 25 years ago, scientists have pushed its limits. Teleportation—outside the Star Trek universe—consists of transmitting a quantum state from one system to another via quantum entanglement. The two quantum systems (two atoms in a laser trap) can be right next to one another, or they can be separated by arbitrarily large distances. Strange as it sounds, quantum teleportation has been widely observed in the lab and even harnessed in emerging quantum technologies. To date, scientists have achieved quantum teleportation across distances as great as 1,400 kilometers. Yet there are other kinds of limitations than physical distance. Until now, quantum teleportation could only happen between, say, atoms that had been in direct contact with each other.

But in a new study, researchers have for the first time accomplished quantum teleportation between two remote nodes that share no direct connection whatsoever. This advance represents an important step toward a quantum Internet that would connect quantum computers together—enabling them to share data and computations a little as today’s cloud-computing frameworks do for classical computers.

“We teleport quantum information, not objects. Teleportation of objects does not work.”
—Ronald Hanson, Delft University of Technology

Quantum physics makes possible a strange phenomenon known as entanglement. Essentially, two or more particles such as photons that get linked or “entangled” can, in theory, influence each other instantly no matter how far apart they are. Entanglement is essential to the workings of quantum computers, which can theoretically solve problems no conventional computer ever could, as well as nigh-unhackable quantum encryption.

To develop a quantum Internet that can link quantum computers together and transmit quantum-encrypted messages, scientists need a way to share quantum information between the nodes of the network. However, such quantum data is fragile. If you wanted to send quantum information encoded in photons, quantum data loss would inevitably happen over great enough lengths of glass fibers.

A potentially better way to send quantum information across large distances is via quantum teleportation. Just like in science fiction, quantum data that undergoes such teleportation essentially disappears one place and reappears someplace else. Since this quantum information does not travel across the intervening space, there is no chance it will get lost.

“The key feature of teleportation is that it enables the reliable transfer of quantum information between nodes, something that in a future Internet will need to happen all the time,” says study senior author Ronald Hanson, a quantum physicist at the Delft University of Technology in the Netherlands.

To accomplish quantum teleportation, one would first entangle, say, two electrons. Then, one of the two electrons—the one to be teleported—would stay in one location while the other electron would be moved to whatever destination is desired.

This is an artist’s impression of the quantum teleportation protocol in a network setting. Quantum information is being teleported between two non-neighboring nodes in the network. Scixel/QuTech

Next, the key details or “quantum state” of the electron to be teleported are analyzed, an act that also destroys its quantum state. Finally, that data is sent to the destination, where it can be used on the other electron to recreate the first one, so that it is indistinguishable from the original. For all intents and purposes, that first electron has teleported. (Since the data is sent using regular signals such as light pulses, quantum teleportation can proceed no faster than the speed of light.)

“We teleport quantum information, not objects,” Hanson notes. “Teleportation of objects does not work.”

Until now, all quantum teleportation required a direct channel between two nodes. This greatly limited the potential complexity of quantum networks.

In the new study, researchers experimented with three nodes, dubbed Alice, Bob and Charlie. They successfully performed quantum teleportation between Charlie to Alice, with the help of intermediate node Bob. This is the first time quantum teleportation was accomplished between two nodes that were not adjacent to one another.

Each node consisted of a microscopic artificial diamond with a defect within it, in which a carbon atom is replaced with a nitrogen atom and the adjacent carbon atom is missing. The spin of single electrons trapped in these “nitrogen-vacancy centers” can encode quantum data as a quantum bit, or qubit.

Optical fibers connected the nodes. Direct links existed between Alice and Bob and between Bob and Charlie, but not between Alice and Charlie. The researchers used these links to set up entanglement between Alice and Bob and between Bob and Charlie.

In addition, in each of Bob and Charlie’s diamonds, a carbon-13 atom served as a “memory qubit.” These memory qubits let these nodes store and then swap their entanglement data, resulting in Alice becoming entangled with Charlie. This enabled the scientists to finally teleport data from Charlie to Alice.

“Teleportation will be the main method for transferring quantum information across a future quantum Internet,” Hanson says.

Performing such quantum teleportation required extraordinarily high-performance quantum components and connections. “I think it is fair to say that this is the most complex quantum-network experiment realized so far, and it needed all elements to operate at or beyond the state of the art,” Hanson says.

For example, “we had to invent a new method for reading out the memory qubits,” Hanson says. “This made the error on that readout drop from 6 percent to below 1 percent.” In addition, “we increased the robustness of the memory qubits by a factor of six by integrating active protection from noise sources into the control sequences.”

In the future, the researchers will seek to extend the distances between nodes, develop more efficient hardware for faster entangling rates, and develop and test a quantum Internet control stack, Hanson says.

The scientists detailed their findings 25 May in the journal Nature.