The first quantum network – with three nodes – is real. This small-scale network, built at Delft Lab, is an important step towards a larger, secure “quantum internet” for creating secure communications and connecting powerful future quantum computers.
Quantitative information can be transmitted over a quantum network. With regular Internet connections and computers, information consists of bits that are 1 or 0. Quantum information consists of qubits, which can be a mixture of 1 and 0. As a result, it contains more information than regular bits. Moreover, you can intertwine qubits, so that they form a special bond with each other. When you measure two interlocking qubits, they always give the same answer instantly, even if you send one of them to the other side of the universe.
Thanks to entanglement, the quantum internet has a number of special and useful properties. For example, it ensures that no one can eavesdrop on your quantum communication unnoticed. By doing this, you are destroying the entanglement. This is useful for sending sensitive information.
Simple quantum communication is already possible with direct connections between two systems. But if you want to build a large-scale quantum internet, you need nodes that connect other nodes. The Delft researchers showed that this could be done by interlacing two qubits and an intermediate quantum node. Their results Thursday noon at Science.
“Each of the three nodes of our network is made up of a diamond chip,” says Sophie Hermans, PhD candidate at the QuTech Research Group in Delft, who conducted the experiment. This is a diamond that has a “bug” in it: a place where a carbon atom has been replaced by a nitrogen atom, and next to it is a hole where a carbon atom is missing. An extra electron trapped in this bug. “These are our working horses, the qubits,” Hermans explains. “To make sure that the sensitive qubits break down as little as possible, we cool them down to -269 degrees Celsius.”
Alice, Bob and Charlie Chips
Delft researchers actually attached two chips, called Bob and Charlie, in the lab (but not yet attached) to a two-meter-long fiber-optic cable. The third chip – Alice – is in a different room and hooked up to Bob with 30 meters of fiber. Bob is the intermediate node and therefore has a second qubit, which acts as a memory. To connect the lattice, Alice and Bob first interlock with each other with the help of photons (light particles) emitted by the electrons that travel through the fiberglass. This synapse is stored in the Popeye’s memory qubit. Now qbit Alice and Bob’s memory are intertwined. The other Qubit Pop then becomes entangled with Charlie, and the network is complete.
“We used the grid after that in two ways,” Hermans says. “First, we linked Pop’s memory qubits to the other qubits, causing the three segments to become entangled.” Such an interlocking network can be used, for example, to accurately synchronize atomic clocks.
In addition, the researchers modified the network by removing Bob and thus “teleportation” of the entanglement with Alice and Charlie. These are not physically related to the fiberglass, but only through their crosslinking. Bob wouldn’t be able to hear the connection between Alice and Charlie unnoticed. This is useful for larger quantum networks. So Bob is a kind of quantum communications provider that connects to others, but can’t listen by itself.
I think it’s amazingly good
Gerhard Rimby Max Planck Institute
“What is important about our experience is that we receive a signal immediately when we have successfully interlaced nodes. This ensures that the network is scalable to include more nodes and more complex processes,” says Hermann. In other experiments with three nodes, for example, only then could you see that entanglement was working.
Uninvolved quantum physicists are excited about the experiment. “I think it’s amazingly good,” says Professor Gerhard Rimby of the German Max Planck Institute for Quantum Optics. “Building a quantum network is one of the biggest challenges in quantum physics research. Connecting multiple nodes is technically difficult because there is a risk that environmental factors might disturb sensitive qubits.”
Hermann does not believe that a quantum internet will replace the current internet. Transmission of quantum information is more difficult and time consuming than normal information. If this is not necessary, the existing network will be simpler. The quantum network could become an additional connection for secure encryption and for communication with future, large, and powerful quantum computers.
A copy of this article also appeared in NRC on the morning of April 16, 2021