“This side of the table is Bob, the other side is Charlie and two doors down is Alice,” says Sophie Hermans, who got her PhD from TU Delft in April. We have just passed through a black curtain into a small laboratory in the basement of the QuTech quantum institute in Delft. The table is littered with mirrors, wires, and Bob and Charlie diamond tiles. There are black partitions around and between the settings that block light during experiments.
“Last year we installed the world’s first quantum network here, connecting Alice and Bob and Bob and Charlie,” says Hermans. Alice, Bob and Charlie are the nodes of the network. Each consists of a diamond chip containing a “bug”: a place where a carbon atom has been replaced by a nitrogen atom, with a hole next to it where a carbon atom is missing. An extra electron gets trapped in this ‘NV center’ which serves as a quantum bit, qubit for short.
Hermans and his colleagues recently taught the network a new trick: teleportation. Quantum information disappears at the sender (Charlie) and appears at the receiver (Alice). Because this doesn’t require a signal to travel from Charlie to Alice, the signal can’t be interrupted or lost along the way, which is what happens when you send quantum information down an optical fiber. This is why teleportation is interesting for a future quantum internet. Hermans and his colleagues published the results of the teleportation last Wednesday in Nature†
The small-scale network between Bob, Charlie, and Alice is an important step toward a larger quantum Internet, one that can ensure secure connections and link future quantum computers.
If someone tries to eavesdrop on our connection, break the mess.
What is a quantum network?
“It is a new type of network through which you send quantum information. With Internet connections and ordinary computers, information consists of bits that are either 1 or 0. Quantum information consists of qubits, which can be a combination of 1 and 0. You can also entangle qubits. If two qubits are entangled, you will no longer be able to see them separately. When you measure them, they always give the same answer right away, regardless of the distance between the two. We can entangle our qubits by shining a laser at an NV hub. The qubit then emits light particles that we can send to another diamond chip through an optical fiber to create an entanglement between the two.”
What can the quantum internet do that the ordinary internet can’t?
“You can use it for guaranteed secure communication. If we have a quantum connection with entangled qubits and make measurements, we find correlated measurement results. If someone tries to eavesdrop on our connection, break the mess. As a result, those correlations disappear. This way we can immediately see if someone is eavesdropping on us. Another application of entanglement is the synchronization of atomic clocks, which is used, for example, for GPS systems”.
Connecting two qubits has been possible for years. Why was a third added only last year?
“Adding a third node is less easy than with a normal network because you can’t copy quantum information and store it poorly, because qubits are fragile. They are easily disturbed. In our network we connect the nodes Alice, Bob and Charlie in one line. As a result, Alice and Charlie do not have a direct fiber optic connection to each other. If you want to entangle them, you’ll need Bob’s help. For this, we first intertwine Alice and Bob. Bob stores that entanglement in his extra memory qubit. So we messed up Bob and Charlie. This mess usually doesn’t work all at once. Each time we try, we slightly disrupt the stored entanglement between Alice’s and Bob’s memory qubit. If it takes a long time to entangle Bob and Charlie, then the entanglement with Alice is broken and we have to start over. That makes setting up a quantum network even more difficult and time-consuming.”
For now, normal internet is faster
However, in 2014 QuTech announced its plan to have an experimental quantum internet between Delft, The Hague, Leiden and Amsterdam by 2020. Why didn’t it work?
“When I started my PhD research in early 2018, the prevailing idea was that we could build a working quantum network in the lab with three nodes in five months and then start taking measurements. In the end it took 2.5 years. Several things turned out to be more difficult than expected and the corona pandemic did not help. If all goes well, there will be a quantum connection between Delft and The Hague this year. The plan is to add nodes later.”
What steps were taken during your promotion?
“In 2020 we finally had the network: a connection between Alice and Charlie through Bob. Alice and Charlie are only connected through quantum entanglement and not through a physical fiber optic. But that connection wasn’t good enough to be used for anything. Last year we investigated where the weak points are in the configuration and we resolved them as much as possible. For example, we improved Bob’s memory qubit by more effectively shielding it from disturbances with electrical pulses. Those enhancements allowed us to entangle Alice and Charlie and teleport quantum information from Charlie to Alice. This quantum network is slow. The teleport only succeeds once every two minutes. This has to be improved for a quantum internet. Personally, I think a different type of qubit is needed to take the next step. I think that in recent years we have squeezed everything that can be extracted from the qubits of the NV center.”
How is the future of the quantum Internet?
“I think it will be an addition to the normal Internet. You can store less information with them than with normal computer bits. And at the moment, the normal internet is faster. For secure communication, you can exchange encryption codes over the quantum internet, for example. It uses this to encrypt your message that you send over the traditional Internet. You could also use it to determine a route with Google Maps, without Google knowing where you are and where you are going. It then sends your location and destination over the quantum internet to Google’s quantum server, which calculates and returns the fastest route. If it’s all done via quantum information, then Google won’t be able to watch at all. But I think it will be decades before everyone can connect to the quantum internet at home.”
A version of this article also appeared in the newspaper on May 30, 2022