Nearly a year ago, Amazon Web Services (AWS) announced it was partnering with Harvard University to test and develop a quantum network. At the end of June this year, AWS opened its labs and released media outlets, including Popular sciencelook at his early models of a quantum repeater, which is similar to a classical amplifier that carries optical signals over long stretches of fiber.
“We are developing the technology for quantum networks. They are not fully baked yet,” said Antia Lamas-Linares, head of the AWS Center for Quantum Networking. “There are many of these technologies that have been partially demonstrated in academic labs that still need quite a bit of development to get to what we call a full-fledged quantum network.”
So what’s the point of this kind of technology? A quantum network can be used to distribute cryptographic keys without going through an intermediary, or to create anonymous broadcasts for multiple users.
The challenges of creating a quantum network
In a quantum network, instead of interacting with classical bits that are one or zero, off or on, there are quantum bits or qubits that can be in a superposition of one and zero at the same time. Computer scientists can entangle these qubits and take advantage of their quantum properties to perform interesting calculations that would be difficult, labor intensive or impossible to do classically.
But as with classical systems, to have a network, the team must be able to generate, move and store the qubits. And a great way to move them is with photons, or bits of light, explains Nicholas Mondrik, quantum researcher at AWS. They travel well, and “you can, with a little ingenuity, encode your qubit into a photon,” he says.
[Related: Chicago now has a 124-mile quantum network. This is what it’s for.]
Light is already used in classic fiber optic systems to transport information over long distances. The problem with this method is that after about 100 miles it starts to get choppy. That’s where optical amplifiers come into play. They can detect when the light is getting weaker and amplify it before sending it down the line. However, the optical amplifier and other devices used to transmit the light signal force that light to choose between one or zero, Mondrik says, and that would destroy the quantum information on the photon.
![Technical photo](https://www.popsci.com/uploads/2023/06/30/IMG_5218-copy-scaled.jpg)
One of the most important innovations AWS has developed is a quantum equivalent of a signal amplifier, called a quantum repeater.
Diamonds are a quantum researcher’s best friend
To make a quantum repeater, they first had to figure out how to make a quantum memory, something that can store a qubit. That way it can capture and process the incoming photon before sending it on its way. The solution: “quantum-grade” synthetic diamonds.
![Technical photo](https://www.popsci.com/uploads/2023/06/30/IMG_5225-copy-scaled.jpg)
“Within the structure of diamonds you sometimes get defects. Sometimes you get diamonds that are not transparent, that have colors and shades. Those things are called color centers and are impurities in the diamond,” says Lamas-Linares. “It turns out that those impurities behave like an artificial atom, and you can use them to store the state of a photon. These interesting colors allow us to communicate with light, store the state of the flying qubit and manipulate that state.”
The way to make the diamond suitable for storing photons is to first “silicon vacancies.” To do that, researchers take a diamond containing as pure carbon as possible and bombard the carbon lattice with silicon atoms. These silicon atoms will knock out a few carbon atoms, take their place and behave like a solid atom in the diamond lattice that can interact with photonic qubits via electrons.
To direct the photon to the electron on the silicon atom, researchers built nanocavities around the silicon vacancy that essentially act like an array of mirrors that direct the light where it needs to go.
![Technical photo](https://www.popsci.com/uploads/2023/06/30/IMG_5215-copy-scaled.jpg)
For this process to work, the team must prevent the diamond structure from vibrating; they do this by cooling it to near absolute zero. The device they use for this is the same chandelier-like dilution refrigerator-vacuum-thermal shield combination used for superconducting quantum computers. But this infrastructure is considerably smaller (about half the size) and the tail attachment is completely different.
“This is where the silicon vacancy, where this diamond memory lives,” says Mondrik. “For silicon vacancies to act like a quantum memory, like a qubit, you have to put them in a strong tunable magnetic field.” Therefore, there are additional structures at the bottom of the chandelier that allow a superconducting magnet to be attached before applying the thermal shield outside.
There’s also a piezoelectric stack that helps researchers steer things, a microwave line that helps them manipulate the qubit, an optical fiber that transmits light into the diamond cavity, and a microscope imaging system that extends from the bottom of the chandelier to the top to show researchers what they are doing.
![the bottom of AWS's chandelier for quantum networks](https://www.popsci.com/uploads/2023/06/30/IMG_5217-copy-scaled.jpg)
But not all science is done in the dilution refrigerator. There are also room temperature workspaces in the lab where qubits are created, measured and characterized.
![Technical photo](https://www.popsci.com/uploads/2023/06/30/IMG_5221-copy-scaled.jpg)
In its current form, the device where all the different components come together appears to be an intricate assembly of wires, metals and lenses. But ultimately, the team wants to compress this technology into a single, customizable piece of hardware that they can drag and drop onto any type of quantum computer.