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Delivering quantum information – a field-deployed quantum network

17 Maggio 2024
Quantum communication and networking technologies enable private information sharing and networked quantum computing – serving as the backbone of a future, secure quantum internet.
In this post, we’ll share the results of joint work by researchers from Harvard University and the AWS Center for Quantum Networking (AWS CQN) to create a multi-node quantum network that distributes, stores, and processes quantum information under the streets of the Boston area.
You’ll learn about the how AWS contributions to state-of-the-art quantum networking technology and what the next steps are for this field.
Quantum networks: the missing link between quantum computers
Quantum computation technology promises to revolutionize fields from materials science to finance by using the unique properties of quantum superposition and entanglement. Quantum computers are rapidly improving in performance and capability but are currently limited to operating as large, isolated processors. This contrasts sharply with classical computers, which are constantly exchanging information across distances measured from centimeters to kilometers, enabling computers to perform tasks together which no single device could accomplish alone.
Quantum computers can benefit from exchanging information between different processors, but must do so in a way which preserves the quantum properties of the information they process.

Accomplishing this requires a new tool: a quantum repeater which corrects for losses and errors that occur as information travels. We’ve covered this technology in a previous blog post, but in short: quantum repeaters are developed using quantum memories – devices that catch individual photons, encoded with quantum information and store this information on a local memory. This memory then does some simple computations to preserve the information stored on it and deliver it on demand, thus acting as a repeater.

Finding the right memory to achieve this task is challenging but a number of teams around the world are rapidly making progress. Now a team of Harvard and AWS scientists have created a 35 km long link that can communicate and store quantum information for more than a second using a unique quantum memory: an atomic defect in diamond called the Silicon Vacancy Center (SiV).

Figure 1 – A map of the quantum link deployed in this demonstration. Photons carrying quantum information entangled with a quantum memory traveled more than 35 km through several neighborhoods in Cambridge and Boston before returning to Harvard where their entanglement was transferred onto a different memory in a separate lab.
This paper demonstrates entanglement of two quantum network nodes over a deployed fiber link under Cambridge, MA. Entanglement of memories using photons distributed using a deployed network is important because it implements the key step that will be executed by a real-world quantum network.
These advances were enabled by (a) the incorporation of quantum frequency conversion to transfer optical signals from the visible to telecommunications domain and; (b) the use of long-lived nuclear memories to store information for extended periods of time. These nuclear memories consist of the atoms composing the material close to the SiV memory qubit (which can be controlled) and interfaces with the SiV qubit using microwave fields.
Using these memories makes it possible to store more complex information consisting of multiple qubits, while also preserving this information for longer periods. This is a unique property of defect qubits like the SiV, which naturally have a large store of nearby nuclear qubits to utilize.
Out of the lab: a field deployed quantum network
A quantum network consists of a set of nodes capable of processing quantum information and a series of links used to exchange information between them. Like in the classical internet, the most common choice for carrying information in a quantum network is light, which can be transported over long distances using existing classical telecom fiber infrastructure, greatly reducing barriers to operationalizing networks. The novel and challenging part of building a quantum network lies in developing a quantum memory or “repeater” node that is capable of efficiently catching, storing, and transferring information initially stored on light. Without these nodes, the unavoidable degradation of an optical signal due to the scattering of light from optical fibers grinds long distance quantum communication to a halt.
Harvard and AWS scientists used a unique system as their quantum network node: optical cavities in diamond which trap light and force it to interact with quantum memories using SiVs. This system, developed at Harvard for over a decade, has a number unique properties that make it a promising quantum memory. Most notably these memories can be created inside integrated photonic devices which can be mass produced using existing nanofabrication technology.

The memory storage and retrieval operations are also heralded – meaning that success or failure can be detected and corrected for at each stage in the operation.

Finally, the system inherently has access to nearby nuclear spin memories which can be used to perform error detection and correction. The primary limitation of this memory is that it requires ultra-low temperatures to function (though recent progress raised the operating temperature by a factor of twenty) and that sufficiently pure diamond is challenging to produce.

Separately, several device-engineering improvements will be needed to speed up the rates and accuracy for the communications.
Despite these challenges, diamond-based quantum memories are unique among solid-state memories because they’ve matured to the point where they have moved beyond demonstrations involving only a single device. Instead, SiVs are ready for deployment and testing in real world networks, allowing us to better understand how these networks will operate and be used.
Figure 2 – A schematic overview of the devices used in the network. SiV located inside one photonic device (lower left) were entangled with photons which traveled through telecom fiber (top) before interacting with a quantum memory in a different location (right). The end result was entanglement generated between two spatially separated quantum memories.
Operation of a SiV-based quantum network begins by encoding quantum information into a qubit (in the form of a single photon), and bouncing that photon off a quantum memory sitting inside a laboratory at Harvard.

When the photon interacts with the quantum memory, it becomes entangled with the memory – meaning that measurements performed on either the photon or the memory would provide information on (and thus modify) the state of the other.

However, instead of measuring the photon (and thus extracting the information), the photon undergoes quantum frequency conversion from visible frequency (where the quantum memory operates) to telecom frequency (where losses in optical fiber are minimized). The (now telecom-frequency) photon then makes a round trip through an underground fiber network before returning to Harvard, where it is converted back to visible frequency.

This process, known as heralding, lets the network know when it has succeeded (or failed) in generating entanglement between nodes, which can then be used for subsequent applications. The result is a pair of entangled quantum memories which, though located in the same building, were entangled by a photon which traveled more than 35 kilometers.

Entanglement can then be stored for more than a second. This storage time is significant because it is sufficient for light to be able to travel more than 300,000 km- or in other words, to go around the world 7.5 times. This means memory storage time is not a limiting factor in developing global-scale quantum networks using these memories.
A world wide web: scaling quantum networks

While this demonstration shows the promise of the underlying technology, this network still needs to make device engineering improvements before it can perform at a commercially relevant bandwidth and accuracy (known as fidelity). Most important among these is moving to multiplexed use of photonic devices, which use many quantum memories operating in parallel to speed up the rate of communication.

Currently each node in the deployed network communicates at roughly 1 Hz rates and uses only a single quantum memory, even though thousands of memories are present on a single diamond chip. This limitation stems from the fact that optically interfacing to multiple memories at cryogenic temperatures is hard – a problem which can be addressed by a technology recently developed by AWS and Harvard.
By using most of the cavity devices on a chip (which can each contain many memories) at once, the rate of communication could be increased by a factor of 1000 or more, dramatically increasing the utility of the network.
The fidelity of the communication also needs to be increased from a current record of roughly 95% to more than 99% if we’re to enable the most interesting quantum network applications like networked quantum computing, blind quantum computing, and networked sensing.
This fidelity is currently limited mainly by the properties of optical devices used to interface the SiV quantum memories with light. These devices are fabricated using a custom nanofabrication technology which limits their reliability. The AWS CQN is developing fabrication techniques that overcome these performance limits – enabling ultra-high-fidelity entangling operations. Integrating these new fabrication approaches with new technologies for improving operating temperature, controlling communication frequency, and reducing optical loss should be sufficient to prepare these quantum network nodes for industry use.
Preparing for quantum secured communication today
Developing, integrating, and scaling all the required technologies for a global quantum network will take time – but quantum communication technology is rapidly scaling towards commercial deployment.
If you’d like to learn more you can read other blog posts, listen to the talk from Re:Invent 2023, or reach out to the Quantum Solutions Lab (QSL) to see how AWS is pushing this technology forward.
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