Quantum network technology

prof. Dr. Stephanie Wehner, Director of the European Quantum Internet Alliance, explains how quantum network technology and a quantum internet can revolutionize communication and connectivity

The Internet, an intricate network that connects devices around the world with classical communication, has profoundly shaped our world. We are now on the verge of a new type of internet. Imagine an evolution of the internet, one that intertwines the principles of quantum mechanics with our existing digital technology. This is the vision of a quantum internet – an innovation that promises to redefine our understanding of communication and connectivity.

Together with our traditional ‘classical’ internet, a quantum internet would connect quantum devices worldwide. Such a network would unlock possibilities that are fundamentally unattainable through classical communication alone. Take Quantum Key Distribution (QKD), for example, a striking application of quantum communication. QKD allows two distant nodes to create an encryption key that is guarded by the immutable principles of quantum mechanics.

This enables secret communications that are future-proof; that is, it is even secure against eavesdroppers equipped with a large-scale quantum computer, now or in the future. In a world where data security is paramount, this quantum advantage could be hugely beneficial and is now commercially available in metropolitan areas.

However, beyond secure communication, we already know of a range of transformative applications that highlight the potential of quantum network technology. The potential is huge, and the vision of a quantum internet is to build a universal quantum network that can be programmed to run any type of future quantum network application.

What is a Quantum Network?

At the heart of these applications are quantum bits, or ‘qubits’. Unlike classical bits that exist as ‘0’ or ‘1’, qubits can simultaneously inhabit a state of ‘0’ and ‘1’. Intriguingly, it is impossible to copy arbitrary qubits. Any attempt at duplication can be detected, making them an ideal tool for secure communication. Two qubits can also be entangled, with entanglement forming an inherently private connection that cannot be shared with anything else.

A quantum network allows the transfer of qubits or, more generally, the creation of entanglement between nodes in the network (Figure 1). Such nodes can be simple photonic devices that measure only one qubit at a time, or more advanced devices.

Unlike quantum computing, where real-world value can only be extracted once one has built a quantum computer that outperforms classical (super)computers, the path to user benefits is more gradual in the realm of quantum networks. Simple photonic devices can already unlock applications such as quantum-secure communication in metropolitan areas.

Today, quantum communication is commercially available in metropolitan areas (short distances up to 100 km in fiber) when limited to simple use cases enabled by QKD. No long-distance quantum networks are currently deployed that enable end-to-end quantum communication and thus end-to-end quantum secure communication.

Ongoing R&D efforts around the world are working to advance quantum networks in three directions: (1) distance – to connect users in different metropolitan areas and beyond by leveraging end-to-end quantum communications; (2) functionality – to enable applications beyond secure communications; and (3) accessibility – to create cheaper devices. Global reach could ultimately be achieved using a combination of quantum repeaters in ground-based fiber optic networks and quantum satellites.

Both are the subject of ongoing R&D efforts. Stages of functionality have been identified (Figure 2) for the development of a quantum internet, with each stage unlocking a larger class of potential user applications. ⁽¹⁾

What can you do with quantum network technology?

Using the stages of functionality as a guideline, let’s briefly discuss applications and potential use cases of quantum network technology.

Stage preparation and measurement

This phase includes QKD, which addresses the critical challenge of securing communications in transit and using keys to authenticate access.

It is interesting to note that many devices that can use QKD can in principle also be used to provide an advantage in other security-sensitive domains, including, for example, password identification or privacy-preserving analytics.

Entanglement generation stage

This phase unlocks versions of the aforementioned use cases in security with the added guarantee that the quantum devices are not trusted – a feature known as device independence in quantum cryptography.

In addition, this stage enables all use cases that exploit the fact that entanglement allows for stronger correlations ⁽⁴⁾ when measuring the qubit than classically allowed. It has been found that there are practical applications for this, so that bridge players can gain an advantage from a distance. ⁽³⁾

On a more speculative note, it might be interesting to investigate whether pre-shared entanglement generated using a quantum network can improve efficiency in other tasks that require coordination, such as high-frequency trading.

Quantum memory phase

This stage can be reached if the devices connected to the quantum network are quantum processors, that is, quantum computers that can store and manipulate single qubits. Examples of possible use cases in this phase are secure quantum computing in the cloud. ⁽²⁾

To emphasize the breadth of possible use cases, we also note that a Quantum Internet can combine remote sensors for higher resolution imaging. ⁽⁵⁾ This has potential applications in astronomy, obtaining sharper celestial images, geological exploration and identifying potential materials in the ground.

Few qubit fault-tolerant phase

This stage differs from the previous ones in that the quality of the qubits in the quantum processor is very high – in particular, their quality is protected by fault-tolerant quantum computing.

For example, the Quantum Internet could reduce the communication requirements for solving specific tasks. ⁽⁶⁾ This could have the potential to enable, for example, faster appointment scheduling across multiple calendars, comparisons of data stored on different network sites, or faster image processing in image recognition tasks. Other examples in this phase include enabling proofs of data deletion. ⁽⁷⁾

Quantum internet alliance

The Quantum Internet Alliance (QIA) is a partnership of currently 40 members, including leading actors from academia and industry in Europe, to build a prototype Quantum Internet. This prototype network will connect two metropolitan networks through a long-haul backbone.

QIA also provides a platform for Quantum Internet Innovation with connectivity capabilities.

References

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2. NPJ Quantum Information, 3(1), 2016
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4. Rev. Mod. Physically. 86, 419 (2014)
5. Physically. Rev. Lett. 109, 070503, 2012
6. Rev. Mod. Physically. 82, 66, 2010
7. IEEE ISIT 2019, https://doi.org/10.1109/ISIT.2019.8849661