Progress in quantum networks

A hallmark of the second quantum revolution is the notion of secure communication,^1,2 giving rise to a fundamentally secure global quantum network³ and quantum internet.^4,5 Progress has been rapid⁶ since the initial table-top quantum key distribution (QKD) demonstration to today's deployment across metropolitan fiber^7–10 and free-space connections,¹¹ reaching globally by ground to satellite links.^12–14 State-of-the-art demonstrations include over 500 km links across optical fibers¹⁵ and up to 4600 km by an integrated space-to-ground network.¹⁶ There are presently many initiatives to further advance the global quantum communications network with space-based solutions as well as efforts to make ground stations portable.¹⁷ These solutions are primarily based on traditional encoding schemes such as qubit polarization states, whereas the promise of high-dimensional encoding such as with spatial modes of light¹⁸ (see Fig. 1) are far less developed, for instance, reaching <100 km ranges in fiber^19,20 and < 1 km in free space,²¹ but with non-linear solutions to teleportation offering a promising future.²² 

As a consequence of these advances, the mooted global quantum network is ever closer to reality, having matured from theoretical concepts that struggled for acceptance to secure inter-continental links via satellite today. The race to emerge first with the core technology is seeing investments into quantum networks and associated science and technology reaching unprecedented levels.²³ In the “Quantum Networks: Past, Present and Future” Special Topic in AVS Quantum Science, we gather original research, industry perspectives, and reviews that give a past, present, and future perspective on the topic.

Advances in existing technology for quantum networks are addressed from the perspectives of quantum architecture²⁴ and information handling²⁵ and protocols.²⁶ The vision is to consider a quantum network that seamlessly integrates with classical hardware and software, with a versatile architecture that allows for multiple experimental platforms, removing “quantum” to a background task.²⁴ Using QKD as an example, the authors show that a network that manages tasks in a time efficient manner is feasible and argue for the need to include quantum networks standards (presently being formulated) into future classical architectures. The work also highlights the need for quantum technologists to consider the needs and formulation of networks in industry, arguing that presently the focus is on hardware development and less on hardware integration. Beyond QKD, a quantum network will have to connect many quantum devices that may differ substantially in form (sensors, computers, etc.), all sharing information through lossy channels, placing stringent requirements on quantum repeaters and their very nature.²⁵ The authors review the available quantum repeater technology and its ability to connect information nodes in a quantum network. They argue for the need to have many nodes with error correction built into the quantum repeater and/or many nodes that simply relay entanglement with a decay in fidelity, balancing direct transmission of information with entanglement distribution schemes. The point made is the need to think about the network as a whole and not just all the parts. Given how far the present generation of quantum repeater technology is, limited in distance as it is, there is a need to consider theoretically how best to distribute entanglement with the current technology in a complete network. Khatri suggests the use of Markov decision making to model protocols used in networks for the near-term to overcome information loss over long distances, while technology develops.²⁶ The idea proposed is to view the quantum network as a graph with the aim of using local vertex links and elementary links and to establish virtual links between distance nodes by entanglement distribution. Practical considerations of existing quantum networks include the small number of nodes and the many imperfections in both the links and the devices. Markov decision making on the elementary links is shown to improve the network as a whole. The result is a predictive tool for how best to exploit a physical imperfect network and how to predict the performance of that network, e.g., key rates for QKD. This theme is continued in a simulation of a quantum network using open source software, showing that the protocol used for entanglement management heavily affects the performance of the network.²⁷ For instance, one can relax the constraints on qubit lifetime but at the expense of high demand on synchronous linking across working nodes (all nodes in a link must work together) or one can preserve the entanglement for a long time while searching for the best working connection across the network, allowing each node to “come online” in due course. The authors analyze the scenarios given in the imperfect networks and finite coherence times, showing the trade-off on demands and the impact on performance and the important considerations in a real-world quantum network.

Most modern classical communications networks are based on optical fibers and so too is there interest in quantum networks that exploit optical fibers. This topic is considered from the perspective of orbital angular momentum (OAM), an exciting basis for quantum information that is both reviewed²⁸ and exploited for random number generation.²⁹ Wang et al. review the state-of-the-art in OAM entanglement and QKD transport down fiber,²⁸ revealing that this exciting prospect is very much in its infancy, with short reach, low key rates, and still requiring custom optical fibers. While the OAM offers a natural basis for high-dimensional encoding, it can also be used for multiple qubits to realize a multi-dimensional quantum transport. A key issue in all spatial mode entanglement in fiber is modal crosstalk, precisely what Zahidy et al. exploit to produce quantum randomness in an optical fiber.²⁹ Using a ring core fiber and OAM, they show that the intrinsic randomness of the coupling can be used as a source for quantum random numbers. They enter a quantum OAM state at an angle into the fiber and show that the resulting OAM superposition can be used as a source of randomness, since the amplitudes of each mode represent probabilistic outcomes. This is an interesting avenue to explore for compact, fast, and integrated quantum random number generation.

Although quantum networks are reaching a high level of maturity, the fundamental science behind them and to exploit them is still ongoing. This collection considers three interesting cases: imaging,³⁰ a feat not yet achieved across a quantum network, indistinguishability of photons,³¹ the core of any entanglement swapping and teleportation scheme, and quantum stationary light pulses,³² the core of quantum memories. Continuing with the OAM theme, Kumar et al. show how to image an OAM photon in flight, as it propagates through free-space, showing the interference of a photon with itself.³⁰ The work develops all the tools needed for estimating the quantum transport of OAM in free-space quantum links. In a comprehensive review, Lal et al. consider the fundamental properties of quantum light and how it can give rise to indistinguishable photons, crucial in many quantum information protocols, pointing out the need for entanglement and indistinguishability simultaneously in quantum network applications.³¹ Finally, in an editor's pick paper³² from the collection, Kim et al. show the first demonstration of using the so-called stationary light pulse effect to trap a free-propagating single photon into a cold atomic ensemble, showing that the quantum properties are preserved in the process. This is a tremendous achievement that may have profound impact on future quantum memories as well as photon–photon interactions.

Quantum networks are steadily gaining in reach, now covering thousands of kilometers across free-space, optical fiber, and space-based links. Given the investment in quantum science and technology, one can expect rapid progress as science translates to technology, and technology matures to advanced generation devices, with a concomitant need for a quantum workforce to keep this progress on track. Presently, there is little connectivity across regions, yet standardization and integration will be needed if the global quantum network is truly to be realized, promising a seamlessly connected network that is fast and secure.

The authors have no conflicts to disclose.

Andrew Forbes: Writing – original draft (equal); Writing – review & editing (equal).