Quantum networks may provide new capabilities for information processing and transport, potentially transformative for science, economy and natural science uses. These capabilities, provably impossible for existing “classical” physics based networking technologies, are of key interest to many U.S. Department of Energy (DOE) mission areas, such as climate and Earth system science, astronomy, materials discovery, and life sciences, etc.

In August of 2021, the Advanced Scientific Computing Research (ASCR) division of the US Department of Energy’s Office of Science announced a funding award for several quantum information system projects in support of the U.S. National Quantum Initiative. One of these projects is QUANT-NET (Quantum Application Network Testbed for Novel Entanglement Technology), a collaboration between Berkeley Lab, UC Berkeley, University of Innsbruck, and Caltech.

QUANT-NET research is focused on building a software-controlled quantum computing network, linking Berkeley Lab and UC Berkeley. ESnet executive director Inder Monga is the project principal investigator. The idea for QUANT-NET was born out of the 2020 DOE Quantum Internet Blueprint workshop, where representatives from DOE national laboratories, universities, industry, and other U.S. agencies came together to define a roadmap for building the first nationwide quantum Internet.

In this post, Dr. Wenji Wu, an ESnet networking researcher who is part of the QUANT-NET team, describes what future capabilities quantum networking may provide and why researchers believe quantum networks will transform scientific activities. 

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Why Quantum Networks?

In the past thirty years, significant progress has been made in the fields of quantum technologies. The combination of quantum mechanics and information science forms a new area – quantum information science (QIS). In the broad context of QIS, quantum networks have an important role for the physical implementation of quantum computing, communication, and metrology. Quantum networks are envisioned to achieve novel capabilities that are provably impossible using classical networks and could be transformative to science, the economy, and national security. These novel capabilities range from cryptography, sensing and metrology, distributed systems, to secure quantum cloud computing. 

A few examples of this include: 

• Secure Quantum Communication: Quantum networks take advantage of the laws of quantum physics (i.e., superposition and entanglement) to transmit information, potentially achieving a level of privacy and security that is impossible to achieve with today’s Internet. See Figure 1a.
• A Quantum Network of Clocks: Recent research shows that a quantum network of atomic clocks can result in a substantial boost of the overall precision if multiple clocks are properly connected by quantum mechanical means. Compared to a single clock, the ultimate precision will improve as much as 1/K, where K is the number of clocks. If the same clocks are connected via a classical network, the precision scales as much as 1/SQRT(K). Ultimately, a quantum network of atomic clocks can surpass the Standard Quantum Limit (SQL) to reach the ultimate precision allowed by quantum theory — the Heisenberg limit. See Figure 1b.
• Upscaling Quantum Computing: An individual quantum computer is typically limited in size. Connected by quantum networks, multiple quantum computers can work together as one big quantum computer to address larger problems. See Figure 1c.

Figure 1a: Secure quantum communication (credit: Chen et al. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.070501).	Figure 1b: A quantum network of clocks (credit: Komar, Peter et al. “A quantum network of clocks.” Nature Physics 10.8 (2014:582-587).	Figure 1c: Upscale quantum computing (credit: Thor Swift, Berkeley Lab).

Quantum Network Basics

Quantum networks are distributed systems of quantum systems, which are able to exchange quantum bits (qubits) and generate and distribute entangled quantum states. As illustrated by Figure 2, a quantum network conceptually consists of three essential quantum components: 

1. Quantum nodes, which are physical quantum systems (e.g., trapped ions, quantum dots, Nitrogen-vacancy centers) connected to the quantum network. Well-characterized matter qubits are typically defined and created from these physical quantum systems. Quantum information is generated, processed, and stored locally by matter qubits in quantum nodes.  Matter qubits, often referred to as stationary qubits, are typically isolated from the surrounding environment to minimize decoherence and facilitate various quantum operations. 
2. Quantum channels, which connect physically separated quantum components in the quantum network and transfer quantum states faithfully from place to place using the flying qubits. Optical fibers and free-space communications are typically implemented as quantum channels because they have a reduced chance of decoherence and loss. Photons with polarization or time-bin encoding are the flying qubit of choice. The implementation of quantum channels also requires that information encoded in a stationary qubit is reliably transferred to a flying qubit, and vice versa. 
3. Quantum repeaters, which allow the end-to-end generation of quantum entanglement, and thus, the end-to-end transmission of qubits by using quantum teleportation. Quantum repeaters typically implement entanglement-related operations such as entanglement swapping and entanglement purification.

Figure 2: A quantum network consists of three essential quantum systems

In quantum networks, qubits cannot be copied due to the no-cloning theorem, which forbids the creation of identical copies of an arbitrary unknown quantum state. Therefore, qubits can not be physically transmitted over long distances without being hindered by the effects of signal loss and decoherence inherent to most transport mediums such as optical fiber. However, qubits can share a special relation known as entanglement. Entangled qubits have interesting non-local properties, even if they are located at distant nodes. Consuming an entangled qubit pair, a data qubit can be sent deterministically to a remote node. Entanglement is the fundamental building block of quantum networks. 

As illustrated in Figure 3, key entanglement-related operations include: 

• Entanglement Purification: Multiple low-quality entanglements can be purified into a high-quality entanglement. 
• Entanglement Swapping: Long-distance entanglement can be built from shorter segments, with flying qubits transmitted locally.
• Teleportation: to enable the end-to-end transmission of qubits.

Figure 3: Key entanglement-related operations

Classic networks typically concern the performance metrics such as bandwidth, throughput, and latency. Likewise, quantum networks care for performance metrics related to quantum operations. Critical quantum quality metrics include entanglement generation rate, decoherence rate, and fidelity. In quantum networks, fidelity is a key indicator to characterize the quality of quantum states or operations. In general, a minimum fidelity (F_min) is required to support quantum operations.

It is envisioned that quantum networks will operate in parallel with classic networks. Quantum networks are not meant to replace classic networks but rather to supplement them with quantum capabilities.

Current Status

Today, quantum networks are in their infancy. Like the Internet, quantum networks are expected to undergo different stages of research and development until they reach their full functionality. There are many promising R&D efforts underway looking to develop quantum network technologies. The DOE unveiled a quantum Internet blueprint in 2020 to accelerate research in quantum science and technology, with the emphasis on the creation of a quantum Internet.