While few details accompanied last week’s official announcement of U.S. plans for a nation-wide quantum internet, many of the priorities and milestones had been worked out during a February workshop and are now available in subsequent reports. The Department of Energy is leading the effort which is part of the U.S. Quantum Initiative passed in 2019.

The race to harness quantum information science – whether through computing, communications, or sensing – has become a global competition. In many ways quantum communications is the furthest along in development and its promise of near absolute security is extremely alluring. DOE’s 17 National Laboratories are intended to serve as the backbone of the U.S. quantum internet effort.

As noted in the official announcement, “Crucial steps toward building such an internet are already underway in the Chicago region, which has become one of the leading global hubs for quantum research. In February of this year, scientists from DOE’s Argonne National Laboratory in Lemont, Illinois, and the University of Chicago entangled photons across a 52-mile “quantum loop” in the Chicago suburbs, successfully establishing one of the longest land-based quantum networks in the nation. That network will soon be connected to DOE’s Fermilab in Batavia, Illinois, establishing a three-node, 80-mile testbed.”

Turning early prototypes into a scaled-up nationwide effort involves tackling many technical challenges. One thorny problem, for example, is development of robust repeater technology, which among other things requires reliable quantum memory technology and prevention of signal loss. Interestingly, satellites may play a role as a bridge according to the report:

“A quantum Internet will not exist in isolation apart from the current classical digital networks. Quantum information largely is encoded in photons and transmitted over optical fiber infrastructure that is used widely by today’s classical networks. Thus, at a fundamental level, both are supported by optical fiber that implements lightwave channels. Unlike digital information encoded and transmitted over current fiber networks, quantum information cannot be amplified with traditional mechanisms as the states will be modified if measured.

“While quantum networks are expected to use the optical fiber infrastructure, it could be that special fibers may enable broader deployment of this technology. At least in the near term, satellite-based entanglement “bridges” could be used to directly connect transcontinental and transatlantic Q-LANs. Preliminary estimates indicate that entangled pairs could be shared at rates exceeding 106 in a single pass of a Medium Earth Orbit (MEO) satellite. Such a capability may be a crucial intermediate step, while efficient robust repeaters are developed (as some estimates predict more than 100 repeaters would be needed to establish a transatlantic link).”

The report from the workshop spells out four priorities along with five milestones. (The event was chaired by Kerstin Kleese van Dam, Brookhaven National Laboratory; Inder Monga, Energy Sciences Network; Nicholas Peters, Oak Ridge National Laboratory; and Thomas Schenkel, Lawrence Berkeley National Laboratory).

Here are the four priorities identified in the report:

• Provide the Foundational Building Blocks for a Quantum Internet. “Today’s quantum networking experiments rely on a set of devices with limited functionality and performance. However, it can be inferred from classical networks that in order to create wide-area, operational quantum networks, we need more capable devices with additional functionality. These new devices will need to satisfy suitable requirements for reliability, scalability, and maintenance. Potential network devices may include space-to-ground connections; high-speed, low-loss quantum switches; multiplexing technologies and transducers for quantum sources; as well as transduction from optical and telecommunications regimes to quantum computer-relevant domains, including microwaves.”
• Integrate Multiple Quantum Networking Devices. “Generally, all key quantum network components remain at laboratory-level readiness to date and have yet to be run operationally in a full network configuration. Moving forward will require overcoming critical challenges toward achieving cascaded operation and connectivity, among them unifying operational properties, achieving high-repetition rates (GHz), and devising quantum memory buffers and detectors to compensate for cascading operation losses.”
• Create Repeating, Switching, and Routing for Quantum Entanglement. Multi-hop networks require a means of strengthening and repeating signals along with selecting paths through the network. While physical and software solutions are used in classical networks, an equivalent has not been found for quantum networks. Challenges include different forms of quantum entanglement swapping, and quantum teleportation protocols over multiple users, as well as coordination and integration of traditional networks with quantum networks technologies for optimal control and operations.
• Enable Error Correction of Quantum Networking Functions. A fundamental difference for quantum networks arises from the fact that entanglement, whose long-distance generation is an essential network function, is inherently present at the network’s physical layer. This differs from classical networking, where shared states typically are established only at higher layers. In this context, solutions must be found to guarantee network device fidelity levels capable of supporting entanglement distribution and deterministic teleportation, as well as quantum repeater schemes that can compensate for loss and allow for operation error correction.

Some of the test cases being discussed are fascinating such as one across Long Island, NY:

“For example, there would be considerable value in expanding on the current results gleaned from the Brookhaven Lab–SBU–ESnet collaboration, which in April 2019 achieved the longest distance entanglement distribution experiment in the United States by covering approximately 20 km. Integral to the testbed are room-temperature quantum network prototypes, developed by SBU’s Quantum Information Technology (QIT) laboratory, that connect several quantum memories and qubit sources. The combination of these important results allowed the Brookhaven–SBU– ESnet team to design and implement a quantum network prototype that connects several locations at Brookhaven Lab and SBU.

“By using quantum memories to enhance the swapping of the polarization entanglement of flying photon pairs, the implementation aims to distribute entanglement over long distances without detrimental losses. The team has established a quantum network on Long Island, N.Y., using ESnet’s and Crown Castle fiber infrastructure, which encompasses approximately 120-km fiber length connecting Brookhaven Lab, SBU, and Center of Excellence in Wireless and Information Technology (CEWIT) at SBU campus locations.

“As a next step, the team plans to connect this existing quantum network with the Manhattan Landing (MAN- LAN) in New York City, a high-performance exchange point where several major networks converge. This work would set the stage for a nationwide quantum-protected information exchange network. Figure 3:3 depicts the planned network configuration.”

Here are milestones called out in the report:

• Milestone 1: Verification of Secure Quantum Protocols over Fiber Networks Prepare and Measure Quantum Networks. In this quantum network prototype, end users receive and measure quantum states, but entanglement is not necessarily involved. Users can have their password verified without revealing it, and two end users can share a private key known only to them. Applications to be achieved in this kind of network include quantum key distribution (QKD) between non-trusted nodes with (comparatively) higher tolerance on timing fluctuations, qubit loss, and errors.
• Milestone 2: Inter-campus and Intra-city Entanglement Distribution Entanglement Distribution Networks. In this type of quantum network, any two end users can obtain entangled states, requiring end-to-end creation of quantum entanglement in a deterministic or heralded fashion, as well as local measurements. These networks provide the most robust quantum encryption possible by enabling implementation of device-independent protocols, such as measurement device- independent QKD and two-party cryptography. The tolerance for fluctuations, loss, and errors is lower than the previous class (Milestone 1). Initial integrations of classic and quantum networks exists.
• Milestone 3: Intercity Quantum Communication using Entanglement Swapping Quantum Memory Networks. In this type of quantum network, any two end users (nodes) can obtain and store entangled qubits and teleport quantum information to each other. End nodes can perform measurements and operations on the qubits they receive. The minimum memory storage requirements are determined by the time for round trip classical communications. This quantum network stage enables limited cloud quantum computing in the sense that it allows a node with the ability to prepare and measure single qubits to connect to a remote quantum computing server.
• Milestone 4: Interstate Quantum Entanglement Distribution using Cascaded Quantum Repeaters Network Connectivity. Classic and quantum networking technologies have been integrated. Successful concatenation of quantum repeaters and quantum error corrected communication with respect to loss and operational errors over continental-scale distances, will pave the way for operational entanglement distribution networks covering longer distances, enabling a first-ever quantum Internet to be created.

A fifth broad milestone – the Cross-cutting milestone: Build a Multi-institutional Ecosystem – emphasizes the importance of federal agency cooperation and coordination and names DOE, NSF, NIST, DoD, NSA, and NASA as key players. “While pursuing these alliances, critical opportunities for new directions and spin-off applications should be encouraged by robust cooperation with quantum communication startups and large optical communications companies. Early adopters can deliver valuable design metrics.”

It’s a clearly ambitious agenda. Stay tuned.

Link to announcement, https://www.hpcwire.com/off-the-wire/doe-unveils-blueprint-for-the-quantum-internet-in-event-at-university-of-chicago/

Link to slide deck, https://science.osti.gov/-/media/ascr/ascac/pdf/meetings/202004/Quantum_Internet_Blueprint_Update.pdf?la=en&hash=8C076C1BEB7CA49A3920B1A3C15AA531B48BDD72

Link to full report, https://www.energy.gov/sites/prod/files/2020/07/f76/QuantumWkshpRpt20FINAL_Nav_0.pdf