Cisco Bridges Classical and Quantum Networks

In the drive to make a practical quantum computer, researchers are developing bigger and better quantum networks-ones with capabilities that will complement and enhance quantum computing. Put another way, building a functioning quantum network that can exchange many qubits securely, over long distances, could be a useful end goal completely apart from the quantum-computer race.

In that vein, Cisco launched a quantum-networking software system on 25 September. The networking giant's technology could help to bring about more powerful quantum sensors, secure position verification, and quantum-enhanced imaging tech-to list just three of a range of emerging, noncomputing applications for quantum networks.

The team has a hybrid purpose in mind as well, says Ramana Kompella, vice president and head of research at Cisco in San Jose, Calif.: quantum networks that can work with classical computers and conventional computer networks.

This is a very fascinating field for us because until now, classical computing didn't have access to a quantum network," Kompella says. But imagine if you had access to a quantum network, what can you actually enable in terms of new capabilities?" Kompella has an answer to his own question. We can secure classical networking with the help of quantum signals by detecting eavesdroppers on long-distance fiber-optic communications," he says.

How Does Quantum Entanglement Secure Networks?

To do so, Kompella says, the system relies on the fact that quantum signals shared across their sensitive network are connected together via quantum entanglement. We inject entangled photons into the optical fiber," he says. And if the attacker tries to tap the fiber, they end up disturbing the entanglement, which allows us to detect them."

Kompella adds that entanglement exchanged over network distances has other classical computing applications in high-frequency trading and fintech, as well as maybe you can drive ultraprecise time synchronization with the help of entanglement-based networks," he says.

Cisco's quantum-networking system is built on top of a practical quantum-networking chip the company introduced in May, which uses existing fiber-optic lines, generates up to 200 million entangled photon pairs per second, and operates at standard telecom wavelengths.

But the new component Cisco recently introduced is software. The compiler the company has now launched enables a coder to write in IBM's Python-based Qiskit quantum-computer language. And the Cisco compiler takes care of technical networking details like optimizing the connections between quantum processors and fine-tuning error-correction strategies.

We hide the physical layer complexity," says Reza Nejabati, Cisco's head of quantum research, which allows the algorithm developers to play with the number of processors and how the processors are connected together to optimize their algorithms."

The compiler takes that high-level goal, breaks it up, and then drives the networking side of the equation," Kompella adds.

Hoi-Kwong Lo, a professor of electrical and computer engineering at the University of Toronto, says that Cisco is championing an underappreciated portion of the larger quantum-technology world.

Investment is a key issue," Lo says. While there have been billions of research funding annually invested in quantum-computing startups, investments in quantum-networking startups have been falling behind."

According to Ronald Hanson, a professor of nanoscience at Delft University of Technology, in the Netherlands, Cisco's work is a key next step. But it's only a next step.

What Cisco is presenting now is not really first of its kind," Hanson says. But the fact that Cisco is working on several of these different elements of the quantum network combined with their classical networking expertise and strengths makes the progress interesting and will push the quantum networking industry as a whole."

What Will It Take for Quantum Networks to Scale?

The biggest limitation on Cisco's system at present, says Nejabati, is the physical distance limit that a single photon can travel before being absorbed by the optical fiber itself.

Our hardware and software technology allows us to...go up to a hundred kilometers with a very-high-quality, high-performance network," Nejabati says.

Lo says physics-in particular a law called the no-cloning theorem," stating that individual quantum bits can never be perfectly replicated-makes large-scale quantum networks especially tricky to realize.

The big challenge is to build quantum repeaters," Lo says. Optical fibers are lossy, and to overcome the distance limit, we need quantum repeaters."

Lo's group, for one, is investigating encoding a qubit's signal not onto another individual photon but rather onto a cluster of entangled photons. IEEE Spectrum has tracked Lo's group's initial work on this method in 2015 and their proof-of-principle experimental test in 2019.

On the other hand, says Hanson, making quantum repeaters isn't the only way forward for next-generation quantum-networking tech.

Just sharing photons is in our mind not the most interesting tech, since many use cases remain out of reach," Hanson says. Instead, our goal is to create...entanglement on demand: by combining entanglement distribution via photonic channels with long-lived quantum memories-a buffer of entangled qubits ready to be consumed."

This way, Hanson says, quantum entanglement can be stored like energy in a battery or terabits on a hard drive, and tapped into when users on either end of the network want to share quantum information.

Buffered entanglement will unlock an interesting range of applications beyond [quantum cryptography] that have the promise to bring real value," Hanson says. It will be interesting to see when Cisco will make the step to that technology for their networks."