Networking for Practical
Quantum Applications

By: Scott Packard, Shane Sexton

Like artificial intelligence, large language models, and the metaverse, quantum technology is very much a topic of current interest. Quantum technology promises to leverage quantum mechanics to achieve previously inconceivable goals in computational complexity, speed and security in communications, and precision and accuracy in sensing and measurement. Quantum mechanics help us understand how the smallest particles in our universe behave, often contrary to our classical understanding of the visible world. 

Quantum technology has been heralded as revolutionary; however, like the other technologies mentioned, it can be challenging for the average person to separate fact from fiction, and hyperbole from truth. Much of quantum technology remains in the early stages of development as researchers, scientists, and engineers attempt to leverage quantum mechanics to achieve great leaps forward.

So, suppose we have yet to realize practical quantum hardware. Why should we consider efforts to connect incipient processors, sensors, or communications equipment into a quantum version of the classical internet? Here, we will explain what composes a quantum network, why they are important, the key considerations for designing and installing one, and what quantum networks can do for us both today and in the future. We will also introduce you to the first commercial quantum network built by Chattanooga’s energy and connectivity provider, EPB, with the help of Qubitekk. Telecommunications professionals will benefit from understanding this emerging technology and the role existing fiber networks play in designing, installing, and operating quantum networks.

What is a Quantum Network?

A quantum network is a type of telecommunications network that uses quantum phenomena to transmit information securely and efficiently. Unlike classical networks that use electrical or light signals to transmit data, quantum networks employ quantum properties of light such as photonic superposition, coherence, and entanglement.

Quantum networks comprise five key elements: a physical layer, quantum sources, quantum detectors, coherence controls, and electronics and software. The physical layer consists of the medium over which quantum bits—or qubits—are transmitted. The primary means is over terrestrial optical fiber. Free-space optical transmission—photon transmission through air or space—is also possible with an appropriate transmitter and receiver, however.

Quantum sources are devices that generate quantum states or particles with specific properties. The exact nature of these states or particles depends on the type of quantum source. For example, a single-photon source generates individual photons, while an entangled photon source produces pairs of photons that are interconnected such that a change in the state of one photon will instantly change the state of the other, no matter how far apart they are. These quantum states are critical for many applications in quantum computing, sensing, and communications.

Quantum detectors are devices designed to measure or detect quantum phenomena, such as individual particles or specific quantum states. A single-photon detector is a type of quantum detector, specifically designed to measure and detect single photons—the elementary particles of light. There are different types of single-photon detectors, including photomultiplier tubes (PMTs), avalanche photodiodes (APDs), and superconducting nanowire single-photon detectors (SNSPDs). Each type has its own advantages and limitations in terms of sensitivity, efficiency, and response time, which makes them suitable for different applications.

Coherence controls improve the stability, integrity, and usability of quantum information transmitted over quantum networks. Quantum states are delicate and can be easily disturbed or destroyed—known as decoherence--which leads to a loss of quantum information. Controlling and extending coherence improves network performance. Lastly, like classical networks, quantum networks require electronics and software to configure and control the network. Timing is critical to distinguishing correlated qubits from random noise. Control plane software enables maintenance and configuration of the network for maximal usability.

Why are Quantum Networks Relevant?

Quantum networks provide two main benefits: 1) connectivity for quantum processors, sensors and communications systems and 2) unparalleled security for information in transit. Given the ubiquitous nature of today’s internet, the first benefit will be easily understood. Networking quantum hardware magnifies computational power, information sharing, detection sensitivity, precision, and accuracy. Secondly, quantum networks also offer an unprecedented level of security that is virtually impossible to breach. Moreover, attempts to do so are immediately detectable. Information is encoded in a way that it cannot be intercepted or manipulated without detection,