By Paula Reinman and Joe Lukens

For the past two years, Joe Lukens, Marconi Society Young Scholar and Research Scientist at Oak Ridge National Laboratory, has been interested in frequency-based quantum information processing as an approach to making the quantum Internet a reality. By using ideas and models from the current Internet, Lukens believes that we can bring the benefits of the quantum Internet to people more quickly and in a more scalable way. He recently co-authored a paper in Optica by OSA outlining an approach to do just that.

Creating a More Practical Quantum Internet

While the classical Internet is built to transmit bits to different locations, the quantum Internet transmits quantum bits, or qubits, the basic unit of information in a quantum computer. If you have a small, low-power quantum computer in one location, you can connect it to a larger quantum computer elsewhere and transmit qubits between them. The quantum Internet would allow this on a global scale.

This is easier said than done. “Qubits are so squirrely,” says Lukens. “They degrade if they interact with their environment. You cannot copy a qubit without messing it up. You cannot amplify a qubit in order to send it further. These are the intrinsic challenges of quantum information.”

Frequency encoding leverages well-understood tools used in the classical Internet, such as pulse shapers and modulators, to control bits going through the system. By making the quantum Internet compatible with the classical Internet, we can make the quantum Internet more practical.

Why Do We Need a Quantum Internet?

While many more use cases for the quantum Internet will no doubt emerge when it is available, just like they did in the classical Internet, there are some immediate applications:

• Security– When we have a quantum Internet, we can realize secure information between nodes, with security based on quantum mechanics rather than computational complexity. Quantum sensing can, in principle, let you detect quantities of dangerous chemicals more quickly and effectively. With entangled quantum sensors at different locations, we may even be able to detect new physics, such as dark matter.
• Quantum computing – Although high in its hype cycle right now, quantum computing has real potential to solve problems that cannot be done efficiently with today’s technology. One example is Shor’s algorithm, which is an efficient way to factor large numbers. Public key cryptography exists today to let us communicate securely on the Internet precisely because of the difficulty of this problem. Quantum computing will make solving problems like Shor’s algorithm much more accessible and will become a disruptive technology in many fields.
• Quantum simulation – When problems become very large and cannot be solved on traditional computers, quantum computers will be able to efficiently simulate other quantum systems. This could be very useful in basic sciences to help us understand particle physics, quantum chemistry, and more generally expand our understanding of the universe on scales ranging from atoms to galaxies.

What Was the Discovery?

All the applications above fall under the general umbrella of “quantum information processing”—using quantum systems to encode and process data. While quantum information processing can be achieved with any quantum particle (atoms, ions, electrons, superconducting circuits), single particles of light—or photons—are the best for traveling over long distances and connecting devices in the emerging quantum Internet. In the case of Lukens and his co-authors, the focus is on how to encode, manipulate, and measure information carried in some property of the individual photons.

One of the novel aspects of Lukens’ research is encoding this information in the photon’s color, or frequency, which means that the frequency of the photon corresponds to 0 or 1 – say the photon is 0 at red and 1 at green. Each frequency forms a “bin” in which a photon can exist, so that each photon can be described as a frequency-bin qubit. This is very similar to the principles of WDM, a widespread technology used in classical optical communications. WDM has great tools to control and use frequencies at high data rates, and it is only recently becoming clear how we can apply these ideas in quantum computing and make full use of these existing tools.

In order to develop large quantum networks with frequency bins, we will need to be able to apply quantum gates (basic logic operations) in parallel in optical fiber. Lukens and his co-authors discovered that they could have two different qubits and that they could apply two different gates, even on the same fiber.

The ability to have different gates on the same fiber and control the qubits in parallel is a technical achievement that could allow us to connect quantum nodes at different wavelengths, which otherwise would not be able to be linked together. This creates a quantum interconnect which takes quantum systems that are physically far away from each other and connects, or quantum entangles, them. This approach to building the quantum Internet leverages tools and technology from the classical Internet, providing a step forward to the many promising applications of connected quantum devices.

For more detail on the solution created by Lukens and Oak Ridge colleagues Nick Peters, Brian Williams, and Pavel Lougovski, as well as Purdue collaborators Hsuan-Hao Lu and Andy Weiner, check out the full article in Optica.