Bell State Analyst Brings the Internet Closer | Research and technology | March 2022

Oak Ridge, Tennessee, March 9, 2022 – Collaborators from industry and academia – researchers at the Department of Energy’s Oak Ridge National Laboratory (ORNL), Freedom Photonics and Purdue University – have made strides toward a fully quantum Internet. The collaborators designed and demonstrated what they reported as the first Bell state analyzer for frequency container ciphers.

Before information is sent through a quantum network, it must first be encoded into a quantum state. The information contained in qubits — similar to bits in classical computing — becomes entangled, meaning that they live in a state in which they cannot be described independently of each other.

The qubit entanglement peak is referred to as the Bell state.

Therefore, measurement of Bell states is critical for many protocols needed to perform quantum communication and entanglement distribution across a quantum network. While these measurements have been made for years, the team’s method represents a Bell state analyzer developed specifically for frequency-container coding — a quantum communications method that harnesses single photons present at two different frequencies simultaneously.


Joseph Lukins of ORNL directs experiments in the Optical Laboratory. Lukens is part of a collaboration that designed and demonstrated the Bell State Analyzer for Frequency Container Cryptography. The work supports developments in the field of frequency coding. Courtesy of Jason Richards/Oak Ridge National Laboratory, US Department of Energy.


“Measuring these Bell states is fundamental to quantum communication,” said Joseph Lukens, an ORNL research scientist and Eugene Wegner Fellow. “To achieve things like teleportation and entanglement swapping, you need a Bell state analyzer.”

Teleportation is the process of transmitting information from one end to another over a large physical distance, and entanglement exchange refers to the ability to entangle previously unentangled qubit pairs.

Lukens hypothetically suggested: Imagine there are two quantum computers connected to an optical fiber network. Because of the spatial separation, they cannot interact with each other on their own.

“However, he supposes that they can each entangle a single photon locally,” Lukens said. “By sending these two photons down an optical fiber and then doing a Bell-state measurement on them where they meet, the end result will be that the two distant quantum computers are now entangled — even though they never interact. This so-called entanglement switch is a critical ability to build complex quantum networks. .”

When there are four complete Bell states, the analyzer can distinguish between only two states at any given moment. Measuring the other two cases will add a layer of complexity that was not necessary until now.

Lukens said the team built the analyzer using simulations and showed an accuracy of 98%, with the remaining 2% error due to unavoidable noise from the random setup of test photons rather than the analyzer itself. Precision enables the basic communication protocols needed for frequency packets.

Lukens and his team demonstrated for the first time in 2020 how single-frequency container volumes can be fully controlled as needed to transmit information over a quantum network.

Using a technology developed at ORNL known as a quantum frequency processor, the researchers demonstrated widely applicable quantum gates, or the Boolean operations needed to perform quantum communication protocols. In these protocols, researchers must be able to manipulate the photons in a way defined by the user, often in response to measurements made on particles elsewhere in the network.

Whereas the conventional operations used in classical computers and communication technologies operate on digital zeros and ones individually, quantum gates work on a simultaneous superposition of zeros and ones. This property keeps quantum information secure as it passes – a phenomenon required for real quantum networks.

While frequency coding and entanglement appear in many systems and are naturally compatible with optical fibres, using these phenomena to perform data processing and processing operations has traditionally proven to be difficult.

With the Bell state analyzer complete, Lukens and colleagues look forward to expanding on the full entanglement exchange experiment, which will be the first of its kind in frequency coding.

Work is planned as part of ORNL’s Quantum Accelerated Internet Testing Project, which was recently awarded by the Department of Energy.

The search was published in Visual (www.doi.org/10.1364/optica.443302).