Bell State Analyzer brings the quantum internet closer | Research & Technology | March 2022

OAK RIDGE, Tenn., March 9, 2022 – Collaborators from industry and academia – researchers from the Department of Energy’s Oak Ridge National Laboratory (ORNL), Freedom Photonics and Purdue University – have made progress towards a fully quantum Internet. The collaborators have designed and demonstrated what they claim to be Bell’s first state analyzer for frequency coding.

Before information can be sent over a quantum network, it must first be encoded into a quantum state. Information is contained in qubits – analogous to bit in classical computing – which are entangled, meaning they reside in a state in which they cannot be described independently of each other.

The peak of qubit entanglement is called the Bell state.

The measurement of Bell states is therefore essential for many protocols needed to perform quantum communication and to distribute entanglement over a quantum network. Although these measurements have been made for years, the team’s method represents a Bell state analyzer developed specifically for frequency encoding – a method of quantum communication that exploits single photons residing in two different frequencies simultaneously.


ORNL’s Joseph Lukens conducts experiments in an optical laboratory. Lukens is part of a collaboration that designed and demonstrated a Bell state analyzer for frequency coding. The work supports advances 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 communications,” said Joseph Lukens, ORNL researcher and Eugene Wigner Fellow. “To do things like teleportation and swapping tangles, you need a Bell state analyzer.”

Teleportation is the act of sending information from one part to another over a significant physical distance, and entanglement exchange refers to the ability to entangle previously unentangled pairs of qubits.

Lukens proposed a hypothesis: Imagine that there are two quantum computers connected by a fiber optic network. Due to their spatial separation, they cannot interact with each other on their own.

“However, suppose they can each be entangled with a single photon locally,” Lukens said. “By sending these two photons down the optical fiber and then performing a Bell state measurement on them where they meet, the end result will be that the two distant quantum computers are now entangled – even if they don’t never interacted. This so-called entanglement swapping is an essential capability for building complex quantum networks.

When there are four Bell states in total, the parser can only distinguish two of them at any given time. Measuring the other two states would add a layer of complexity that has so far not been necessary.

The team designed the analyzer using simulations and demonstrated 98% fidelity, with the remaining 2% error rate being the result of unavoidable noise from random preparation of test photons rather than the parser itself, Lukens said. Precision enables the fundamental communication protocols needed for frequency intervals.

Lukens and his team demonstrated for the first time in 2020 how single-frequency bin qubits can be fully controlled as needed to transfer information across a quantum network.

Using a technology developed at ORNL known as the Quantum Frequency Processor, the researchers demonstrated widely applicable quantum gates, or the logical operations needed to run quantum communication protocols. In these protocols, researchers need to be able to manipulate photons in a user-defined way, often in response to measurements made on particles elsewhere in the array.

While traditional operations used in classical computers and communication technologies operate on individually digital zeros and ones, quantum gates operate on simultaneous superpositions of zeros and ones. This feature protects quantum information as it passes through – a phenomenon necessary to achieve a true quantum network.

While frequency encoding and entanglement appear in many systems and are naturally compatible with fiber optics, using these phenomena to perform data manipulation and processing operations has traditionally proven difficult.

With Bell’s state analyzer completed, Lukens and his colleagues are looking to expand to a full entanglement swap experiment, which would be the first of its kind in frequency coding.

The work is planned as part of ORNL’s Quantum-Accelerated Internet Testbed project, recently awarded by the Department of Energy.

The research has been published in Optical (www.doi.org/10.1364/optica.443302).

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