Bell State Analyzer Brings Quantum Internet Closer

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 have designed and demonstrated what they report is the first Bell state analyzer for frequency bin coding.

Before information can be sent over a quantum network, it must first be encoded into a quantum state. The information is contained in qubits — analogous to the bit in classical computing — that become entangled, meaning they reside in a state in which they cannot be described independently of one another.

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

Measuring Bell states, therefore, is critical to many of the protocols necessary to perform quantum communication and to distribute entanglement across a quantum network. While these measurements have been performed for years, the team’s method represents a Bell state analyzer developed specifically for frequency bin coding — a quantum communications method that harnesses single photons residing in two different frequencies simultaneously.

ORNL’s Joseph Lukens runs experiments in an optics lab. Lukens is part of a collaboration that has designed and demonstrated a Bell state analyzer for frequency bin coding. The work supports advancements in the area of frequency encoding. Courtesy of Jason Richards/Oak Ridge National Laboratory, U.S. Department of Energy.

“Measuring these Bell states is fundamental to quantum communications,” said Joseph Lukens, an ORNL research scientist and Eugene Wigner fellow. “To achieve things such as teleportation and entanglement swapping, you need a Bell state analyzer.”

Teleportation is the act of sending information from one party to another across a significant physical distance, and entanglement swapping refers to the ability to entangle previously unentangled qubit pairs.

Lukens proposed a hypothetical: Imagine there are two quantum computers connected by a fiber optic network. Because of their spatial separation, they can’t interact with one another on their own.

“However, suppose they can each be entangled with a single photon locally,” Lukens said. “By sending these two photons down 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 though they never interacted. This so-called entanglement swapping is a critical capability for building complex quantum networks.”

When there are four total Bell states, the analyzer can only distinguish between two at any given moment. 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 have demonstrated 98% fidelity, with the remaining 2% error rate being the result of unavoidable noise from the random preparation of the test photons rather than the analyzer itself, Lukens said. The accuracy enables the fundamental communication protocols necessary for frequency bins.

Lukens and his team first demonstrated in 2020 how single-frequency bin qubits can be fully controlled as needed to transfer 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 logical operations necessary for performing quantum communications protocols. In these protocols, researchers must be able to manipulate photons in a user-defined way, often in response to measurements performed on particles elsewhere in the network.

Whereas the traditional operations used in classical computers and communications technologies operate on digital zeros and ones individually, quantum gates operate on simultaneous superpositions of zeros and ones. This characteristic keeps the quantum information protected as it passes through — a phenomenon required to realize true quantum networking.

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 the Bell state analyzer completed, Lukens and colleagues are looking to expand to a complete entanglement swapping experiment, which would be the first of its kind in frequency encoding.

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

The research was published in Optica (www.doi.org/10.1364/optica.443302).