Quantum Communication Enhanced by Diamond-Based Repeaters: A Breakthrough at MIT

In the field of quantum communication, the vulnerability of qubit transfers can be likened to the message distortions experienced in the childhood game of telephone. Researchers are currently exploiting imperfections in diamonds to develop quantum repeaters. These devices act as intermediaries between quantum systems, facilitating more dependable data transmission, with a broad spectrum of applications, from artificial intelligence to satellite-based navigation systems.

This innovative approach for the storage and transmission of quantum information over unstable links could lay the groundwork for scalable quantum networks.

The well-known game of telephone operates on a straightforward concept: an initial message is whispered from one player to the next, continuing down the line until the final recipient pronounces it aloud to the group. Typically, the message at the end differs substantially from the original, having been distorted along the way.

This phenomenon of transmission errors from origin to destination is also prevalent in quantum systems. As qubits—quantum equivalents of classical bits—navigate through a channel, they can experience state degradation or complete loss. This is especially pronounced over extended distances as qubits are inherently unstable, subject to the principles of quantum mechanics governing subatomic particles.

At this nanoscale level, even minor interactions with the external environment can lead to a loss of qubit coherence, altering the data they hold. This is analogous to the game of telephone where the original and received messages differ.

A prototype quantum repeater, located centrally on a gold-plated copper assembly and connected to green printed circuit boards, features eight optical memories that store qubits in a silicon atom within a diamond. Credit: Glen Cooper

Quantum Networking: Challenges and Opportunities

Scott Hamilton, who leads the Optical and Quantum Communications Technology Group at MIT’s Lincoln Laboratory, part of the Communications Systems R&D area, states, “One major obstacle in quantum networking is how to efficiently transfer these fragile quantum states between multiple quantum systems. Our group is actively investigating this issue.”

Hamilton adds that modern quantum computing chips house roughly 100 qubits. However, thousands or even billions of qubits are needed for a fully operational quantum computer, which would unleash unparalleled computational capabilities for diverse applications including artificial intelligence, cybersecurity, healthcare, and manufacturing. Connecting these chips to form a single computer may offer a viable path forward.

For sensing technologies, connecting quantum sensors to exchange quantum data could lead to enhanced capabilities and performance that transcend individual sensors. For example, a common quantum reference among multiple sensors could improve the localization of radio-frequency emission sources.

Space and defense organizations are keen on connecting quantum sensors across extended ranges for satellite-based navigation and timing systems or inter-satellite atomic clock networks. Quantum satellites could form part of a quantum network structure that connects ground-based stations, creating a genuinely global quantum internet.

However, present-day communication systems, reliant on bit-measuring detectors and amplifiers that replicate bits, are incompatible with quantum networks. This is because qubits cannot be duplicated or measured without destroying their quantum states; they exist in a superposition of states rather than fixed states of zero or one.

To address this, researchers are striving to create quantum versions of classical amplifiers, known as quantum repeaters, to mitigate transmission and interconnection losses. These devices segment the transmission into shorter, more manageable lengths to reduce loss.

Quantum Repeaters: The Next Frontier in Quantum Communication

“Quantum repeaters are crucial for enabling quantum networks to effectively transmit information across lossy links, but a fully functional quantum repeater has yet to be developed,” says Hamilton.

Unlike classical repeaters that use a ‘copy and paste’ mechanism, quantum repeaters operate through quantum entanglement. This phenomenon involves strong correlation between two particles, regardless of the distance that separates them. Knowledge of one particle’s state in an entangled pair instantly informs you of the other’s state.

Entangled qubits facilitate quantum teleportation, allowing for the transfer of quantum information between distant systems without the need to physically move particles. The quantum repeater plays a pivotal role in generating end-to-end quantum entanglement and, consequently, end-to-end qubit transmission via teleportation.

Ben Dixon, a researcher in the Optical and Quantum Communications Technology Group, outlines the process: “Initially, pairs of specific entangled qubits, known as Bell states, are generated and sent in opposite directions to two distinct quantum repeaters that capture and store these qubits. Subsequently, one of the repeaters performs a two-qubit measurement between the sent and stored qubit and a separate, arbitrary qubit intended for transmission across the link for the purpose of connecting remote quantum systems. Measurement outcomes are communicated to the second repeater, which then transforms the stored Bell state qubit into the arbitrary qubit. Finally, this new qubit can be integrated into the remote quantum system, thus connecting the two.”

Advances in Quantum Memory

For quantum repeaters to retain entangled states, they need a storage mechanism. In 2020, researchers at Harvard University showed that a qubit could be held in a single silicon atom in a diamond. This option has proven to be an attractive solution for quantum memory.

Like any other valence electron, the outermost electron on a silicon atom can point either up or down. Its direction, known as its spin, is comparable to the binary ones and zeros used in classical computers.

The ability to manipulate silicon’s valence electron with visible light enables the transfer and storage of a photonic qubit into the electron’s spin state. Collaborators at MIT demonstrated that this basic operation could be performed with multiple waveguides, successfully generating silicon vacancies in all of them.

Lincoln Laboratory is currently applying quantum engineering techniques to develop a quantum memory module with enhanced capabilities, suitable for functioning as a quantum repeater. This includes custom diamond growth, the creation of a silicon-nanophotonics interposer for controlling the silicon-vacancy qubit, and system integration at the cryogenic temperatures required for long-term storage. The current setup includes two memory modules, each capable of holding eight optical qubits.

Real-World Testing and Outcomes

To validate these technologies, the team is using an optical-fiber test bed, which includes a 50-kilometer-long telecommunications network fiber currently linking three nodes: Lincoln Laboratory, MIT campus, and Harvard. Local industries can also access this fiber as part of the Boston-Area Quantum Network (BARQNET).

Hamilton notes, “Our objective is to translate cutting-edge research from our academic partners into practical applications that can be tested over real-world channels.”

The team has already made strides, in collaboration with MIT and Harvard, by being the first to show a quantum interaction with a nanophotonic quantum memory over a deployed telecommunications fiber. Their test results were highly encouraging, achieving best or near-best metrics in terms of distance, efficiency, fidelity, and scalability when compared with other leading global efforts.

The Lincoln Laboratory team is now concentrating on adding multiple quantum memories at each node and incorporating more nodes into their quantum network test bed. Future work will focus on system-level exploration of quantum networking protocols, in addition to the ongoing materials science research being conducted by their academic partners at Harvard and MIT.

The work on the nanophotonic quantum memory module has been recognized with a 2023 R&D 100 Award.

Frequently Asked Questions (FAQs) about Quantum Repeaters

More about Quantum Repeaters

• MIT Lincoln Laboratory’s Optical and Quantum Communications Technology Group
• Boston-Area Quantum Network (BARQNET) Overview
• Quantum Repeaters: A Comprehensive Review
• Diamond Defects and Quantum Information Processing
• Quantum Communication and Networking Technologies
• Scott Hamilton Research Profile
• Quantum Entanglement: A Primer
• Challenges in Quantum Networking