Toshiba Demonstrates Breakthrough in Long-distance Quantum Communications

Toshiba’s Cambridge Research Laboratory demonstrated quantum communications over optical fibers exceeding 600 km in length. The work leverages a technique called dual-band stabilization, and it marks a significant advance in quantum-encrypted communication, particularly for the secure transfer of information between metropolitan areas.

How to transmit quantum bits over long optical fibers is among the challenges in building a quantum internet; small changes in ambient conditions, such as fluctuations in temperature, cause the fibers to expand and contract. This scrambles the fragile qubits encoded as a phase delay of a weak optical pulse in the fiber.
Diagram describing Toshiba's dual band stabilization method for quantum communication. Courtesy of Toshiba.

Diagram describing Toshiba’s dual-band stabilization method for quantum communication. Courtesy of Toshiba.

Toshiba introduced a novel dual-band stabilization technique that sends two optical reference signals at different wavelengths to minimize phase fluctuations in long fibers. The first wavelength cancels the various fluctuations as they happen, and the second wavelength, which is the same wavelength as the optical qubits, is used for fine adjustment of the phase. With this method, the Cambridge team found that it’s possible to hold the optical pulse of a quantum signal constant to within a fraction of a wavelength, with a precision of tens of nanometers, even after propagation through hundreds of kilometers of fiber.

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The first application of this method will be in long distance quantum key distribution (QKD). Current commercially available QKD systems are limited to around 100 to 200 km of fiber. In 2018, Toshiba published a paper in Nature Photonics that proposed the Twin Field QKD protocol as a way to extend that distance, and tested its resilience to optical loss using short fibers and attenuators. With the introduction of the dual-band stabilization technique, the team demonstrated Twin Field QKD on longer fibers of more than 600 km.

“With the new techniques we have developed, further extensions of the communication distance for QKD are possible, and our solutions can be applied to other quantum communications protocols and applications,” said Mirko Pittaluga, first author of the paper describing the results.

The development follows news in 2020 of the first industrial quantum secure network in the U.K., built by U.K. communications company BT and Toshiba. The system transmits data between the National Composites Centre and the Centre for Modelling & Simulation. Toshiba’s multiplexing compatibility enables the data and quantum keys to be transmitted along the same fiber, eliminating the need for dedicated infrastructure for key distribution. The researchers believe the combination of multiplexed QKD using existing infrastructure for shorter distances and Twin Field QKD for longer distances could pave the way for a commercially viable global quantum-secure network.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-021-00811-0).

Published: June 2021

Glossary

quantum: The term quantum refers to the fundamental unit or discrete amount of a physical quantity involved in interactions at the atomic and subatomic scales. It originates from quantum theory, a branch of physics that emerged in the early 20th century to explain phenomena observed on very small scales, where classical physics fails to provide accurate explanations. In the context of quantum theory, several key concepts are associated with the term quantum: Quantum mechanics: This is the branch of...
quantum key distribution: Quantum key distribution (QKD) is a method of secure communication that utilizes principles from quantum mechanics to establish a shared secret key between two parties, typically referred to as Alice and Bob, while detecting any potential eavesdropping attempts by a third party, commonly known as Eve. The fundamental principle behind QKD is the use of quantum properties, such as the superposition principle and the no-cloning theorem, to enable the distribution of cryptographic keys in a...
optical fiber: Optical fiber is a thin, flexible, transparent strand or filament made of glass or plastic used for transmitting light signals over long distances with minimal loss of signal quality. It serves as a medium for conveying information in the form of light pulses, typically in the realm of telecommunications, networking, and data transmission. The core of an optical fiber is the central region through which light travels. It is surrounded by a cladding layer that has a lower refractive index than...
qubit: A qubit, short for quantum bit, is the fundamental unit of information in quantum computing and quantum information processing. Unlike classical bits, which can exist in one of two states (0 or 1), qubits can exist in multiple states simultaneously, thanks to a quantum property known as superposition. This unique feature enables quantum computers to perform certain types of calculations much more efficiently than classical computers. Key characteristics of qubits include: Superposition: A...

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