The Cambridge Research Laboratory of Toshiba Europe announced recently the first demonstration of quantum communi­cations over optical fibers exceeding 600 kilo­meters in length. The breakthrough will enable long distance quantum-secured information transfer between metropolitan areas and is a major advance towards building the future Quantum Internet. This future global network of quantum computers will be connected by long distance quantum communi­cation links. It is expected to allow the ultrafast solution of complex optimi­zation problems in the cloud, a more accurate global timing system and highly secure communi­cations across the globe. Several large government initiatives to build a quantum internet have been announced, for example in the US, EU and China. 

One of the most difficult techno­logical challenges in building the quantum internet, is the problem of how to transmit quantum bits over long optical fibres. Small changes in the ambient conditions, such as temperature fluctuations, cause the fibres to expand and contract, thereby scrambling the fragile qubits, which are encoded as a phase delay of a weak optical pulse in the fiber. Now, Toshiba has demonstrated record distances for quantum communi­cations by introducing a novel dual band stabili­zation technique. This sends two optical reference signals, at different wavelengths, for mini­mizing the phase fluc­tuations on long fibres. The first wavelength is used to cancel the rapidly varying fluc­tuations, while the second wavelength, at the same wavelength as the optical qubits, is used for fine adjustment of the phase. After deploying these new techniques, Toshiba found it is possible to hold the optical phase of a quantum signal constant to within a fraction of a wavelength, with a precision of 10s of nanometers, even after propagation through 100s of kilometers of fiber. Without cancelling these fluc­tuations in real time, the fibre would expand and contract with temperature changes, scrambling the quantum information.

The first appli­cation for dual band stabi­lization will be for long distance Quantum Key Distribution (QKD). Commercial QKD systems are limited to around 100-200 kilometers of fiber. In 2018, Toshiba proposed the Twin Field QKD protocol as a way to extend the distance, and tested its resilience to optical loss using short fibres and attenuators. By intro­ducing the dual band stabili­zation technique, Toshiba has now implemented Twin Field QKD on long fibers and demonstrated QKD over 600 kilometers, for the first time. “This is a very exciting result,” says Mirko Pittaluga. “With the new techniques we have developed, further extensions of the communi­cation distance for QKD are still possible and our solutions can also be applied to other quantum communi­cations protocols and applications”.

Andrew Shields, Head of the Quantum Technology Division at Toshiba Europe remarks, “QKD has been used to secure metro­politan area networks in recent years. This latest advance extends the maximum span of a quantum link, so that it is possible to connect cities across countries and continents, without using trusted inter­mediate nodes. Implemented along with Satellite QKD, it will allow us to build a global network for quantum secured communications.” Taro Shimada, Corporate Senior Vice President and Chief Digital Officer of Toshiba Corporation reflects, “With this success in Quantum Technology, Toshiba is willing to further expand its quantum business with rapid speed. Our vision is a platform for quantum information technology services, which will not only enable secure communi­cation on a global scale, but also transformational techno­logies such as cloud-based quantum computing and distributed quantum sensing.”

The work was partially funded by the EU through the H2020 project, OpenQKD. The team is now engineering the proposed solutions to simplify their future adoption and deployment. This latest development follows the announce­ment last year that BT and Toshiba had installed the UK's first industrial quantum-secure network. Transmitting data between the National Composites Centre (NCC) and the Centre for Modelling & Simulation (CFMS), Toshiba's multiplexing compa­tibility allows the data and the quantum keys to be transmitted on the same fibre, eliminating the need for costly dedicated infra­structure for key distribution. The combined arrival of multiplexed QKD using existing infra­structure for shorter distances, alongside Twin Field QKD for longer distances, paves the way for a commer­cially viable global quantum-secure network.

QKD allows users to securely exchange confi­dential information over an untrusted communi­cation channel. It does that by distributing to the intended users a common secret key that can be used to encrypt, and thus protect, the information exchanged over the communication channel. The security of the secret key rests upon the funda­mental properties of individual quantum systems which are encoded and transmitted for the key generation. In the event that these photons are intercepted by an undesignated user, quantum physics guarantees that the intended users can perceive the eavesdropping, and consequently protect the communi­cation.

Unlike other existing security solutions, the security of quantum cryptography derives directly from the laws of physics we use to describe the world around us, and for this reason, it is secure against any future advances in mathematics and computing including the advent of quantum computers. In light of this, QKD is expected to become an essential tool for protecting operation-critical communi­cations for businesses and governments. (Source: Toshiba)

Reference: M. Pittaluga et al.: 600-km repeater-like quantum communications with dual-band stabilization, Nat. Phot. 15, 530 (2021); DOI: 10.1038/s41566-021-00811-0

Link: Quantum Information Group, Cambridge Research Lab, Toshiba Europe Ltd, Cambridge, UK