NUS scientist part of international team which has achieved breakthrough in high-rate quantum secure communication

A quantum information scientist from the National University of Singapore (NUS) is part of an international team of experimental and theoretical scientists, that has recently achieved a significant breakthrough in high-rate quantum secure communication.

Assistant Professor Charles Lim, who is from the Department of Electrical and Computer Engineering at NUS Faculty of Engineering as well as Centre for Quantum Technologies at NUS, is working with researchers from Duke University, Ohio State University and Oak Ridge National Laboratory.

Quantum computers can break today’s most prevalent encryption technologies in minutes. Recent progress in quantum computing means that large-scale quantum computers are now becoming a reality and this threat is no longer theoretical. If successfully implemented, these computers could be exploited to decrypt any organisation’s trade secrets, confidential communication, and sensitive data retrospectively or remotely. 

However, Quantum key distribution (QKD) presents a solution to the threats posed to cybersecurity by quantum computing.

QKD utilises quantum entanglement to enable the establishment of secret keys between two or more parties in an untrusted network, which can then be used to encrypt and decrypt messages, ensuring that they can be deciphered only by authorised individuals or entities. Moreover, if we perform a measurement on one of two entangled objects, the entanglement correlation is broken. If this measurement is done by an eavesdropper then the end-users detect no entanglement, and the eavesdropper is revealed.

(Quantum entanglement is a phenomenon when pairs or groups of particles cannot be described independently of the others. The physical properties of the particles are correlated and the correlation persists regardless of the distance separating them.)

Unlike conventional encryption techniques, the security of QKD is mathematically unbreakable. Messages and data encrypted using QKD keys are completely secure against any attacks on the communication channel.

The press release notes that QKD technology is relatively mature today and there are several companies selling QKD systems. Very recently, researchers from China successfully distributed QKD keys to two ground stations located 1200 kilometres apart.

However, practical QKD systems still face some inherent limitations. One major limitation is the secret key throughput — current QKD systems are only able to transmit 10,000 to 100,000 secret bits per second.

This limitation is largely because many QKD systems are still using low-dimensional information basis, such as the polarisation basis (which is binary, either vertical or horizontal), to encode quantum information. This has been a major bottleneck affecting the use of quantum secure communication on a wider scale. For practical applications, such systems need to be able to generate secret key rates in the order of megabits per second to meet today’s digital communication requirements.  

In the study, the research team developed a QKD system based on time and phase bases which allows for more secret bits to be packed into a single photon. Notably, the team had achieved two secret bits in a single photon, with a secret key rate of 26.2 megabits per second.

Encoding quantum information in the time and phase bases is highly robust against typical optical channel disturbances, while also being scalable in the information dimension. In this approach, secret bits are encoded in the arrival time of single photons, while the complementary phase states — for measuring information leakages — are encoded in the relative phases of the time states. In principle, this could allow one to pack arbitrarily many bits into a single photon and generate extremely high secret key rates for QKD.

However, implementing such high-dimensional systems is technically challenging and there are limited tools for quantifying the practical security of high-dimensional QKD. 

To overcome these problems for their QKD system, the researchers used a novel combination of security proof techniques developed by Asst Prof Lim and an interferometry technique by Professor Daniel Gauthier’s research group from Duke University and Ohio State University. Asst Prof Lim was involved in the protocol design of the QKD system, and also in proving the security of the protocol using quantum information theory.

These theoretical and experimental techniques have resolved some of the major challenges for high-dimensional QKD systems based on time-bin encoding, and can potentially be used for image and video encryption, as well as data transfer involving large encrypted databases.

The findings of the study were published online in scientific journal Science Advances on 24 November 2017.

Moving forward, the team will be exploring ways to generate more bits in a single photon using time-bin encoding. This will help advance the development of commercially viable QKD systems for ultra-high rate quantum secure communication.

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