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“Toolboxes” Designed for Quantum Cyber Security

Efficient “toolboxes” containing theoretical tools and protocols for quantifying the security of high-speed quantum communication have been created by a quantum information scientist from the National University of Singapore (NUS). Assistant Professor Charles Lim is part of a global team of experimental and theoretical scientists from Duke University, Ohio State University and Oak Ridge National Laboratory that has lately accomplished a great breakthrough in high-rate quantum secure communication.

Quantum computers are robust machines that can break present day’s most widespread encryption technologies within minutes. Remarkably, latest progress in quantum computing has shown that this threat is real and not merely theoretical, and large-scale quantum computers are nowadays becoming a reality. If successfully executed, these computers could be broken into to decrypt any organization’s confidential communication, trade secrets, and sensitive data retrospectively or remotely.

Quantum key distribution (QKD) is an upcoming quantum technology that enables the formation of secret keys between two or more parties in an untrusted network. Notably, unlike conventional encryption methods, the security of QKD is mathematically resilient — it is based exclusively on the proven laws of nature. As such, messages and data encrypted using QKD keys are fully secure against any attacks on the communication channel. For this reason, QKD is extensively viewed as the solution that will totally resolve the security risks posed by future quantum computers.

Today, QKD technology is moderately mature and there are currently several companies selling QKD systems. Recently, researchers from China have managed to spread QKD keys to two ground stations situated 1200 km apart. However, regardless of these key developments and advances, practical QKD systems still face some intrinsic limitations. One big limitation is the secret key throughput — present QKD systems are only able to convey 10,000 to 100,000 secret bits per second. This limitation is mostly because of the choice of quantum information basis: a number of QKD systems are still using low-dimensional information basis, such as the polarization basis, to encode quantum data.

Poor secret key rates arising from current QKD implementations have 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.

Asst. Prof 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.

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

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

Time-bin encoding

Encoding quantum information in the time and phase bases is a potential approach that is very robust against usual optical channel disturbances and nevertheless scalable in the information dimension. In this method, 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. This encoding method, in principle, could allow one to pack randomly many bits into a single photon and produce very high secret key rates for QKD. However, executing such high-dimensional systems is technically hard and tools for quantifying the practical security of high-dimensional QKD are inadequate.

To surpass these issues for their QKD system, the researchers used an innovative combination of security proof methods formulated by Asst. Prof Lim and an interferometry method by Professor Daniel Gauthier’s research group from Duke University and Ohio State University. Asst. Prof Lim participated in creating the protocol design of the QKD system as well as establishing the security of the protocol using quantum information theory.

Our newly developed 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. This will help pave the way for high-dimensional quantum information processing.

Asst. Prof 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.

Next steps

Going forward, the team will be looking at ways to produce more bits in one photon using time-bin encoding. This will help progress the development of commercially feasible QKD systems for ultra-high rate quantum secure communication.

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