Quantum key distribution enables unhackable communication,
but it is limited at the moment by low key throughput.
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
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.
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
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
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
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