Research Topics | Charles Lim NUS

Quantum Cryptography Theory & Mathematical Toolboxes for Security

Quantum cryptography is the art of using quantum mechanical effects, like the celebrated quantum no-cloning principle, to achieve information security over an untrusted environment. For a non-expert overview of quantum cryptography, please see our NUS news article.

In this area, our group is primarily interested in the security of quantum key distribution (QKD) and quantum random number generation (QRNG). Our expertise is in the development of security proof techniques for practical quantum cryptosystems. In the recent years we have also started working on device-independent quantum cryptography.

In order to have a universal toolbox for computing secure key rates, we take an numerical approach, using mathematical methods like semi-definite programming and entropy inequalities to derive tight and reliable security bounds. You can find some of our latest results below:

Device-independent quantum key distribution with random key basis (published in Nature Communications and presented at both QCRYPT 2020 and AQIS 2020); video from QCRYPT.
Computing secure key rates for quantum key distribution with untrusted devices (accepted as a contributed talk at both QCRYPT 2019 and QIP 2020); video from QIP.
Versatile security analysis of measurement-device-independent quantum key distribution (accepted as a contributed talk at QCRYPT 2019).
Characterising the correlations of prepare-and-measure quantum networks (accepted at QCRYPT 2018) video from QCRYPT.

Beside these topics, we are also constantly thinking of new protocols for quantum cryptography. Some recent results along the lines of two-way classical communication and coherent-state quantum key distribution can be found here:

For more information of our group research, please feel free to contact Charles or any of the group members.

Applied Quantum Cryptography & Quantum Integrated Photonics

A key feature of our research is the tight interplay between the theory and practice of quantum information. In our group, theorists, experimentalists, and engineers work closely together to develop and engineer new quantum technologies and devices. Key areas of focus include quantum key distribution systems, randomness generation devices, and sensing.

Security countermeasures

For example, in our latest experimental paper (see QCRYPT video), we proposed the idea of optical power limiter, a passive optical device that limits the amount of light energy entering a quantum cryptosystem. Such a device is essential for the practical security of QKD as it deters side-channel attacks based on injection of bright light (e.g., blinding and Trojan-horse attacks).

Applications beyond QKD

Another example is our ongoing three-node MDI-QKD network experiment, which will be used to implement symmetric private information retrieval (SPIR), a cryptographic protocol that enables both database privacy and user privacy (see our SPIR+QKD proposal; ETSI workshop video presentation).

Quantum photonics chipsets

One of our strongest interests lies in integrated photonics for quantum sciences. In this direction, we explore how to make the best of today Silicon Photonics Foundry's capabilities, studying how quantum effects can be achieved with standard devices (PDKs). Our key targets are self-testing quantum random number generators (QRNG) chipsets and QKD chipsets.