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Our research is focused on generating single and entangled photons and manipulating the photon-matter interaction. Single photons are wavepackets of light energy quanta and no-cloning property. Entangled photons, on the other hand, are photon pairs with “spooky action at a distance” quoted by Einstein. With unique quantum properties, they are both essential to realizing quantum computation, quantum communication, and quantum teleportation, and to understanding quantum mechanics.

We seek to develop applications with single and entangled photons and to study the analogue between the photonic system and other physical systems by manipulating their quantum property. We use optical methods and lasers in experiments and build theoretical models to explain our observations. Our material systems include solid-state, semiconductor, or cold atom systems. This research area leads to applications as well as fundamental interests.

Research Highlights:

Field Test of Quantum Key Distribution

Using an optical fiber link between National Tsing Hua University and National Chiao Tung University, we recently demonstrated Taiwan's first outdoor quantum key distribution (QKD). Secure keys are generated by sending single photons encoded with bits 0 and 1. Encrypted communication with these keys promise unconditional security based on the laws of physics and is also impossible for eavesdroppers to keep a transcript of communication.

Key members of our QKD team: Yung-Cheng Kao (高永成), Sheng-Hsuan Huang (黃聖軒), Chih-Hsiang Wu (吳至翔), Chin-Hsuan Chang (張進宣)


Revival of Quantum Interference, Entanglement, and Nonlocality

Quantum interference and entanglement, apart from the fundamental interest, are at the heart of quantum computing and quantum communication. However, these quantum properties are easily degraded by the imperfections or limitations of the experiments. By manipulating the quantum wavepacket, we demonstrate the revival of quantum interference, entanglement, and nonlocality that would otherwise be destroyed by the distinguishability of the photons. Our study shows that these quantum features can achieve full recovery if the wavepacket manipulation is properly designed.

Phys. Rev. Lett. 123, 143601 (2019)


Shaping and Purifying Single Photons from Semiconductor Nanocrystals

Colloidal quantum dots (or semiconductor nanocrystals) are promising single-photon emitters at room temperature. However, their single-photon purity is poor due to the spectrally broad bi-exciton emission. We demonstrate single-photon purification by manipulating the temporal envelope of the single photons. The purified single photons have a purity comparable to their cryogenic-temperature counterparts. Moreover, the single-photon purity does not vary with the pumping power or between different quantum dots.

Phys. Rev. Lett. 119, 143601 (2017)


Light-Matter Interaction at Single-Photon Level

Efficient light-matter interaction at the single-photon level is essential to quantum computation and quantum communication. Such interaction requires single photons of subnatural linewidth and high spectral brightness. We demonstrate a subnatural-linewidth single-photon source with the highest spectral brightness reported to date. The interaction between the single photons and atoms is also demonstrated by the controlled absorption of the single photons in an atomic vapor.

Phys. Rev. A 96, 023811 (2017)