What is it about?

The biphoton is a pair of single photons. One photon of a pair is utilized as a heralding photon. The second photon of the same pair is then employed in a quantum process or operation started by the heralding photon. Thus, the second photon is called heralded single photons, and qubits made of heralded single photons are convenient for the applications in quantum technology. We report a biphoton source with a temporal width as long as 13 micro-seconds and a spectral linewidth as narrow as 50 kHz. The temporal width is the longest, and the spectral linewidth is the narrowest up to date. This is also the first biphoton result that obtains a linewidth below 100 kHz, reaching a new milestone.

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Why is it important?

A quantum operation employing heralded single photons of a narrower linewidth can achieve a better efficiency. For example, a narrower linewidth of input photons can result in a higher storage efficiency for quantum memory, a greater success rate for quantum phase gates, and a larger conversion efficiency for quantum frequency converters. As another example, while the entanglement swapping is a key process in the quantum repeater protocol for long-distance quantum communication, the deterministic entanglement swapping with the fidelity of 79% has been demonstrated with ion qubits, which operated at a narrow-linewidth transition.


The effect of electromagnetically induced transparency (EIT) has recently been observed in superconducting qubits or artificial atoms driven by narrow-linewidth microwaves. One can foresee that narrow-linewidth photonic qubits, which are converted to and from microwave coherences based on the EIT effect, will be employed in a quantum network that links superconducting-qubit-based quantum computers.

National Tsing Hua University

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This page is a summary of: Temporally ultralong biphotons with a linewidth of 50 kHz, APL Photonics, December 2022, American Institute of Physics, DOI: 10.1063/5.0102393.
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