Chromatic Photon Effect for Quantum Networks

Illustration of a quantum mechanical interference, the Hong-Ou-Mandel effect: A yellow and an orange photon hit a frequency mixer from the right and emerge always together in the same color, here two yellow photons. (Source: M. Kues & A. Khodadad Kashi)

It’s another step on the road to developing quantum information processing appli­cations. A key experiment succeeded in going beyond the previously defined limits for photon appli­cations. Anahita Khodadad Kashi and Michael Kues from the Institute of Photonics and the Cluster of Excellence PhoenixD at Leibniz Univer­sity Hannover have demons­trated a novel interference effect. The scientists have thus shown that new colour-coded photonic networks can be tapped, and the number of photons involved can be scaled.

“This discovery could enable new benchmarks in quantum communi­cation, compu­tational operations of quantum computers as well as quantum measure­ment techniques and is feasible with existing optical telecommunication infrastructure,” says Kues. The decisive experiment was success­fully performed in the newly established Quantum Photonics Labora­tory (QPL) of the Institute of Photonics and the Hannover Centre for Optical Technologies at Leibniz University Hannover. Anahita Khodadad Kashi succeeded in quantum-mechani­cally interfering inde­pendently generated pure photons with different colours, i.e. frequencies. Khodadad Kashi detected a Hong-Ou-Mandel effect.

Hong-Ou-Mandel inter­ference is a funda­mental effect of quantum optics that forms the basis for many quantum infor­mation processing appli­cations – from quantum computing to quantum metrology. The effect describes how two photons behave when they collide on a spatial beam splitter and explains the phenomenon of quantum mechanical inter­ference. The researchers have now realised a frequency beam­splitter using tele­communications components and demonstrate the Hong-Ou-Mandel effect for the first time between two inde­pendently generated photons in the frequency domain.

In contrast to other dimensions, such as the polari­zation or the photon’s position, the frequency is much less susceptible to inter­ference. “Our approach allows flexible confi­gurability and access to high-dimensional systems, which may lead to large-scale controllable quantum systems in the future,” says Kues. This two-photon inter­ference phenomenon can serve as a foundation for a quantum internet, non-classical communication and quantum computers. In other words, the results could be used for frequency-based quantum networks. Another notable feature of the new discovery is that this increase in perfor­mance could be used with existing infra­structure, i.e. standard fibre optic connections for connecting to the internet. The use of quantum techno­logies at home could thus theo­retically be made possible in the future.

“I was very pleased that our experiment was able to demons­trate the Hong-Ou-Mandel effect in the frequency domain,” says Khodadad Kashi. In the future, Kashi and Kues will continue their research on the topic of spectral Hong-Ou-Mandel interference. “I would like to extend the current experiment to exploit the demons­trated effect for quantum infor­mation processing,” says Khodadad Kashi. (Source: U. Hannover)

Reference: A. Khodadad. Kashi & M. Kues: Spectral Hong–Ou–Mandel Interference between Independently Generated Single Photons for Scalable Frequency‐Domain Quantum Processing, Laser & Phot. Rev., online 18 March 2021; DOI: 10.1002/lpor.202000464

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