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Two-Cavity System Generates High-Quality Single Photons Efficiently

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CAMBRIDGE, Mass., May 17, 2019 — A new dual-cavity design for a quantum emitter, from the Massachusetts Institute of Technology (MIT), emits more high-quality single photons for carrying quantum information at room temperature than existing methods. The two-cavity system could make the development of quantum computers more practical.

A dual cavity design for a quantum emitter, MIT.
MIT researchers have designed a new single-photon emitter that generates, at room temperature, more of the high-quality photons that could be useful for practical quantum computers, quantum communications, and other quantum devices. Courtesy of MIT.

While classical computers process and store information in bits of either 0s or 1s, quantum bits (qubits) can be 0 and 1 simultaneously. To create qubits, it is necessary to produce single photons with identical quantum properties, known as “indistinguishable” photons. To improve the indistinguishability of photons, emitters funnel light through an optical cavity where the photons bounce back and forth, a process that helps match their properties to the cavity. Generally, the longer photons stay in the cavity, the more they match.

In large cavities, quantum emitters generate photons spontaneously, resulting in only a small fraction of photons staying in the cavity, thus making the process of generating indistinguishable photons less efficient. Smaller cavities extract higher percentages of photons, but the photons are lower-quality.

To achieve a better solution to single-photon emission, the MIT team split one cavity into two and gave each cavity a designated task. A small cavity handles the efficient extraction of photons, while an attached large cavity stores the photons long enough to boost indistinguishability.

“What we found is that in this architecture, we can separate the roles of the two cavities: The first cavity merely focuses on collecting photons for high efficiency, while the second focuses on indistinguishability in a single channel,” said researcher Hyeongrak Choi. “One cavity playing both roles can’t meet both metrics, but two cavities achieve both simultaneously.”

The small cavity was attached to an emitter composed of an optical defect in a diamond. Light produced by the defect collected in the small cavity, and because of the cavity’s light-focusing structure, photons were extracted at a fast rate. The small cavity then channeled the photons into a second, larger cavity, where they reached a high level of indistinguishability. The photons exited through a partial mirror formed by holes connecting the larger cavity to a waveguide.

The researchers’ coupled cavity demonstrated the ability to generate photons with around 95% indistinguishability, compared to a single cavity generating photons with around 80% indistinguishability. The coupled cavity system also reduced the quality factor (Q-factor) requirement to levels achievable with present-day technology, the researchers said. The coupled cavity demonstrated about 3× higher efficiency than a single cavity, even when its Q-factor was only about 100th the Q-factor of the single-cavity system. The results were obtained using numerical and closed-form analytical models with strong emitter dephasing.

The coupled cavities can be tuned to optimize for efficiency versus indistinguishability, and to consider any constraints on the Q-factor, depending on the application. That’s important, Choi said, because today’s emitters that operate at room temperature can vary greatly in quality and properties.

Next, the researchers will test the ultimate theoretical limit of multiple cavities. One cavity would still handle the initial extraction efficiently, but to achieve optimal indistinguishability it would be linked to multiple cavities for photons of different sizes. But there will most likely be a limit, Choi said. “With two cavities, there is just one connection, so it can be efficient. But if there are multiple cavities, the multiple connections could make it inefficient. We’re now studying the fundamental limit for cavities for use in quantum computing.”

The research was published in Physical Review Letters (
May 2019
Smallest amount into which the energy of a wave can be divided. The quantum is proportional to the frequency of the wave. See photon.
quantum optics
The area of optics in which quantum theory is used to describe light in discrete units or "quanta" of energy known as photons. First observed by Albert Einstein's photoelectric effect, this particle description of light is the foundation for describing the transfer of energy (i.e. absorption and emission) in light matter interaction.
Research & TechnologyeducationAmericasMITquantumquantum emitterssingle photonsquantum opticsoptical cavityCommunicationsquantum computingindistinguishable photons

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