When the electromagnetic environment around matter changes, the radiation characteristics and energy level structure of matter will be modulated. Placing an emitter in an optical cavity, if the rate of energy exchange between the emitter and the cavity is greater than the system loss rate, then the emitter and the cavity is in the strong coupling regime. Specifically, strong coupling with a single emitter (SE) is an important means to achieve quantum entanglement, storage, and processing of quantum information. However, due to the small physical size of a SE compared to its transition wavelength, the interaction between the emitter and photons is sufficient, making it difficult to achieve strong coupling. Existing methods mainly rely on improving the quality factor or optical density of states of the cavity. A high quality factor allows each photon to bounce between the cavity mirrors for many times before being lost, thus improving the coupling strength. However, it requires a high vacuum and low temperature environment to suppress the phonon- and thermal reservoir-induced dissipation and frequency diffusion. By increasing the optical density of states, each photon is localized in a very small space, thereby increasing the field intensity and improving the coupling strength. But this strategy normally relies on plasmonic systems and are intrinsically associated with high-loss. The small cavity volume also sets challenges for precise positioning of SEs.
In a new paper (https://doi.org/10.1038/s41377-024-01508-z) published in Light Science & Applications, a team of scientists, led by Professor Hong-Bo Sun from State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, China, and co-workers have proposed a new protocol to improve the SE-cavity coupling strength by using waveguide-assisted energy quantum transfer (WEQT) to extend the interaction cross-section. Multiple ancillary emitters bridged by an optical waveguide are introduced to provide an additional coupling channel. The energy quantum of the cavity is collected through its coupling to these ancillae and delivered to the target emitter, permitting individual addressing of each emitter.
“Since the coupling strength between a SE and the cavity is very small, the first idea that comes to mind is to use multiple emitters. Even if the coupling strength between each emitter and the optical cavity is small, the overall coupling strength should not be underestimated. Although collective effects are terrifying, quantum technology often requires manipulating individual emitters. The SE-cavity coupling strength does not change when focusing on an individual emitter, and several quantum effects, such as the Rabi oscillation and photon blockade of a SE cannot be achieved in collective coupling.”
“To exploit the collective effect, N ancillary emitters, say B, are placed in the same optical cavity with a SE, say A. Now all emitters are coupled with the cavity. In order to enhance the coupling strength between A and the cavity, all emitters are coupled to a one-dimensional waveguide. Under specific arrangements and coupling strength settings, the energy coupled from the cavity to B will be transferred to A and then coupled to the cavity. Therefore, the presence of B and waveguide provides an additional and indirect coupling channel between A and the optical cavity, which is the connotation of "waveguide assisted energy quantum transfer". In this system, the status of A and B is not equal. The introduction of B is just to improve the interaction cross-section between A and the optical cavity.”
“It is noted that the parasitic loss cannot be ignored in this coupling channel, i.e., if an energy quantum transferred from the cavity to B is dissipated before it is coupled to A or back to the cavity, the dissipation rate of the cavity will be increased. Similar consideration applies to the dissipation rate of A. We designed a photon gate by controlling the dissipation.” they added.
“The WEQT effect provides a new protocol to collect and deliver energy quantum between different emitters. The tunable coupling releases the rigorous design and fabrication of extreme cavities and provide new degrees of freedom for on-chip quantum manipulation. It is anticipated that the concept of ancilla-assisted photonic quantum devices can be scaled up for application of various integrated photonics and quantum computing systems.” the scientists forecast.
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References
DOI
10.1038/s41377-024-01508-z
Original Source URL
https://doi.org/10.1038/s41377-024-01508-z
Funding information
This work was supported by the National Key Research and Development Program of China under Grant No. 2020YFA0715000; the National Natural Science Foundation of China under Grant Nos. grant 62075111 and 61960206003; the Tsinghua University Initiative Scientific Research Program; and Tsinghua-Foshan Innovation Special Fund under Grant No. 2021THFS0102.
About Light: Science & Applications
The Light: Science & Applications will primarily publish new research results in cutting-edge and emerging topics in optics and photonics, as well as covering traditional topics in optical engineering. The journal will publish original articles and reviews that are of high quality, high interest and far-reaching consequence.