Researchers at QuTech, Delft University of Technology and the Kavli Institute of Nanoscience Delft have demonstrated coherent cooperativity exceeding unity for tin-vacancy (SnV) centers embedded within diamond photonic crystal cavities, a crucial step beyond previous observations of only Purcell enhancement. The team achieved this by fabricating free-standing photonic crystal cavities using a precise technique, enabling a level of control over coherent optical coupling previously difficult to achieve for quantum protocols. Detailed characterization of 327 cavities revealed an average quality factor exceeding 11,000, with single SnVs modulating cavity transmission with an extinction contrast reaching 98.8(4)% on resonance. These results, the researchers say, establish these nanophotonic devices as highly efficient, coherent interfaces, providing a fundamental hardware building block for the realization of sophisticated and scalable long-distance quantum networks.
Tin-Vacancy Centers Embedded in Photonic Crystal Cavities
A remarkable 98.8(4)% extinction contrast was observed. This crucial distinction signifies a capacity for robust entanglement generation, essential for building practical quantum networks. The team fabricated these specialized structures using a precise method for creating free-standing photonic crystal cavities. This fabrication process was critical in enabling the observed level of performance, allowing for strong light confinement and interaction with the embedded SnV centers. Quality factors of 11,000, indicating minimal energy loss within the cavities and maximizing their potential for enhancing the SnV’s optical properties, were measured. Two cavities were examined in greater detail, exhibiting quality factors reaching Q = 25,400 and Purcell-reduced lifetimes corresponding to cooperativities up to C = 20.3(11). The ability to strongly modulate cavity transmission with the SnV centers is a key indicator of the strong coupling achieved.
Researchers observed that the single SnVs were able to alter the light passing through the cavities, with the highest measured extinction contrast reaching 98.8(4)%. This precise control over light transmission is vital for creating the single photons needed to encode and transmit quantum information. SnV linewidth measurements confirmed the presence of above-unity coherent cooperativities in both devices, with a peak value of Ccoh = 8.3(12). The researchers state that these results open the door to using cavity-coupled SnV centers as efficient, coherent light-matter interfaces for future quantum networks. This advancement builds upon previous work demonstrating Purcell enhancement, but represents a substantial step forward. As Nina Codreanu, Tim Turan, and Daniel Bedialauneta Rodriguez, lead authors on the study, explain, achieving coherent cooperativity is essential for executing high-fidelity quantum entanglement generation protocols.
Fabrication of High Quality Factor PCC Structures
The pursuit of robust quantum networks hinges on efficiently interfacing stationary qubits with flying photons; diamond tin-vacancy (SnV) centers are promising candidates, but realizing their full potential demands increasingly sophisticated nanophotonic structures. While initial experiments demonstrated Purcell enhancement of SnV centers within photonic crystal cavities (PCCs), achieving coherent coupling, essential for complex quantum protocols, proved a significant hurdle. Central to this advance was the fabrication of free-standing PCCs utilizing a precise technique. This method allowed for the creation of cavities exhibiting exceptionally high quality factors, exceeding Q = 11,000 on average across 327 characterized cavities. The team focused on minimizing energy loss within the cavities, achieving values up to Q = 25,400 for individual cavities, rather than simply aiming for high Q.
This level of control is critical, as it directly impacts the efficiency of light-matter interaction and the fidelity of quantum operations. The researchers report that the interaction speed was enhanced. Beyond simply enhancing the interaction, the team demonstrated coherent cooperativity, a crucial distinction from mere Purcell enhancement, with values up to C = 20.3(11). The single SnVs exhibited a remarkable ability to modulate cavity transmission, achieving an extinction contrast up to 98.8(4)% on resonance. This precise control over light transmission, previously difficult to achieve, is a key requirement for building efficient quantum interfaces. Measurements of SnV linewidths confirmed the achievement of above-unity coherent cooperativities, with the highest recorded value reaching Ccoh = 8.3(12).
Measured Cooperativity and Extinction Contrast of SnVs
Their recent work focuses on achieving robust light-matter interaction, a critical step toward building scalable quantum systems. Unlike previous demonstrations limited to Purcell enhancement, simply increasing the rate of light emission, the team has demonstrated modulation of SnV centers, signifying a leap in control over the quantum properties of these emitters. This enhanced control stems from a meticulous fabrication process. The researchers employed a precise technique to create free-standing photonic crystal cavities, structures designed to trap and amplify light interacting with the SnV centers. Quality factors exceeding Q = 11,000 were achieved. This level of precision is essential for maintaining the delicate quantum states needed for information transfer. Beyond cooperativity, the team also achieved an impressive 98.8(4)% extinction contrast in cavity transmission by modulating the SnV centers on resonance. This high contrast demonstrates a remarkable degree of control over the light passing through the cavity, a crucial requirement for creating well-defined quantum signals.
The significance of coherent cooperativity lies in its ability to facilitate diverse entanglement generation protocols. The team believes this work is a building block for long-distance quantum networks. The ability to reliably generate and control entanglement is paramount for secure communication and distributed quantum computing, and this work represents a substantial step toward realizing those goals.
Demonstration of Above-Unity Coherent Coupling
This achievement moves beyond simply enhancing light-matter interaction, a phenomenon known as Purcell enhancement, to achieving coherent control, a crucial step for building robust quantum communication channels. This precise method was essential for creating structures capable of supporting the strong light-matter interactions observed. This level of performance is critical for minimizing decoherence, a major obstacle in quantum information processing. Crucially, the researchers observed that the single SnV centers strongly modulate the cavity transmission with an extinction contrast up to 98.8(4)% on resonance. This signifies that the rate of coherent interaction between the SnV center and the cavity photons exceeds the rate at which the quantum information is lost due to spontaneous emission and dephasing. The team reports demonstrating a significant advancement in the field, paving the way for more efficient and reliable quantum communication systems.
