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arXiv:1008.0594 · Physical Review Letters

Quantum Light from a Whispering-Gallery-Mode Disk Resonator

Josef Urban Fürst, Dmitry Strekalov, Dominique Elser, Andrea Aiello, Ulrik L. Andersen, Christoph Marquardt, Gerd Leuchs

Published Mar 15, 2011• 172 citations

Abstract

Optical parametric down-conversion has proven to be a valuable source of nonclassical light. The process is inherently able to produce twin-beam correlations along with individual intensity squeezing of either parametric beam, when pumped far above threshold. Here, we present for the first time the direct observation of intensity squeezing of -1.2 dB of each of the individual parametric beams in parametric down-conversion by use of a high quality whispering-gallery-mode disk resonator. In addition, we observed twin-beam quantum correlations of -2.7 dB with this cavity. Such resonators feature strong optical confinement and offer tunable coupling to an external optical field. This work exemplifies the potential of crystalline whispering-gallery-mode resonators for the generation of quantum light. The simplicity of this device makes the application of quantum light in various fields highly feasible.

DOI: 10.1103/physrevlett.106.113901

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Optical parametric down-conversion has proven to be a valuable source of nonclassical light. The process is inherently able to produce twin-beam correlations along with individual intensity squeezing of either parametric beam, when pumped far above threshold. Here, we present for the first time the direct observation of intensity squeezing of -1.2 dB of each of the individual parametric beams in parametric down-conversion by use of a high quality whispering-gallery-mode disk resonator. In addition, we observed twin-beam quantum correlations of -2.7 dB with this cavity. Such resonators feature strong optical confinement and offer tunable coupling to an external optical field. This work exemplifies the potential of crystalline whispering-gallery-mode resonators for the generation of quantum light. The simplicity of this device makes the application of quantum light in various fields highly feasible.

Squeezed vacuum states enable optical measurements below the quantum limit and hence are a valuable resource for applications in quantum metrology and also quantum communication. However, most available sources require high pump powers in the milliwatt range and large setups, which hinders real world applications. Furthermore, degenerate operation of such systems presents a challenge. Here, we use a compact crystalline whispering gallery mode resonator made of lithium niobate as a degenerate parametric oscillator. We demonstrate about 1.4 dB noise reduction below the shot noise level for only 300

μ\text{W}

of pump power in degenerate single mode operation. Furthermore, we report a record pump threshold as low as 1.35

μ\text{W}

. Our results show that the whispering gallery based approach presents a promising platform for a compact and efficient source for nonclassical light.

Single-photon sources play a crucial role in quantum information processing. Among the various methods for generating single photons, spontaneous parametric down-conversion (SPDC) is one of the most widely used. Numerous studies have demonstrated the implementation of SPDC sources across different platforms. Among these, crystalline whispering-gallery-mode resonators (WGMRs) represent an intriguing platform for realizing SPDC-based sources. WGMR-based SPDC sources have been shown to be capable of efficiently interacting with alkali atoms due to the narrow and tunable emission bandwidths. Moreover, they operate with relatively low pump power, which is advantageous for large-scale integration and power-constrained applications. In this work, we further demonstrate the versatility of WGMR-based SPDC sources through three key experiments. First, we generate polarization-entangled photon pairs using an interferometric scheme, achieving a Clauser–Horne–Shimony–Holt (CHSH) inequality parameter of 𝑆 =2.54 ±0.06, violating the classical bound by over eight standard deviations. Second, we observe Hong–Ou–Mandel interference with visibilities of 74 ±5% and 76 ±4% for photons from one and two independent WGMRs, respectively, using an in-coupled pump power as low as 50 nW. Finally, we demonstrate photon--ion interaction between a WGMR-generated photon and a ¹⁴⁷Yb⁺ ion with a success probability of 8%. These results highlight the potential of WGMR-based SPDC sources for scalable quantum networks and hybrid quantum systems.