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arXiv:2310.16589 · npj Quantum Information

Polarization-entangled photons from a whispering gallery resonator

Sheng-Hsuan Huang, Thomas Dirmeier, Golnoush Shafiee, Kaisa Laiho, Dmitry Strekalov, Gerd Leuchs, Christoph Marquardt

Published Sep 13, 2024• 5 citations
arXivHTMLPDF

Abstract

Abstract Crystalline whispering gallery mode resonators (WGMRs) have been shown to facilitate versatile sources of quantum states that can efficiently interact with atomic systems. These features make WGMRs an efficient platform for quantum information processing. Here, we experimentally show that it is possible to generate polarization entanglement from WGMRs by using an interferometric scheme. Our scheme gives us the flexibility to control the phase of the generated entangled state by changing the relative phase of the interferometer. The S value of Clauser–Horne–Shimony–Holt’s inequality in the system is 2.45 ± 0.07, which violates the inequality by more than six standard deviations.

DOI
10.1038/s41534-024-00876-z

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Abstract Crystalline whispering gallery mode resonators (WGMRs) have been shown to facilitate versatile sources of quantum states that can efficiently interact with atomic systems. These features make WGMRs an efficient platform for quantum information processing. Here, we experimentally show that it is possible to generate polarization entanglement from WGMRs by using an interferometric scheme. Our scheme gives us the flexibility to control the phase of the generated entangled state by changing the relative phase of the interferometer. The S value of Clauser–Horne–Shimony–Holt’s inequality in the system is 2.45 ± 0.07, which violates the inequality by more than six standard deviations.
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.
Quantum information is the next frontier in communications. to realize quantum communications, the quantum mechanical properties of today's best communication medium, light, must be harnessed in a scalable and efficient manner. Whispering-gallery mode resonators (WGMRs), a type of optical cavity, have advantages over traditional designs that can enhance processes used in the generation of nonclassical (quantum) states of light. In particular they reduce the power threshold for intensity-dependent nonlinear phenomena. One such process, second harmonic generation, can reduce the shot noise of light below the standard quantum limit. This dissertation explores the theoretical analysis and experimental tests of noise reduction through second harmonic generation in a crystalline whispering-gallery mode resonator. We also observe the generation of another nonlinear optical process, hyper-Raman scattering, at modest optical powers inside a crystalline WGMR. The change in optical properties of vanadium dioxide due to an optically-induced phase transition is also studied as a potential Q-switching material in a WGMR-type cavity.
Whispering Gallery Mode Resonators (WGMRs) made ofχ2\chi^{2}χ2nonlinear dielectric materials have been shown to be a versatile source of quantum states [1]–[4]. Here, we investigate this type of source and demonstrate a Hong-Ou-Mandel (HOM) interference between heralded photon pairs that are produced in two spontaneous parametric downconversion (SPDC) processes within a single WGMR [5]. The WGMR is pumped by a continuous-wave laser with only 100 nW in total from the clockwise (CW) and counterclockwise (CCW) directions in two degen-erate counter-propagating modes. Hence, the heralded photon pairs from the two directions are indistinguishable in frequency. The indistinguishability of spatial and polarization modes is achieved by using standard optical elements. We measure the Glauber correlation function,gsi(2)(t)g_{si}^{(2)}(t)gsi(2)​(t), from both directions, and find that the two functions are almost identical. As a result, the photon pairs generated from both directions are highly indistinguishable in the temporal domain (See Fig. 1(a)). The signal photons emitted in both directions are combined on a beam splitter, while the idler photons are coupled directly into detectors. The HOM interference fringe is extracted by detecting four-fold coincidence counts. The highest measured HOM visibilities reach a value above 74% (See Fig. 1(b)). We also show that the visibilities change when the value ofgsi(2)(0)g_{si}^{(2)}(0)gsi(2)​(0)varies, and it agrees well with the theoretical prediction provided by Ref. [6]. (See Fig. 1(c)). Finally, we measure the Glauber correlation function from two different WGMRs (See Fig. 1(d)) showing that the two functions are almost identical. This indicates that it is possible to match the temporal mode of heralded photon pairs from different WGMRs. Together with the extremely low pump power requirement, WGMRs could be a potential platform for large-scale quantum applications.
Efficient generation of harmonics in nonlinear optical resonators, particularly in whispering-gallery-mode (WGM) resonators, has led to successes such as parametric oscillators, narrow-band single-photon sources, and versatile frequency converters. A subtle problem, though, has strongly limited efficiency and hindered precise out-coupling of the generated light. Implementing their own prior theory and a polarization-selective birefringent coupling prism, the authors demonstrate an improvement of more than an order of magnitude in the amount of out-coupled light. The results are particularly interesting for applications in quantum optics relying on WGMs.
We present a theoretical investigation of a whispering gallery mode (WGM) resonator coupled to a Mach-Zehnder interferometer (MZI) and show a bimodal coincidence transmission spectrum when the input state is an indistinguishable photon pair. This is due to the doubled WGM phase shift experienced by the path-entangled state in the interferometer. Further, we model the noise in a WGM resonance shift measurement comparing photon pairs with a coherent state. At least a four-fold improvement in the signal-to-noise ratio (SNR) is possible, with clear implications for quantum-enhanced WGM sensing.