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arXiv:2412.15760 · APL Photonics

Indistinguishable MHz-narrow heralded photon pairs from a whispering gallery resonator

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

Published May 1, 2025• 2 citations
arXivHTMLPDF

Abstract

The Hong–Ou–Mandel interference plays a vital role in many quantum optical applications where indistinguishability of two photons is important. Such photon pairs are commonly generated as the signal and idler in the polarization-degenerate spontaneous parametric downconversion (SPDC). To scale this approach to a larger number of photons, we demonstrate how two independent signal photons radiated into different spatial modes can be rendered conditionally indistinguishable by a heralding measurement performed on their respective idlers. We use the SPDC in a whispering gallery resonator, which is already proven to be versatile sources of quantum states. Its extreme conversion efficiency allowed us to perform our measurements with only 50 nW of in-coupled pump power in each propagation direction. The Hong–Ou–Mandel interference of two counterpropagating signal photons manifested itself in the fourfold coincidence rate, where the detection of two idler photons heralds a pair of signal photons with a desired temporal overlap. We achieved the Hong–Ou–Mandel dip contrast of 74% ± 5%. Importantly, the optical bandwidth of all involved photons is of the order of a MHz and is continuously tunable. This, on the one hand, makes it possible to achieve the necessary temporal measurement resolution with standard electronics and, on the other hand, creates a quantum state source compatible with other candidates for qubit implementation, such as optical transitions in solid-state or vaporous systems. We also discuss the possibility of generating photon pairs with similar temporal modes from two different whispering gallery resonators.

DOI
10.1063/5.0263312

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The Hong–Ou–Mandel interference plays a vital role in many quantum optical applications where indistinguishability of two photons is important. Such photon pairs are commonly generated as the signal and idler in the polarization-degenerate spontaneous parametric downconversion (SPDC). To scale this approach to a larger number of photons, we demonstrate how two independent signal photons radiated into different spatial modes can be rendered conditionally indistinguishable by a heralding measurement performed on their respective idlers. We use the SPDC in a whispering gallery resonator, which is already proven to be versatile sources of quantum states. Its extreme conversion efficiency allowed us to perform our measurements with only 50 nW of in-coupled pump power in each propagation direction. The Hong–Ou–Mandel interference of two counterpropagating signal photons manifested itself in the fourfold coincidence rate, where the detection of two idler photons heralds a pair of signal photons with a desired temporal overlap. We achieved the Hong–Ou–Mandel dip contrast of 74% ± 5%. Importantly, the optical bandwidth of all involved photons is of the order of a MHz and is continuously tunable. This, on the one hand, makes it possible to achieve the necessary temporal measurement resolution with standard electronics and, on the other hand, creates a quantum state source compatible with other candidates for qubit implementation, such as optical transitions in solid-state or vaporous systems. We also discuss the possibility of generating photon pairs with similar temporal modes from two different whispering gallery resonators.
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.
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.
Hong–Ou–Mandel (HOM) interferometry with entangled photons exhibits distinctive quantum features. By introducing frequency entanglement (discrete-color entangled states) into HOM interference, the characteristic HOM dip is modulated by sinusoidal fringes, which significantly enhances the sensitivity of HOM sensors. The frequency-entangled photon sources demonstrated to date rely on non-resonant parametric down-conversion (PDC), which limits the photon coherence length and, consequently, restricts the sensing dynamic range to the millimeter scale. In this work, we demonstrate narrowband frequency-entangled photon source based on resonant PDC in a crystalline whispering gallery mode resonator. The MHz-level spectral bandwidth of photons enables a meter-scale dynamic range. With highly nondegenerate frequency-entangled photon pairs featuring a 96 THz frequency detuning, we observe high-contrast quantum beating with sub-picosecond resolution in the HOM experiment. Our WGMR-based frequency-entangled photon source has potential applications in quantum metrology and quantum information processing.
Hong-Ou-Mandel (HOM) interferometry with entangled photons exhibits distinctive quantum features. By introducing frequency entanglement (discrete-color entangled states) into HOM interference, the characteristic HOM dip is modulated by sinusoidal fringes, which significantly enhance the sensitivity of HOM sensors. The frequency-entangled photon sources demonstrated to date rely on non-resonant parametric down-conversion (PDC), which limits the photon coherence length and, consequently, restricts the sensing dynamic range to the sub-millimeter scale. In this work, we demonstrate narrowband frequency-entangled photon source based on resonant PDC in a crystalline whispering gallery mode resonator. The MHz-level spectral bandwidth of photons enables a meter-scale dynamic range. With highly nondegenerate frequency-entangled photon pairs featuring a 96 THz frequency detuning, we observe high-contrast quantum beating with sub-picosecond resolution in the HOM experiment. Our WGMR-based frequency-entangled photon source has potential applications in quantum metrology and quantum information processing.
We implement an ultrafast pulsed type-II parametric down conversion source in a periodically poled KTP waveguide at telecommunication wavelengths with almost identical properties between signal and idler. As such, our source resembles closely a pure, genuine single mode photon pair source with indistinguishable modes. We measure the joint spectral intensity distribution and second order correlation functions of the marginal beams and find with both methods very low effective mode numbers corresponding to a Schmidt number below 1.16. We further demonstrate the indistinguishability as well as the purity of signal and idler photons by Hong-Ou-Mandel interferences between signal and idler and between signal/idler and a coherent field, respectively. Without using narrowband spectral filtering, we achieve a visibility for the interference between signal and idler of 94.8% and determine a purity of more than 80% for the heralded single photon states. Moreover, we measure raw heralding efficiencies of 20.5% and 15.5% for the signal and idler beams corresponding to detector-loss corrected values of 80% and 70%. (C) 2013 Optical Society of America
Quantum information protocols tend to involve more and more entangled photon pairs, for enabling advanced quantum tasks. To compensate for the low success efficiency of these protocols, one can increase the repetition rate of the emitted pairs, up to current telecom standard (40 GHz). In this context, we propose a realization of ultra-fast photon pair sources, intended for either heralded quantum state engineering or entanglement swapping at telecom wave- length. In this paper, we present preliminary results consisting of a two-photon interference experiment using four photons where the achieved raw visibility of the Hong-Ou-Mandel (HOM) dip exceeds 98.6 %. The interfering photons are generated through spontaneous parametric down conversion process (SPDC) in two independent, separate, periodically poled lithium niobate (PPLN) waveguides.