In-Line Fiber-Integrated Photon-Pair Generation from van der Waals Crystals

Abstract

Miniaturized quantum light sources that operate directly in optical fibers are an attractive platform for optical quantum technologies. However, most miniaturized spontaneous parametric down- conversion (SPDC) sources still rely on objective-lens-based free-space pumping and collection, which limits compactness, robustness, and direct compatibility with fiber-based systems. Here we demonstrate a lens-free in-line SPDC photon-pair source by integrating a van der Waals NbOI2 flake directly onto the end facet of an optical fiber. In this configuration, the generated photon- pairs are efficiently collected into optical fibers, eliminating the need for bulk free-space collection optics. Despite the limited numerical aperture of the single-mode fiber, efficient photon-pair collection with high purity, characterized by a coincidence-to-accidental ratio of up to ~4600, is achieved in an ultracompact configuration. These results establish van der Waals ferroelectric materials as a promising platform for fiber-integrated quantum light sources and provide a pathway toward compact, alignment-free quantum photonic devices.

Figures (4)

• Figure 1: NbOI$_2$ properties for SPDC process. (a) Crystal structure of layered NbOI$_2$ and definition of the laboratory axes ($x$,$y$,$z$) used in the measurements and their relation to the crystallographic axes ($a$,$b$,$c$). (b) Schematic illustration of the second-order Peierls distortion, in which Nb atoms displace along either the $+b$ or $-b$ direction, breaking inversion symmetry and giving rise to a nonzero ${\chi}^{(2)}$. (c) Wavelength-dependent refractive index $n(\lambda)$ and (d) extinction coefficient $\kappa(\lambda)$ along the principal axes, obtained from spectroscopic ellipsometry. Calculated SPDC generation rate $R_{\mathrm{CC}}$ as a function of crystal thickness $L$: (e) without considering material absorption and (f) including absorption based on the complex refractive index. See the main text for further details.
• Figure 2: Preparation of the fiber-integrated NbOI$_2$ device. (a) Surface profile of the NbOI$_2$ flake used in this work, showing a thickness of $\sim$410 nm. Inset shows the optical image of the flake and the scan direction used for the measurement. (b) Optical microscope image of the fiber facet showing the core and cladding. (c) NbOI$_2$ flake transferred onto the fiber core. Note that the original flake, shown in the inset of (a), we split into two pieces before transfer. (d) NbOI$_2$ on the fiber facet is encapsulated by graphene layers with $\sim$35nm thickness.
• Figure 3: SPDC photon-pair generation from fiber-integrated NbOI$_2$ devices. (a) Experimental schematic for SPDC generation from an NbOI$_2$ flake placed on the fiber input facet. A continuous-wave 405 nm pump laser passes through a half-wave plate (HWP) for polarization control and is focused onto the NbOI$_2$ flake using a 10$\times$ objective lens. The generated SPDC photons are directly coupled into the fiber and analyzed using a separate Hanbury Brown-Twiss (HBT) setup, see the main text for further details. (b,c) Pump polarization dependence of the SPDC coincidence counts for Device 1 (multimode fiber) and Device 2 (single-mode fiber), respectively. (d) Representative second-order correlation $g^{2}(\tau)$ measured for Device 1 as a function of the time delay $\tau$, measured with a bin width of 60 ps over an acquisition time of 30 min. (e) Coincidence counts and accidental counts as a function of pump power for Device 1 and Device 2. The coincidence window of 1.5 ns was used, and the coincidence counts were accumulated for 120 s. The error bars for the accidental counts largely overlap with lines and are therefore not clearly visible. (f) Single-detector count rate as a function of pump power for Device 1 and Device 2. Error bars are smaller than the marker size and not visible.
• Figure 4: Fully fiber-integrated SPDC source based on NbOI$_2$. (a) Experimental configuration in which the pump laser is delivered through an optical fiber. The 405 nm pump passess through a quarter-wave plate (QWP) and a half-wave plate (HWP) for polarization control and is coupled to Fiber 1, which delivers the pump to the NbOI$_2$ crystal. Fiber 1 and Fiber 2 are connected using a standard FC-FC fiber adapter, with the NbOI$_2$ crystal positioned between the two fiber facets. The generated SPDC photons are collected by Fiber 2. (b) Representative second-order correlation $g^{2}(\tau)$ measured for Device 4 as a function of the time delay $\tau$, measured with a bin width of 60 ps over an acquisition time of 30 min. (c) Coincidence counts and accidental counts as a function of pump power for Device 3 and Device 4. The coincidence window of 1.5 ns was used, and the coincidence counts were accumulated for 120 s. The error bars for the accidental counts largely overlap with lines and are therefore not clearly visible. (d) Single-detector count rate as a function of pump power for Device 3 and Device 4. Error bars are smaller than the marker size and not visible.