Realizing a Compact, High-Fidelity, Telecom-Wavelength Source of Multipartite Entangled Photons

TL;DR

The paper addresses the need for high-fidelity multipartite entangled photons at telecom wavelengths for scalable quantum networks. It introduces a compact layered Sagnac source that produces two indistinguishable Bell pairs in parallel layers inside a single ppKTP crystal and fuses them into a four-qubit GHZ state via entanglement fusion, with spectral purification to improve purity. The device achieves a GHZ fidelity of about 0.9473 and a generation rate around 1.7 Hz (with higher-power operation reaching up to ~152 Hz at lower fidelity), supported by thorough characterization including quantum state tomography, joint spectral intensity measurements, and Hong-Ou-Mandel interference. This work provides a practical, scalable route toward deploying multipartite entanglement in real-world quantum networks, with clear paths to larger GHZ states and telecom-network integration.

Abstract

Multipartite entangled states are an essential building block for advanced quantum networking applications. Realizing such tasks in practice puts stringent requirements on the characteristics of the states in terms of fidelity and generation rate, along with a desired compatibility with telecommunication network deployment. Here, we demonstrate a photonic platform design capable of producing high-fidelity Greenberger-Horne-Zeilinger (GHZ) states, at telecom wavelength and in a compact and scalable configuration. Our source relies on spontaneous parametric down-conversion in a layered Sagnac interferometer, which only requires a single nonlinear crystal. This enables the generation of highly indistinguishable photon pairs, leading by entanglement fusion to four-qubit polarization-entangled GHZ states with fidelity up to $(94.73 \pm 0.21)\%$ with respect to the ideal state, at a rate of 1.7Hz. We provide a complete characterization of our source and highlight its suitability for practical quantum network applications.

Figures (6)

• Figure 1: Layered Sagnac GHZ source. Laser pump: A Ti:Sapphire laser (Coherent Mira-HP) with an average power of 3.4 W emits 2 ps pulses at a wavelength of 775 nm with a repetition rate of 76 MHz. Spatial mode shaping: The spatial mode of the laser is shaped into a Gaussian profile. Spatial multiplexer: The pump pulses are split into two parallel beams: the top and bottom layers, horizontally and vertically polarized, respectively. Polarization shaping: Both layers are diagonally polarized, in order to maximize the fidelity of output states with respect to the Bell state. Sagnac interferometer: Photon pairs are probabilistically generated via type-II SPDC in a ppKTP crystal (30mm-long, 46.2 $\mu$m poling period, provided by Raicol) and entangled in polarization in the Sagnac loop, resulting in the output state $(\ket{H}_s\ket{V}_i+e^{i\theta}\ket{V}_s\ket{H}_i)/\sqrt{2}$. Coupling and Filtering: After filtering the single photons with a dichroic mirror, 1100 nm long pass filters and 1.3 nm ultra-narrowband filters, the bottom layer photons are reflected on half-circle shaped mirrors, while the top layer photons are transmitted over them. The photons are coupled to SM fibers with 12 mm focal lens. Fusion station: The mechanical delay on the bottom layer idler photon is fine tuned such that both idler photons (from the top and bottom layers) arrive simultaneously to the FPBS. If each of them is transmitted to different outputs of the FPBS, and conditioned on fourfold coincidences, a GHZ state $(\ket{HHHH}+e^{i\delta}\ket{VVVV})/\sqrt{2}$ is generated. Unitary compensation: Three sets of QWP-HWP-QWP rotate the final state to the $\ket{GHZ}_{\delta=0}$ state. Polarization Analyser and Detection: A set of HWP-QWP-FPBS is used to map each photon's polarization to the spatial degree of freedom. The fibers are connected to superconducting nanowire single-photon detectors (ID281 SNSPD) that, in turn, are linked to a time tagger that allows to count and correlate detection events for the analysis.
• Figure 2: Experimental density matrix of the state produced with our source, estimated via quantum state tomography. The pump power was set to 63 mW (57 mW) in the top (bottom) layer, yielding a rate of 1.7 GHZ states per second for an acquisition time of 600 s per basis. The fidelity to the GHZ state is $\mathscr{F}=(94.73 \pm 0.21)\%$.
• Figure 3: Experimental state fidelity with respect to the GHZ state, $\mathscr{F}(\rho_{\text{exp}},\rho_{GHZ})$, as a function of the four-fold coincidences rate, for density matrices directly estimated with all the four-fold events detected (purple) and with the counts corrected to exclude the high-order emission events (blue). We observe that the two curves meet at a zero rate as expected within the error bars. The lack of perfect convergence is due to the residual imperfections in the higher-order emission correction.
• Figure 4: a) Schematic for (top) SPDC - a pump photon with frequency $\omega_p$ is converted into two photons of lower energy: $\omega_s$ and $\omega_i=\omega_p-\omega_s$, respectively; (bottom) DFG - once the pump $\omega_p$ exits the crystal, the seed, $\omega_s$, stimulates the emission of two photons: one of the same frequency $\omega_s$ and another with the remaining energy, $\omega_i=\omega_p-\omega_s$. b) Joint Spectral Intensity measurement setup. The pump - previously described - and seed - CW at telecom band (tunable) with 20 pm linewidth - lasers are sent to the Sagnac loop polarized vertically. The interaction of the two electromagnetic fields with the ppKTP crystal results in difference frequency generation, whose outcome is composed of two frequencies: the frequency of the seed (vertically polarized), $\omega_s$, and the difference between the pump and the seed frequencies, $\omega_i=\omega_p-\omega_s$, horizontally polarized. The HWP rotated to 45 degrees, together with the PBS, filter out the seed from the idler, reflecting the latter to be later collected by the optical spectrum analyser (EXFO OSA20).
• Figure 5: Experimental results for the joint spectral intensity measurement for (left) unfiltered top, (middle) unfiltered bottom and (right) filtered top layers. The heat map represents the power intensity for different combinations of signal and idler wavelengths (x and y axis, respectively). The grey region corresponds to the seed spectrum that was leaked due to the limited extinction ratio of the PBS. The data was collected by sweeping the wavelength of the seed laser, in steps of 20 pm, and analysing the respective idler spectrum, averaged over 20 scans with a resolution of 20 pm.