Compact and stable source of polarization-entangled photon-pairs based on a folded linear displacement interferometer

The realization of quantum networks requires the development of robust low size, weight and power (SWaP) systems suitable for operation under harsh environments in remote and mobile nodes such as satellites. We present a source of polarization-entangled photon-pairs in a folded linear displacement interferometer based on spontaneous parametric down conversion using a Type-0 periodically poled potassium titanyl phosphate crystal. Featuring a compact and stable double-pass geometry using a corner-cube retroreflector, the source has a detected pair rate of 2.5 M pairs/s/mW with a Bell state fidelity of 94.1% +/- 2.1%. The qualities and demonstrated performance of the source make it suitable for deployment in entanglement-based quantum networks.

💡 Research Summary

The paper presents a compact, robust source of polarization‑entangled photon pairs designed for deployment in harsh, size‑constrained environments such as satellite‑based quantum networks. The authors introduce a Folded Linear Displacement Interferometer (FLDI) that combines a single beam displacer (BD) with a corner‑cube retroreflector (CCR) in a double‑pass configuration. A 405 nm continuous‑wave laser pumps a Type‑0 periodically poled KTP (PPKTP) crystal (1 × 2 × 10 mm, poling period 3.425 µm). The BD splits the pump into orthogonal horizontal (H) and vertical (V) components separated by ~1 mm. An achromatic half‑wave plate (a‑HWP) set at 45° flips the polarization of the pump on the first pass; after passing through the crystal, the pump and any generated photon pairs are reflected by the CCR and traverse the crystal a second time. On the return pass the a‑HWP flips the polarization of the down‑converted photons, so that the clockwise path produces VV pairs while the counter‑clockwise path produces HH pairs. The two paths recombine at the BD, yielding the maximally entangled state |Φ⟩ = ( |HH⟩ + e^{iφ}|VV⟩ )/√2, where φ is controlled by a liquid‑crystal variable retarder (LCVR) acting on the pump’s relative phase.

The experimental setup is divided into four functional blocks: pump preparation, beam steering, entanglement generation, and detection. The pump power and polarization are regulated with a half‑wave plate and polarizing beam splitter; a second half‑wave plate and the LCVR set the input state. The pump is focused into the crystal with a 200 mm focal‑length plano‑convex lens. After the crystal, a dichroic mirror (DM1) separates the pump from the SPDC photons, and a long‑pass filter removes residual pump light. Because the BD introduces a slight path‑length mismatch for the two polarizations, a second, shorter BD (0.73 mm) is inserted before the long‑pass filter to ensure temporal overlap. The non‑degenerate SPDC photons (signal ≈ 780 nm, idler ≈ 842 nm) are separated by a second dichroic mirror (DM2). Polarizers in each arm allow full quantum state tomography, and the photons are coupled into single‑mode fibers and detected with silicon avalanche photodiodes (Excelitas SPCM‑AQRH‑14‑FC). Coincidences are recorded with a 20 ns window using a TimeTagger.

Optimization of the quasi‑phase‑matching (QPM) condition was performed by scanning both the crystal temperature and the pump wavelength (controlled via laser diode temperature). At a crystal temperature of 25.5 °C and a pump wavelength of 404.6 nm, the signal spectrum peaks at 781.3 nm and the idler at 845.6 nm. A 10 nm band‑pass filter centered at 780 nm isolates the signal, ensuring that each detected signal photon has a corresponding idler photon.

Performance metrics were measured as a function of pump power (≤ 1.5 mW to avoid detector saturation). The source exhibits a detected pair rate of 2.5 M pairs · s⁻¹ · mW⁻¹, a coincidence‑to‑accidental ratio (CAR) that remains high over the power range, and a brightness that decreases only slowly with increasing pump power when spectral filtering is applied. Heralding efficiencies stay roughly constant (~20 % without filtering, higher with filtering), indicating that the source’s intrinsic efficiency is not compromised by the filtering scheme.

Polarization correlation measurements were carried out by fixing the signal polarizer (P1) at H, V, D (45°), and A (135°) while sweeping the idler polarizer (P2) through 180°. The observed visibilities were 98.8 % (H), 97.0 % (V), 89.3 % (D), and 91.2 % (A). From these data the fidelity to the Bell state |Φ⁺⟩ was calculated as 94.1 % ± 2.1 %, corresponding to a quantum bit error rate (QBER) of 5.9 %, well within the thresholds required for secure quantum key distribution.

A key advantage of the FLDI design is its insensitivity to tip‑tilt misalignment, inherited from the CCR. Experiments deliberately misaligned the CCR in angle (up to ±0.85°) and lateral position; the coincidence rate and heralding efficiency degraded only modestly (to ~1 M pairs · s⁻¹ and ~20 % efficiency) and could be recovered with minor adjustments to the fiber coupling. This robustness demonstrates that the source can tolerate the mechanical disturbances typical of launch and in‑orbit operation.

Overall, the FLDI source occupies a footprint of only ~9.5 cm in length, a dramatic reduction compared with conventional Sagnac or double‑LDI configurations, while delivering pair production rates suitable for satellite‑to‑ground quantum links. The authors argue that further miniaturization (e.g., integrated optics, custom packaging) could enable deployment on CubeSat platforms, opening the path toward scalable, space‑based quantum networks.