Generating quantum entanglement from sunlight

Abstract

Energy consumption is becoming a serious bottleneck for integrating quantum technologies within the existing global information infrastructure. In photonic architectures, considerable energy overheads stem from using lasers, whose high coherence was long considered indispensable for quantum state preparation. Here, we demonstrate that natural, incoherent sunlight can successfully produce quantum-entangled states via spontaneous parametric down-conversion. We detect polarization-entangled photon pairs with a concurrence of $0.905\pm0.053$ and a Bell state fidelity of $0.939\pm0.027$. Importantly, the system violates Bell's inequality with $S=2.5408\pm0.2171$, exceeding the classical threshold of 2, while maintaining generation rates comparable to laser-based setups. These findings pave the way for sustainable quantum applications in resource-limited environments like interplanetary missions.

Figures (6)

• Figure 1: Schematics of the experimental setup The setup consists of a sunlight concentration module and an entangled-photon source. The sunlight concentration module collects direct sunlight via a $1~\mathrm{m} \times 1.4~\mathrm{m}$ Fresnel lens and concentrates the spectrally filtered light into a multimode fiber (MMF). In the entangled-photon source, the sunlight is collimated and filtered to 405 nm $\pm$ 0.75 nm (inset) to pump a ppKTP crystal inside a PSI. We have carefully balanced the PSI to compensate for most spatial and temporal distinguishabilities between the SPDC processes in two directions. The resulting SPDC field is split and detected by MMF-coupled APDs to characterize two-photon entanglement and test for violation of local realism.
• Figure 2: Time-correlation histogram of sunlight-pumped SPDC measured in joint polarization bases VH and HV, respectively In the titles of subplots, we note the average pump power measured over the time of data acquisition. (A-C) display results measured on three separate days. The bar plots display the experimentally measured photon coincidences, while the grey shade indicates the coincidence time window. The photon pairs show clear temporal correlation with negligible accidental coincidences.
• Figure 3: Real and imaginary parts of the density matrix of the two-photon state produced from sunlight-pumped SPDC. These results are calculated using quantum state tomography based on polarization correlation measurements in 16 different projection bases, which include those that have been displayed in Fig. \ref{['fig:2']}. These results confirm polarization entanglement with a concurrence of $C =0.905\pm0.053$, a purity of $P =0.919\pm0.045$, and a fidelity of $F = 0.939\pm0.027$ to the target Bell state (Eqn. \ref{['eqn1']}).
• Figure 4: Bell parameter measurement using the CHSH formalism. Idler photons are projected into different linear polarizations while signal photons are projected into the (A) rectilinear bases and (B) (anti-)diagonal bases. We denote $\theta_{s(i)}$ as the angle of linear polarization with respect to the horizontal polarization, so that $\theta_{s} = 0^\circ, 45^\circ, 90^\circ, 135^\circ$ represents projecting signal photons in the horizontal (H), diagonal (D), vertical (V), and anti-diagonal (A) bases, respectively. These results lead to a violation of local realism with a Bell-CHSH parameter of $S = 2.5408 \pm 0.2171 > 2$.
• Figure S1: Real-life photographs of experimental setups used for the sunlight-pumped generation of polarization-entangled photon pairs The entire setup consists of two components: a sunlight concentration module (right) and an entangled-photon source (left) housed inside a protective and darkening tent, which also contains the associated electronics. Inset: Optical setup of the entangled-photon source, showing the optical arrangement and avalanche photodiodes (APDs) used for photon-pair generation. A hinged door (not shown) is also installed as part of the enclosure, which, when closed, covers the front and top sides of the optical setup.