Eleven Photon-Triplets Per Second Created Using New Light-Mixing Technique

Gaspar Mougin-Trichon at CNRS and colleagues efficiently produce entangled photons using a difference-frequency-mixing process within a KTiOPO4 crystal pumped at 532nm. The experiment, supported by a semiclassical model, achieved a maximal flux of 11.6 photon-triplets per second, representing a key step towards brighter and more reliable sources for quantum applications.

Enhanced triplet generation via difference-frequency-mixing surpasses quantum model predictions

A photon-triplet flux of 11.6 per second has been achieved, exceeding previous limitations of approximately 2.2 triplets per second with similar setups. This represents a key advance because reliable, multi-photon sources were previously unattainable at this rate. This breakthrough enables more complex quantum experiments, particularly those requiring brighter entangled states for applications like quantum computing and cryptography. Experimental data confirms the accuracy of a semiclassical model in predicting triplet generation, surpassing the performance of purely quantum models by nearly an order of magnitude. The significance of this lies in the ability to move beyond simple entangled photon pair generation towards more complex entangled states, crucial for advanced quantum protocols.

Difference-frequency-mixing in a KTP crystal efficiently converts input laser beams into these entangled triplets, paving the way for more efficient quantum light sources. The process relies on non-linear optical phenomena where the interaction of multiple photons within the crystal results in the creation of new photons with different energies and momenta, while maintaining quantum correlations. Specifically, a pump beam at 532nm (green light) and a signal beam at 1491nm (infrared light) are combined within the type II phase-matched KTP crystal. Type II phase matching ensures that energy and momentum are conserved, allowing for efficient down-conversion and the generation of the triplet state. Increasing the energy of the stimulation beam from 3.04 μJ to 11.2 μJ, while maintaining a pump energy of 19.3 μJ, boosted triplet generation from 2.2 to 11.6 per second. This corresponds to 1.16 triplets per pulse, given the 10Hz repetition rate. Quantum efficiency reached 2.23 × 10−14 relative to the pump photons, and 1.38 × 10−14 relative to stimulation photons. These efficiency figures, while seemingly small, are representative of the challenges inherent in multi-photon entanglement generation and demonstrate a substantial improvement over previous attempts. The quantum efficiency calculation considers the number of generated triplets relative to the total number of pump and signal photons incident on the crystal, providing a measure of the process’s effectiveness.

The KTP crystal was chosen for its high non-linear coefficient and ability to be phase-matched at the desired wavelengths, optimising the difference-frequency-mixing process. Phase matching is critical; it ensures that the generated photons constructively interfere, maximising the efficiency of triplet generation. Deviations from perfect phase matching lead to destructive interference and reduced output. The experimental setup involved precise alignment of the laser beams through the KTP crystal, careful control of the beam polarisation, and a coincidence counting system to detect the entangled photon triplets. Coincidence counting is a technique used to identify correlated photons by detecting them within a very short time window, confirming their entanglement. The coincidence window is carefully calibrated to minimise background noise and ensure accurate triplet detection. Although this flux rate represents a sharp improvement, sustained, high-repetition rates remain necessary for practical quantum computing applications, requiring further optimisation of the experimental setup. Current limitations include the thermal effects within the KTP crystal at higher repetition rates, which can degrade the phase matching and reduce efficiency. Addressing these thermal issues through improved cooling mechanisms is a key area for future research.

Entangled photons are fundamental to progress in quantum communication and computing, yet creating complex multi-photon states remains a significant hurdle. Quantum key distribution, for example, relies on the secure transmission of information encoded in entangled photons. More complex quantum algorithms require the manipulation of multiple entangled qubits, necessitating brighter and more reliable multi-photon sources. The French team’s work builds upon previous efforts focused on generating pairs of entangled photons, extending the technique to produce triplets, groups of three linked photons. Generating triplets, as opposed to pairs, introduces additional complexity in terms of maintaining entanglement and accurately characterising the quantum state. The three photons are entangled in such a way that the measurement of one instantaneously influences the state of the others, regardless of the distance separating them. This non-local correlation is a key feature of quantum entanglement and is exploited in various quantum technologies.

Scaling to the repetition rates needed for practical quantum technologies presents a challenge, but this work validates a semiclassical model alongside quantum predictions, informing future designs. The achieved flux of 11.6 photon-triplets per second validates a semiclassical model, incorporating both quantum and classical physics, as accurately predicting this complex behaviour. Purely quantum models, which treat light solely as discrete photons, failed to accurately capture the observed triplet generation rates. The semiclassical model accounts for the classical electromagnetic fields of the laser beams, providing a more complete description of the interaction within the KTP crystal. Accurately modelling multi-photon entanglement is vital for developing more sophisticated quantum technologies, and this confirmation is significant. The process involves combining two laser beams within a potassium titanyl phosphate crystal, a technique known as difference-frequency-mixing, to create a new, lower-energy beam containing the triplets. Further research will focus on increasing the repetition rate, improving the quantum efficiency, and exploring the generation of even more complex entangled states, such as ququartets (four entangled photons), to unlock the full potential of quantum technologies.

The researchers successfully generated photon-triplets, achieving a flux of 11.6 triplets per second using a potassium titanyl phosphate crystal and two laser beams. This demonstrates a method for creating groups of three entangled photons, which are linked such that measuring one instantly influences the others. The study confirms that a semiclassical model, combining both quantum and classical physics, accurately predicts the rate of triplet generation, offering a more complete understanding than purely quantum approaches. The authors intend to focus on increasing the rate of triplet production and exploring the creation of even larger entangled states.