Experimental entanglement swapping through single-photon χ⁽²⁾ nonlinearity.

Area of Science:

• Quantum Information Science
• Nonlinear Optics
• Photonics

Background:

• Photonic quantum information processing relies on nonlinear photon-photon interactions for essential operations like two-qubit gates and entanglement swapping.
• The weak interaction between single photons has historically hindered the all-photonic realization of these crucial quantum operations.

Purpose of the Study:

• To demonstrate entanglement swapping using sum-frequency generation (SFG) in a nonlinear optical waveguide.
• To address the stringent signal-to-noise ratio (SNR) requirements for entanglement swapping protocols.

Main Methods:

• Utilized sum-frequency generation (SFG) in a χ(2)-nonlinear optical waveguide for single-photon interactions.
• Employed an ultralow-dark-count superconducting single-photon detector for a high-SNR entanglement heralder.
• Leveraged ultrafast telecom entangled photon-pair sources operating in the GHz range to enhance system clock speed.

Main Results:

• Achieved a high signal-to-noise ratio (SNR) for the entanglement heralder, meeting the protocol's demands.
• Confirmed a lower bound of 0.770(76) for the fidelity of the swapped entangled state, exceeding the classical limit of 0.5.
• Demonstrated the feasibility of entanglement swapping using all-single-photonic nonlinear interactions.

Conclusions:

• The developed SFG-based entanglement heralder and detection system satisfy the high SNR requirements for photonic entanglement swapping.
• The successful demonstration of entanglement swapping with high fidelity highlights the potential of nonlinear optical waveguides.
• These findings pave the way for advancements in long-distance quantum communication and scalable photonic quantum computation.