On-demand semiconductor source of entangled photons which simultaneously has high fidelity, efficiency, and indistinguishability (1903.06071v1)

Abstract: An outstanding goal in quantum optics and scalable photonic quantum technology is to develop a source that each time emits one and only one entangled photon pair with simultaneously high entanglement fidelity, extraction efficiency, and photon indistinguishability. By coherent two-photon excitation of a single InGaAs quantum dot coupled to a circular Bragg grating bullseye cavity with broadband high Purcell factor up to 11.3, we generate entangled photon pairs with a state fidelity of 0.90(1), pair generation rate of 0.59(1), pair extraction efficiency of 0.62(6), and photon indistinguishability of 0.90(1) simultaneously. Our work will open up many applications in high-efficiency multi-photon experiments and solid-state quantum repeaters.

• The paper demonstrates a semiconductor-based entangled photon source that achieves high state fidelity (0.90(1)), efficient extraction (0.62(6)), and near-unity indistinguishability.
• It employs an InGaAs quantum dot coupled with a circular Bragg grating cavity, realizing a Purcell factor up to 11.3 to enhance photon emission.
• Coherent two-photon excitation with low-power π pulses minimizes multi-excitation effects, advancing scalable quantum communication and computing applications.

On-demand Semiconductor Source of Entangled Photons with High Fidelity, Efficiency, and Indistinguishability

The paper presents a semiconductor-based source of entangled photons that achieves simultaneous high fidelity, efficiency, and indistinguishability—a significant milestone in quantum optics and scalable photonic quantum technologies. The authors utilize an InGaAs quantum dot coupled with a circular Bragg grating bullseye cavity, achieving a Purcell factor of up to 11.3. By employing coherent two-photon excitation, the setup produces entangled photon pairs with state fidelity of 0.90(1), a pair generation rate of 0.59(1), a pair extraction efficiency of 0.62(6), and indistinguishability of 0.90(1).

The research details a source meeting four key criteria for entangled photon generation:

1. Entanglement Fidelity: The photons should approximate a maximally entangled Bell state.
2. On-demand Generation: The source must emit a distinct pair of entangled photons at specific times.
3. Extraction Efficiency: Efficient collection of emitted photons is crucial.
4. Indistinguishability: The photons should be indistinguishable in all degrees of freedom.

Historically, achieving these conditions simultaneously has posed substantial challenges. The authors utilize a circular Bragg grating cavity, enhancing both the extraction efficiency and entanglement fidelity without sacrificing one for the other.

Experimentally, the authors demonstrate that self-assembled InGaAs quantum dots have suitable properties for photon emission with near-unity quantum efficiency. Leveraging a broadband Purcell enhancement technique—notably, a circular Bragg grating architecture—the paper reports an emission linewidth nearly transform-limited. This is critical in eliminating dephasing and other potential sources of inefficiency.

Furthermore, the paper discusses the device's functionality using a coherent two-photon excitation scheme, with a pump pulse resonant with the virtual biexciton state, allowing efficient excitations with minimal power. The setup achieves first Rabi oscillations at $\pi$ pulses with a lower power threshold compared to non-resonant excitation methods. This low power requirement is pivotal in reducing higher-order multi-excitation pathways, which are otherwise detrimental in such systems.

The strong Purcell coupling results in radiative lifetimes for the XX and X photons of 66.4 ps and 126.7 ps, respectively. The reported Purcell factor indicates a considerable enhancement compared to standard quantum dot emissions, providing enhanced extraction and collection efficiencies. Notably, the construction supports a high degree of photon indistinguishability with a distinct advantage over traditional parametric down-conversion methods, which often face trade-offs in purity and efficiency.

The authors further elaborate on photon entanglement fidelity through polarization-resolved cross-correlation measurements across multiple polarizations. The results demonstrate a significant overlap with the ideal entangled state, reaching a fidelity metric of 0.90(1).

In terms of future implications, the development of a high-efficiency, on-demand, and pure entangled photon source has far-reaching impacts. This could benefit quantum communication protocols, including quantum key distribution and quantum teleportation, where the current technological bottleneck has been sources that do not concurrently achieve all outlined criteria. Additionally, this source could prove pivotal for quantum computing and metrology, presenting opportunities for integration with solid-state quantum repeaters.

In summary, the research underscores an advancement in the quest to develop viable quantum light sources, marking measurable progress toward realizing practical quantum information processing systems. The extension of this work to include entanglement swapping and application in multi-photon systems will likely remain an exciting area of future investigation.