Single-photon nonlinear optics with a quantum dot in a waveguide

Abstract: Strong nonlinear interactions between photons enable logic operations for both classical and quantum-information technology. Unfortunately, nonlinear interactions are usually feeble and therefore all-optical logic gates tend to be inefficient. A quantum emitter deterministically coupled to a propagating mode fundamentally changes the situation, since each photon inevitably interacts with the emitter, and highly correlated many-photon states may be created . Here we show that a single quantum dot in a photonic-crystal waveguide can be utilized as a giant nonlinearity sensitive at the single-photon level. The nonlinear response is revealed from the intensity and quantum statistics of the scattered photons, and contains contributions from an entangled photon-photon bound state. The quantum nonlinearity will find immediate applications for deterministic Bell-state measurements and single-photon transistors and paves the way to scalable waveguide-based photonic quantum-computing architectures.

• The paper demonstrates that a quantum dot in a photonic-crystal waveguide can achieve deterministic single-photon nonlinear interactions.
• It reports an experimental coupling efficiency of 85% and a critical photon flux per lifetime of 0.81, underscoring low-power operation.
• The findings highlight potential energy-efficient optical switches with 0.17 attojoule switching energy and photon-photon bound states.

Overview of Single-Photon Nonlinear Optics with a Quantum Dot in a Waveguide

The paper entitled "Single-photon nonlinear optics with a quantum dot in a waveguide" explores the capabilities of a quantum dot embedded in a photonic-crystal waveguide to exert substantial nonlinear optical effects at the single-photon level. This study is significant for addressing one of the prominent challenges in optical physics: achieving strong nonlinear interactions using weak photon fields, which is essential for both classical and quantum-information technologies. The research demonstrates how this setup can be leveraged for deterministic interactions between single photons and quantum emitters, thereby suggesting potential enhancements in photonic quantum computing architectures.

The authors present an experimental investigation where a quantum dot acts as a '1D atom', coupling strongly with a propagating photonic mode. A critical feature of this system is its ability to achieve nonlinear responses at power levels corresponding to a single photon. This could have practical implications by providing all-optical logic gates with significantly improved efficiency relative to existing platforms.

Key Numerical Results

• The experimental setup achieved a coupling efficiency of $\beta = 85\%$ and demonstrated a critical photon flux per lifetime of $n_c = 0.81$, indicating the nonlinearity is at the single-photon level.
• The characteristic switching energy of the system was measured to be approximately $0.17$ attojoule, demonstrating the potential for extremely energy-efficient optical switches.
• Photon correlation measurements indicated a photon-photon bound-state contribution to the two-photon component transmission probability of $\sim 70\%$.

Significant Experimental Findings

The paper details how single-photon interference and the generation of bound photon states are facilitated by this system. When illuminating the quantum dot with resonant light, single-photon components were reflected, while multi-photon components had enhanced transmission probability. This nonlinearity was characterized by transmission spectra and the intensity autocorrelation function of the transmitted light.

Furthermore, the intrinsic nonlinearity of the quantum dot enabled the generation of photon-photon bound states, as evidenced by observing photon bunching in the autocorrelation function of the transmitted light. At low power, this system efficiently reflected single-photon levels, demonstrating the quantum-dot waveguide's potential for high-fidelity photon-photon interactions.

Implications and Future Directions

The research has several implications for advancing quantum-information processing technology. The capability to operate at the single-photon level brings efficient, energy-saving designs for optical switches and logic gates closer to realization. Such advancements could push the limits of current photonic architectures, enabling a new horizon of resource-efficient quantum-information protocols, providing a pathway towards scalable, quantum-dot based photonic circuits.

In terms of future developments, the prospect of integrating multiple quantum dots within a photonic-crystal waveguide architecture could open up avenues for quantum simulations and state engineering of complex quantum systems. By enhancing spectral and spatial control over quantum dot emissions, the design of integrated photonic circuits capable of sophisticated processing tasks at the fundamental photon level is within reach.

This study lays a robust foundation for future explorations into scalable quantum optical technologies, providing a practical platform for single-photon experiments that may ultimately drive innovations in both information processing and quantum computing sectors.