A chiral one-dimensional atom exhibits unequal interaction with photons propagating in opposite directions. This asymmetry has potential applications in quantum technologies, such as single-photon routers, circulators, and quantum gates. In a non-chiral system, an atom couples equally to right and left-propagating modes. However, in a chiral system, the atom's interaction depends on the propagation direction, leading to non-reciprocity (T₁₂ ≠ T₂₁). Two cases are of interest: β = 1 (perfect transparency) and β = ½ (absorption in one direction, transmission in the other). Previous implementations have used single emitters in nano-engineered waveguides. This paper presents a different approach using a quantum dot in a low-volume, one-sided microcavity coupled to single-mode optical fibers. Chirality is induced by applying a magnetic field. The microcavity boosts light-matter interaction, allowing for tunable β-factors. This approach aims to achieve the β = ½ condition, resulting in a 'single-photon diode' where photons are transmitted in one direction and absorbed in the other. The challenges include achieving the precise β-factor and a high-quality quantum dot. The authors claim to achieve an isolation of 10.7 dB, the highest reported with a single quantum emitter, enabling the observation of optical nonlinearities at ultralow power (100 pW). The quantum nature of this nonlinearity is validated by photon bunching.

The paper cites several previous works on chiral quantum optics. These include implementations using Rb atoms in the evanescent field of a dielectric nanofiber and semiconductor quantum dots in waveguides. The authors highlight the advantages of their approach compared to these existing methods, focusing on the controllable tuning of the β-factor using a microcavity and the high efficiency of fiber coupling. They acknowledge that semiconductor systems can achieve high β-factors in nano-beam or photonic-crystal waveguides, but their method offers superior control and efficiency.

The experimental setup consists of a single-mode optical fiber waveguide, a quantum dot embedded in a low-volume one-sided microcavity, and another single-mode fiber. Chirality is achieved by applying a magnetic field (2.0 T) to a neutral quantum dot, selecting a specific circularly polarized transition (σ⁺). The microcavity, comprising a highly reflective bottom mirror and a less reflective top mirror, allows precise control over the photon-emitter coupling (β-factor) by adjusting the quantum dot's lateral position relative to the anti-node of the cavity mode. The microcavity's frequency is tuned by adjusting the distance between the mirrors. The polarisation of the light is controlled using a polarising beam-splitter (PBS) and a quarter-wave plate (λ/4) to achieve spin-momentum locking. Transmission measurements are performed using coherent laser light at low powers (single-photon regime). The β-factor is tuned by adjusting the quantum dot's lateral position, targeting the critical coupling condition (β = 0.5). Transmission is measured in both forward (1→2) and backward (2→1) directions as a function of quantum dot and microcavity detuning. To demonstrate the quantum nature of the non-reciprocity, the power dependence and photon statistics of the output are characterized. Power-dependent transmission measurements are performed in the backward direction to observe saturation effects of the two-level quantum system. Second-order autocorrelation function, g⁽²⁾(τ), measurements of the backward-propagating light are carried out at different input powers to verify photon bunching.

The authors achieved a non-reciprocal transmission with an isolation of 10.7 dB (T₁₂/T₂₁ ≈ 11.9), the highest reported value for a single quantum emitter. This was achieved at the critical coupling condition (β ≈ 0.5), where transmission in the forward direction is high (≈0.82) and significantly suppressed in the backward direction (≈0.07). The high overall efficiency (56%) allowed observation of nonlinear effects at input powers as low as 100 pW. The nonlinearity arises from the saturation of the two-level quantum dot. Power-dependent transmission measurements in the backward direction showed a critical power (P_c) of approximately 13 pW, consistent with theoretical expectations. The quantum nature of the nonlinearity was confirmed through g⁽²⁾(τ) measurements which revealed strong photon bunching (g⁽²⁾(0) ≈ 101) at low input powers (5 pW) in the backward direction, indicating suppression of the single-photon component. The g⁽²⁾(τ) measurements were consistent with a theoretical model, validating the single-emitter origin of the observed non-reciprocity and the saturation behavior. In the forward direction, g⁽²⁾(τ) remained close to unity, further highlighting the non-reciprocal nature of the system.

The results demonstrate a highly efficient and strongly non-reciprocal single-photon diode based on a quantum dot in an open microcavity. The close agreement between experimental data and the theoretical model confirms the validity of the approach and the understanding of the underlying physics. The low critical power and strong photon bunching suggest potential applications in quantum information processing, including the creation of exotic photonic states and the simulation of many-body dynamics. The observed non-reciprocity can be dynamically controlled, opening possibilities for optical switches and transistors. Future work could focus on eliminating the microcavity mode splitting to further improve the performance and achieve even higher isolation.

The authors successfully demonstrated a single-photon diode using a quantum dot in an open microcavity, achieving a record-high isolation of 10.7 dB. The system's performance is well-described by a theoretical model, validating the understanding of the underlying chiral quantum optics. The demonstrated strong non-reciprocity, coupled with ultralow power nonlinearity and high efficiency, opens promising avenues for future quantum technologies, including optical switches and transistors, and the generation of two-photon bound states. Future improvements could be achieved by minimizing microcavity mode splitting, potentially leading to improved performance and applications in single-photon phase-shifting and deterministic two-photon quantum gates.

The study primarily focuses on the demonstration of a highly efficient single-photon diode at a specific operating point (β ≈ 0.5). Further research is needed to explore the device's performance over a wider range of parameters and operating conditions. The residual mode-splitting in the microcavity introduces some insertion loss (1.5 dB) which could be minimized in future iterations of the device. The current experiments were limited to low photon flux levels, and the scalability to higher flux rates for practical applications needs to be investigated.