Researchers have achieved coherent reflection of single photons from a semiconductor quantum dot integrated with a nanowire waveguide, overcoming a longstanding challenge in isolating light interacting with the quantum dot from stray electromagnetic fields. This advance uses a technique called “extinction” to enable efficient transfer of photons to the reflection port, ensuring each interacts with the quantum dot.
Resonance fluorescence, where a single two-level system is irradiated with monochromatic light, has fascinated physicists for more than a century. Its non-linear quantum mechanical behavior is evident in the fact that the TLS can only scatter a single photon at a time, a phenomenon known as photon antibunching, which was predicted by Carmichael and Walls and observed by Kimble et al. Such a high-efficiency platform opens applications in few-photon quantum nonlinear optics and resource states for universal photon-based quantum computing.
Waveguide-Quantum Dot System for Strong Coupling
The ability to confine light interaction to a single quantum dot within a nanowire waveguide has enabled observation of coherent photon reflection, a phenomenon predicted over a century ago. The optical system designed for this experiment relies on a 93(T):3 non-polarizing beamsplitter to direct a laser focused onto the nanowire quantum dot, while simultaneously collecting reflected photons. Suppression of coupling into the reflection port of any back-scattered light that does not interact with the quantum dot confirms the efficiency of directing photons for interaction.
This is evidenced by measurements of , a metric quantifying the probability of detecting multiple photons at the same time. A value of 0.054 ± 0.001 was observed for an incident photon rate of = 1. This contrasts sharply with a of 0.20 ± when the incident photon rate increased to = 10, demonstrating a clear reduction in multi-photon scattering.
The spectral response of the reflected signal at resonance further confirms the coherent nature of the single-photon reflection. Analysis using a Fabry-Perot etalon revealed that the majority of the reflected signal maintains coherence with the input drive, a hallmark of the quantum system’s behavior. The researchers write, highlighting the dominance of single-photon interactions.
This coherent reflection arises from the quantum dot imposing a π phase shift on the input drive, similar to a classical harmonic oscillator, but this limitation is due to non-linear saturation within the quantum dot, the reflection is effectively limited to one photon per radiative lifetime. This restriction leads to destructive interference of any multi-photon components attempting to contribute to the detected signal. The experiment demonstrates this by observing the interference response function (IRF) at varying photon rates; = 0.001, = 1, and = 10.
Resonance Fluorescence and Single-Photon Antibunching
This observation confirms a prediction made decades ago by Carmichael and Walls regarding the behavior of two-level systems, and was initially observed by Kimble and colleagues in atomic fluorescence. The current work establishes near-unity mode-matching to the nanowire, providing a clear pathway to access the reflected signal and isolate it from background electromagnetic noise.
Researchers measured this effect, achieving a of 0. 02 ± 0. 01, a value indicating a strong suppression of multi-photon events. This combination of antibunching and coherent reflected light realizes what the team terms a ‘quantum mirror’, a device that reflects single photons while blocking additional ones arriving within the quantum dot’s radiative lifetime.
In a one-dimensional atom, all of the incident electromagnetic field is perfectly reflected off the TLS when treated as a classical oscillating dipole in the ideal limit with no additional dephasing processes. In the quantum regime, the quantized nature of the TLS restricts it to reflect only one photon per radiative lifetime. Alternatively, multiphoton states are transmitted through the one-dimensional atom, analogous to a beamsplitter that sorts an input state based on its photon number.
Previous observations of resonance fluorescence in the Heitler regime revealed elastically scattered photons, but in this nanowire quantum dot system, almost all elastic scattering directs into the reflection port, effectively mimicking a mirror. The researchers note that this “mirror” is fundamentally limited by the quantized nature of the TLS, capable of reflecting only single photons before saturation occurs.
This squeezing, associated with self-homodyning of the TLS fluorescence, was modeled by the researchers, who found that pure dephasing reduces the effect but does not impact the measured antibunching. The quantum dot can also be conceptualized as reflecting photons spaced by its radiative lifetime, but any arrival of two photons within that timeframe results in the transmission of one while the TLS interacts with the other.
This contrast between continuous and discrete pictures has spurred recent debate and is now used to create new methods for generating non-classical light through multiphoton engineering of resonance fluorescence from bulk quantum dots. Such experiments require excitation of the TLS with antibunched light, which can be achieved in this nanowire quantum dot system through delayed coherent feedback, rerouting the TLS’s resonance fluorescence back to itself.
Alternatively, the team notes that maturing on-demand single-photon sources could also be used for non-classical excitation of the TLS. “We envision that our system will enable access to a few-photon nonlinearities to realize single-photon waveplates and photon–photon gates,” as originally proposed by Kimble.
Quantum Dot Reflection Demonstrates ‘Quantum Mirror’
Accessing the reflection port of a nanophotonic waveguide presented a significant hurdle in observing single-photon reflection, previously obscured by stray electromagnetic fields; researchers overcame this challenge with a technique called “extinction,” enabling clean signal isolation. The team achieved near-unity mode-matching, establishing clear access to the reflection port of the quantum dot, and ensuring all interacting photons engaged with the artificial atom. in the fluorescence from sodium atoms.
It was envisioned by Kimble et al. that a TLS coupled strongly to a single electromagnetic mode could be used for deterministic photon-photon interactions. Modeling the resonance fluorescence from the one-dimensional atom using a master equation with an input-output formalism, the researchers verified the high efficiency of their optical setup and validated the observed quantum mirror effect. The model accounted for the influence of the waveguide on the quantum dot, demonstrating a strong correlation between predicted and experimental results.
Analysis revealed that almost all elastic scattering from the quantum dot occurs into the reflection port, further reinforcing the mirror-like behavior. “Almost all of the back-scattered light can be treated as reflection off the TLS,” the team reports, highlighting the efficiency of the photon transfer to the reflection port.
The spectral overlap between the excitation and reflected light is notable, yet the photon statistics of the reflected light remain distinctly non-classical, as evidenced by power-dependent reflection intensity measurements and second-order correlations. This observation emphasises the quantum mechanical nature of the reflection process, differentiating it from classical mirror behavior. The team’s current work focuses on the stationary state response of the quantum dot. But future research will explore the dynamics of the reflection and transmission processes using pulsed excitation.
The transparent growth substrate allows for easy access to the transmission port of the quantum dot, promising complete access to the photon dynamics of the waveguide-QED system. Resolving these dynamics will require pulsed excitation of the system, opening avenues for the study of non-Markovian dynamics in quantum systems. The researchers anticipate that this platform will facilitate investigations into the behavior of quantum systems under more complex conditions, expanding the understanding of light-matter interactions at the nanoscale.
InAsP Quantum Dot Growth and Waveguide Dimensions
This advancement enabled the observation of coherent single photon reflection, a key step toward realizing complex quantum photonic circuits. This mode-matching allows for efficient transfer of photons from the input to the reflection port while having all interacted with the quantum dot. This precise alignment ensures that nearly all back-scattered light originates from the quantum dot itself, rather than from spurious sources. This theoretical framework accurately predicted the observed reflection characteristics, validating the experimental setup and analysis.
By inputting coherent light and probing the reflected signal, the team demonstrated a quantum mirror effect, where photons are reflected with high fidelity. The team believes this work will enable access to few-photon nonlinearities, ultimately realizing single-photon waveplates and photonic quantum gates.
g^(2)(0) of 0.02 Validates Single-Photon Reflection
This low value indicates a strong suppression of multiple photon events, validating the single-photon nature of the reflected light. Measurements revealed that even with an average incident photon number of 0.001, the system maintained a of 0.02, a result achieved without the need for corrections typically employed to filter out unwanted noise. This coherence is particularly evident in the Heitler regime, where emission from the quantum dot is dominated by coherent scattering under weak excitation conditions.
The team’s nanophotonic waveguide-QED system provides a platform where non-linear photon transport is efficiently confined to a single spatial mode, allowing for precise accounting of every photon within the system. This precise mode-matching, developed as part of the research, is important for enabling efficient access to non-linear quantum optics at the few-photon level.
The system’s ability to confine and control photons in this manner opens new avenues for exploring fundamental light-matter interactions and developing advanced quantum technologies. The observed dynamics, where a single photon is reflected within the 0.894 nanosecond radiative decay time of the quantum dot, demonstrate the speed and efficiency of this process.
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