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Single-photon source with ultranarrow linewidth manipulated by coherent fields

Hebin Zhang, Gao‐xiang Li

Published Mar 22, 2021• 1 citation
OpenAlex

Abstract

Recent works [PhysRevLett.125.170402 (2020) and PhysRevLett.125.043603 (2020)] verify that the perfect single-photon character is a global property of resonance fluorescence including all the spectral components, and then the absence of some components can spoil the single-photon character, which considerably limits the application of filtering scheme to obtain single-photon source (SPS) with subnatural linewidth. Here, we note that the linewidth of fluorescent photon is not always limited by the intrinsic linewidth of the emitter, and provide a scheme where the linewidth of the fluorescent single photon can be much smaller than the natural linewidth of emitter by manipulating external coherent fields. By simulating the response of the filter or detection setup with a bandwidth approaching and even much smaller than the natural linewidth of emitter, the realization of ultranarrow SPS can be confirmed. Besides, we demonstrate that this scheme of ultranarrow SPS can be implemented based on various real physical platforms.

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Recent works [PhysRevLett.125.170402 (2020) and PhysRevLett.125.043603 (2020)] verify that the perfect single-photon character is a global property of resonance fluorescence including all the spectral components, and then the absence of some components can spoil the single-photon character, which considerably limits the application of filtering scheme to obtain single-photon source (SPS) with subnatural linewidth. Here, we note that the linewidth of fluorescent photon is not always limited by the intrinsic linewidth of the emitter, and provide a scheme where the linewidth of the fluorescent single photon can be much smaller than the natural linewidth of emitter by manipulating external coherent fields. By simulating the response of the filter or detection setup with a bandwidth approaching and even much smaller than the natural linewidth of emitter, the realization of ultranarrow SPS can be confirmed. Besides, we demonstrate that this scheme of ultranarrow SPS can be implemented based on various real physical platforms.
Abstract Resonance fluorescence—the light emitted when exciting resonantly a two-level system—is a popular quantum source as it seems to inherit its spectral properties from the driving laser and its statistical properties from the two-level system, thus providing a subnatural-linewidth single-photon source (SPS). However, these two qualities do not actually coexist in resonance fluorescence, since an optical target detecting these antibunched photons will either be spectrally broad itself and not benefit from the spectrally narrow source, or match spectrally with the source but in this case the antibunching will be spoiled. We first explain this failure through a decomposition of the field-emission and how this gets affected by frequency resolution. We then show how to restore the sought joint subnatural linewidth and antibunched properties, by interfering the resonance fluorescence output with a coherent beam. We finally discuss how the signal that is eventually generated in this way features a new type of quantum correlations, with a plateau of antibunching which suppresses much more strongly close photon pairs. This introduces a new concept of perfect SPS.
Subnatural-linewidth single photons are of vital importance in quantum optics and quantum information science. According to previous research, it appears difficult to utilize resonance fluorescence to generate single photons with subnatural linewidth. Here we propose a universally applicable approach to generate fluorescent single photons with subnatural linewidth, which can be implemented based on Λ-shape and similar energy structures. Further, the general condition to obtain fluorescent single photons with subnatural linewidth is revealed. The single-photon linewidth can be easily manipulated over a broad range by external fields, which can be several orders of magnitude smaller than the natural linewidth. Our study can be easily implemented in various physical platforms with current experimental techniques and will significantly facilitate the research on the quantum nature of resonance fluorescence and the technologies in quantum information science.
Spectral filtering of resonance fluorescence is widely employed to improve single photon purity and indistinguishability by removing unwanted backgrounds. For filter bandwidths approaching the emitter linewidth, complex behavior is predicted due to preferential transmission of components with differing photon statistics. We probe this regime using a Purcell-enhanced quantum dot in both weak and strong excitation limits, finding excellent agreement with an extended sensor theory model. By changing only the filter width, the photon statistics can be transformed between antibunched, bunched, or Poissonian. Our results verify that strong antibunching and a subnatural linewidth cannot simultaneously be observed, providing new insight into the nature of coherent scattering.
The observation of quantum-dot resonance fluorescence enabled a new solid-state approach to generating single photons with a bandwidth approaching the natural linewidth of a quantum-dot transition. Here, we operate in the small Rabi frequency limit of resonance fluorescence--the Heitler regime--to generate subnatural linewidth and high-coherence quantum light from a single quantum dot. The measured single-photon coherence is 30 times longer than the lifetime of the quantum-dot transition, and the single photons exhibit a linewidth which is inherited from the excitation laser. In contrast, intensity-correlation measurements reveal that this photon source maintains a high degree of antibunching behavior on the order of the transition lifetime with vanishing two-photon scattering probability. Generating decoherence-free phase-locked single photons from multiple quantum systems will be feasible with our approach.
Resonance fluorescence has played a major role in quantum optics with predictions and later experimental confirmation of nonclassical features of its emitted light such as antibunching or squeezing. In the Rayleigh regime where most of the light originates from the scattering of photons with subnatural linewidth, antibunching would appear to coexist with sharp spectral lines. Here, we demonstrate that this simultaneous observation of subnatural linewidth and antibunching is not possible with simple resonant excitation. Using an epitaxial quantum dot for the two-level system, we independently confirm the single-photon character and subnatural linewidth by demonstrating antibunching in a Hanbury Brown and Twiss type setup and using high-resolution spectroscopy, respectively. However, when filtering the coherently scattered photons with filter bandwidths on the order of the homogeneous linewidth of the excited state of the two-level system, the antibunching dip vanishes in the correlation measurement. Our observation is explained by antibunching originating from photon-interferences between the coherent scattering and a weak incoherent signal in a skewed squeezed state. This prefigures schemes to achieve simultaneous subnatural linewidth and antibunched emission.
Single-photon purity is one of the most important key metrics of many quantum states of light. For applications in photonic quantum technologies, e.g. quantum communication and linear optical quantum computing, a minimization of the multi-photon error rate is required because of its error-introducing nature. Ultimately, the purity of state-of-the-art single-photon sources was found to be limited by spontaneous emission and subsequent reexcitation during the interaction with the driving field. Here, we demonstrate that even this fundamental limit to the single-photon purity can be overcome due to the distinct spectro-temporal properties of the individual photons forming multi-photon errors. For driving pulses shorter than the emitter lifetime, we find that photons emitted during the pulse exhibit a significantly broader spectral shape than the emitter's natural linewidth. Thus, we can selectively filter out the majority of this instantaneously emitted photon by employing narrowband spectral filters, which reduces the measured degree of second-order coherence at zero time delay by almost one order of magnitude. This enables a significant suppression of the multi-photon error rate without detrimental effects on the desired single-photon emission.