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Physical Review A

Controlling purity, indistinguishability, and quantum yield of an incoherently pumped two-level system by spectral filters

Ivan V. Panyukov, Vladislav Yu. Shishkov, E. S. Andrianov

Published Aug 10, 2023• 1 citation
Publisher

Abstract

Dephasing processes significantly impact the performance of deterministic single-photon sources. Dephasing broadens the spectral line and suppresses the indistinguishability of the emitted photons, which is undesirable for many applications, primarily for quantum computing. We consider a light emitted by a two-level system with a pulsed incoherent pump in the presence of the spectral filter. The spectral filter allows control of the second-order autocorrelation function, indistinguishability, and quantum yield. We show that narrow spectral filters can increase the indistinguishability of the emitted light while undermining the quantum yield. The influence of the spectral filter on the second-order autocorrelation function depends on the duration of the pump. When the pump pulse is long compared to the lifetime of the two-level system, the narrow spectral filters lead to a rapid increase in the second-order autocorrelation function. In this limit, the statistics of the light from the two-level system inherit the statistics of the incoherent pump. In the case of the short duration of the pump pulse, it is possible to preserve single-photon properties to some degree for the sublifetime width of the spectral filter. Moreover, the single-photon properties of the light manifest themselves differently, depending on the response time of the quantum system affected by this light. In particular, in the case of long response time, the spectral filter with a sublifetime width can provide the near-zero second-order autocorrelation function.

DOI
10.1103/physreva.108.023711

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Influence of spectral filtering on the quantum nature of light

Krista Joosten, Gerard Nienhuis

We study the two-time correlation function of spectrally filtered light from a quantum mechanical light source. The bunching and anti-bunching properties of the photo-statistics of a field display the quantum nature of the source. We study whether these properties are affected by the presence of filters that are modelled by dielectric slabs. We quantize the radiation field in the presence of the filters and express the correlation function in terms of source operators. As a quantum source we take the resonance fluorescence of a driven two-level atom. We find that the spectral correlations depend on the linewidth of the filters. When the pass time of the filter is much longer than the time scale on which the source evolves, the presence of the filter can conceal the quantum nature of the source. However, quantum mechanical correlations between opposite side bands survive even for very narrow filters.

Journal of Optics B Quantum and Semiclassical Optics•Apr 1, 2000•44 citations
Dephasing processes significantly impact the performance of deterministic single-photon sources. Dephasing broadens the spectral line and suppresses the indistinguishability of the emitted photons, which is undesirable for many applications, primarily for quantum computing. We consider a light emitted by a two-level system with a pulsed incoherent pump in the presence of the spectral filter. The spectral filter allows control of the second-order autocorrelation function, indistinguishability, and quantum yield. We show that narrow spectral filters can increase the indistinguishability of the emitted light while undermining the quantum yield. The influence of the spectral filter on the second-order autocorrelation function depends on the duration of the pump. When the pump pulse is long compared to the lifetime of the two-level system, the narrow spectral filters lead to a rapid increase in the second-order autocorrelation function. In this limit, the statistics of the light from the two-level system inherit the statistics of the incoherent pump. In the case of the short duration of the pump pulse, it is possible to preserve single-photon properties to some degree for the sublifetime width of the spectral filter. Moreover, the single-photon properties of the light manifest themselves differently, depending on the response time of the quantum system affected by this light. In particular, in the case of long response time, the spectral filter with a sublifetime width can provide the near-zero second-order autocorrelation function.
Dephasing processes significantly impact the performance of deterministic single-photon sources. Dephasing broadens the spectral line and suppresses the indistinguishability of the emitted photons, which is undesirable for many applications, primarily for quantum computing. We consider a light emitted by a two-level system with a pulsed incoherent pump in the presence of the spectral filter. The spectral filter allows control of the second-order autocorrelation function, indistinguishability, and quantum yield. We show that narrow spectral filters can increase the indistinguishability of the emitted light while undermining the quantum yield. The influence of the spectral filter on the second-order correlation function depends on the duration of the pump. When the pumping pulse is long compared to the lifetime of the two-level system, the narrow spectral filters lead to a rapid increase in the second-order autocorrelation function. In this limit, the statistics of the light from the two-level system inherit the statistics of the incoherent pump. In the case of the short duration of the pump pulse, it is possible to preserve single-photon properties to some degree for the sub-lifetime width of the spectral filter. Moreover, when the light emitted by the single-photon source is used to control a quantum system, e.g., cavity, the single-photon properties of the light manifest themselves differently, depending on the response time of the quantum system. In particular, in the case of long response time, the spectral filter with sub-lifetime width can provide the near-zero second-order autocorrelation function.
Abstract Light with zero second‐order autocorrelation function, g(2)(0), emitted by single‐photon source (SPS), for example, two‐level system (TLS), is usually considered as being in a Fock state with one photon in a certain electromagnetic mode of free space. However, real SPSs have finite linewidths and excite all modes lying within the linewidth. It is shown that the zero value of g(2)(0) of light emitted by an incoherently pumped TLS is a consequence of the quantum interference of electromagnetic modes lying within the linewidth. Applying the quantum regression theorem for out‐of‐time‐ordered correlation functions, an analytical expression for g(2)(0) is obtained for the light emitted by a TLS and passed through a spectral filter. It is shown that narrowing the spectral width of the band‐pass filter inevitably leads to an increase in g(2)(0). is found and it is demonstrated that certain spectral filters lead to its nonmonotonic time dependence. These results open up the possibility of controlling the statistics of the emitted light using a spectral filter, which can find applications in quantum communication and cryptography.
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.
High-quality quantum states lie at the heart of advanced quantum information processing. The degree of photon indistinguishability is critical for applications from photonic quantum computation to precision metrology. The two-photon Hong–Ou–Mandel (HOM) interference effect provides a rigorous quantification method, with its visibility serving as the ultimate benchmark for source quality. Generally, coherent pumping is widely regarded as indispensable for the preparation of quantum sources. As a result, incoherent light sources have seen limited applications in the current quantum technologies. We generate an indistinguishable two-photon state by incoherent light generated by frequency doubling of amplified spontaneous emission light. The theoretical analysis indicates that phase randomization of the pumping does not affect the coincidence visibility in two-photon intensity interference. Moreover, temporal incoherence further enhances the symmetry of the generated spectrum in second-harmonic generation. In the experiment, the incoherently pumped photon sources exhibit a heralding efficiency of ∼60% and a coincidence-to-accidental ratio exceeding 15,000. The observed HOM interference fringes show the visibility of 99.1% without any spectrum filtering, confirming the near-ideal indistinguishability of the photons. Our study reveals the role of temporal coherence in second-order nonlinear interactions. It provides a potential approach to use an easily accessible incoherent light for engineering high-quality quantum sources.
Quantum emitters promise to emit exactly one photon when pumped by a laser pulse. However, even in ideal systems, re-excitation during the laser pulse duration causes the consecutive emission of two photons, thus limiting the single-photon purity. Although the probability and characteristics of re-excitation are largely determined by the excitation scheme, until now, only resonant driving has been studied. Here, we demonstrate qualitative differences for phonon-assisted excitation and resolve the distinct temporal and spectral profile of each of the photons. These uncover correlations between the emission time and wavelength, resulting in an asymmetric two-photon spectrum. On a fundamental level, we show that this spectrum provides direct access to the Rabi frequency of an incoherently driven quantum dot. On the application side, we leverage the asymmetry to selectively suppress multiphoton noise, ensuring high single-photon purity regardless of the laser pulse length and thus enhancing implementations across quantum cryptography and quantum computing.
We study the two-time correlation function of spectrally filtered light from a quantum mechanical light source. The bunching and anti-bunching properties of the photo-statistics of a field display the quantum nature of the source. We study whether these properties are affected by the presence of filters that are modelled by dielectric slabs. We quantize the radiation field in the presence of the filters and express the correlation function in terms of source operators. As a quantum source we take the resonance fluorescence of a driven two-level atom. We find that the spectral correlations depend on the linewidth of the filters. When the pass time of the filter is much longer than the time scale on which the source evolves, the presence of the filter can conceal the quantum nature of the source. However, quantum mechanical correlations between opposite side bands survive even for very narrow filters.