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arXiv:quant-ph/0510061 · Physical Review A

Theory of single-photon control from a two-level system source

Yong He, Eli Barkai

Published Jul 28, 2006• 10 citations
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Abstract

We investigate the theory of a few photons on demand from a single two-level emitter excited by a square laser pulse. Using the generalized optical Bloch equations, we find exact expressions for the probability of generating zero, one, two, and three photons, the average $⟨N⟩$, and Mandel's $Q$ parameter, which exhibit rich physical behaviors. The exact solution is used to design control fields that yield an optimal single- or two-photon source, under the constraints of finite laser-field strength and finite interaction time.

DOI
10.1103/physreva.74.011803

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arXiv:quant-ph/0309197•Journal of Physics B Atomic Molecular and Optical Physics•Jun 8, 2004•13 citations
We investigate the theory of a few photons on demand from a single two-level emitter excited by a square laser pulse. Using the generalized optical Bloch equations, we find exact expressions for the probability of generating zero, one, two, and three photons, the average ⟨N⟩⟨N⟩⟨N⟩, and Mandel's QQQ parameter, which exhibit rich physical behaviors. The exact solution is used to design control fields that yield an optimal single- or two-photon source, under the constraints of finite laser-field strength and finite interaction time.
We consider the theory of single photon on demand from a two level atom or molecule source. Using optical Bloch equations and the generating function formalism we investigate three approaches to single photon control: (i) the square laser pulse; (ii) the square modulation of absorption frequency; and (iii) the rapid adiabatic following approach investigated in the experiments of Brunel et al., Phys. Rev. Lett., 1999, 83, 2722. We discuss the conditions for obtaining the maximum of the probability of emission of a single photon and a pair of photons, under the constrains of finite field strength and finite interaction time with excitation field. We obtain analytical expression for the probability of emitting zero, one, and two photons for the square pulse, and discuss semi-classical and strongly quantum limiting cases. Numerical results obtained from the generating function formalism are compared with experimental results showing that the two level system approach is suitable for the description of cryogenic temperature single molecules, and that experiments were conducted very close to the optimal conditions.
Summary form only given. On-demand single-photon sources that produce a single photon as a response to a deterministic input are widely regarded as critical light sources for quantum key distribution, quantum repeaters, and quantum information science generally [1]. The wavepackets from such a source must have low probability to contain more than one photon, exist in a pure state so they may interfere with one another, and have a spatio-temporal profile that is appropriate for their application [2]. A very popular source for generating high quality single-photon wavepackets is the quantum two-level system, an artificial atom with one ground and one excited state, often implemented using solid-state quantum dots embedded within microcavities [3]. The two-level system is excited with a short resonant pulse of width τFWHM that prepares the system in its excited state, called a π-pulse, where it subsequently decays and emits a single photon with almost unity probability. Because the typical pulse lengths are orders of magnitude shorter than the lifetime of the system, it is often not appreciated that there is finite chance for an ideal two-level system to emit more than one photon at a time.
We investigate the reduction in the probability of single-photon emission from a single-emitter system caused by intersystem crossing. The physical model is that a single-emitter system with three levels is excited by a square pulse laser, and then a single photon is emitted at a controllable time. We find that the intersystem crossing always reduces the probability of single-photon emission. We obtain quantitative and analytical expressions for the probabilities of zero-, one-, and two-photon emission and Mandel's Q parameter as the function of the intersystem crossing rate under the conditions of finite field strength and finite interaction time with the laser pulse. We discuss the conditions for obtaining the maximal probability of single-photon emission in the presence of intersystem crossing. We find that one can obtain a high probability of single-photon emission from the single-emitter system using square pulse excitation in comparison with continuous wave excitation. Mandel's Q parameter exhibits a transition from a sub-Poissonian behavior to a super-Poissonian behavior due to the influence of intersystem crossing. The calculated Mandel's Q parameter and second-order correlation function of photon emission considering the coherent effect are in better agreement with the experimental data.
Due to their excellent optical properties, quantum dots are promising for applications in photonic quantum technologies. For on-demand single-photon generation, a two-level system given by an excitonic transition is typically excited with a resonant laser pulse of area π. This prepares the two-level system in its excited state from where it spontaneously emits a single photon. However, emission that occurs already during the presence of the laser pulse allows for re-excitation and, thus, multi-photon emission which limits the single-photon purity [1]. In contrast, when exciting the system with a pulse of area 2π, the system is expected to be returned to the ground state. However, in this case emission during the presence of the pulse is most likely to occur when the system is in its excited state – exactly after an area of π has been absorbed. This restarts the Rabi oscillation with a pulse area of π remaining in the pulse which leads to re-excitation with near-unity probability and the emission of a second photon within the excited state lifetime [2,3]. Finally, we present the generation of single photons with ultra-low multi-photon probability [4]. Using two-photon excitation of the bi-exciton suppresses re-excitation and improves the single photon purity by several orders of magnitude for short pulses. [1] K. A. Fischer, et al., New J. Phys. 18, 113053 (2016) [2] K. A. Fischer, et al., Nature Physics 13, 649-654 (2017) [3] K. A. Fischer, et al., Quantum Sci. Technol. 3, 014006 (2017) [4] L. Hanschke et al., arxiv: 1801.01672 (2018)