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Journal of the Optical Society of America B

Generation of frequency-tunable squeezed single photons from a single quantum dot

Parvendra Kumar, Agnikumar G. Vedeshwar

Published Nov 13, 2018• 2 citations
Publisher

Abstract

We demonstrate the generation of frequency-tunable squeezed single photons from a continuous wave laser-driven single-quantum-dot system using a rigorous theoretical formalism, the so-called polaron master equation theory for accurately incorporating the exciton–phonon coupling. We investigate the squeezing for both resonant and off-resonant excitations and show that even for an appropriate off-resonant excitation, the degree of quadrature squeezing can be equal to or even greater than the maximum value obtained for a resonant excitation due to the increased purity of the quantum dot system. We further demonstrate, without losing the degree of squeezing, the tunability of the central frequency of squeezed photons by adjusting the detuning. We show that the detrimental effects due to the exciton–phonon coupling remain negligible up to the phonon bath temperature of 20 K and become quite appreciable beyond this temperature, particularly at higher phonon cutoff frequencies.

DOI
10.1364/josab.35.003055

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We theoretically demonstrate the enhanced and dephasing-immune squeezing in the resonance fluorescence of a single quantum dot (QD) confined to a pillar microcavity and driven by a continuous wave laser. We employ a formalism based on polaron master equation theory for incorporating the influence of exciton-phonon coupling quite accurately in the dot-cavity system. We show a significant enhancement of squeezing due to cavity coupling of the QD as compared to that of an ideal single two-level system in free space. Particularly, we show a fourfold enhancement in squeezing as compared to that of a single QD without cavity coupling. We further demonstrate the persistence of squeezing even when the pure dephasing becomes greater than the radiative decay rate. These features are attributed to the cavity-enhanced coherence causing partial reduction of the deteriorating effects of phonon-induced incoherent rates. We also show that the deteriorating effects of phonon-induced incoherent rates on squeezing can be partially circumvented by properly adjusting the detunings.
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We theoretically propose a scheme for the quadrature squeezing of the cavity field via dissipative processes. The effects of the electron-phonon interaction (EPI) on the squeezing are investigated, where the cavity is off-resonantly coupled with a coherently driven quantum dot (QD) which is allowed to interact with an acoustic-phonon reservoir. Under certain conditions, the participation of the phonon induced by both the EPI and the off-resonant coupling of the cavity with the QD enables some dissipative processes to occur resonantly in the dressed-state basis of the QD. The cavity-mode photons emitted or absorbed during the phonon-mediated dissipative processes are correlated, thus leading to the squeezing of the cavity field. A squeezed vacuum reservoir for the cavity field is built up due to the EPI plus the off-resonant coupling between the cavity and the QD. The numerical results obtained with an effective polaron master equation derived using second-order perturbation theory indicate that, in low temperature limit, the degree of squeezing is maximal but the increasing temperature of the phonon reservoir could hinder the squeezing and degrade the degree of the squeezing of the cavity field. In addition, the presence of the photonic crystal could enhance the quadrature squeezing of the cavity field.
We demonstrate theoretically that the single-photon purity of photons emitted from a quantum dot exciton prepared by phonon-assisted off-resonant excitation can be significantly higher in a wide range of parameters than that obtained by resonant preparation for otherwise identical conditions. Despite the off-resonant excitation, the brightness stays on a high level. These surprising findings exploit the fact that the phonon-assisted preparation is a two-step process where phonons first lead to a relaxation between laser-dressed states while high exciton occupations are reached only with a delay to the laser pulse maximum by adiabatically undressing the dot states. Due to this delay, possible subsequent processes, in particular multiphoton excitations, appear at a time when the laser pulse is almost gone. The resulting suppression of reexcitation processes increases the single-photon purity. Due to the spectral separation of the signal photons from the laser frequencies this enables the emission of high quality single photons not disturbed by a laser background while taking advantage of the robustness of the phonon assisted scheme.