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Physical Review A

Entangled-photon generation from a quantum dot in a microcavity through pulsed laser irradiation

Kazunori Shibata, Hiroshi Ajiki

Published Apr 22, 2014• 3 citations
Publisher

Abstract

We theoretically investigate fast and highly efficient entangled-photon generation by irradiating a quantum dot (QD) embedded in a microcavity with short laser pulses. We evaluate the optimized width and field strength of the pulses for achieving a high population of an excited state as an entangled-photon source. We also calculate the emission time of the entangled photon pair as a function of the cavity quality factor $Q$. The shortest emission time reaches $3/(4g)$ under the condition $Q={\ensuremath{\omega}}_{0}/(4g)$, where $g$ is the coupling frequency between the cavity photon and excited states of the QD and ${\ensuremath{\omega}}_{0}$ is the excitation frequency of the QD. The QD-cavity system has a potential for fast and efficient generation of high-quality entangled-photon pairs.

DOI
10.1103/physreva.89.042319

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We theoretically investigate fast and highly efficient entangled-photon generation by irradiating a quantum dot (QD) embedded in a microcavity with short laser pulses. We evaluate the optimized width and field strength of the pulses for achieving a high population of an excited state as an entangled-photon source. We also calculate the emission time of the entangled photon pair as a function of the cavity quality factor QQQ. The shortest emission time reaches 3/(4g)3/(4g)3/(4g) under the condition Q=\ensuremathω0/(4g)Q={\ensuremath{\omega}}_{0}/(4g)Q=\ensuremathω0​/(4g), where ggg is the coupling frequency between the cavity photon and excited states of the QD and \ensuremathω0{\ensuremath{\omega}}_{0}\ensuremathω0​ is the excitation frequency of the QD. The QD-cavity system has a potential for fast and efficient generation of high-quality entangled-photon pairs.
Theoretical study on entangled photon generation from a quantum dot (QD) embedded in microcavity is briefly reviewed within a strong coupling regime. In this system, entangled photon pairs can be generated through the cascade photon emission via dressed states in the cavity quantum electrodynamics (CQED). Excitation levels of a QD are modeled by a V-type three-level system and four-level system including a biexciton. In contrast to the entangled-photon generation until now, all four Bell states can be generated from an identical cavity system by simply selecting applied-field polarizations and frequencies as characteristic features of the CQED. The CQED effects play a crucial role in providing a high degree of entanglement: (i) spectral fi ltering can be used to extract entangled photons due to the vacuum Rabi splitting and (ii) non-entangled co-polarized photons are strongly suppressed due to the photon blockade effect.
Entangled photon pairs are a ubiquitous resource in quantum technologies, used in quantum key distribution and quantum networking as well as fundamental tests of non-locality. For scalable quantum networks, pairs that are indistinguishable in all unentangled degrees of freedom are essential, as they enable high-fidelity entanglement swapping across network nodes. To date the most-studied sources of "swappable" entangled photon pairs have been based on spontaneous parametric down-conversion (SPDC) in non-linear crystals. However, the probabilistic nature and unavoidable trade-off between brightness and unwanted multi-photon emission limits their performance in lossy channels. Here, we demonstrate a high-fidelity source of "swappable" entangled photon pairs using a semiconductor quantum dot (QD) coupled to a tunable microcavity. By actively modulating the QD emission between orthogonal polarisation states, delaying one path in a low-loss Herriott cell, and recombining the two on a balanced beam splitter, we generate entangled photon pairs with a fidelity of 96.1±0.596.1\pm0.596.1±0.5 %. We identify and mitigate fidelity-limiting factors, achieving a maximum fidelity of 98.1±0.598.1\pm0.598.1±0.5 % through time-resolved post-selection. The scheme suppresses residual multi-photon events concentrated near the excitation pulse and has only a modest impact on the rate. Furthermore, the photons are mutually indistinguishable, enabling efficient entanglement swapping. Our results establish semiconductor QDs as a viable platform for quantum network-compatible swappable entangled photon pair generation, with feasible entanglement generation rates exceeding 0.5 Gpairs/s.
We propose two schemes to realize a highly efficient solid-state source of photon pairs using four-wave mixing and stimulated Raman adiabatic passage in a single quantum dot embedded in a microcavity. A resonant continuous-wave laser applied between biexciton and exciton states and a pulsed laser applied between a ground state and exciton state are utilized to facilitate coherent pumping. We show in the case of four-wave mixing that, although the probability of generating two photons in a cavity mode is small without cavity damping, two-photon-resonant emission is enhanced by cavity damping within the strong-coupling regime. For strong continuous-wave laser, a single photon from a pulsed laser and two-photon-resonant transition through a strongly coupled cavity mode lead to a (1+2)-type Raman transition through the generated Autler-Townes doublet. We also discuss the spectrum of the generated photon pair and the photon-photon correlations in the generated photon pair.
In this paper, we provide theoretical study on entangled photon generation by means of RHPS in a QD embedded in a microcavity. The emphasis lies on the photon blockade effect on efficient entangled photon generation with high degree of purity. Furthermore, we note that a biexciton plays only a minor role in entangled photon generation because of the characteristic excited states in the cavity QED. This is contrast to entangled photon generation in a semiconductor until now, in which excitation of biexciton is indispensable for the entangled photon generation.
A quantum dot strongly coupled to a photonic crystal has been recently proposed as a source of entangled photon pairs [R. Johne et al., Phys. Rev. Lett. 100, 240404 (2008)]. The biexction decay via intermediate polariton states can be used to overcome the natural splitting between the exciton states coupled to the horizontally and vertically polarized light modes so that high degrees of entanglement can be expected. We investigate theoretically the features of realistic dot-cavity systems, including the effect of the different oscillator strength of excitons resonances coupled to the different polarizations of light. We show that in this case, an independent adjustment of the cavity resonances is needed in order to keep a high entanglement degree. We also consider the case when the biexciton-exciton transition is also strongly coupled to a cavity mode. We show that a very fast emission rate can be achieved allowing the repetition rates close to the terahertz range. Such fast emission should however be paid for by a very complex tuning of the many strongly coupled resonances involved and by a loss of quantum efficiency. Altogether a strongly coupled dot-cavity system seems to be very promising as a source of entangled photon pairs.
Summary form only given. The latest advances in the design and implementation of semiconductor sources of quantum light show their competence to efficiently deliver indistingishable single photons or photon pairs with high degree of entanglement. These achievements combined with the possibility of photon storage show the potential of quantum dots to become building blocks of a quantum network. Here we investigated the possibility to extend the use of micro-pillar cavities to extraction of photon pairs. Particularly, we are interested in quantum dots excited resonantly using a method of two-photon resonant excitation of the biexciton. We theoretically modelled the system consisting of a two-photon resonantly excited quantum dot placed in a micro-pillar cavity. We estimated the coupling strength required to achieve cavity mediated photon pair emission. We related this finding to the device design parameters, which indicate a feasible design. Controlling the parameters as cavity bandwidth and coupling strength we can achieve difference emission dynamics ranging from individual emission of biexciton and exciton photons to a cavity mediated bi-photon emission.