Limits to coherent scattering and photon coalescence from solid-state quantum emitters

Abstract

. Limits to coherent scattering and photon coalescence from solid-state quantum emitters. Physical Review B, 95(20), [201305] University of Bristol -Explore Bristol Research General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms The desire to produce high-quality single photons for applications in quantum information science has lead to renewed interest in exploring solid-state emitters in the weak excitation regime. Under these conditions it is expected that photons are coherently scattered, and so benefit from a substantial suppression of detrimental interactions between the source and its surrounding environment. Nevertheless, we demonstrate here that this reasoning is incomplete, as phonon interactions continue to play a crucial role in determining solid-state emission characteristics even for very weak excitation. We find that the sideband resulting from non-Markovian relaxation of the phonon environment is excitation strength independent. It thus leads to an intrinsic limit to the fraction of coherently scattered light and to the visibility of two-photon coalescence at weak driving, both of which are absent for atomic systems or within simpler Markovian treatments.

Intrinsic and environmental effects on the interference properties of a high-performance quantum dot single-photon source

“…In particular, as previously mentioned, phonon interactions in QDs have been shown to lead to two primary mechanisms through which photons can lose coherence. The first is a coupling induced by displacement of the vibrational lattice due to the changes in the charge configuration of the QD [24,38,39]. During radiative recombination of an exciton, this can lead to the emission or absorption of a lattice excitation in addition to an emitted photon.…”

“…To describe the effect of phonon interactions on the exciton dynamics, we shall use a polaron master-equation approach [33,39,47,51]. Here we apply the unitary transformation…”

“…This leads to a displaced representation of the phonon environment, providing an optimized basis for a perturbative description of the QD dynamics. Importantly, this transformation naturally captures the non-Markovian relaxation behavior of the phonon environment during exciton recombination [24,39,47].…”

“…Phonon-induced dephasing, on the other hand, has a characteristic timescale of picoseconds [33][34][35][36][37] and impacts the spectral properties of the photons in two principle ways. The first is the emergence of a broad phonon sideband, related to the relaxation of the vibrational lattice of the host material during photon emission [24,38,39]. The second is a broadening of the zero phonon line due to virtual phonon processes-here a phonon in the material scatters off the QD, driving a virtual transition to an excited electronic state, and leading to a random phase change of the emitted photon [27,40,41].…”

Imaginary echoes

“…Highly directional emission is crucial to the overall efficiency of the source, and is typically achieved by either placing the QD in a waveguide with low out-of-plane scattering 15,16 , or by coupling resonantly to an optical cavity mode 6-9,12,13 . Nevertheless, the solid-state nature of QDs leads to strong coupling between the electronic degrees of freedom and their local environment; fluctuating charges 17 , nuclear spins 18,19 , and lattice vibrations 20-23 all lead to a suppression of photon coherence and a resulting reduction in indistinguishability 11,[24][25][26][27] . While early experiments were indeed limited by these factors 6-9 , improvements in fabrication and resonant excitation techniques have steadily increased photon indistinguishability to levels now exceeding 99% in resonantly coupled QD-cavity systems 12,13 .…”