Towards quantum-dot arrays of entangled photon emitters

Abstract

To make photonic quantum information a reality 1,2 , a number of extraordinary challenges need to be overcome. One of the outstanding challenges is the achievement of large arrays of reproducible "entangled" photon generators, maintaining the compatibility with integration with optical devices and detectors 3,4,5 . Semiconductor quantum dots (QDs) are potentially ideal for this. They allow generating photons on demand 6,7 without relying on probabilistic processes 8,9 . Nevertheless, most QD systems are limited by the intrinsic lack of symmetry, which allows to obtain only a small number (typically 1/100 or worse) of good dots per chip. The recent retraction of Mohan et al. 10 seemed to question the very possibility of matching site-control and high symmetry. Here we show that with a new family of (111) grown pyramidal sitecontrolled InGaAs 1- N  QDs, it is possible to overcome previous difficulties and obtain areas containing as much as 15% of polarization-entangled photon emitters, showing fidelities as high as 0.721±0.043.The idea underlining the principle of entangled photon emission with QDs relies on fundamental quantum physics: particle indistinguishability generates a superposition state when two energetically nearly degenerate quantum levels are populated at the same time. In QDs, entanglement resides in polarization of two photons emitted during the cascaded biexciton-exciton recombination 11 . Here one difficulty arises: when the two excitonic levels are not perfectly degenerate (i.e. there is a fine structure splitting, FSS), the entanglement in the emission persists, but a phase term between the two (linearly) polarized photons proportional to both energy and time is introduced. This results in a relative rotation of the two photon polarizations (not constant in time) making entanglement substantially impossible to be detected in a simple way 12 .All currently reported QD systems allowing entangled photon emission tend to present a large FSS, fundamentally allowing only a few (post-growth selected) QDs on a semiconductor wafer as good sources, while till now no entangled photon emission has been

Indium segregation during III–V quantum wire and quantum dot formation on patterned substrates

“…10,11 A noteworthy advantage of this approach is the fabrication of arrays of devices, as recently demonstrated in Ref. 12, where entangled photon emission from an array of In 0.25 Ga 0.75 As nanostructures was reported.…”

Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods

“…58 However, these methods do not allow the control of the position and even the size of the nanostructures, which is an essential requirement for applications such as single photon emitters. 59 This growth method of InGaN/GaN nanorods provides a template to allow for wafer-scale position controlled growth of InGaN QD arrays as demonstrated by Juska et al 60 for InGaAs QDs grown in inverted pyramidal arrays.…”

Coherence and degree of time-bin entanglement from quantum dots

“…In a more refined operation mode employing biexcitons, the Coulomb-bound four-carrier states containing two electrons and two holes, quantum dots can provide pairs of photons emitted in a fast cascade very similar to the original atomic cascade experiment by Aspect et al [2]. It has been demonstrated that in the absence of the fine structure splitting of the bright exciton levels, such a cascade exhibits polarization entanglement [3][4][5][6][7][8]. Entanglement of photons is a fundamental resource for long distance quantum communications [9,10], where it forms the central part of various quantum communication protocols like teleportation [11] and entanglement swapping [12].…”