Vanishing fine-structure splittings in telecommunication-wavelength quantum dots grown on (111)A surfaces by droplet epitaxy

Abstract

The emission cascade of a single quantum dot is a promising source of entangled photons. A prerequisite for this source is the use of a symmetric dot analogous to an atom in a vacuum, but the simultaneous achievement of structural symmetry and emission in a telecom band poses a challenge. Here we report the growth and characterization of highly symmetric InAs/InAlAs quantum dots self-assembled on C3v symmetric InP(111)A. The broad emission spectra cover the O (λ ∼ 1.3 µm), C (λ ∼ 1.55 µmm), and L (λ ∼ 1.6 µm) telecom bands. The distribution of the fine-structure splittings is considerably smaller than those reported in previous works on dots at similar wavelengths. The presence of dots with degenerate exciton lines is further confirmed by the optical orientation technique. Thus, our dot systems are expected to serve as efficient entangled photon emitters for long-distance fiber-based quantum key distribution.Semiconductor quantum dots (QD) are expected to play a central role in quantum information networks. A noteworthy device based on dots is the solid-state single photon source, which ensures absolute security in quantum key distribution (QKD)1 . Since QDs can confine charged carriers in nanometer-sized regions, recombination enables single photons to appear on demand, i.e., synchronously with a master clock shared in networks 2 . QKD over a 50 km commercial fiber has already been demonstrated with QD photon sources, which emitted at a wavelength of 1.5 µm 3 . The transmission distance in that work was limited purely by the absorption loss of silicate fibers. Exceeding this fundamental limit requires the development of quantum link protocols, which exploit the nonlocality inherent in quantum theory. An efficient source of entangled photon pairs is a key element in the realization of such protocols, examples of which include quantum teleportation 4 and entanglement swapping 5 . The generation of entangled photons with semiconductor QDs is directly linked to the singlet configuration of two excitons (X), which form a biexciton (XX). Eventually, two photons associated with the XX-X cascade show polarization correlations independent of the choice of measurement basis, yielding quantum entanglement in the polarization state. However, a common class of QDs exhibits considerable fine-structure splittings (FSS) 6-10 , which exclude entanglement in emitted photons 11 . Numerous attempts have been made to suppress FSS and recover the symmetry of QDs grown on conventional (001) oriented substrates [12][13][14][15][16][17][18][19] . However, from a practical point of view, the reproducible growth of symmetric dots with (at least) near-zero FSS is highly desirable.A noteworthy strategy for achieving such high QD symmetry is the application of C 3v symmetric (111) makes it possible to grow QDs on (111) substrates. Hence, a great reduction in FSS was observed in these QDs 23,24 , which led to the demonstration of entangled photon emission in pyramidal QDs on patterned (111)B substrates 25 , and the filtering-free violation ...

Universal Growth Scheme for Quantum Dots with Low Fine-Structure Splitting at Various Emission Wavelengths

“…Previous reports have shown a correlation between the emission wavelength and the FSS of the quantum dot for both SK dots [7] and DE dots [8]. However, for the DE QDs studied here, we observe no obvious correlation, as shown in Fig.…”

“…Production of quantum dots with naturally small FSS may be achieved in special cases, by self-assembled growth of InAs quantum dots around 885 nm, where an inversion of the wave-function symmetry is observed [7], or growth on (111) surfaces [8][9][10], aided by the underlying C 3v crystal symmetry which very often results in dot density preventing the spectral isolation of individual lines [11][12][13]. Another approach is the growth of strain-free GaAs=ðAl; GaÞAs QDs in locally etched pits [14].…”

Conditions for entangled photon emission from (111)B site-controlled pyramidal quantum dots

“…2). While vanishing FSS of QDs grown along (111) direction is predicted theoretically 20,42,43 and typically small values were obtained experimentally, 21,44,45 there are a number of effects that can cause deviation from theory. QD alloy disorder is a potential cause of reduced symmetry.…”

Fine structure of electron-hole complexes in trigonal quantum dots

“…In contrast to the structures with C 2v sym metry, in quantum dots with C 3v symmetry, the aniso tropic doublet splitting is forbidden [7,8]. In a number of works, the authors observed a very small (Շ10 μeV) anisotropic splitting in the [111] quantum dots imme diately after the growth (without further treatment) [9][10][11][12], as well as the generation of entangled photon pairs [13,14].…”