Symmetric quantum dots as efficient sources of highly entangled photons: Violation of Bell's inequality without spectral and temporal filtering

Abstract

An ideal emitter of entangled photon pairs combines the perfect symmetry of an atom with the convenient electrical trigger of light sources based on semiconductor quantum dots. Our source consists of strain-free GaAs dots self-assembled on a triangular symmetric (111)A surface. The emitted photons reveal a fidelity to the Bell state as high as 86(±2)% without postselection. We show a violation of Bell's inequality by more than five times the standard deviation, a prerequisite to test a quantum cryptography channel for eavesdropping. Due to the strict nonlocal nature the source can be used for real quantum processing without any postprocessing. The remaining decoherence channel of the photon source is ascribed to random charge and nuclear spin fluctuations in and near the dot.

Statistical properties of exciton fine structure splitting and polarization angles in quantum dot ensembles

“…Firstly, the basic idea can be easily generalized to study the optical properties of high-symmetric QDEs. Although the two bright states are degenerate for QDs with C 3v or D 2d symmetry [40], the random term V 1 can still render the probability of finding QDs with vanishing FSS small, as seen in recent experiments [41][42][43]. For high-symmetric QDEs, δ 0 = 0, thus only two independent parameters are required to fully characterize the statistical properties of FSS and polarization angle.…”

“…We estimate σ κ and σ δ ∼ 1 µeV using the results from Ref. [41,42], which seems to a bit smaller than that in C 2v symmetric QDEs, see Table I. Secondly, the effective model is derived purely from symmetry argument, and is independent of the morphology details of QDs, thus it is also applicable to study the optical properties of other semiconductor nanostructures, e.g., quantum rod and colloid nanocrystals [44][45][46].…”

Single pairs of time-bin-entangled photons

“…It must be said that down-conversion sources still reach better entanglement fidelities. Yet, quantum-dot sources show rapid progress towards purer (polarization-)entangled states [43][44][45][46][47]. A recent approach is to create polarizationentangled photons in a [111]-grown quantum dot embedded in a nanowire [47,48], where the cylindric symmetry reduces the fine-structure splitting [49].…”

The role of phonons for exciton and biexciton generation in an optically driven quantum dot

“…Typical values in InGaAs QDs range from almost zero to a few 100 µeV [32,33]. For schemes that are targeted at the preparation of the biexciton state, in order to subsequently initiate a decay cascade that creates entangled photon pairs [12,15,17] it is a necessary precondition to ideally have a vanishing exchange splitting [15,16,34,35]. Otherwise, a kind of which-path information would be introduced in the decay that prevents a high degree of entanglement [14,36].…”