Polarization-correlated photon pairs from a single quantum dot

Abstract

Polarization correlation in a linear basis, but not entanglement, is observed between the biexciton and single-exciton photons emitted by a single InAs quantum dot in a two-photon cascade. The results are well described quantitatively by a probabilistic model that includes two decay paths for a biexciton through a non-degenerate pair of one-exciton states, with the polarization of the emitted photons depending on the decay path. The results show that spin non-degeneracy due to quantumdot asymmetry is a significant obstacle to the realization of an entangled-photon generation device. [4,5,6], but they can also generate sequences of photons in a radiative cascade [7,8]. In such a cascade, each photon has a unique wavelength, and the photons may also have correlated, or even entangled [9] polarizations.In the two-photon cascade, a biexciton singlet state (two electrons and two holes, 2X) decays to one of two optically active single-exciton states (one electron and one hole, 1X) by emitting one photon, and then to the empty-dot state by emitting a second photon. In theory, the polarization properties of photon pairs emitted through these two decay paths result entirely from properties of the optically-active 1X doublet [10,11,12]. For a symmetric quantum dot, the two 1X states are degenerate, and the two decay paths become "indistinguishable," ideally producing polarization-entangled photons [9]. For an asymmetric quantum dot, the 1X doublet is split through the electron-hole exchange interaction into states that couple to photons having orthogonal linear polarizations [13,14,15]. If this splitting is much larger than the radiative linewidth, then the two paths become "distinguishable," with one decay path producing two horizontally-polarized photons and the other producing two vertically polarized photons, for example. In this case, polarization correlation is expected only in a single, preferred basis. Some additional factors are also important, such as spin flip [16] and decoherence processes [17,18,19] that randomize the intermediate 1X state, and valence-band mixing [20].In this article, we present an experimental study of the polarization correlation properties of photon pairs emitted through biexciton decay in a single InAs quantum dot. While temporal correlations between 1X and 2X photons have previously been seen [21], polarization correlation has not yet been reported, to our knowledge, although a lack of such correlation has been mentioned elsewhere [22]. For our sample, we observe a strong polarization correlation in a linear polarization basis, verifying the theoretical picture described above. However, we do not observe entanglement, suggesting that quantum-dot asymmetry is an obstacle to realizing an entangled photon source.A sample was fabricated containing self-assembled InAs quantum dots [3] grown by molecular-beam epitaxy on a (001) GaAs substrate, capped by 75 nm of GaAs. A high growth temperature increased intermixing between the InAs and surrounding GaAs, shortening the quantum-dot emission wa...

Inversion of exciton level splitting in quantum dots

“…However, polarized cross-correlation measurements on the exciton and biexciton photons have confirmed that a splitting of the intermediate exciton state results in spectral "which path" information, and the emission of only polarization-correlated photon pairs. [12][13][14] In this Brief Report we present the reduction of this exciton level splitting through the zero point, a crucial step toward the generation of entangled photon pairs. The splitting of the exciton level has been measured in many dot systems including InAs/ GaAs, 12 CdSe/ ZnSe, 15 InP/ GaInP, 16 and interface fluctuations in thin GaAs/ AlGaAs quantum wells.…”

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

“…The experimental realization of Rabi rotations between the biexciton state and an exciton state [165], as well as between the biexciton and the ground state [149,150,18,151,166,167], which can be both described in the four-level model, has great potential impact on the field of quantum logic where exciton-biexciton systems have been proposed as two-qubit quantum gates [21,165] or as emitters of entangled photon pairs [11,12,13,14,15,16,17,18]. Rabi rotations between the ground and the biexciton state were also preformed by using two-color excitations [168].…”

“…This compatibility with existing technologies makes QDs attractive candidates for a wide range of applications ranging from optoelectronic devices like QD lasers [1,2,3,4], where the target is an improved performance compared to other laser structures, up to new applications in the fields of quantum cryptography or quantum information processing, where the presence of discrete energy levels is mandatory. Examples of such quantum applications are single-photon sources [5,6,7,8,9,10], sources of entangled photon pairs [11,12,13,14,15,16,17,18], and qubit devices or quantum gates [19,20,21,22,23,24]. The functionality of all these latter applications relies on the preparation of a well-defined quantum state.…”

Single photon emission with high degree of circular polarization from a single quantum dot under zero magnetic field

“…Therefore, suppression of spin flip during whole processes including absorption, capture, relaxation, and radiative recombination is a crucial issue. In case of neutral exciton, long-range electron-hole exchange interaction stems from an anisotropic confinement potential [3][4][5] will lead to a simultaneous spin flip of electron and hole [6]. Singly-charged exciton (trion), on the other hand, is free from the exchange interaction owing to the presence of spin-paired carriers.…”