A source of triggered entangled photon pairs is a key component in quantum information science; it is needed to implement functions such as linear quantum computation, entanglement swapping and quantum teleportation. Generation of polarization entangled photon pairs can be obtained through parametric conversion in nonlinear optical media or by making use of the radiative decay of two electron-hole pairs trapped in a semiconductor quantum dot. Today, these sources operate at a very low rate, below 0.01 photon pairs per excitation pulse, which strongly limits their applications. For systems based on parametric conversion, this low rate is intrinsically due to the Poissonian statistics of the source. Conversely, a quantum dot can emit a single pair of entangled photons with a probability near unity but suffers from a naturally very low extraction efficiency. Here we show that this drawback can be overcome by coupling an optical cavity in the form of a 'photonic molecule' to a single quantum dot. Two coupled identical pillars-the photonic molecule-were etched in a semiconductor planar microcavity, using an optical lithography method that ensures a deterministic coupling to the biexciton and exciton energy states of a pre-selected quantum dot. The Purcell effect ensures that most entangled photon pairs are emitted into two cavity modes, while improving the indistinguishability of the two optical recombination paths. A polarization entangled photon pair rate of 0.12 per excitation pulse (with a concurrence of 0.34) is collected in the first lens. Our results open the way towards the fabrication of solid state triggered sources of entangled photon pairs, with an overall (creation and collection) efficiency of 80%.

The mesoscopic spin system formed by the 10 4 − 10 6 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of many-body spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counter-part as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum-)dynamics of the coupled electron-nuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic, coupled electron-nuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and read-out quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a non-zero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantummechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.

We have studied the electron spin relaxation in semiconductor InAs/GaAs quantum dots by time-resolved optical spectroscopy. The average spin polarization of the electrons in an ensemble of p-doped quantum dots decays down to 1/3 of its initial value with a characteristic time T(Delta) approximately 500 ps, which is attributed to the hyperfine interaction with randomly oriented nuclear spins. We show that this efficient electron spin relaxation mechanism can be suppressed by an external magnetic field as small as 100 mT.

We report on the optical spectroscopy of a single InAs/GaAs quantum dot (QD) doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A 0 whose effective spin J = 1 is significantly perturbed by the QD potential and its associated strain field. The spin interaction with photo-carriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of µeV, but vanishingly small for electrons.PACS numbers: 71.35. Pq, 78.67.Hc,75.75.+a,78.55.Cr The spin state of a single magnetic impurity could be envisaged as a primary building block of a nanoscopic spin-based device [1,2] in particular for the realization of quantum bits [3]. However probing and manipulating such a system require extremely high sensitivity. Several techniques have been successfully developed over the last few years to address a single or few coupled spins: electrical detection [4,5], scanning tunneling microscopy (STM) [6,7,8,9], magnetic resonance force microscopic [10], optical spectroscopy [11]. Recently, Besombes et al. [12,13,14,15] have investigated the spin state of a single Mn +2 ion embedded in a single II-VI self-assembled quantum dot (QD). In this system the magnetic impurity is an isoelectronic center in a 3d 5 configuration with spin S = 5/2. The large exchange interaction between the spin of the photocreated carriers confined inside the dot and the Mn magnetic moment induces strong modifications of the QD photoluminescence (PL) spectrum: 2S + 1 = 6 discrete lines are observed, reflecting the Mn spin state at the instant when the exciton recombines.The case of the Mn ion is different in GaAs, since the impurity is an acceptor in this matrix with a rather large activation energy (113 meV). Two types of Mn centers exist in GaAs, the A 0 and the A − states. In low doped GaAs (below 10 19 cm −2 ), the former is dominant. It corresponds to the 3d 5 + h configuration, where h is a hole bound to the Mn ion with a Bohr radius around 1 nm [16]. When considering a single Mn impurity in InAs QD several issues arise: the impurity configuration, its possible change when photo-carriers are captured, the influence on the binding energy of excitonic complexes, the strength and sign of the effective exchange interaction with each of the carriers (electron or hole) in the QD S-shell. In this Letter, we report the first evidences of a single Mn impurity in an individual InAs QD which enable us to answer most of the above questions. In particular, we find that the formation of excitons, biexciton and trions is weakly perturbed by the impurity center, whereas the effective exchange coupling with the Mn impurity (found in the A 0 configuration) is ferromagnetic for holes (a few 100 µeV's) and almost zero for the electrons.The sample was grown by molecular beam epitaxy on a semi-insulating GaAs [001] substrate. The Mn-doped quantum layer was embedded inbetween an electron reservoir and a Schottky gate. This des...

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We show that the spin state of the resident electron in an n-doped self-assembled InAs-GaAs quantum dot can be written and read using nonresonant, circularly polarized optical pumping. A simple theoretical model is presented and accounts for the remarkable dynamics producing counterpolarized photoluminescence.

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The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2431758͔There is currently great interest in producing entangled photons on demand for applications in quantum information processing. 1 One proposal which spurred much research is using radiative biexciton-exciton cascade in semiconductor quantum dots ͑QDs͒ to produce pairs of polarization entangled photons. 2 In an idealized QD, the bright exciton states ͑M = ±1͒ are degenerate. In this case the two decay paths from the biexciton to the vacuum state via the intermediate single exciton are indistinguishable in energy; thus photons emitted in the radiative cascade are polarization entangled. However, in practice, the rotational symmetry of a self-assembled QD is broken and the electron-hole exchange interaction mixes the bright exciton states into a nondegenerate doublet ͓referred to as a fine-structure splitting ͑FSS͔͒. This leads to an energetically distinguishable recombination path for the biexciton-exciton cascade. Polarization correlations are observed in the linear basis but polarization entanglement is destroyed due to the FSS. 3,4 For photons to be polarization entangled using this scheme, the requirement that the FSS be less than the homogeneous linewidth must be met. The FSS is typically 10-100 eV, while the homogeneous linewidth of self-assembled InGaAs QDs is ϳ1 eV. 5 Techniques used to actively tune the FSS include an inplane electric 6 or magnetic 7 field and an in situ uniaxial stress. 8 Also, QDs which are smaller due to the growth process 9 or subsequent annealing 10 have a smaller FSS. Unfortunately, such QDs are higher in energy and the QD photons become difficult to distinguish from those produced in the wetting layer. Recently, polarization entangled photons have been reported from specific QDs with energetically overlapping bright exciton states 11 and initially nondegenerate states tuned via a magnetic field. 12 However, a robust approach that would allow one to actively tune the FSS from a large value to zero for each QD is still necessary to realize an event ready entangled photon pair source. To this end we further explore the effect of a lateral electric field on the FSS.There are three basic characteristics of an exciton in a lateral electric field attributed to the quantum confined Stark effect, as has been investigated for quantum wells 13 and QDs: 14 a redshift in recombination energy, decreased oscillator strength, and an increase in nonradiative carrier tunneling probability. Additionally, electric fi...