The spin of an electron is a natural two-level system for realizing a quantum bit in the solid state [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] . For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon-related spin relaxation [1][2][3][4][5][6][7] . However, this advantage is offset by the hyperfine interaction between the electron spin and the 10 4 to 10 6 spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds [8][9][10][11][12][13][14] . Spin-echo techniques have been used to mitigate the hyperfine interaction 14,15 , but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this 16,17 but is very difficult to realize in practice 12,18,19 . Exploring materials with zero-spin nuclei is another option, and carbon nanotubes 20 , graphene quantum dots 21 and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin-3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak 22,23 . We demonstrate here high fidelity (about 99 per cent) initialization of a single hole spin confined to a self-assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid-state quantum networks 24 that can intra-convert the spin state with the polarization of a photon.Our scheme to initialize a single hole spin is presented in Fig. 1. The quantum dot contains a single hole. The strong in-built strain in an InAs quantum dot shifts the valence light hole states with spin J 53/2, J z 5 61/2 away from the fundamental gap such that the uppermost valence states have heavy hole character with spin J 53/2, J z 5 63/2. The corresponding hole spin states are represented as X j i and Y j i. A s z -polarized laser drives the Y j i hole to an exciton state with spin S z 5 21/2, XY,; j i, containing a spin-up, spin-down hole pair and a spin-down electron. Unlike the hole spin, the electron spin interacts with the nuclear spins through the contact hyperfine interaction. The electron spin experiences a small magnetic field, ,20 mT (refs 8-12), as a result of the incomplete cancellation of the random nuclear spins in the quantum dot. The component of the magnetic field in the plane, B xy nuclei , causes the electron spin in the excited state to precess with a period of ,1 ns. The coherence of the precession is destroyed by spontaneous emission with a characteristic ...

Semiconductors have uniquely attractive properties for electronics and photonics. However, it has been difficult to find a highly coherent quantum state in a semiconductor for applications in quantum sensing and quantum information processing. We report coherent population trapping, an optical quantum interference effect, on a single hole. The results demonstrate that a hole spin in a quantum dot is highly coherent.

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...

We report the observation of a spin-flip process in a quantum dot whereby a dark exciton with total angular momentum L = 2 becomes a bright exciton with L = 1. The spin-flip process is revealed in the decay dynamics following nongeminate excitation. We are able to control the spin-flip rate by more than an order of magnitude simply with a dc voltage. The spin-flip mechanism involves a spin exchange with the Fermi sea in the back contact of our device and corresponds to the high temperature Kondo regime. We use the Anderson Hamiltonian to calculate a spin-flip rate, and we find excellent agreement with the experimental results.

Energy transfer from semiconductor nanocrystal monolayers to metal surfaces revealed by time-resolved photoluminescence spectroscopy