Photonic cluster states are a resource for quantum computation based solely on single-photon measurements. We use semiconductor quantum dots to deterministically generate long strings of polarization-entangled photons in a cluster state by periodic timed excitation of a precessing matter qubit. In each period, an entangled photon is added to the cluster state formed by the matter qubit and the previously emitted photons. In our prototype device, the qubit is the confined dark exciton, and it produces strings of hundreds of photons in which the entanglement persists over five sequential photons. The measured process map characterizing the device has a fidelity of 0.81 with that of an ideal device. Further feasible improvements of this device may reduce the resources needed for optical quantum information processing.

We demonstrate that the quantum dot-confined dark exciton forms a long-lived integer spin solid state qubit which can be deterministically on-demand initiated in a pure state by one optical pulse. Moreover, we show that this qubit can be fully controlled using short optical pulses, which are several orders of magnitude shorter than the life and coherence times of the qubit. Our demonstrations do not require an externally applied magnetic field, and they establish that the quantum dotconfined dark exciton forms an excellent solid state matter qubit with some advantages over the half-integer spin qubits, such as the confined electron and hole, separately. Since quantum dots are semiconductor nanostructures that allow integration of electronic and photonic components, the dark exciton may have important implications for implementations of quantum technologies consisting of semiconductor qubits.

We use an atomistic model to consider the effect of shape symmetry breaking on the optical properties of self-assembled InAs/GaAs quantum dots. In particular, we investigate the energy level structure and optical activity of the lowest energy excitons in these nanostructures. We compare between quantum dots with two-fold rotational and two reflections (C2v) symmetry and quantum dots in which this symmetry was reduced to one reflection only (Cs) by introducing a facet between the quantum dots and the host material. We show that the symmetry reduction mostly affects the optical activity of the dark exciton. While in symmetric quantum dots, one of the dark exciton eigenstates has a small dipole moment polarized along the symmetry axis (growth direction) of the quantum dot, in non-symmetric ones, the two dark excitons' dipole moments are predominantly cross-linearly polarized perpendicular to the growth direction and reveal pronounced polarization anisotropy. Our model calculations agree quantitatively with recently obtained experimental data.

We perform full time resolved tomographic measurements of the polarization state of pairs of photons emitted during the radiative cascade of the confined biexciton in a semiconductor quantum dot. The biexciton was deterministically initiated using a π-area pulse into the biexciton two-photon absorption resonance. Our measurements demonstrate that the polarization states of the emitted photon pair are maximally entangled. We show that the measured degree of entanglement depends solely on the temporal resolution by which the time difference between the emissions of the photon pair is determined. A route for fabricating an on demand source of maximally polarization entangled photon pairs is thereby provided.The ability to generate entangled photons on-demand is crucial for many future applications in quantum information processing. Devices based on the biexcitonexciton radiative cascade in single semiconductor quantum dot are considered to be one of the best candidates for these applications [1][2][3]. The ability to deterministically excite the biexciton using its two-photon absorption resonance [4,5] makes this avenue even more promising. A remaining challenge, however, is the excitonic fine structure, which splits the two exciton eigenstates thus providing spectral "which-path" information on the radiative cascade and preventing the pairs of emitted photons from being polarization entangled [2]. Various strategies were tried in an attempt to reduce the influence of the fine structure splitting. Spectral [2] and temporal filtering [2,6], which introduce non desired, nondeterministic post selection. Enhancement of the radiative rate using the Purcell effect [3], thereby reducing, but not limiting the effect of exciton precession. Attempts to reduce the fine-structure splitting using heat treatment [7] or growth along the [111] crystalographic direction [8] were reported as well as applications of external stress [9], electric [10] and magnetic fields tuning [11,12]. These efforts, usually result in unwanted loss of emission quantum efficiency [10], and increase in the exciton spin decoherence [6].We present here a novel study of a single semiconductor quantum dot, optically depleted [13] and then resonantly excited on-demand by a π-area pulse to the biexciton two photon absorption resonance [5]. The resulting pairs of biexciton and exciton photons are detected by two superconducting detectors synchronized to the exciting laser pulse. By performing synchronized time resolved polarization tomography of the two emitted photons, we unambigously show that the photons remain maximally polarization entangled during the whole radiative decay, and that the measured degree of entanglement does not depend on the QD source, but rather depends on the temporal resolution by which the time difference between the two photon emissions can be determined. Since during the radiative decay the exciton does not lose coherence, there is no need to eliminate the excitonic fine structure splitting. A relatively simple arrangement [14,15] ...

We use one single, few-picosecond-long, variably polarized laser pulse to deterministically write any selected spin state of a quantum dot confined dark exciton whose life and coherence time are six and five orders of magnitude longer than the laser pulse duration, respectively. The pulse is tuned to an absorption resonance of an excited dark exciton state, which acquires non-negligible oscillator strength due to residual mixing with bright exciton states. We obtain a high fidelity one-to-one mapping from any point on the Poincaré sphere of the pulse polarization to a corresponding point on the Bloch sphere of the spin of the deterministically photogenerated dark exciton.Future technologies based on quantum information processing (QIP) [1][2][3][4][5][6] require the ability to coherently control matter two-level systems, or qubits. Since they are ultrafast and require no contacts, optical means of control are preferred. Spins of charge carriers in semiconductors are promising matter qubits since they complement contemporary leading technologies, specifically those of light sources and detectors. Semiconductor QDs isolate single carriers and can be easily incorporated into nanophotonic devices, thereby providing an excellent interface between single spins and single photons. For these reasons, semiconductor quantum dots (QDs) have been the subject of many recent works, which demonstrated significant progress in optical writing, readout, and control of confined spins [7][8][9][10][11][12][13][14][15][16].In semiconductors, the absorption of a photon results in the promotion of an electron from the full valence band, across the forbidden band-gap, to the empty conduction band, leaving its spin unaltered. The missing valence band electron (or "hole") and the conduction band electron form an electron-hole-pair with opposite spin directions, or a bright exciton (BE). The BE forms an integer spin (total spin 1) qubit in the matter.In a previous work, we demonstrated that, in straininduced self-assembled quantum dots, the polarization of a resonantly tuned, single picosecond optical pulse can be used to deterministically write the bright exciton spin qubit in any desired coherent state [14]. Such a process is not possible for single spins, where a few pulses are required to prepare the spin in an eigenstate and then a three step (Ramsey) rotation is need ed to write the spin state at will [7,8,10], a process that can take a few nanoseconds. Moreover, while full coherent control ("rotation") of the bright exciton can be achieved by one single optical pulse [15,17], single spins require two pulses and free precession in between [7][8][9]12]. However, these advantages of the bright exciton are not very useful, since its lifetime is rather short (sub ns), limited by radiative recombination of the electron-hole pair.Since light barely interacts with the electronic spin, an electron-hole pair with parallel spin directions is almost optically inactive. Such a pair is called a dark exciton (DE). The DE is also a spin integer (t...