In semiconductors, the T2* coherence time of a single confined spin is limited either by the fluctuating magnetic environment (via the hyperfine interaction), or by charge fluctuations (via the spin-orbit interaction). We demonstrate that both limitations can be overcome simultaneously by using two exchange-coupled electron spins that realize a single decoherence-avoiding qubit. Using coherent population trapping, we generate a coherent superposition of the singlet and triplet states of an optically active quantum dot molecule, and show that the corresponding T2* may exceed 200 ns.

We report on the fabrication and characterization of a scannable Fabry-Perot microcavity, consisting of a curved micromirror at the end of an optical fiber and a planar distributed Bragg reflector. Furthermore, we demonstrate the coupling of single organic molecules embedded in a thin film to well-defined resonator modes. We discuss the choice of cavity parameters that will allow sufficiently high Purcell factors for enhancing the zero-phonon transition between the vibrational ground levels of the electronic excited and ground states.

Two-electron charged self-assembled quantum dot molecules exhibit a decoherence-avoiding singlet-triplet qubit subspace and an efficient spin-photon interface. We demonstrate quantum entanglement between emitted photons and the spin-qubit after the emission event. We measure the overlap with a fully entangled state to be (69.5 ± 2.7) %, exceeding the threshold of 50 % required to prove the non-separability of the density matrix of the system. The photonic qubit is encoded in two photon states with an energy difference larger than the timing resolution of existing detectors. We devise a novel heterodyne detection method, enabling projective measurements of such photonic color qubits along any direction on the Bloch sphere. [4][5][6], teleportation from a photonic to a spin qubit [7] and heralded distant spin entanglement using QDs in Voigt geometry [8]. However, the magnetic field configuration to achieve efficient spin measurement [9] is incompatible with the configuration for coherent manipulation.The moleclular states |S and |T 0 in optically active quantum dot molecules (QDMs) in Faraday geometry emerge as a promising alternative effective qubit for quantum information processing since (i) they exhibit a decoherence-avoiding clock-transition that is insensitive to fluctuations in both electric and magnetic fields [10], (ii) the spin polarized triplet states (|T + and |T − ) of the ground-state manifold exhibit cycling optical transitions [11], and (iii) the qubit states exhibit equal coupling strength to common optically excited trion states, essential for maximal spin-photon entanglement. In this letter, we experimentally determine the amount of entanglement obtained from the spontaneous emission from such an excited state.S-T 0 qubits in QDMs Our experiment is carried out on a single InGaAs self-assembled QDM, consisting of two QDs separated by a 9 nm GaAs tunneling barrier along the growth direction [12]. The QDM is embedded in a Schottky diode, formed by a semi-transparent metallic top gate and an n-doped layer, which is used to control the charge state of the QDM and the optical transition energies vie the quantum confined Stark effect [13]. A distributed Bragg reflector (DBR) below the doped layer forms a weak planar microcavity together with the top gate, enhancing the collection efficiency though a combination of emission profile modification and Purcell effect [9]. Thanks to engineered confinement energies of the two QDs, the QDM can be brought into the (1,1)-regime [14,15], where each QD is charged with a single electron. In this regime, the singlet state (|S ) is split from the triplet states (|T 0 , |T + and |T − ) by the exchange splitting, which is gate-voltage tunable and has a minimum value of J = 97 GHz in our device. The triplet states are split by 15.5 GHz from each other by an external magnetic field of 2 T that is applied along the growth direction (Faraday geometry). The relevant level scheme is outlined in fig. 1 (a). Under these conditions, |S and |T 0 can be compared to atomic clock ...

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Terahertz time-domain spectroscopy is shown to provide a convenient and rapid means to measure the conductivity of individual layers in semiconductor heterostructures such as terahertz quantum cascade lasers. By modeling the complex transmission at terahertz frequencies, the electron density and the in-plane momentum scattering time of the active regions and doped contact layers were determined for both GaAs/AlGaAs and InGaAs/InAlAs epilayers. The measured temperature dependence of the electron scattering rate revealed the significance of impurity and LO phonon scattering. The implications for laser operation at room temperature are discussed by considering the changes in absorption and resonant tunneling current with temperature.