Entanglement has a central role in fundamental tests of quantum mechanics as well as in the burgeoning field of quantum information processing. Particularly in the context of quantum networks and communication, a main challenge is the efficient generation of entanglement between stationary (spin) and propagating (photon) quantum bits. Here we report the observation of quantum entanglement between a semiconductor quantum dot spin and the colour of a propagating optical photon. The demonstration of entanglement relies on the use of fast, single-photon detection, which allows us to project the photon into a superposition of red and blue frequency components. Our results extend the previous demonstrations of single-spin/single-photon entanglement in trapped ions, neutral atoms and nitrogen-vacancy centres to the domain of artificial atoms in semiconductor nanostructures that allow for on-chip integration of electronic and photonic elements. As a result of its fast optical transitions and favourable selection rules, the scheme we implement could in principle generate nearly deterministic entangled spin-photon pairs at a rate determined ultimately by the high spontaneous emission rate. Our observation constitutes a first step towards implementation of a quantum network with nodes consisting of semiconductor spin quantum bits.

A quantum interface between a propagating photon used to transmit quantum information and a long-lived qubit used for storage is of central interest in quantum information science. A method for implementing such an interface between dissimilar qubits is quantum teleportation. Here we experimentally demonstrate transfer of quantum information carried by a photon to a semiconductor spin using quantum teleportation. In our experiment, a single photon in a superposition state is generated using resonant excitation of a neutral dot. To teleport this photonic qubit, we generate an entangled spin-photon state in a second dot located 5 m away and interfere the photons from the two dots in a Hong-Ou-Mandel set-up. Thanks to an unprecedented degree of photon-indistinguishability, a coincidence detection at the output of the interferometer heralds successful teleportation, which we verify by measuring the resulting spin state after prolonging its coherence time by optical spin-echo.

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 demonstrate the fabrication of single-crystalline diamond nanopillars on a (111)-oriented chemical vapor deposited diamond substrate. This crystal orientation offers optimal coupling of nitrogen-vacancy (NV) center emission to the nanopillar mode and is thus advantageous over previous approaches. We characterize single native NV centers in these nanopillars and find one of the highest reported saturated fluorescence count rates in single crystalline diamond in excess of 106 counts per second. We show that our nano-fabrication procedure conserves the preferential alignment as well as the spin coherence of the NVs in our structures. Our results will enable a new generation of highly sensitive probes for NV magnetometry and pave the way toward photonic crystals with optimal orientation of the NV center's emission dipole.

Single-shot read-out of individual qubits is typically the slowest process among the elementary single-and two-qubit operations required for quantum information processing. Here, we use resonance fluorescence from a single-electron charged quantum dot to read-out the spin-qubit state in 800 nanoseconds with a fidelity exceeding 80%. Observation of the spin evolution on longer timescales reveals quantum jumps of the spin state: we use the experimentally determined waitingtime distribution to characterize the quantum jumps.PACS numbers: 03.67. Lx, 73.21.La, A fundamental difficulty in quantum information processing is the need for isolation of individual quantum systems from their noisy environment on the one hand, and the requirement for information extraction by selective coupling of qubits to classical (noisy) detectors on the other hand [1]. The requisite one-and two-qubit operations, as well as initialization of each qubit can be carried out by using classical out-of-equilibrium external fields, such as lasers or microwaves; the lack of a need for heralding the successful completion of these operations ensures that they can be accomplished in short timescales. In contrast, quantum measurements are typically slow since information extraction by a classical observer is in many cases hindered by the need to protect the qubit from the external fluctuations. While ingenious schemes for fast qubit measurements have been developed, the timescales required for a high fidelity qubit measurement remains at least an order of magnitude longer than those required for coherent operations in practically all quantum information processing schemes [2][3][4]. In the case of spin qubits in optically active quantum dots (QD), the predicament is even more striking since while optical excitation allows for fast turn on/off of light-matter interaction enabling spin read-out, it at the same time allows for an additional fast channel for spin relaxation. In fact, with the exception of a slow coupled QD scheme requiring a designated read-out QD [5], it has not been possible to carry out single-shot spin measurements on isolated optically active spin qubits [6].In this Letter, we overcome the predicament underlying single-shot spin read-out by enhancing the collection efficiency of resonance fluorescence (RF) from spindependent recycling transitions that are ubiquitous to single-electron charged QDs. The photon collection efficiency of 0.45% that we achieve allows us to obtain a single-shot spin read-out fidelity exceeding 80% in a measurement time of 800 ns. This result corresponds to an enhancement of the spin read-out time by almost three orders of magnitude as compared to the prior measurements on coupled QDs [5]. Continuous monitoring of the spin state enabled by single-shot read-out reveals quantum jumps of the observed spin stemming either from the finite T 1 spin lifetime or spin pumping induced by the resonant read-out laser. A theoretical analysis of quantum jumps using the waiting time distribution (W (τ )) was presented ear...