We propose an interface between the spin of a photon and the spin of an electron confined in a quantum dot embedded in a microcavity operating in the weak-coupling regime. This interface, based on spin selective photon reflection from the cavity, can be used to construct a CNOT gate, a multiphoton entangler and a photonic Bell-state analyzer. Finally, we analyze experimental feasibility, concluding that the schemes can be implemented with current technology. DOI: 10.1103/PhysRevLett.104.160503 PACS numbers: 03.67.Àa, 42.50.Pq, 78.67.Hc Hybrid quantum information systems hold great promise for the development of quantum communication and computing since they allow exploiting different quantum systems at the best of their potentials. For example, in order to build a quantum network [1], photons are excellent candidates for long-distance transmission while quantum states of matter are preferred for local storage and processing. Hybrid (photon-matter) systems can also be used to effectively enable strong nonlinear interactions between single photons [2][3][4]. Several systems have been identified as candidates for local matter qubits, for example, atoms [5,6], ions [7], superconducting circuits [8,9], and semiconductor quantum dots [10][11][12], and their coupling strengths to optical modes have been investigated.Quantum information protocols based on cavity QED often require the system to operate in the strong-coupling regime [2,[13][14][15], where the vacuum Rabi frequency of the dipole g exceeds both the cavity and dipole decay rates. However, in the bad cavity limit, where g is smaller than the cavity decay rate, the coupling between the radiation and the dipole can drastically change the cavity reflection and transmission properties [16][17][18], allowing quantum information schemes to operate in the weak-coupling regime. We exploit this regime, using spin selective dipole coupling, for a system consisting of a single electron charged self-assembled GaAs=InAs quantum dot in a micropillar resonator [19,20]. The potential of this system has also been recognized in [21]. We first show that this specific system can lead to a quantum CNOT gate with the confined electron spin as the control qubit and the incoming photon spin as the target qubit. We apply the CNOT gate to generate multiphoton entangled states. We then construct a complete two-photon Bell-state analyzer (BSA). Complete deterministic BSA is an important prerequisite for many quantum information protocols like superdense coding, teleportation, or entanglement swapping. It cannot be performed with linear optics only [22], while it can be done using nonlinear optical processes [23] (with low efficiency) or employing measurement-based nonlinearities in nondeterministic schemes [24]. Deterministic complete BSA has been shown in a scheme which is conceptually different from the one presented here, exploiting entanglement in two or more degrees of freedom of two photons [25,26]. We conclude with a discussion on the experimental feasibility of the proposed ...

We present results on a new fullerene-based anchoring group for molecular electronics. Using lithographic mechanically controllable break junctions in vacuum we have determined the conductance and stability of single-molecule junctions of 1,4-bis(fullero[c]pyrrolidin-1-yl)benzene. The compound can be self-assembled from solution and has a low-bias conductance of 3 x 10(-4) G0. Compared to 1,4-benzenedithiol the fullerene-anchored molecule exhibits a considerably lower conductance spread. In addition, the signature of the new compound in histograms is more significant than that of 1,4-benzenediamine, probably owing to a more stable adsorption motif. Statistical analyses of the breaking of the junctions confirm the stability of the fullerene-gold bond.

We have investigated electrical transport through the molecular model systems benzenedithiol, benzenediamine, hexanedithiol and hexanediamine. Conductance histograms under different experimental conditions indicate that measurements using mechanically controllable break junctions in vacuum are limited by the surface density of molecules at the contact. Hexanedithiol histograms typically exhibit a broad peak around 7·10 −4 G 0 . In contrast to recent results on scanning tunneling microscope (STM)-based break junctions in solution we find that the spread in single-molecule conductance is not reduced by amino anchoring groups. Histograms of hexanediamine exhibit a wide peak around 4·10 −4 G 0 . For both benzenedithiol and benzenediamine we observe a large variability in low-bias conductance. We attribute these features to the slow breaking of the lithographic mechanically controllable break junctions and the absence of a solvent that may enable molecular readsorption after bond breaking. Nevertheless, we have been able to acquire reproducible current-voltage characteristics of benzenediamine and benzenedithiol using a statistical measurement approach. Benzenedithiol measurements yield a conductance gap of about 0.9 V at room temperature and 0.6 V at 77 K. In contrast, the current-voltage characteristics of benzenediamine-junctions typically display conductance gaps of about 0.9 V at both temperatures.