We introduce a simple optical gate to fuse arbitrary size polarization entangled W-states to prepare larger W-states. The gate requires a polarizing beam splitter (PBS), a half wave plate (HWP) and two photon detectors. We study numerically and analytically the necessary resource consumption for preparing larger W-states by fusing smaller ones with the proposed fusion gate. We show analytically that resource requirement scales at most sub-exponentially with the increasing size of the state to be prepared. We numerically determine the resource cost for fusion without recycling where W-states of arbitrary size can be optimally prepared. Moreover, we introduce another strategy which is based on recycling and outperforms the optimal strategy for non-recycling case.

We introduce an elementary optical gate for expanding polarization entangled W states, in which every pair of photons are entangled alike. The gate is composed of a pair of 50:50 beamsplitters and ancillary photons in the two-photon Fock state. By seeding one of the photons in an n-photon W state into this gate, we obtain an (n + 2)-photon W state after post-selection. This gate gives a better efficiency and a simpler implementation than previous proposals for W-state preparation.

A diamond nano-crystal hosting a single nitrogen vacancy (NV) center is optically selected with a confocal scanning microscope and positioned deterministically onto the subwavelength-diameter waist of a tapered optical fiber (TOF) with the help of an atomic force microscope. Based on this nano-manipulation technique we experimentally demonstrate the evanescent coupling of single fluorescence photons emitted by a single NV-center to the guided mode of the TOF. By comparing photon count rates of the fiber-guided and the free-space modes and with the help of numerical FDTD simulations we determine a lower and upper bound for the coupling efficiency of (9.5 ± 0.6)% and (10.4 ± 0.7)%, respectively. Our results are a promising starting point for future integration of single photon sources into photonic quantum networks and applications in quantum information science.PACS numbers: 03.67. 42.50.Ex, 78.67.Bf Efficient collection of single photons radiated by a single solid state quantum emitter -like the nitrogen vacancy (NV) center in diamond 1 -is an important prerequisite for future applications in applied physical and quantum information science, like ultra-sensitive fluorescence spectroscopy and linear optical quantum computation 2-4 . A standard technique for fluorescence collection is confocal microscopy. However, when applied to defect centers in bulk diamond, total internal reflection limits the collection efficiency to few percent. Recently, the collection efficiency of NV-fluorescence has been increased by one order of magnitude by combining confocal microscopy with solid immersion lenses (SILs) 5-7 , respectively photonic nanowires 8 . In the latter system the improvement is based on efficient coupling of NV-fluorescence photons to the strongly confined mode (HE 11 ) 9-11 of diamond nanowires. For defect centers in diamond nano-crystals, tapered optical fibers (TOFs) 12 with a subwavelength diameter waist are a particularly attractive alternative platform. Due to the strong evanescent field at the surface, such TOFs promise coupling efficiencies up to 36%13,14 and approaching unity when combined with Bragg-grating cavities 15,16 . Until now, evanescent coupling of fluorescence photons to a single guided mode of a TOF has been achieved for a) email: lars.liebermeister@physik.uni-muenchen.de b) email: markusweber@lmu.de various solid state quantum emitters 17-20 , molecules 21 , and laser-cooled atomic vapors 22 . To bring these emitters into the strong evanescent optical field at the surface of the nano-fiber several non-deterministic deposition techniques like dip-coating 17,18 , picoliter-dispensers 19,20 , and optical surface traps 23 have been applied. However, for real applications in quantum information science, e. g., the photonic quantum-bus mediated coupling of NVcenters in a lattice 24 , deterministic positioning of single solid state quantum emitters onto the submicron waist of a TOF with nm position control is desirable. In this letter we demonstrate significant steps towards deterministic coup...