High-fidelity projective read-out of a solid-state spin quantum register.

Initialization and read-out of coupled quantum systems are essential ingredients for the implementation of quantum algorithms. Single-shot read-out of the state of a multi-quantum-bit (multi-qubit) register would allow direct investigation of quantum correlations (entanglement), and would give access to further key resources such as quantum error correction and deterministic quantum teleportation. Although spins in solids are attractive candidates for scalable quantum information processing, their single-shot detection has been achieved only for isolated qubits. Here we demonstrate the preparation and measurement of a multi-spin quantum register in a low-temperature solid-state system by implementing resonant optical excitation techniques originally developed in atomic physics. We achieve high-fidelity read-out of the electronic spin associated with a single nitrogen-vacancy centre in diamond, and use this read-out to project up to three nearby nuclear spin qubits onto a well-defined state. Conversely, we can distinguish the state of the nuclear spins in a single shot by mapping it onto, and subsequently measuring, the electronic spin. Finally, we show compatibility with qubit control: we demonstrate initialization, coherent manipulation and single-shot read-out in a single experiment on a two-qubit register, using techniques suitable for extension to larger registers. These results pave the way for a test of Bell's inequalities on solid-state spins and the implementation of measurement-based quantum information protocols.

Defect Control and n-Doping of Encapsulated Graphene by Helium-Ion-Beam Irradiation.

We study with Raman spectroscopy the influences of He(+) bombardment and the environment on beam-induced defects in graphene encapsulated in hexagonal boron nitride (h-BN). We show for the first time experimentally the autonomous behavior of the D' defect Raman peak: in contrast to the D defect peak, the D' defect peak is sensitive to the local environment. In particular, it saturates with ion dose in the encapsulated graphene. Electrical measurements reveal n-type conduction in the BN-encapsulated graphene. We conclude that unbound atoms ("interfacials") between the sp(2)-layers of graphene and h-BN promote self-healing of the beam-induced lattice damage and that nitrogen-carbon exchange leads to n-doping of graphene.

Optical angular momentum conversion in a nanoslit: reply.

We respond to a Comment on our Letter [Opt. Lett.37, 4946 (2012)], in which we reported on the spin-to-orbital optical angular momentum conversion of a circular nanoslit in a thin metal layer. We claimed, in an unfortunately worded sentence, that the conversion efficiency was independent of the slit's dichroism, which the Comment pointed out was incorrect. We acknowledge this and reiterate our original intention that as long as the dichroism is not too large, then it has little effect on the conversion efficiency in our system.

Optical angular momentum conversion in a nanoslit.

We demonstrate partial conversion of circularly polarized light into orbital angular momentum-carrying vortex light with opposite-handed circular polarization. This conversion is accomplished in a novel manner using the birefringent properties of a circular subwavelength slit in a thin metal film. Our technique can be applied over a very wide range of frequencies and even allows the creation of anisotropic vortices when using a slit without circular symmetry.

A subwavelength slit as a quarter-wave retarder.

We have experimentally studied the polarization-dependent transmission properties of a nanoslit in a gold film as a function of its width. The slit exhibits strong birefringence and dichroism. We find, surprisingly, that the transmission of the polarization parallel to the slit only disappears when the slit is much narrower than half a wavelength, while the transmission of the perpendicular component is reduced by the excitation of surface plasmons. We exploit the slit's dichroism and birefringence to realize a quarter-wave retarder.

Nanopillar growth by focused helium ion-beam-induced deposition.

A 25 keV focused helium ion beam has been used to grow PtC nanopillars on a silicon substrate by beam-induced decomposition of a (CH(3))(3)Pt(C(P)CH(3)) precursor gas. The ion beam diameter was about 1 nm. The observed relatively high growth rates suggest that electronic excitation is the dominant mechanism in helium ion-beam-induced deposition. Pillars grown at low beam currents are narrow and have sharp tips. For a constant dose, the pillar height decreases with increasing current, pointing to depletion of precursor molecules at the beam impact site. Furthermore, the diameter increases rapidly and the total pillar volume decreases slowly with increasing current. Monte Carlo simulations have been performed with realistic values for the fundamental deposition processes. The simulation results are in good agreement with experimental observations. In particular, they reproduce the current dependences of the vertical and lateral growth rates and of the volumetric deposition efficiency. Furthermore, the simulations reveal that the vertical pillar growth is due to type-1 secondary electrons and primary ions, while the lateral outgrowth is due to type-2 secondary electrons and scattered ions.

Controlling supercurrents and their spatial distribution in ferromagnets.

Spin-triplet Cooper pairs induced in ferromagnets form the centrepiece of the emerging field of superconducting spintronics. Usually the focus is on the spin-polarization of the triplets, potentially enabling low-dissipation magnetization switching. However, the magnetic texture which provides the fundamental mechanism for generating triplets also permits control over the spatial distribution of supercurrent. Here we demonstrate the tailoring of distinct supercurrent pathways in the ferromagnetic barrier of a Josephson junction. We combine micromagnetic simulations with three-dimensional supercurrent calculations to design a disk-shaped structure with a ferromagnetic vortex which induces two transport channels across the junction. By using superconducting quantum interferometry, we show the existence of two channels. Moreover, we show how the supercurrent can be controlled by moving the vortex with a magnetic field. This approach paves the way for supercurrent paths to be dynamically reconfigured in order to switch between different functionalities in the same device.

Simultaneous scanning tunneling microscopy and synchrotron X-ray measurements in a gas environment.

A combined X-ray and scanning tunneling microscopy (STM) instrument is presented that enables the local detection of X-ray absorption on surfaces in a gas environment. To suppress the collection of ion currents generated in the gas phase, coaxially shielded STM tips were used. The conductive outer shield of the coaxial tips can be biased to deflect ions away from the tip core. When tunneling, the X-ray-induced current is separated from the regular, 'topographic' tunneling current using a novel high-speed separation scheme. We demonstrate the capabilities of the instrument by measuring the local X-ray-induced current on Au(1 1 1) in 800 mbar Ar.

Imaging and nanofabrication with the helium ion microscope of the Van Leeuwenhoek Laboratory in Delft.

Although helium ion microscopy (HIM) was introduced only a few years ago, many new application fields are emerging. The connecting factor between these novel applications is the unique interaction of the primary helium ion beam with the sample material at and just below its surface. In particular, the HIM secondary electron signal stems from an area that is extremely well localized around the point of incidence of the primary beam. This makes the HIM well suited for both high-resolution imaging and high-resolution nanofabrication. Another advantage in nanofabrication is the low ion backscattering fraction, which leads to a weak proximity effect. The subnanometer probe size and the unique beam-materials interactions have opened new areas of research. This review presents a selection of studies conducted on a single instrument. The selection encompasses applications ranging from imaging to nanofabrication and from fundamental academic research to applied industrial developments.

TEM study of locally coated nanopore fabricated by ion-beam-induced deposition in a thin membrane.

We studied the formation of locally coated sub-10-nm nanopores fabricated by ion-beam milling and ion-beam-induced deposition (IBID) in a thin silicon nitride membrane. Two typical precursor gases representing conductive ((CH(3))(3)Pt(CpCH(3)), CPC for short) and insulating (tetra ethyl oxysilane, TEOS for short) material deposition are used. Three-dimensional electron tomography, EDX and EELS analysis are used to measure the changes in chemical composition and shape of the pores after their formation and at various stages of pore shrinkage. The formation and shrinkage are shown to be due to a shifting competition between IBID and material sputtering during ion-beam exposure. The chemical distribution at the rim of the nanopore is dependent on the precursor gases used: CPC forms a thin carbon layer with small embedded Pt particles at the top and inner surfaces of the nanopore, whereas TEOS forms SiO(x)C(y) with Ga particles dispersed at the rim of the nanopore.

Sculpting nanoelectrodes with a transmission electron beam for electrical and geometrical characterization of nanoparticles.

A method to produce metal electrodes with a gap of a few nanometers with a highly focused electron beam in a transmission electron microscope (TEM) is described. With this method the electrical and geometrical characterization of the same particle is possible. The I-V characteristics of a gold particle trapped between such electrodes showed the expected single-electron tunneling behavior, with a Coulomb gap corresponding to the geometry of the particle as observed with high-resolution TEM.

Polarization tomography of metallic nanohole arrays.

We report polarization tomography experiments on metallic nanohole arrays with square and hexagonal symmetry. As a main result we find that a fully polarized input beam is partly depolarized after transmission through a nanohole array. This loss of polarization coherence is found to be anisotropic; i.e., it depends on the polarization state of the input beam. The depolarization is ascribed to a combination of two factors: (i) the nonlocal response of the array as a result of surface-plasmon propagation and (ii) the non-plane-wave nature of a practical input beam.