The spin states of an electron bound to a single phosphorus donor in silicon show remarkably long coherence and relaxation times, which makes them promising building blocks for the realization of a solid-state quantum computer. Here we demonstrate, by high-fidelity (93%) electrical spin readout, that a long relaxation time T 1 of about 2 s, at B ¼ 1.2 T and TE100 mK, is also characteristic of electronic spin states associated with a cluster of few phosphorus donors, suggesting their suitability as hosts for spin qubits. Owing to the difference in the hyperfine coupling, electronic spin transitions of such clusters can be sufficiently distinct from those of a single phosphorus donor. Our atomistic tight-binding calculations reveal that when neighbouring qubits are hosted by a single phosphorus atom and a cluster of two phosphorus donors, the difference in their electron spin resonance frequencies allows qubit rotations with error rates E10 À 4 . These results provide a new approach to achieving individual qubit addressability.

We demonstrate high-fidelity electron spin read-out of a precision placed single donor in silicon via spin selective tunneling to either the D(+) or D(-) charge state of the donor. By performing read-out at the stable two electron D(0)↔D(-) charge transition we can increase the tunnel rates to a nearby single electron transistor charge sensor by nearly 2 orders of magnitude, allowing faster qubit read-out (1 ms) with minimum loss in read-out fidelity (98.4%) compared to read-out at the D(+)↔D(0) transition (99.6%). Furthermore, we show that read-out via the D(-) charge state can be used to rapidly initialize the electron spin qubit in its ground state with a fidelity of F(I)=99.8%.

This work reports an electronic and micro-structural study of an appealing system for optoelectronics: tungsten disulfide (WS) on epitaxial graphene (EG) on SiC(0001). The WS is grown via chemical vapor deposition (CVD) onto the EG. Low-energy electron diffraction (LEED) measurements assign the zero-degree orientation as the preferential azimuthal alignment for WS/EG. The valence-band (VB) structure emerging from this alignment is investigated by means of photoelectron spectroscopy measurements, with both high space and energy resolution. We find that the spin-orbit splitting of monolayer WS on graphene is of 462 meV, larger than what is reported to date for other substrates. We determine the value of the work function for the WS/EG to be 4.5 ± 0.1 eV. A large shift of the WS VB maximum is observed as well, due to the lowering of the WS work function caused by the donor-like interfacial states of EG. Density functional theory (DFT) calculations carried out on a coincidence supercell confirm the experimental band structure to an excellent degree. X-ray photoemission electron microscopy (XPEEM) measurements performed on single WS crystals confirm the van der Waals nature of the interface coupling between the two layers. In virtue of its band alignment and large spin-orbit splitting, this system gains strong appeal for optical spin-injection experiments and opto-spintronic applications in general.

Real-time sensing of (spin-dependent) single-electron tunneling is fundamental to electrical readout of qubit states in spin quantum computing. Here, we demonstrate the feasibility of detecting such single-electron tunneling events using an atomically planar charge sensing layout, which can be readily integrated in scalable quantum computing architectures with phosphorus-donor-based spin qubits in silicon (Si:P). Using scanning tunneling microscopy (STM) lithography on a Si(001) surface, we patterned a single-electron transistor (SET), both tunnel and electrostatically coupled to a coplanar ultrasmall quantum dot, the latter consisting of approximately four P donors. Charge transitions of the quantum dot could be detected both in time-averaged and single-shot current response of the SET. Single electron tunneling between the quantum dot and the SET island on a time-scale (τ ∼ ms) two-orders-of-magnitude faster than the spin-lattice relaxation time of a P donor in Si makes this device geometry suitable for projective readout of Si:P spin qubits. Crucial to scalability is the ability to reproducibly achieve sufficient electron tunnel rates and charge sensitivity of the SET. The inherent atomic-scale control of STM lithography bodes extremely well to precisely optimize both of these parameters.

By exhibiting a measurable bandgap and exotic valley physics, atomically-thick tungsten disulfide (WS2) offers exciting prospects for optoelectronic applications. The synthesis of continuous WS2 films on other two-dimensional (2D) materials would greatly facilitate the implementation of novel all-2D photoactive devices. In this work we demonstrate the scalable growth of WS2 on graphene and hexagonal boron nitride (h-BN) via a chemical vapor deposition (CVD) approach. Spectroscopic and microscopic analysis reveal that the film is bilayer-thick, with local monolayer inclusions. Photoluminescence measurements show a remarkable conservation of polarization at room temperature peaking 74% for the entire WS2 film. Furthermore, we present a scalable bottomup approach for the design of photoconductive and photoemitting patterns.

We report the superlubric sliding of monolayer tungsten disulfide (WS 2) on epitaxial graphene (EG) grown on silicon carbide (SiC). Single-crystalline WS 2 flakes with lateral size of hundreds of nanometers are obtained via chemical vapor deposition (CVD) on EG. Microscopic and diffraction analyses indicate that the WS 2 /EG stack is predominantly aligned with zero azimuthal rotation. The present experiments show that, when perturbed by a scanning probe microscopy (SPM) tip, the WS 2 flakes are prone to slide over the graphene surfaces at room temperature. Atomistic force field-based molecular dynamics simulations indicate that, through local physical deformation of the WS 2 flake, the scanning tip releases enough energy to the flake to overcome the motion activation barrier and trigger an ultralow-friction rototranslational displacement, that is superlubric. Experimental observations show that, after sliding, the WS 2 flakes come to rest with a rotation of n/3 with respect to graphene. Moreover, atomically resolved measurements show that the interface is atomically sharp and the WS 2 lattice is strain-free. These results help to shed light on nanotribological phenomena in van der Waals (vdW) heterostacks, and suggest that the applicative potential of the WS 2 /graphene heterostructure can be extended by novel mechanical prospects.

An atomistic method of calculating the spin-lattice relaxation times (T1) is presented for donors in silicon nanostructures comprising of millions of atoms. The method takes into account the full band structure of silicon including the spin-orbit interaction. The electron-phonon Hamiltonian, and hence the deformation potential, is directly evaluated from the strain-dependent tight-binding Hamiltonian. The technique is applied to single donors and donor clusters in silicon, and explains the variation of T1 with the number of donors and electrons, as well as donor locations. Without any adjustable parameters, the relaxation rates in a magnetic field for both systems are found to vary as B 5 in excellent quantitative agreement with experimental measurements. The results also show that by engineering electronic wavefunctions in nanostructures, T1 times can be varied by orders of magnitude.PACS numbers: 71.55. Cn, 03.67.Lx, 85.35.Gv, 71.70.Ej Due to the extremely long spin coherence times, in some cases exceeding seconds [1,2], and the existing industrial fabrication infrastructure, silicon is well-suited to be an outstanding platform for semiconductor quantum computer technology [3][4][5][6][7][8]. Qubits hosted by donors in silicon [3] have some added advantages as they are readily available few-electron systems with a rich electronic structure and can form identical qubits [9]. In the last few years, several key experimental milestones have been achieved in dopant based quantum computing, including the demonstration of electron [10] and nuclear [11] spin qubits, single spin read-out and initialization [12,13], and the observation of spin blockade and exchange towards two qubit coupling [14,15]. Recent advances in Scanning Tunneling Microscope (STM) lithography has enabled placement of single donors with atomic scale precision [16], with the result that various functional units such as quantum wires [17], single electron transistors (SET) [13,18], and quantum dots [19] can all be realized in-plane with densely packed donor islands. The STM approach provides the fabrication precision needed to develop test-bed quantum chips for the demonstration of quantum algorithms in a solid-state quantum computer.One of the two most important timescales for a spin qubit is the spin-lattice relaxation time (T 1 ). Recent experiments have measured T 1 times in a single donor and in a few-donor cluster indicating shorter T 1 times in the latter [12,13]. Previous theoretical works exist in the literature qualitatively describing two different spin relaxation mechanisms in a bulk donor system [20,21]. However, a comprehensive quantitative theory which combines all the different mechanisms under a unified framework and accounts for the local inhomogeneous environment of the donors in a realistic nanostructure is still lacking. Moreover, there is no theoretical work yet to explain the measured T 1 times in densely packed donor clusters. In this letter, we present a comprehensive approach to compute the T 1 times in single donors and...

One of the major issues in graphene-based optoelectronics is to scale-up high-performing devices. In this work, we report an original approach for the fabrication of efficient optoelectronic devices from scalable tungsten disulfide (WS)/graphene heterostructures. Our approach allows for the patterned growth of WS on graphene and facilitates the realization of ohmic contacts. Photodetectors fabricated with WS on epitaxial graphene on silicon carbide (SiC) present, when illuminated with red light, a maximum responsivity R ∼220 A W, a detectivity D* ∼2.0 × 10 Jones and a -3 dB bandwidth of 250 Hz. The retrieved detectivity is 3 orders of magnitude higher than that obtained with graphene-only devices at the same wavelength. For shorter illumination wavelengths we observe a persistent photocurrent with a nearly complete charge retention, which originates from deep trap levels in the SiC substrate. This work ultimately demonstrates that WS/graphene optoelectronic devices with promising performances can be obtained in a scalable manner. Furthermore, by combining wavelength-selective memory, enhanced responsivity and fast detection, this system is of interest for the implementation of 2d-based data storage devices.