A two-qubit gate between phosphorus donor electrons in silicon.

Electron spin qubits formed by atoms in silicon have large (tens of millielectronvolts) orbital energies and weak spin-orbit coupling, giving rise to isolated electron spin ground states with coherence times of seconds1,2. High-fidelity (more than 99.9 per cent) coherent control of such qubits has been demonstrated3, promising an attractive platform for quantum computing. However, inter-qubit coupling-which is essential for realizing large-scale circuits in atom-based qubits-has not yet been achieved. Exchange interactions between electron spins4,5 promise fast (gigahertz) gate operations with two-qubit gates, as recently demonstrated in gate-defined silicon quantum dots6-10. However, creating a tunable exchange interaction between two electrons bound to phosphorus atom qubits has not been possible until now. This is because it is difficult to determine the atomic distance required to turn the exchange interaction on and off while aligning the atomic circuitry for high-fidelity, independent spin readout. Here we report a fast (about 800 picoseconds) [Formula: see text] two-qubit exchange gate between phosphorus donor electron spin qubits in silicon using independent single-shot spin readout with a readout fidelity of about 94 per cent on a complete set of basis states. By engineering qubit placement on the atomic scale, we provide a route to the realization and efficient characterization of multi-qubit quantum circuits based on donor qubits in silicon.

High-fidelity initialization and control of electron and nuclear spins in a four-qubit register.

Single electron spins bound to multi-phosphorus nuclear spin registers in silicon have demonstrated fast (0.8 ns) two-qubit SWAP gates and long spin relaxation times (~30 s). In these spin registers, when the donors are ionized, the nuclear spins remain weakly coupled to their environment, allowing exceptionally long coherence times. When the electron is present, the hyperfine interaction allows coupling of the spin and charge degrees of freedom for fast qubit operation and control. Here we demonstrate the use of the hyperfine interaction to enact electric dipole spin resonance to realize high-fidelity ( F = 10 0 - 6 + 0 %) initialization of all the nuclear spins within a four-qubit nuclear spin register. By controllably initializing the nuclear spins to ⇓ ⇓ ⇓ , we achieve single-electron qubit gate fidelities of F = 99.78 ± 0.07% (Clifford gate fidelities of 99.58 ± 0.14%), above the fault-tolerant threshold for the surface code with a coherence time of T 2 * = 12 µ s .

Development and validation of HIV-1 Multiplex Serology.

By inducing immunosuppression in infected patients, human immunodeficiency virus-1 (HIV-1) generates a favorable environment for opportunistic infections and the development of several human cancers. In order to detect individual serum or plasma HIV-1 antibody status for epidemiological studies, high-throughput HIV-1 Multiplex Serology was developed. Seven HIV-1 antigens were recombinantly expressed in E. coli as N-terminal glutathione-S-transferase (GST) fusion proteins that are bound to glutathione-coupled sets of beads with distinct fluorescent color. Combining all bead sets in a suspension array allowed for simultaneous detection of antibodies targeting structural, regulatory and accessory proteins expressed during HIV-1 infection. HIV-1 Multiplex Serology was validated with 244 reference sera whose HIV-1 serostatus had been pre-determined by screening microparticle immunoassay and confirmatory line immunoassay. The multifunctional protein GAG emerged as an excellent marker to determine HIV-1 serostatus with a specificity of 99% (95% CI 96%-100%) and sensitivity of 100% (95% CI 88%-100%). Seropositivity for multiple HIV-1 antigens appeared to be characteristic for HIV-1 infected individuals (median number of antigens recognized in reference assay positive sera: 4; median number of antigens recognized in reference assay negative sera: 0), indicating a broad immune response targeting also regulatory and accessory proteins which may be useful for the identification of antibody patterns specific for infection-associated disease stages. HIV-1 Multiplex Serology performs similarly to conventional HIV-1 serology but eliminates the need for a two-step screening approach with subsequent confirmation assay. Thus, this high-throughput method will facilitate large-scale epidemiological studies of the role of HIV-1 in cancer development.