Bounds to electron spin qubit variability for scalable CMOS architectures.

Spins of electrons in silicon MOS quantum dots combine exquisite quantum properties and scalable fabrication. In the age of quantum technology, however, the metrics that crowned Si/SiO2 as the microelectronics standard need to be reassessed with respect to their impact upon qubit performance. We chart spin qubit variability due to the unavoidable atomic-scale roughness of the Si/SiO2 interface, compiling experiments across 12 devices, and develop theoretical tools to analyse these results. Atomistic tight binding and path integral Monte Carlo methods are adapted to describe fluctuations in devices with millions of atoms by directly analysing their wavefunctions and electron paths instead of their energy spectra. We correlate the effect of roughness with the variability in qubit position, deformation, valley splitting, valley phase, spin-orbit coupling and exchange coupling. These variabilities are found to be bounded, and they lie within the tolerances for scalable architectures for quantum computing as long as robust control methods are incorporated.

High-fidelity spin qubit operation and algorithmic initialization above 1 K.

The encoding of qubits in semiconductor spin carriers has been recognized as a promising approach to a commercial quantum computer that can be lithographically produced and integrated at scale1-10. However, the operation of the large number of qubits required for advantageous quantum applications11-13 will produce a thermal load exceeding the available cooling power of cryostats at millikelvin temperatures. As the scale-up accelerates, it becomes imperative to establish fault-tolerant operation above 1 K, at which the cooling power is orders of magnitude higher14-18. Here we tune up and operate spin qubits in silicon above 1 K, with fidelities in the range required for fault-tolerant operations at these temperatures19-21. We design an algorithmic initialization protocol to prepare a pure two-qubit state even when the thermal energy is substantially above the qubit energies and incorporate radiofrequency readout to achieve fidelities up to 99.34% for both readout and initialization. We also demonstrate single-qubit Clifford gate fidelities up to 99.85% and a two-qubit gate fidelity of 98.92%. These advances overcome the fundamental limitation that the thermal energy must be well below the qubit energies for the high-fidelity operation to be possible, surmounting a main obstacle in the pathway to scalable and fault-tolerant quantum computation.

Jellybean Quantum Dots in Silicon for Qubit Coupling and On-Chip Quantum Chemistry.

The small size and excellent integrability of silicon metal-oxide-semiconductor (SiMOS) quantum dot spin qubits make them an attractive system for mass-manufacturable, scaled-up quantum processors. Furthermore, classical control electronics can be integrated on-chip, in-between the qubits, if an architecture with sparse arrays of qubits is chosen. In such an architecture qubits are either transported across the chip via shuttling or coupled via mediating quantum systems over short-to-intermediate distances. This paper investigates the charge and spin characteristics of an elongated quantum dot-a so-called jellybean quantum dot-for the prospects of acting as a qubit-qubit coupler. Charge transport, charge sensing, and magneto-spectroscopy measurements are performed on a SiMOS quantum dot device at mK temperature and compared to Hartree-Fock multi-electron simulations. At low electron occupancies where disorder effects and strong electron-electron interaction dominate over the electrostatic confinement potential, the data reveals the formation of three coupled dots, akin to a tunable, artificial molecule. One dot is formed centrally under the gate and two are formed at the edges. At high electron occupancies, these dots merge into one large dot with well-defined spin states, verifying that jellybean dots have the potential to be used as qubit couplers in future quantum computing architectures.

On-demand electrical control of spin qubits.

Once called a 'classically non-describable two-valuedness' by Pauli, the electron spin forms a qubit that is naturally robust to electric fluctuations. Paradoxically, a common control strategy is the integration of micromagnets to enhance the coupling between spins and electric fields, which, in turn, hampers noise immunity and adds architectural complexity. Here we exploit a switchable interaction between spins and orbital motion of electrons in silicon quantum dots, without a micromagnet. The weak effects of relativistic spin-orbit interaction in silicon are enhanced, leading to a speed up in Rabi frequency by a factor of up to 650 by controlling the energy quantization of electrons in the nanostructure. Fast electrical control is demonstrated in multiple devices and electronic configurations. Using the electrical drive, we achieve a coherence time T2,Hahn ≈ 50 µs, fast single-qubit gates with Tπ/2 = 3 ns and gate fidelities of 99.93%, probed by randomized benchmarking. High-performance all-electrical control improves the prospects for scalable silicon quantum computing.

Caged circular siRNAs for photomodulation of gene expression in cells and mice.

By means of RNA interference (RNAi), small interfering RNAs (siRNAs) play important roles in gene function study and drug development. Recently, photolabile siRNAs were developed to elucidate the process of gene silencing in terms of space, time and degree through chemical modification of siRNAs. We report herein a novel type of photolabile siRNA that was synthesized through cyclizing two ends of a single stranded RNA with a photocleavable linker. These circular siRNAs became more resistant to serum degradation. Using reporter assays of firefly/Renilla luciferase and GFP/RFP, the gene silencing activities of caged circular siRNAs for both genes were evaluated in HEK293 cells. The results indicated that the target genes were successfully photomodulated using these caged circular siRNAs that were formed by caged circular antisense guide RNAs and their linear complementary sense RNAs. Using the caged circular siRNA targeting GFP, we also successfully achieved photomodulation of GFP expression in mice. Upon further optimization, this new type of caged circular siRNA is expected to be a promising tool for studying gene therapy.

Bioorthogonal Metabolic DNA Labelling using Vinyl Thioether-Modified Thymidine and o-Quinolinone Quinone Methide.

Bioorthogonal metabolic DNA labeling with fluorochromes is a powerful strategy to visualize DNA molecules and their functions. Here, we report the development of a new DNA metabolic labeling strategy enabled by the catalyst-free bioorthogonal ligation using vinyl thioether modified thymidine and o-quinolinone quinone methide. With the newly designed vinyl thioether-modified thymidine (VTdT), we added labeling tags on cellular DNA, which could further be linked to fluorochromes in cells. Therefore, we successfully visualized the DNA localization within cells as well as single DNA molecules without other staining reagents. In addition, we further characterized this bioorthogonal DNA metabolic labeling using DNase I digestion, MS characterization of VTdT as well as VTdT-oQQF conjugate in cell nuclei or mitochondria. This technique provides a powerful strategy to study DNA in cells, which paves the way to achieve future spatiotemporal deciphering of DNA synthesis and functions.

Vitamin E-Labeled Polyethylenimine for in vitro and in vivo Gene Delivery.

A series of Vitamin E (vitE)-labeled PEIs (PEI-vitEn) were synthesized and showed excellent complexation ability with plasmid DNA (pDNA). The cellular uptake of PEI-vitEn/pDNA complexes was greatly enhanced with the increase of vitE labeling, which is much better than that of control PEI25 in three different cell lines. PEI-vitE6 showed the best performance in gfp pDNA delivery and following GFP expression in HEK-293A cells. In addition, in vivo gene delivery in living mice also confirmed that PEI-vitE6 showed low toxicity and efficiently delivered gfp pDNA to the cells of liver and lung tissues for gene expression.

Sensitive Detection of Single-Nucleotide Mutation in the BRAF Mutation Site (V600E) of Human Melanoma Using Phosphate-Pyrene-Labeled DNA Probes.

A series of novel nucleotide phosphoramidites were rationally designed and synthesized and were then site-specifically incorporated in DNA oligonucleotide probes with pyrene-modified phosphate. These oligodeoxynucleotide (ODN) probes almost have no inherent fluorescence emission with pyrene modification at 3' phosphate of corresponding nucleotides as a result of the photoinduced electron-transfer quenching effect of nucleobases (thymidine ∼ cytidine > guanosine ≫ adenosine). However, strong fluorescence emission was observed only with the perfectly matched duplex for the probes with pyrene modified at 3' phosphate of thymidine and cytidine. These rationally designed ODN probes successfully worked as "turn on" fluorescence oligonucleotide sensors for single-nucleotide polymorphism (SNP) and were used for detecting a single BRAF mutation site (V600E) of human melanoma.

N-dots as a photoluminescent probe for the rapid and selective detection of Hg2+ and Ag+ in aqueous solution.

Green fluorescent nitrogen-rich quantum dots (N-dots) have been synthesized under mild conditions (50 °C in methanol) and applied as a turn-off chemosensor for the rapid and selective detection of Hg2+ and Ag+ in aqueous solutions. Distinguishing between Hg2+ and Ag+ was also successfully achieved simply through the addition of ethylene diamine tetraacetic acid (EDTA). These N-dots-Hg and N-dots-Ag could also be used as turn-on chemosensors for biothiols through their binding competition for Hg2+ and Ag+ with N-dots.

Synthesis of site-specifically phosphate-caged siRNAs and evaluation of their RNAi activity and stability.

A complete set of new photolabile nucleoside phosphoramidites were synthesized, then site-specifically incorporated into sense or antisense strands of siRNA for phosphate caging. Single caging modification was made along siRNA strands and their photomodulation of gene silencing were examined by using the firefly luciferase reporter gene. Several key phosphate positions were then identified. Furthermore, multiple caging modifications at these key positions led to significantly enhanced photomodulation of gene silencing activity, suggesting a synergistic effect. The caging group on both the terminally phosphate-caged siRNA and the single-stranded caged RNA has comparatively high stability, whereas hydrolysis of the caged group from the internally caged siRNA was observed, irrespective of the presence of Mg(2+). Molecular dynamic simulations demonstrated that enhanced hydrolysis of the caging group on internally phosphate-caged siRNAs was due to easy fragmentation of the caging group upon formation of the pentavalent intermediate of the phosphotriester with attack by water. The caging group in the terminally phosphate-caged siRNA or single-stranded caged RNA prefers to form π-π stacks with nearby nucleobases. In addition to providing explanations for previous observations, this study sheds further light on the design of caged oligonucleotides and indicates the direction of future development of nucleic acid drugs with phosphate modifications.