Silicon has many attractive properties for quantum computing, and the quantum dot architecture is appealing because of its controllability and scalability. However, the multiple valleys in the silicon conduction band are potentially a serious source of decoherence for spin-based quantum dot qubits. Only when these valleys are split by a large energy does one obtain well-defined and long-lived spin states appropriate for quantum computing. Here we show that the small valley splittings observed in previous experiments on Si/SiGe heterostructures result from atomic steps at the quantum well interface. Lateral confinement in a quantum point contact limits the electron wavefunctions to several steps, and enhances the valley splitting substantially, up to 1.5 meV. The combination of electronic and magnetic confinement produces a valley splitting larger than the spin splitting, which is controllable over a wide range. These results improve the outlook for realizing spin qubits with long coherence times in silicon-based devices.The fundamental unit of quantum information is the qubit. Qubits can be constructed from the quantum states of physical objects like atomic ions [1], quantum dots [2,3,4,5,6,7] or superconducting Josephson junctions [8]. A key requirement is that these quantum states should be well-defined and isolated from their environment. An assemblage of many qubits into a register and the construction of a universal set of operations, including initialization, measurement, and single and multi-qubit gates, would enable a quantum computer to execute algorithms for certain difficult computational problems like prime factorization and database search far faster than any conventional computer [9].The solid state affords special benefits and challenges for qubit operation and quantum computation. State-ofthe-art fabrication techniques enable the positioning of electrostatic gates with a resolution of several nanometers, paving the way for large scale implementations. On the other hand, the solid state environment provides numerous pathways for decoherence to degrade the computation [10]. Spins in silicon offer a special resilience against decoherence because of two desirable materials properties [11,12]: a small spin-orbit coupling and predominately spin-zero nuclei. Isotopic purification could essentially eliminate all nuclear decoherence mechanisms.Silicon, however, also has a property that potentially can increase decoherence. Silicon has multiple conduction band minima or valleys at the same energy. Unless this degeneracy is lifted, coherence and qubit operation will be threatened. In strained silicon quantum wells there are two such degenerate valleys [13] whose quantum numbers and energy scales compete directly with the spin degrees of freedom. In principle, sharp confinement potentials, like the quantum well interfaces, couple these two valleys and lift the degeneracy, providing a unique ground state if the coupling is strong enough [14,15]. Theoretical analyses for noninteracting electrons in perfectly f...

We demonstrate real-time recording of chemical vapor fluc-tuations from 22m away with a fast Fourier-transform infrared (FTIR) spectrometer that uses a laser-like infrared probing beam generated from two 10-fs Ti:sapphire lasers. The FTIR's broad 9-12 microm spectrum in the "molecular fingerprint" region is dispersed by fast heterodyne self-scanning, enabling spectra at 2cm-1 resolution to be recorded in 70 micros snapshots. We achieve continuous acquisition at a rate of 950 IR spectra per second by actively manipulating the repetition rate of one laser. Potential applications include video-rate chemical imaging and transient spectroscopy of e.g. gas plumes, flames and plasmas, and generally non-repetitive phenomena such as those found in protein folding dynamics and pulsed magnetic fields research.

Spins based in silicon provide one of the most promising architectures for quantum computing. Quantum dots are an inherently scalable technology. Here, we combine these two concepts into a workable design for a silicon-germanium quantum bit. The novel structure incorporates vertical and lateral tunneling, provides controlled coupling between dots, and enables single electron occupation of each dot. Precise modeling of the design elucidates its potential for scalable quantum computing. For the first time it is possible to translate the requirements of faulttolerant error correction into specific requirements for gate voltage control electronics in quantum dots. We demonstrate that these requirements are met by existing pulse generators in the kHzMHz range, but GHz operation is not yet achievable. Our calculations further pinpoint device features that enhance operation speed and robustness against leakage errors. We find that the component technologies for silicon quantum dot quantum computers are already in hand.Quantum computing offers the prospect of breaking out of the classical von Neumann paradigm that dominates present-day computation. It would enable huge speedups of certain very hard problems, notably factorization 1 . Constructing a quantum computer (QC) presents many challenges, however. Chief among these is scalability: the 10 6 qubits needed for simple applications 2 far exceed the potential of existing implementations. This requirement points strongly in the direction of Si-based electronics for QC. Silicon devices offer the advantage of long spin coherence times, fast operation, and a proven record of scalable integration.Specific Si-based qubit proposals utilize donor-bound nuclear 3,4 or electronic 5 spins as qubits. However, quantum dots can also be used to house electron spins 6-8 , and they have the advantage that the electrostatic gates controlling qubit operations are naturally aligned to each qubit. These proposals describe an intriguing possibility. Our aim here is to describe a new SiGe qubit design, and, just as importantly, to carry out detailed modeling of a specific design for the first time. Modeling provides a proof of principle, pinpoints problem areas, and suggests new directions.The fundamental goal of our design is the ability to reduce the electron occupation of an individual dot precisely to one, as in vertically coupled structures. It may be possible to use the spin of multi-electron quantum dots as qubits, but single occupation is clearly desirable. The spin state "up" = 0 or "down" = 1 , stores the quantum bit of information. At the same time, it is necessary to have tunable coupling between neighboring dots. This is achieved by controlled movement of electrons along the quantum well that contains two dots. The solution is to draw on two distinct quantum dot technologies: lateral and vertical tunneling quantum dots 9 .

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