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...

Strain plays a critical role in the properties of materials. In silicon and silicon-germanium, strain provides a mechanism for control of both carrier mobility and band offsets. In materials integration, strain is typically tuned through the use of dislocations and elemental composition. We demonstrate a versatile method to control strain by fabricating membranes in which the final strain state is controlled by elastic strain sharing, that is, without the formation of defects. We grow Si/SiGe layers on a substrate from which they can be released, forming nanomembranes. X-ray-diffraction measurements confirm a final strain predicted by elasticity theory. The effectiveness of elastic strain to alter electronic properties is demonstrated by low-temperature longitudinal Hall-effect measurements on a strained-silicon quantum well before and after release. Elastic strain sharing and film transfer offer an intriguing path towards complex, multiple-layer structures in which each layer's properties are controlled elastically, without the introduction of undesirable defects.

The spins of localized electrons in silicon are strong candidates for quantum information processing because of their extremely long coherence times and the integrability of Si within the present microelectronics infrastructure. This paper reviews a strategy for fabricating single electron spin qubits in gated quantum dots in Si/SiGe heterostructures. We discuss the pros and cons of using silicon, present recent advances, and outline challenges.

We report on the fabrication and characterization of quantum dot devices in a Schottky-gated silicon/silicon-germanium two-dimensional electron gas (2DEG). The dots are confined laterally inside an etch-defined channel, while their potential is modulated by an etch-defined 2DEG gate in the plane of the dot. For the first time in this material, Schottky top gates are used to define and tune the tunnel barriers of the dot. The leakage current from the gates is reduced by minimizing their active area. Further suppression of the leakage is achieved by increasing the etch depth of the channel. The top gates are used to put the dot into the Coulomb blockade regime, and conductance oscillations are observed as the voltage on the side gate is varied.

The spins of localized electrons in silicon are strong candidates for quantum information processing because of their extremely long coherence times and the integrability of Si within the present microelectronics infrastructure. This paper reviews a strategy for fabricating single electron spin qubits in gated quantum dots in Si/SiGe heterostructures. We discuss the pros and cons of using silicon, present recent advances, and outline challenges.

We report the fabrication and electrical characterization of a single electron transistor in a modulation doped silicon/silicon-germanium heterostructure. The quantum dot is fabricated by electron beam lithography and subsequent reactive ion etching. The dot potential and electron density are modified by laterally defined side gates in the plane of the dot. Low temperature measurements show Coulomb blockade with a single electron charging energy of 3. Silicon-germanium modulation doped field-effect transistors ͑MODFETs͒ are potentially attractive devices for high-speed, low noise communications applications, where low cost and compatibility with complementary metaloxide-semiconductor logic are desirable.1 Because the silicon quantum well containing the electrons is strained by up to 2%, the electron mobility of these structures is as much as a factor of five larger than that of unstrained silicon fieldeffect transistors ͑FET͒ at room temperature, offering the prospect of high speed operation. At low temperatures, electron mobilities as high as 5.2ϫ10 5 cm 2 /V s have been reported, 2,3 raising the possibility of lithographically patterned quantum devices.Development of quantum devices in silicon MODFETs is of particular interest, because silicon is unique among the elemental and binary semiconductors in that it has an abundant nuclear isotope of spin zero. Silicon also has very small spin orbit coupling. Together, these two features provide only weak channels for electron spin relaxation; the electron spin dephasing time T 2 for phosphorus-bound donors has been measured to be as long as 3 ms at 7 K. 4 Kane has pointed out the advantages of nuclear spins in silicon for quantum computation, 5 and his scheme has been extended to electrons in SiGe heterostructures.6 Following Loss and DiVincenzo, 7 specific schemes have been proposed for spin-based quantum computation in silicon-germanium electron quantum dots. 8,9Here we demonstrate a quantum dot fabricated in a layered silicon/silicon-germanium ͑Si/SiGe͒ heterostructure that includes a strained Si quantum well containing a twodimensional electron gas ͑2DEG͒. Even with recent advances in the growth of high mobility SiGe modulationdoped heterostructures, producing lithographically defined n-type quantum dots with periodic Coulomb blockade has been challenging. The fabrication of highly isolated Schottky top gates is particularly difficult. 10,11 Due to the lattice mismatch between layers of different Ge fraction, misfit dislocations must be present to relieve the strain in the SiGe buffer layer. Misfit dislocations terminate in threading arms running up to the heterostructure surface, and these threading arms may play a role in forming a conductive path between top Schottky contacts and the 2DEG later. We have avoided this problem by fabricating a dot with highly isolated side gates formed from the 2DEG itself.The Si/SiGe heterostructure used here was grown by ultrahigh vacuum chemical vapor deposition.2 The 2DEG sits near the top of 80 Å of strained Si grown on a s...

A technique is reported for measuring and mapping the maximum internal temperature of a structural epoxy resin with high spatial resolution via the optically detected shape transformation of embedded gold nanorods (AuNRs). Spatially resolved absorption spectra of the nanocomposites are used to determine the frequencies of surface plasmon resonances. From these frequencies the AuNR aspect ratio is calculated using a new analytical approximation for the Mie-Gans scattering theory, which takes into account coincident changes in the local dielectric. Despite changes in the chemical environment, the calculated aspect ratio of the embedded nanorods is found to decrease over time to a steady-state value that depends linearly on the temperature over the range of 100-200 °C. Thus, the optical absorption can be used to determine the maximum temperature experienced at a particular location when exposure times exceed the temperature-dependent relaxation time. The usefulness of this approach is demonstrated by mapping the temperature of an internally heated structural epoxy resin with 10 μm lateral spatial resolution.

While numerous flow sensor architectures mimic the natural cilia of crickets, locusts, bats, and fish, the prediction of sensor output for given flow conditions based on the sensor properties has not been achieved. Challenges include difficulty in determining the electromechanical properties of the sensors, limited working knowledge of the boundary layer, low sensitivity to small hair deflections, and lack of models for large deflections. Within this work, hair sensors are fabricated using piezoresistive arrays of carbon nanotubes (CNTs) without traditional microelectromechanical processing. While correlating the CNT array electromechanical properties to synthesis conditions remains a challenge, a consistent, proportional, and predictable response to steady, boundary-determined air flow is obtained using theory and measurement for various lengths of hairs. The moment sensitivity is shown to scale inversely with the CNT length and stiffness to a typical maximum of 1.3 ± 0.4% resistance change nN −1 m −1 . The normalized CNT piezoresistivity is constant (1.1 ± 0.2) for a majority of the more than two dozen sensors examined despite the orders-of-magnitude variability in both sensitivity and CNT compressive modulus. The sensor sensitivity and noise both distinctly change as the flow transitions from steady and laminar to turbulent, suggesting the sensor may be capable of detecting flow transitions.