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 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...

No abstract

Using picosecond millimetre-wave impulses we probe the electronic structure and dynamics of electrons in a single quantum dot—the artificial atom—which is essential for understanding their applications such as computational elements and detectors. Although dc transport shows strong elastic cotunnelling in the Coulomb blockade (CB) regime, this effect is suppressed under picosecond impulses whose energies are commensurate with those of the dot. Under non-zero bias we find excited-state resonances in the induced complex photoconductance as well as strong deviations of the dot capacitance compared to equilibrium values according to the ‘orthodox model’ of CB, a direct signature of the relaxation times of single-electron tunnelling.