Determination of the complex microwave photoconductance of a single quantum dot

A small quantum dot containing approximately 20 electrons is realized in a two-dimensional electron system of an AlGaAs/GaAs heterostructure. Conventional transport and microwave spectroscopy reveal the dot's electronic structure. By applying a coherently coupled two-source technique, we are able to determine the complex microwave induced tunnel current. The amplitude of this photoconductance resolves photon-assisted tunneling (PAT) in the non-linear regime through the ground state and an excited state as well. The out-of-phase component (susceptance) allows to study charge relaxation within the quantum dot on a time scale comparable to the microwave beat period.Comment: 5.5 pages, 6 figures, accepted by Phys. Rev. B (Jan. B15 2001

Adiabatic steering and determination of dephasing rates in double dot qubits

We propose a scheme to prepare arbitrary superpositions of quantum states in double quantum--dots irradiated by coherent microwave pulses. Solving the equations of motion for the dot density matrix, we find that dephasing rates for such superpositions can be quantitatively infered from additional electron current pulses that appear due to a controllable breakdown of coherent population trapping in the dots.Comment: 5 pages, 4 figures. To appear in Phys. Rev.

Adiabatic Transfer of Electrons in Coupled Quantum Dots

We investigate the influence of dissipation on one- and two-qubit rotations in coupled semiconductor quantum dots, using a (pseudo) spin-boson model with adiabatically varying parameters. For weak dissipation, we solve a master equation, compare with direct perturbation theory, and derive an expression for the `fidelity loss' during a simple operation that adiabatically moves an electron between two coupled dots. We discuss the possibility of visualizing coherent quantum oscillations in electron `pump' currents, combining quantum adiabaticity and Coulomb blockade. In two-qubit spin-swap operations where the role of intermediate charge states has been discussed recently, we apply our formalism to calculate the fidelity loss due to charge tunneling between two dots.Comment: 13 pages, 8 figures, to appear in Phys. Rev.

Non-Markovian dynamics of double quantum dot charge qubits due to acoustic phonons

We investigate the dynamics of a double quantum dot charge qubit which is coupled to piezoelectric acoustic phonons, appropriate for GaAs heterostructures. At low temperatures, the phonon bath induces a non-Markovian dynamical behavior of the oscillations between the two charge states of the double quantum dot. Upon applying the numerically exact quasiadiabatic propagator path-integral scheme, the reduced density matrix of the charge qubit is calculated, thereby avoiding the Born-Markov approximation. This allows a systematic study of the dependence of the Q-factor on the lattice temperature, on the size of the quantum dots, as well as on the interdot coupling. We calculate the Q-factor for a recently realized experimental setup and find that it is two orders of magnitudes larger than the measured value, indicating that the decoherence due to phonons is a subordinate mechanism.Comment: 5 pages, 7 figures, replaced with the version to appear in Phys. Rev.

Stratigraphic and Ecologic Implications of Late Precambrian Microfossils from Utah

Abundant and well preserved microfossils have been discovered in black shales and siltstones of the glaciogenic upper Precambrian Mineral Fork Formation near Salt Lake City, Utah. The rocks are interpreted to have been deposited close to an ice margin in a shallow, restricted marine embayment during glacial retreat. Several distinct morphotypes are discernible, but following Moorman (1974), we interpret most of these as stages in the life cycle of a single planktonic, endosporulating alga, Bavlinella faveolata (Shepeleva) Vidal. The low taxonomic diversity of this assemblage, coupled with the large population size and almost complete dominance by Bavlinella, suggests an ecologically stressed environment. The source of this stress was probably the melting glacier. On the basis of stratigraphic position and lithological correlation with a radiometrically dated sequence in Washington, the Mineral Fork Formation has been considered to be about 800 m.y. old; however, most well dated occurrences of Bavlinella are in rocks of Vendian (650-570 m.y.) age. The presence of Bavlinella faveolata in Mineral Fork strata raises significant questions concerning both the stratigraphic range of this presumed Vendian index fossil and the timing of Late Precambrian glaciation in the North American Cordillera

Photon-Assisted Transport Through Ultrasmall Quantum Dots: Influence of Intradot Transitions

We study transport through one or two ultrasmall quantum dots with discrete energy levels to which a time-dependent field is applied (e.g., microwaves). The AC field causes photon-assisted tunneling and also transitions between discrete energy levels of the dot. We treat the problem by introducing a generalization of the rotating-wave approximation to arbitrarily many levels. We calculate the dc-current through one dot and find satisfactory agreement with recent experiments by Oosterkamp et al. . In addition, we propose a novel electron pump consisting of two serially coupled single-level quantum dots with a time-dependent interdot barrier.Comment: 16 pages, Revtex, 10 eps-figure

Steering of a Bosonic Mode with a Double Quantum Dot

We investigate the transport and coherence properties of a double quantum dot coupled to a single damped boson mode. Our numerically results reveal how the properties of the boson distribution can be steered by altering parameters of the electronic system such as the energy difference between the dots. Quadrature amplitude variances and the Wigner function are employed to illustrate how the state of the boson mode can be controlled by a stationary electron current through the dots.Comment: 10 pages, 6 figures, to appear in Phys. Rev.

Molecular states in carbon nanotube double quantum dots

We report electrical transport measurements through a semiconducting single-walled carbon nanotube (SWNT) with three additional top-gates. At low temperatures the system acts as a double quantum dot with large inter-dot tunnel coupling allowing for the observation of tunnel-coupled molecular states extending over the whole double-dot system. We precisely extract the tunnel coupling and identify the molecular states by the sequential-tunneling line shape of the resonances in differential conductance.Comment: 5 pages, 4 figure

Single-electron quantum dot in Si/SiGe with integrated charge-sensing

Single-electron occupation is an essential component to measurement and manipulation of spin in quantum dots, capabilities that are important for quantum information processing. Si/SiGe is of interest for semiconductor spin qubits, but single-electron quantum dots have not yet been achieved in this system. We report the fabrication and measurement of a top-gated quantum dot occupied by a single electron in a Si/SiGe heterostructure. Transport through the quantum dot is directly correlated with charge-sensing from an integrated quantum point contact, and this charge-sensing is used to confirm single-electron occupancy in the quantum dot.Comment: 3 pages, 3 figures, accepted version, to appear in Applied Physics Letter

Introduction: Oligocene to Pleistocene Eustatic Change at the New Jersey Continental Margin--a Test of Sequence Stratigraphy

The emergence of seismic/sequence stratigraphy since the late 1970s has led to a revolution in stratigraphy and a renewal of interest in the stratigraphic response to eustasy. Two arguments were advanced in support of the eustatic interpretation. One involved widespread seismic evidence for the existence of regional unconformities (sequence boundaries) characterized by apparently abrupt basinward shifts in onlap, which were interpreted to imply relatively rapid falls of sea level with amplitudes of up to several hundred meters. The second was based on the purported global synchroneity of these unconformities, which, if correct, would be difficult to explain by other than a eustatic mechanism. The primary goals of Leg 174A are the following: 1. To date as precisely as possible sequence boundaries of Oligocene—Pleistocene age, and to compare this stratigraphic record with the timing of glacial-eustatic changes inferred from deep-sea d18O variations; 2. To place constraints on the amplitudes and rates of sea-level change that may have been responsible for unconformity development; 3. To assess the relationships between depositional facies and sequence architecture; and 4. To provide a baseline for future scientific ocean drilling that will address the effects and timing of sea-level changes on this and other passive continental margins. An additional goal for Leg 174A is technical. The leg represents the first attempt by scientific ocean drilling in almost 30 years to sample a thickly sedimented continental margin in water depths less than 150 m. Two sites (Sites 1071 and 1072) are located on the outer part of the continental shelf in water depths of 88-90 and 98-100 m, respectively. An additional site (Site 1073) is located on the uppermost continental slope, part of the Hudson Apron, in a water depth of 639 m