We study single-electron tunneling in a two-junction device in the presence of microwave radiation. We introduce a model for numerical simulations that extends the Tien-Gordon theory for photon-assisted tunneling to encompass correlated single-electron tunneling. We predict sharp current jumps which reflect the discrete photon energy h f, and a zero-bias current whose sign changes when an electron is added to the central island of the device. Measurements on split-gate quantum dots show microwave-induced features that are in good agreement with the model. An oscillating potential with frequency f changes the energy F. of an electron state into a set of energies E+nhf with n=0,~1,~2, . . . . These so-called sideband energies can lead to electron tunneling that involves the emission (n&0) and absorption (n)0) of photons. The formation of sidebands has been important for many time-dependent transport studies addressing issues such as photon-assisted tunneling of quasiparticles in superconducting junctions, the tunneling time, and time-dependent resonant tunneling. Recent theoretical work has begun to focus on the effects of an oscillating potential on transport through small capacitance devices where the charging energy regulates the tunnel processes. For instance, Bruder and Schoeller have calculated the photoresponse of a two-level system. However, little theoretical work exists on realistic systems, e.g. , quantum dots with many quantum levels, and no experiments have been reported except at very low frequencies. "In this paper, we present numerical simulations and experiments on photon-assisted tunneling through a quantum dot. We assume a continuous single-particle density of states, i.e. , a metallic system with an equivalent single electron circuit shown in the inset of Fig. 1 . Our model extends theTien-Gordon theory' to include the correlated tunneling of single electrons through a two-junction device. This model predicts discrete photon features, and is in agreement with measurements on split-gate quantum dot devices irradiated by 19-GHz microwaves.We model the microwaves as an oscillating potential V cos(2mft) of the central island relative to the source and drain leads. To allow for an asymmetry in the ac coupling, we model the microwave amplitude by two parameters a; =eV;/h f, i =s,d where V, and Vd are the ac voltage drops across the source and drain junctions. Analogous to the Tien-Gordon description, we write the tunnel rate I; in the presence of microwaves in terms of the rate 1; without microwaves as

We have used gated GaAs/AlGaAs heterostructures to explore nonlinear transport between spin-resolved Landau level (LL) edge states over a submicron region of two-dimensional electron gas (2DEG). The current I flowing from one edge state to the other as a function of the voltage V between them shows diode-like behavior-a rapid increase in I above a well-defined threshold Vt under forward bias, and a slower increase in I under reverse bias. In these measurements, a pronounced influence of a current-induced nuclear spin polarization on the spin splitting is observed, and supported by a series of NMR experiments. We conclude that the hyperfine interaction plays an important role in determining the electronic properties at the edge of a 2DEG.

We present the multilevel fabrication and measurement of a Coulomb-blockade device displaying tunable negative differential resistance (NDR). Applications for devices displaying NDR include amplification, logic, and memory circuits. Our device consists of two Al/Al$_{x}$O$_{y}$ islands that are strongly coupled by an overlap capacitor. Our measurements agree excellently with a model based on the orthodox theory of single-electron transport.Comment: 3 pages, 3 figures; submitted to AP

We have measured the nonlinear transport properties of two GaAs/Al x Ga 1Ϫx As quantum dots connected in series. At high source-drain bias the Coulomb oscillations develop a sharp overstructure. The behavior of this overstructure is studied as a function of the electrostatic potentials of the dots. The structure is shown to arise from the modulation of interdot tunneling that occurs as the quantum levels in the two dots are aligned and dealigned.

We present results on microwave-assisted transport through quantum dots. First, the important energy/frequency scales are discussed. Then, measurements of the current versus gate voltage characteristics in the presence of microwaves are presented. At finite source-drain bias, microwave-induced features are observed, and at zero source-drain bias, an oscillating photocurrent is observed. A model of photon-assisted transport is discussed that can account for the experimental observations.

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We investigate the ground state properties of a system containing two superconducting islands coupled capacitively by a wire. The ground state is a macroscopic superposition of charge states, even though the islands cannot exchange charge carriers. The ground state of the system is probed by measuring the switching current of a Bloch transistor containing one of the islands. Calculations based on superpositions of charge states on both islands show good agreement with the experiments. The ability to couple quantum mechanical charge fluctuations in two neighboring devices using a wire is relevant for realizing quantum computation with this kind of circuit. DOI: 10.1103/PhysRevB.67.144512 PACS number͑s͒: 73.23.Hk, 74.50.ϩr, 85.25.Hv Quantum phenomena in artificially fabricated structures has received much attention lately largely due to the interest in performing quantum computation in such systems. If quantum states can be manipulated in an artificially fabricated circuit, there is hope that the circuit could be increased in complexity to a size where it may be able to perform useful functions. Quantum coherence in fabricated structures has been discussed for the charge states in quantum dots 1 and for nuclear spin states of impurity atoms embedded in silicon. 2 Measurements have been performed using charge states on a single superconducting island 3-6 and flux states in a circuit containing a superconducting loop. [7][8][9] In this paper, we show that the ground state of a system containing two superconducting islands that are capacitively coupled by a wire, can be in a superposition of spatially distinct charge states. This type of coupling is of importance for realizing complex quantum circuits with mesoscopic charge devices. Figure 1 shows a scanning electron microscope ͑SEM͒ photo of the sample and the circuit schematic. The two square superconducting islands labeled L and R play a central role in this circuit. They are spaced 3 m apart and are coupled by a wire that contains two capacitors in series. There is no exchange of charge carriers between the two superconducting islands; the interaction between the islands is purely electrostatic. Each island can exchange charge with its superconducting leads through small-capacitance Josephson junctions. Together with the leads and the left gate electrode, the island L forms a Bloch transistor that was current biased by an external current source. Single Bloch transistors have been studied in detail and their behavior is well understood. [10][11][12][13][14][15] The leads of island R are joined in a small loop, transforming the island into a Cooper-pair box with tunable Josephson energy, one of the promising candidates for the realization of a charge qubit. 3,16 The state of this circuit can be conveniently described by the charge states of the two islands ͉n L ,n R ͘. Here n L is the number of excess Cooper pairs on the left island and n R is the number of excess Cooper pairs on the right island. For certain values of the gate voltages and the applied flux, t...

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