Double Dot System Research Articles

Random telegraph signal in a metallic double-dot system.

In this work, we investigate the dynamics of a single electron surface trap, embedded in a self-assembly metallic double-dot system. The charging and discharging of the trap by a single electron is manifested as a random telegraph signal of the current through the double-dot device. We find that we can control the duration time that an electron resides in the trap through the current that flows in the device, between fractions of a second to more than an hour. We suggest that the observed switching is the electrical manifestation of the optical blinking phenomenon, commonly observed in semiconductor quantum dots.

Phonon influence on the measurement of spin states in double quantum dots using the quantum point contact

We study the influence of phonon scattering on the noise characteristics of a quantum point contact coupled to a two-electron system in a double quantum dot, as proposed for a singlet-triplet measurement scheme in a double-dot system. We point out that at low temperatures phonon-induced relaxation to the ground state suppresses transitions to doubly occupied singlet states which are the source of detectable current fluctuations in this measurement scheme. Thus, for a relatively strong electron-phonon interaction present in the system, the two configurations display the same noise characteristics. In this way, coupling to phonons reduces the distinguishability between the singlet and triplet configurations. Under such conditions, the proposed measurement scheme is no longer valid even though the times of the measurement-induced decoherence of an initial singlet-triplet superposition and of the localization into the singlet or triplet subspace remain essentially unchanged.

Precision control of charge coherence in parallel double dot systems through spin-orbit interaction

In terms of the exact quantum master equation solution for open electronic systems, the coherent dynamics of two charge states described by two parallel quantum dots with one fully polarized electron on either dot is investigated in the presence of spin-orbit interaction. We demonstrate that the double dot system can stay in a dynamically decoherence free space. The coherence between two double dot charge states can be precisely manipulated through a spin-orbit coupling. The effects of the temperature, the finite bandwidth of lead, and the energy deviations during the coherence manipulation are also explored.

Reconfigurable ground states in connected double-dot system

Controlling the polarity of a vortex core in patterned magnetic arrays of double-dot systems was recently shown via a nonlinear spin dynamics manipulation procedure. In the present work, we utilize the microwave absorption amplitude to monitor this polarity switching transition as the excitation field is increased from the linear to nonlinear regime and back. A representative phase diagram is constructed for the two coupled-modes of the double dot system as a function of excitation frequency and amplitude.

Parity qubits and poor man's Majorana bound states in double quantum dots

We study a double quantum dot connected via a common superconducting lead and show that this system can be tuned to host one Majorana bound state (MBS) on each dot. We call them "poor man's Majorana bound states" since they are not topologically protected, but otherwise share the properties of MBS formed in topological superconductors. We describe the conditions for the existence of the two spatially separated MBS, which include breaking of spin degeneracy in the two dots, with the spins polarized in different directions. Therefore, we propose to use a magnetic field configuration where the field directions on the two dot form an angle. By control of this angle the cross Andreev reflection and the tunnel amplitudes can be tuned to be approximately equal, which is a requirement for the formation of the MBS. We show that the fermionic state encoded in the two Majoranas constitutes a parity qubit, which is non-local and can only be measured by probing both dots simultaneously. Using a many-particle basis for the MBS, we discuss the role of interactions and show that inter-dot interactions always lift the degeneracy. We also show how the MBS can be probed by transport measurements and discuss how the combination of several such double dot systems allows for entanglement of parity qubits and measurement of their dephasing times.

Kondo temperature and screening extension in a double quantum dot system

In this work we use the Slave Boson Mean Field Approximation at finite U to study the effects of spin-spin correlations in the transport properties of two quantum dots coupled in series to metallic leads. Different quantum regimes of this system are studied in a wide range of parameter space. The main aspects related to the interplay between the half-filling Kondo effect and the antiferromagnetic correlation between the quantum dots are reviewed. Slave boson results for conductance, local density of states in the quantum dots, and the renormalized energy parameters, are presented. As a different approach to the Kondo physics in a double dot system, the Kondo cloud extension inside the metallic leads is calculated and its dependence with the inter-dot coupling is analyzed. In addition, the cloud extension permits the calculation of the Kondo temperature of the double quantum dot. This result is very similar to the corresponding critical temperature $T_c$, as a function of the parameters of the system, as obtained by using the finite temperature extension of the Slave Boson Mean Field Approximation.

Zero bias anomalies in the Kondo regime of single and double quantum dots

The zero bias anomaly of the differential conductance of mesoscopic systems is a fingerprint of the electron-electron interaction. The common example is the peak of the differential conductance in Kondo mesoscopic systems.We show that in complex mesoscopic systems, in particular in the side-coupled double dot, the dip of the differential conductance at zero bias is also possible due to simultaneous effects of interference and correlations. The external parameters that control this effect are the temperature and the gate potential on the lateral dot. We argue that even in single dots, in a multiple lead configuration, the shift from suppression to enhancement of dI/dV with increasing bias is allowed if the bias applied on the leads is not symmetric. The differential conductance exhibits a peak-dip crossover, the effect being controlled by the strength of the asymmetry and the ratio of the dot-lead couplings.The scaling of the differential conductance for the double-dot system is also discussed. The results are obtained using an extended Anderson model, the Keldysh transport formalism and the equation of motion technique extended to the non-equilibrium.

The role of the indirect tunneling processes and asymmetry in couplings in orbital Kondo transport through double quantum dots

A system of two quantum dots attached to external electrodes is considered theoretically in the orbital Kondo regime. In general, the double dot system is coupled via both Coulomb interaction and direct hopping. Moreover, the indirect hopping processes between the dots (through the leads) are also taken into account. To investigate the system’s electronic properties we apply the slave-boson mean field (SBMF) technique. With the help of the SBMF approach the local density of states for both dots and the transmission (as well as linear and differential conductance) is calculated. We show that Dicke- and Fano-like line shapes may emerge in the transport characteristics of the double dot system. Moreover, we observed that these modified Kondo resonances are very susceptible to the change of the indirect coupling’s strength. We have also shown that the Kondo temperature becomes suppressed with increasing asymmetry in the dot–lead couplings when there is no indirect coupling. Moreover, when the indirect coupling is turned on the Kondo temperature becomes suppressed. By allowing a relative sign of the nondiagonal elements of the coupling matrix with left and right electrodes, we extend our investigations to become more generic. Finally, we have also included the level renormalization effects due to indirect tunneling, which are mostly neglected.

Large-deviation analysis for counting statistics in double-dot Aharonov-Bohm interferometer

The Coulomb correlation and quantum coherence in a double-dot Aharonov-Bohm interferometer can result in two distinct transport channels: a fast channel and a slow one, while their coupling is tunable by changing the magnetic flux passing through an interference loop. However, these effects cannot be manifested by the conventional transport current. In this work, employing the large-deviation method which was originally developed in the nonequilibrium statistical mechanics, we perform a large-deviation analysis for the transport through this double-dot interferometer system and reveal a clear dynamical phase transition behavior.

Symmetric Double Quantum Dot Energy States in a High Magnetic Field

The dynamical Green's function and energy spectrum of a 2D symmetric quantum double-dot system on a planar host in a normal magnetic field are analyzed here, representing the two dots by Dirac delta function potentials. The proliferation of energy levels due to Landau quantization is examined in detail.

On-demand single-electron transfer between distant quantum dots

Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit. Highly controlled tunnelling between neighbouring dots has been demonstrated, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times-a cumulative distance of 0.25 mm-without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices.

Nuclear dynamics during Landau-Zener singlet-triplet transitions in double quantum dots

We consider nuclear-spin dynamics in a two-electron double dot system near the intersection of the electron spin singlet $S$ and the lower energy component ${T}_{+}$ of the spin triplet. The electron spin interacts with nuclear spins and is influenced by the spin-orbit coupling. Our approach is based on a quantum description of the electron spin in combination with the coherent semiclassical dynamics of nuclear spins. We consider single and double Landau-Zener passages across the $S$-${T}_{+}$ anticrossings. For linear sweeps, the electron dynamics is expressed in terms of parabolic cylinder functions. The dynamical nuclear polarization is described by two complex conjugate functions ${\ensuremath{\Lambda}}^{\ifmmode\pm\else\textpm\fi{}}$ related to the integrals of the products of the singlet and triplet amplitudes ${\mathrm{c\ifmmode \tilde{}\else \~{}\fi{}}}_{S}^{*}{\mathrm{c\ifmmode \tilde{}\else \~{}\fi{}}}_{{T}_{+}}$ along the sweep. The real part $P$ of ${\ensuremath{\Lambda}}^{\ifmmode\pm\else\textpm\fi{}}$ is related to the $S$-${T}_{+}$ spin-transition probability, accumulates in the vicinity of the anticrossing, and for long linear passages coincides with the Landau-Zener probability ${P}_{\mathit{LZ}}=1\ensuremath{-}{e}^{\ensuremath{-}2\ensuremath{\pi}\ensuremath{\gamma}}$, where $\ensuremath{\gamma}$ is the Landau-Zener parameter. The imaginary part $Q$ of ${\ensuremath{\Lambda}}^{+}$ is specific for the nuclear-spin dynamics, accumulates during the whole sweep, and for $\ensuremath{\gamma}\ensuremath{\gtrsim}1$ is typically an order of magnitude larger than $P$. $P$ and $Q$ also show critically different dependences on the shape and the duration of the sweep. $Q$ has a profound effect on the nuclear-spin dynamics, by (i) causing intensive shakeup processes among the nuclear spins and (ii) producing a high nuclear spin generation rate when the hyperfine and spin-orbit interactions are comparable in magnitude. Even in the absence of spin-orbit coupling, when the change in the the total angular momentum of nuclear spins is less than $\ensuremath{\hbar}$ per single Landau-Zener passage, the change in the global nuclear configuration might be considerably larger due to the nuclear-spin shakeups. We find analytical expressions for the back action of the nuclear reservoir represented via the change in the Overhauser fields the electron subsystem experiences.

Coulomb oscillations in the Fano-Kondo effect and zero-bias anomalies in a double-dot mesotransistor

We investigate theoretically the transport properties of the side-coupled double quantum dots in connection with the experimental study of Sasaki et al. [Phys. Rev. Lett. 103, 266806 (2009)]. The setup consists of connecting the Kondo dot directly to the leads, while the side dot provides an interference path which affects the Kondo correlations. We analyze the oscillations of the source-drain current due to the periodical Coulomb blockade of the many-level side dot at the variation of the gate potential applied on it. The Fano profile of these oscillations may be controlled by the interdot coupling, level spacing of the side dot, and also by the temperature. The nonequilibrium conductance of the double-dot system exhibits zero-bias anomaly which, besides the usual enhancement, may show also a suppression (a diplike aspect) which occurs around the Fano zero. In the same region, the weak temperature dependence of the conductance indicates the suppression of the Kondo effect. Scaling properties of the nonequilibrium conductance in the Fano-Kondo regime are discussed. Since the Kondo temperature of the single-impurity Anderson model is no longer the proper scaling parameter, we look for an alternative specific to the double dot. The extended Anderson model, Keldysh formalism, and equation-of-motion technique are used.

Impurity effects on semiconductor quantum bits in coupled quantum dots

We theoretically consider the effects of having unintentional charged impurities in laterally coupled two-dimensional double (GaAs) quantum-dot systems, where each dot contains one or two electrons and a single charged impurity. Using molecular orbital and configuration interaction methods, we calculate the effect of the impurity on the two-electron energy spectrum of each individual dot as well as on the spectrum of the coupled-double-dot two-electron system. We find that the singlet-triplet exchange splitting between the two lowest-energy states, both for the individual dots and the coupled-dot system, depends sensitively on the location of the impurity and its coupling strength (i.e. the effective charge). A strong electron-impurity coupling breaks down the equality of the two doubly occupied singlets in the left and the right dots, leading to a mixing between different spin singlets. As a result, the maximally entangled qubit states are no longer fully obtained in the zero-magnetic-field case. Moreover, a repulsive impurity results in a triplet-singlet transition as the impurity effective charge increases or the impurity position changes. We comment on the impurity effect in spin qubit operations in the double-dot system based on our numerical results.

Charge state readout and hyperfine interaction in a few-electron InGaAs double quantum dot

A laterally defined InGaAs double quantum dot with an integrated charge readout sensor is realized in an InGaAs/InP heterostructure. The charge states of the double quantum dot are measured with the use of the charge readout sensor in the few-electron regime in which the current is too weak to be observable by direct measurements of electron transport through the double dot. We also measure the leakage current of the double quantum dot in the Pauli spin-blockade few-electron regime and study the singlet-triplet state mixing by the hyperfine coupling to the nuclear spins. The measurements of the leakage current in the weak external-magnetic-field region and for weak interdot couplings allow us to extract an effective nuclear magnetic field in the double-dot system. We also study spin relaxation and transport processes in the Pauli spin-blockade region at large external magnetic fields and observe transport through the excited triplet state in the few-electron double quantum dot.

Superconducting proximity effect in interacting double-dot systems

We study subgap transport from a superconductor through a double quantum dot with large on-site Coulomb repulsion to two normal leads. Non-local superconducting correlations in the double dot are induced by the proximity to the superconducting lead, detectable in non-local Andreev transport that splits Cooper pairs in locally separated, spin-entangled electrons. We find that the $I$--$V$ characteristics are strongly asymmetric: for a large bias voltage of certain polarity, transport is blocked by populating the double dot with states whose spin symmetry is incompatible with the superconductor. Furthermore, by tuning gate voltages one has access to splitting of the Andreev excitation energies, which is visible in the differential conductance.

Superperiodic conductance in a molecularly wired double-dot system self-assembled in a nanogap electrode

We examined charge transport properties of two gold nanoparticles (GNPs) in a nanogap transistor with a gap width of ∼10 nm. The GNPs connected to each other and to outer electrodes through a small number of dithiolated terthiophene wire molecules as a tunneling barrier. The transport property measured at 11 K was analyzed based on the theory of double-dot single-electron transistors and inelastic cotunneling. The results clearly show mutual Coulomb interactions between the two GNPs. Moreover, we found the appearance of superperiodic conductance, because of differences in the charging energy of the two GNPs.

Fabrication of quantum-dot devices in graphene

We describe our recent experimental results on the fabrication of quantum-dot devices in a graphene-based two-dimensional system. Graphene samples were prepared by micromechanical cleavage of graphite crystals on a SiO2/Si substrate. We performed micro-Raman spectroscopy measurements to determine the number of layers of graphene flakes during the device fabrication process. By applying a nanofabrication process to the identified graphene flakes, we prepared a double-quantum-dot device structure comprising two lateral quantum dots coupled in series. Measurements of low-temperature electrical transport show the device to be a series-coupled double-dot system with varied interdot tunnel coupling, the strength of which changes continuously and non-monotonically as a function of gate voltage.

Universal Conductance Enhancement and Reduction of the Two-Orbital Kondo Effect

We investigate theoretically the linear and nonlinear conductance through a nanostructure with two-fold degenerate single levels, corresponding to the transport through nanostructures such as a carbon nanotube, or double dot systems with capacitive interaction. It is shown that the presence of the interaction asymmetry between orbits/dots affects significantly the profile of the linear conductance at finite temperature, and, of the nonlinear conductance, particularly around half-filling, where the two-particle Kondo effect occurs. Within the range of experimentally feasible parameters, the SU(4) universal behavior is suggested, and comparison with relevant experiments is made.

Gate-defined graphene double quantum dot and excited state spectroscopy

A double quantum dot is formed in a graphene nanoribbon device using three top gates. These gates independently change the number of electrons on each dot and tune the interdot coupling. Transport through excited states is observed in the weakly coupled double dot regime. We extract from the measurements all relevant capacitances of the double dot system, as well as the quantized level spacing.