Double Quantum Dot Research Articles

Nonlocal quantum heat engines made of hybrid superconducting devices

We discuss a quantum thermal machine that generates power from a thermally driven double quantum dot coupled to normal and superconducting reservoirs. Energy exchange between the dots is mediated by electron-electron interactions. We can distinguish three main mechanisms within the device operation modes. In the Andreev tunneling regime, energy flows in the presence of coherent superposition of zero- and two-particle states. Despite the intrinsic electron-hole symmetry of Andreev processes, we find that the heat engine efficiency increases with increasing coupling to the superconducting reservoir. The second mechanism occurs in the regime of quasiparticle transport. Here we obtain large efficiencies due to the presence of the superconducting gap and the strong energy dependence of the electronic density of states around the gap edges. Finally, in the third regime there exists a competition between Andreev processes and quasiparticle tunneling. Altogether, our results emphasize the importance of both pair tunneling and structured band spectrum for an accurate characterization of the heat engine properties in normal-superconducting coupled dot systems.

Delayed response to the photovoltaic performance in a double quantum dots photocell with spatially correlated fluctuation

A viable strategy for enhancing photovoltaic performance is to comprehend the underlying quantum physical regime of charge transfer in a double quantum dots (DQD) photocell. This work explored the photovoltaic performance dependent spatially correlated fluctuation in a DQD photocell. The effects of spatially correlated fluctuation on charge transfer and output photovoltaic efficiency were explored in a proposed DQD photocell model. The results revealed that the charge transport process and the time to peak photovoltaic efficiency were both significantly delayed by the spatially correlated fluctuation, while the anti-spatially correlated fluctuation reduced the output peak photovoltaic efficiency. Further results revealed that the delayed response could be suppressed by gap difference and tunneling coefficient within two dots. Subsequent investigation demonstrated that the delayed response was caused by the spatial correlation fluctuation slowing the generative process of noise-induced coherence, which had previously been proven to improve the quantum photovoltaic performance in quantum photocells. And the reduced photovoltaic properties were verified by the damaged noise-induced coherence owing to the anti-spatial correlation fluctuation and a hotter thermal ambient environment. The discovery of delayed response generated by the spatially correlated fluctuations will deepen the understanding of quantum features of electron transfer, as well as promises to take our understanding even further concerning quantum techniques for high efficiency DQD solar cells.

Josephson transport through parallel double coupled quantum dots at infinite-U limit

Josephson transport through parallel double coupled quantum dots at infinite-U limit

Transport through the T-shaped double-quantum-dot structure with side-coupled Majorana bound states

We perform theoretical investigation of the transport properties in the T-shaped double-quantum-dot structure, by considering Majorana bound states (MBSs) to be coupled to the dot in the main channel. Calculation results show that in the linear transport regime, the side-coupled dot covers the influence of the MBS on the transport behaviors, independent of the Coulomb interaction in the QDs. In the absence of superconducting pairing potential in the QDs, tuning the side-coupled dot level to the Fermi energy can erase the contribution of MBS to the conductance, due to the occurrence of antiresonance. Otherwise, the linear conductance exhibits two distinct results determined by the inter-MBS coupling manners. In the case of Majorana zero mode, the linear conductance value keeps equal to e 2 2 h when the side-coupled dot departs from the Fermi energy. Alternatively in the case of Majorana nonzero mode, the linear conductance is always analogous to the MBS-absent case. On the other hand, when the superconducting pairing potential is incorporated, the linear conductance is also independent of the QD-MBS coupling. Especially in the case of Majorana nonzero mode, the MBSs are decoupled from the QDs. These findings provide new information for understanding the interplay between the MBSs and the quantum dots. • We study transport in T-shaped double quantum dots, by considering MBSs to be coupled to the main dot. • Tuning the side-coupled dot level to the Fermi level can erase the contribution of MBS to the conductance. • For Majorana zero mode, the linear conductance is equal to e 2 /2h when the side-coupled dot departs from the Fermi level. • For Majorana nonzero mode, the linear conductance is always analogous to the MBS-absent case. • If superconducting pairing potential exists in QDs, MBSs are decoupled from QDs for Majorana nonzero mode.

The spatial electric field effect on the impurity binding energy and self-polarization in a double quantum dot

The spatial electric field effect on the impurity binding energy and self-polarization in a double quantum dot

Cooper pair splitter in a photonic cavity: Detection of Andreev scatterings

We simulated the radiative response of the cavity quantum electrodynamics (QED) coupled to the double quantum dot Cooper pair splitter and analyzed its spectral dependence to get insight into dynamics of the Cooper pair transfers. The model is confined to the energy subspace where two entangled electrons are transferred to two normal electrodes through the interdot singlet state on two proximitized quantum dots. Our research is focused on the Andreev scatterings in the subgap regime, for which the local charge susceptibility $\mathrm{\ensuremath{\Pi}}({\ensuremath{\omega}}_{p})$ is derived, by means of Keldysh Green functions, in a whole bias voltage range. In particular, in the large voltage limit, the spectrum of $\mathrm{\ensuremath{\Pi}}({\ensuremath{\omega}}_{p})$ is expressed by a simple analytical formula, which shows various dissipation processes related with photon-induced transitions between the Andreev bound states.

Double heterojunction CQDs/CeO2/BaFe12O19 magnetic separation photocatalysts: Construction, structural characterization, dye and POPs removal, and the interrelationships between magnetism and photocatalysis

This novel double heterojunction carbon quantum dots (CQDs)/CeO2/BaFe12O19 magnetic separation photocatalysts (CCBFOMSPs) were successfully prepared via a polyacrylamide gel method and low temperature sintering technology combined with a hydrothermal method. The CQDs components in CCBFOMSPs were identified by neutron powder diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy. The charge carriers could transfer and separation between the valence state components (CeO2 and BaFe12O19) and CQDs facilely, inhibiting the recombination of electron-hole pairs significantly by Hall effect. The photocatalytic activity of CCBFOMSPs for the degradation of mixed dyes include methyl orange (MO), rhodamine B (RhB) and methylene blue (MB) and tetrabromobisphenol A (TBBPA) were remarkably improved by regulating the electromagnetic interaction between the BaFe12O19 contents and the photo-generated electrons. Capturing experiment showed that the active species of hole (hVB+), hydroxyl radical (•OH) and superoxide radical (•O2−) played dominant roles in mixed dyes degradation. Attractively, the effect shed new light on wide applications of the double heterojunction magnetic separation photocatalysts.

Real-Time Observation of Charge-Spin Cooperative Dynamics Driven by a Nonequilibrium Phonon Environment.

We report on experimental observations of charge-spin cooperative dynamics of two-electron states in a GaAs double quantum dot located in a nonequilibrium phonon environment. When the phonon energy exceeds the lowest excitation energy in the quantum dot, the spin-flip rate of a single electron strongly enhances. In addition, originated from the spatial gradient of phonon density between the dots, the parallel spin states become more probable than the antiparallel ones. These results indicate that spin is essential for further demonstrations of single-electron thermodynamic systems driven by phonons, which will greatly contribute to understanding of the fundamental physics of thermoelectric devices.

Normal mode splitting and four-wave mixing in Kerr-nonlinear optical system containing coupled double quantum dot

We theoretically investigate the four wave mixing (FWM) and normal mode splitting (NMS) in coupled double quantum dots cavity system in the presence of a third-order Kerr nonlinear medium. The FWM generated in the system has narrow spectral width and this can be tuned by nonlinear medium strength, the coupling strength between two coupled quantum dots and pump strength. Further, we study NMS that can also be tuned by various physical parameters and show that energy exchange between different modes is possible. The proposed model have potential applications in the field of quantum information processing.

Double quantum dot–metal nanoparticle systems under strong coupling

This work uses the Green function to model the emission spectra from a hybrid metal nanoparticle (MNP) coupled with a double quantum dot (DQD), considering higher-order plasmonic mode contribution. It calculates the quantum dot (QD) energy states and momenta, i.e., this work differs from other strong-coupling systems by considering the material entities. A Fano-shape spectrum is shown with peaks depending on interference between the fields with the DQD and MNP. A prominent effect of the pump field appears as it interferes with other fields (probe and MNP polarization field). The MNP–DQD distance and MNP radius control the peak height and its position in the spectrum. The importance of the probe field in controlling the peak frequency and its height is demonstrated. The transition energy with momenta controls the spectra. An approximated relation is presented. High strength in the DQD–MNP and a more strong contribution are obtained compared to QD–MNP.

Optical susceptibility of metal nanoparticle-double quantum dot hybrid system

This work is modeling the linear optical susceptibility of a double quantum dot (DQD)–metal nanoparticle (MNP) hybrid system using density matrix equations that consider the interaction between excitons and surface plasmons. The wetting layer (WL)-QD interaction is considered.The absorption of the hybrid DQD-MNP system increases by five times compared to the bare DQD system. The absorption peak is reducing in a similar ratio of increasing MNP radius. A Mollow triplet with asymmetric sidebands appears and decreases with increasing MNP radius, which is compatible with recent experimental evidence. The susceptibility peaks at resonance for a small distance between the hybrid MNP-DQD. The susceptibility is increased by one order for the DQD system compared to a single QD. A similar result is obtained under WL-QD detuning compared to the case of the probe between DQD states. Small DQDs give high absorption and dispersion, offering more shifts from resonance.

Swap Test with Quantum Dot Charge Qubits

We propose the implementation of the swap test using a charge qubit in a double quantum dot. The swap test is a fundamental quantum subroutine in quantum machine learning and other applications for estimating the fidelity of two unknown quantum states by measuring an auxiliary qubit. Our proposal uses a controlled three-qubit gate, which is natural to quantum dot charge qubits. It allows us to implement a swap test with a circuit depth of six layers, and an estimated gate time of less than 3 ns, that is below the coherence time of double quantum dots. This work paves the way for enhancing the toolbox of quantum machine learning developments in semiconductor qubits.

Fano effect in a thermally induced transport through a triple quantum dot within the Coulomb blockade regime

In the present work we theoretically study thermoelectric transport and heat transfer in a junction including a T-shaped double quantum dot coupled to nonmagnetic electrodes and supplemented with a third dot in a parallel configuration. We focus on the combined effect of Coulomb interactions and quantum interference occurring in the T-shaped portion of the considered triple quantum dot on the thermoelectric electron transport. The transport through the system is studied within the Coulomb blockade regime in the limit of strong intra-dot interactions between electrons beyond the linear response regime. It is shown that under these conditions both charge and heat transfer through the considered system may be significantly affected by the quantum interference and inter-dot Coulomb interactions.

The effect of intrinsic decoherence on quantum dynamics of two coupled double quantum dot systems

Double quantum dot (DQD) system is an ideal candidate for qubit in quantum information theory. Hence, we will investigate the two couple DQDs in the presence of intrinsic decoherence. We derive the energy spectrum and the corresponding eigenstates. Using the concurrence as a measure of entanglement for a bipartite pure state, the entanglement of the energy eigenstates is obtained and shows that the energy eigenstates reduce to the Bell states for Δ1=Δ2=0. The effects of the physical parameters such as tunneling rate between two QDs, Coulomb coupling, and intrinsic decoherence on the entanglement and the dynamics of DQD system will be examined. Furthermore, fidelity is used to measure how close the dynamically evolved state is to the initial state. It is found that for a given Γ and the smaller (larger) values of Δ2 (coulomb potential), the DQD system is more robust to the destructive effect of the intrinsic decoherence.

In situ Tuning of the Electric-Dipole Strength of a Double-Dot Charge Qubit: Charge-Noise Protection and Ultrastrong Coupling

Semiconductor quantum dots in which electrons or holes are isolated via electrostatic potentials generated by surface gates are promising building blocks for semiconductor-based quantum technology. Here, we investigate double-quantum-dot (DQD) charge qubits in GaAs capacitively coupled to high-impedance superconducting quantum interference device array and Josephson-junction array resonators. We tune the strength of the electric-dipole interaction between the qubit and the resonator in situ using surface gates. We characterize the qubit-resonator coupling strength, the qubit decoherence, and the detuning noise affecting the charge qubit for different electrostatic DQD configurations. We find all quantities to be systematically tunable over more than one order of magnitude, resulting in reproducible decoherence rates Γ2/2π<5 MHz in the limit of high interdot capacitance. In the opposite limit, by reducing the interdot capacitance, we increase the DQD electric-dipole strength and, therefore, its coupling to the resonator. Employing a Josephson-junction array resonator with an impedance of approximately 4 kΩ and a resonance frequency of ωr/2π∼5.6 GHz, we observe a coupling strength of g/2π∼630 MHz, demonstrating the possibility to operate electrons hosted in a semiconductor DQD in the ultrastrong-coupling regime (USC). The presented results are essential for further increasing the coherence of quantum-dot-based qubits and investigating USC physics in semiconducting QDs.6 MoreReceived 26 October 2021Revised 3 April 2022Accepted 26 May 2022DOI:https://doi.org/10.1103/PhysRevX.12.031004Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum information architectures & platformsQuantum information processingQuantum information with hybrid systemsQuantum information with solid state qubitsQuantum Information

The impact of hydrostatic pressure and temperature on the binding energy, linear, third-order nonlinear, and total optical absorption coefficients and refractive index changes of a hydrogenic donor impurity confined in GaAs/AlxGa1−xAs double quantum dots

The impact of hydrostatic pressure and temperature on the binding energy, linear, third-order nonlinear, and total optical absorption coefficients and refractive index changes of a hydrogenic donor impurity confined in GaAs/AlxGa1−xAs double quantum dots

Measurement of Tunnel Coupling in a Si Double Quantum dot Based on Charge Sensing

Tunnel coupling is a key parameter in coupled semiconductor quantum dots, and is a crucial ingredient in various device applications, such as exchange gates and spin shuttling in quantum information processing. A widely used charge-sensing technique to extract the tunnel coupling of a double quantum dot accounts for only the ground orbital state in each dot, but the authors show that in a Si double dot, valley-orbit coupling must be included in the analysis. With their more complete model, one can not only extract the $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}y$ (ground-state) tunnel coupling more accurately, but also obtain information on $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}y$ (ground-to-excited-state) tunnel coupling.

Optical response in a double quantum dot molecule inside a nonlinear photonic crystal cavity

In this work, we theoretically analyze optical bistability and absorption in coupled double quantum dot (CDQDs) cavity system in the presence of a third-order Kerr nonlinear χ(3) medium. We find that the optical bistability can be tuned by the coupling strength between the two quantum dots, nonlinear medium strength and other system parameters. The possibility of making the system an efficient optical switch is also studied. We further show that the system parameters can also tune the absorption spectrum which shows both positive and negative absorption (emission). Such a study of the CDQDs system should be helpful for various applications in optical logic devices.

Thermal transport driven by Coulomb interactions in quantum dots: Enhancement of thermoelectric and heat currents

We investigate thermal transport in a serial asymmetric double quantum dot (DQD) coupled to two electron reservoirs with different temperatures. The inter- and intra-Coulomb interactions are taken into account in a Coulomb blockade DQD where the electron sequential tunneling via four different master equation approaches is considered. In the absence of Coulomb interactions, a neglectable thermoelectric and heat currents is found identifying as the Coulomb blockade DQD regime. In the presence of Coulomb interactions, intra- and inter-Coulomb interactions, crossings energies between the intra- and the inter-dot many-body electron states are observed. The crossings induce extra channels in the energy spectrum of the DQD that enhance thermoelectric and heat currents. The extra channels form several peaks in the thermoelectric and heat currents in which intensity and position of the peaks depend on strength of the inter- and intra-dot Coulomb interactions. In addition, the problem of coherences and incoherences are studied using different approaches to the master equation, which are the first order von-Neumann, the Redfield, a first order Lindblad, and the Pauli methods. We find that all methods give almost similar thermal transport when the role of the coherences is irrelevant in the DQD.

Quantum-correlated photons generated by nonlocal electron transport

Since the realization of high-quality microwave cavities coupled to quantum dots, one can envisage the possibility to investigate the coherent interaction of light and matter in semiconductor quantum devices. Here we study a parallel double quantum dot device operating as single-electron splitter interferometer, with each dot coupled to a local photon cavity. We explore how quantum correlation and entanglement between the two separated cavities are generated by the coherent transport of a single electron passing simultaneously through the two different dots. We calculate the covariance of the cavity occupations by use of a diagrammatic perturbative expansion based on Keldysh Green's functions to the fourth order in the dot-cavity interaction strength, taking into account vertex diagrams. In this way, we demonstrate the creation of entanglement by showing that the classical Cauchy-Schwarz inequality is violated if the energy levels of the two dots are almost degenerate. For large level detunings or a single dot coupled to two cavities, we show that the inequality is not violated.