Double Quantum Dot Research Articles

Finite-frequency noise, Fano factor, ΔT-noise and cross-correlations in double quantum dots.

A theoretical study on electrical current fluctuations in a double quantum dot connected to electronic reservoirs is presented, with the aim of deriving the finite-frequency noise, the Fano factor and the ΔT-noise. We establish a general expression for the noise in terms of Green functions in the double quantum dot and self-energies in the reservoirs. This result is then applied to model double quantum dots in various situations. For a non-interacting double quantum dot, we have highlighted several interesting features in the physical properties of this system. In particular, we have demonstrated the possibility of obtaining a significant reduction in zero-frequency noise and Fano factor either when the system is placed in a given operating regime, or when a temperature gradient is applied between the two reservoirs, resulting in a negative ΔT-noise being generated. In addition, in the vicinity of honeycomb vertices, a sign change is observed in the finite-frequency cross-correlator between the two reservoirs, in contrast to what is obtained for the zero-frequency cross-correlator, which remains negative throughout the (ε1,ε2)-plane, ε1, ε2 being the level energies in each of the two dots. By using an approximate first-level numerical approach, we finally study how the finite-frequency noise in a double quantum dot evolves under the influence of Coulomb interactions.

Decoherence in a crystal-phase defined double quantum dot charge qubit strongly coupled to a high-impedance resonator

Decoherence in a crystal-phase defined double quantum dot charge qubit strongly coupled to a high-impedance resonator

CNOT Quantum Gate Based on Spatial Photonic Qubits Under Resonant Electro-Optical Control

A theoretical model of a quantum node that implements the two-qubit CNOT operation with use of photonic qubits with spatial encoding is considered. Each qubit is represented by a pair of modes supporting an arbitrary superposition of single-photon states. The active element of the node is a single or double quantum dot with a tunable frequency, which coherently exchanges an energy quantum with the modes. The spectral characteristics of the quantum node elements are simulated. The probability of implementation of a controlled inversion of the qubit state is calculated depending on the system parameters.

Nonlocal correlations and entanglement in two coupled double quantum dots under external magnetic field

Abstract Double quantum dot (DQD) systems are highly regarded as promising candidates for advancements in the fields of quantum computing and quantum information processing. In this work, we investigate the behaviour of quantum correlations (QCs) in a system of two DQDs, AlGaAs/GaAs under various parameters such as temperature, energy offsets, coupling strength, and magnetic fields. We employ uncertainty-induced nonlocality and Bell nonlocality to quantify QC and logarithmic negativity to measure entanglement in the considered physical system. The findings demonstrate that QCs are significantly influenced by noisy equilibrium temperature and system parameters. In particular, lower temperatures enhance QCs, whereas higher temperatures result in a decrease due to thermal effects. In addition, the magnetic fields have a profound influence on the conditions of resonance. We also show that maximum QCs occur at resonance conditions with no energy offset between each DQD. However, introducing an energy offset or too-strong coupling reduces these correlations. Finally, our findings provide an understanding of the behaviour of entanglement and quantum nonlocality in DQD systems, offering potential implications for quantum technologies.

Electronic correlations in parallel-coupled double quantum dot system: An exact analytical approach

Exact eigenstates of the parallel coupled double quantum dots attached to the non-interacting leads taken in the zero-bandwidth limit (effectively quadruple quantum dots) are analytically obtained in each particle and spin subspace. The ground state of the half-filled system is determined within a four-dimensional subspace of the twenty-dimensional Hilbert space. The effects of tunable parameters, such as quantum dot energy levels, interdot tunneling, ondot and interdot Coulomb interactions, on spin-spin correlation and dot occupancies are analyzed. In the parameter space defined by interdot tunneling and ondot Coulomb interaction, the tunnel-coupled dots exhibit both ferromagnetic and antiferromagnetic correlations, suggesting suitability for singlet-triplet qubits in quantum computing. A critical dependency of interdot tunneling on ondot Coulomb interaction leads to a transition from ferromagnetic to antiferromagnetic correlation as interdot tunneling increases. These correlations persist even without interdot tunneling via indirect exchange through the leads, with the interdot Coulomb interaction significantly influencing this dependency.

Long-distance photon-mediated and short-distance entangling gates in three-qubit quantum dot spin systems

Superconducting resonator couplers will likely become an essential component in modular semiconductor quantum dot (QD) spin qubit processors, as they help alleviate crosstalk and wiring issues as the number of qubits increases. Here, we focus on a three-qubit system composed of two modules: a two-electron triple QD resonator coupled to a single-electron double QD. Using a combination of analytical techniques and numerical results, we derive an effective Hamiltonian that describes the three-qubit logical subspace and show that it accurately captures the dynamics of the system. We examine the performance of short-range and long-range entangling gates, revealing the effect of a spectator qubit in reducing the gate fidelities in both cases. We further study the competition between nonadiabatic errors and spectator-associated errors in short-range operations and quantify their relative importance across practical parameter ranges for short and long gate times. We also analyze the impact of charge noise together with residual coupling to the spectator qubit on intermodule entangling gates and find that for current experimental settings, leakage errors are the main source of infidelities in these operations. Our results help pave the way toward identifying optimal modular QD architectures for quantum information processing on semiconductor chips. Published by the American Physical Society 2024

Linear and Nonlinear Optical Properties of Symmetric and Asymmetric Double Triangular Quantum Dots Withinside the Presence of Magnetic Field

AbstractLinear and third‐order nonlinear optical absorption coefficients and relative refractive index changes in symmetric and asymmetric double triangular quantum dots are examined theoretically. The dependence of these optical properties on the magnetic field is examined. After calculating energies and wave functions within the effective mass and parabolic band approaches, analytical expressions of linear and nonlinear optical properties are obtained using the compact density matrix approach and iterative method. Numerical calculations are presented for typical GaAs/AlGaAs material. The results show that the magnetic field causes different effects on the and transitions. Moreover, the calculated results also reveal that the resonance frequency and nonlinear contribution are different in symmetric and asymmetric structures. As a result, it is concluded that the magnetic field plays a vital and important role in the electronic and optical properties of the system and can be used to tune the inter‐subband transitions and change the corresponding optical sensitivities.

Scalable on-chip multiplexing of silicon single and double quantum dots

Owing to the maturity of complementary metal oxide semiconductor (CMOS) microelectronics, qubits realized with spins in silicon quantum dots (QDs) are considered among the most promising technologies for building scalable quantum computers. For this goal, ultra-low-power on-chip cryogenic CMOS (cryo-CMOS) electronics for control, read-out, and interfacing of the qubits is an important milestone. We report on-chip interfacing of tunable electron and hole QDs by a 64-channel cryo-CMOS multiplexer with less-than-detectable static power dissipation. We analyze charge noise and measure state-of-the-art addition energies and gate lever arm parameters in the QDs. We correlate low noise in QDs and sharp turn-on characteristics in cryogenic transistors, both fabricated with the same gate stack. Finally, we demonstrate that our hybrid quantum-CMOS technology provides a route to scalable interfacing of a large number of QD devices, enabling, for example, variability analysis and QD qubit geometry optimization, which are prerequisites for building large-scale silicon-based quantum computers.

Double quantum dot Andreev molecules: Phase diagrams and critical evaluation of effective models

Double quantum dot Andreev molecules: Phase diagrams and critical evaluation of effective models

Probing the two-electron-interaction-induced three new hopping processes in a double quantum-dot system revealed by finite-frequency shot noise

In the serially coupled double quantum dot (DQD) system, the two-electron-interaction-induced electron hopping processes and spin-exchange hopping processes play a crucial role in manipulating the spin degree of freedom of localized electrons in the DQD system. Consequently, from the perspective of electron transport, how to detect the new hopping processes induced by the two-electron interaction has become an increasingly important issue. Here, the influences of the three new hopping processes predicted by the generalized Hubbard model on the finite-frequency shot noise of electron transport through a serially coupled DQD system are studied. It is demonstrated that in the energy-level detuning case the criterion for the existence of the electron-occupation-modulated hopping processes is that the maximum number of peaks and dips of the finite-frequency shot noise equals eight while in the energy-level alignment case the criteria for the existence of the two-electron-interaction-induced three new hopping processes, and the antiparallel-electron-spin-exchange hopping and electron-pair hopping processes, and the electron-occupation-modulated hopping processes are that the maximum numbers of peaks and dips equal eight, seven, and six, respectively. Therefore, these results provide an alternative way to detect the three new hopping processes predicted by the generalized Hubbard model in the serially coupled DQD system.

Flux-tunable Kitaev chain in a quantum dot array

Connecting quantum dots through Andreev bound states in a semiconductor-superconductor hybrid provides a platform to create a Kitaev chain. Interestingly, in a double quantum dot, a pair of poor man’s Majorana zero modes can emerge when the system is fine-tuned to a sweet spot, where superconducting and normal couplings are equal in magnitude. Control of the Andreev bound states is crucial for achieving this, usually implemented by varying its chemical potential. In this work, we propose using Andreev bound states in a short Josephson junction to mediate both types of couplings, with the ratio tunable by the phase difference across the junction. Now a minimal Kitaev chain can be easily tuned into the strong coupling regime by varying the phase and junction asymmetry, even without changing the dot-hybrid coupling strength. Furthermore, we identify an optimal sweet spot at \piπ phase, enhancing the excitation gap and robustness against phase fluctuations. Our proposal introduces a new device platform and a new tuning method for realizing quantum-dot-based Kitaev chains.

Supercurrent and Superconducting Diode Effect in Parallel Double Quantum Dots with Rashba Spin-Orbit Interaction.

We study theoretically the supercurrent and the superconducting diode effect (SDE) in a structure comprising parallel-coupled double quantum dots (DQDs) sandwiched between two superconductor leads in the presence of a magnetic flux. The influence of the Rashba spin-orbit interaction (RSOI), which induces a spin-dependent phase factor in the dot-superconductor coupling strength, is taken into account by adopting the nonequilibrium Green's function technique. This RSOI-induced phase factor serves as a driving force for the supercurrent in addition to the usual superconducting phase difference, and it leads to the system's left/right asymmetry. Correspondingly, the magnitude of the positive and negative critical currents become different from each other: the so-called SDE. Our results show that the period, magnitude, and direction of the supercurrents depend strongly on the RSOI-induced phase factor, dots' energy levels, interdot coupling strengths, and the magnetic flux. In the absence of magnetic flux, the diode efficiency is negative and may approach -2, which indicates the perfect diode effect with only negative flowing supercurrent in the absence of a positive one. Interestingly enough, both the sign and magnitude of the diode efficiency can be efficiently adjusted with the help of magnetic flux, the dots' energy levels and the interdot coupling strength and thus provide a controllable SDE by rich means, such as gate voltage or host materials of the system.

Fully analytical equation of motion approach for the double quantum dot in the Coulomb blockade regime

Fully analytical equation of motion approach for the double quantum dot in the Coulomb blockade regime

Transport diagrams of germanium double quantum dots/Si barriers using photocurrent measurement

We reported transport diagrams of self-assembled germanium (Ge) double quantum-dots (DQDs) using direct current measurement under illumination at wavelength (λ) of 850 nm and at the base temperature of 4.5 K. Ge DQDs with a coupling-barrier of Si, tunneling-barriers of Si3N4, and self-aligned p+-Si reservoirs were fabricated in a self-organized CMOS approach. Charge transport through the Ge-DQDs is facilitated by photon-assisted tunneling. Characteristic gate-controlled hexagonal-shaped cells over a wide range of hole occupancy are acquired thanks to hard-wall confinement. Large dimensions (ΔVG > 200 mV) of hexagonal-shaped cells are favored for the operation of charge states, indicating that our Ge DQDs system is less susceptible to shot noises arising from external voltage sources. Estimated intra-QD and inter-QD charging energies are EC,R/EC,L = 48.9 meV/42.7 meV and ECm = 7.8 meV, respectively.

A singlet-triplet hole-spin qubit in MOS silicon

Holes in silicon quantum dots are promising for spin qubit applications due to the strong intrinsic spin-orbit coupling. The spin-orbit coupling produces complex hole-spin dynamics, providing opportunities to further optimise spin qubits. Here, we demonstrate a singlet-triplet qubit using hole states in a planar metal-oxide-semiconductor double quantum dot. We demonstrate rapid qubit control with singlet-triplet oscillations up to 400 MHz. The qubit exhibits promising coherence, with a maximum dephasing time of 600 ns, which is enhanced to 1.3 μs using refocusing techniques. We investigate the magnetic field anisotropy of the eigenstates, and determine a magnetic field orientation to improve the qubit initialisation fidelity. These results present a step forward for spin qubit technology, by implementing a high quality singlet-triplet hole-spin qubit in planar architecture suitable for scaling up to 2D arrays of coupled qubits.

Spin and orbital Yu-Shiba-Rusinov states and delocalization control of magnetic order in parallel double quantum dot Josephson junctions

Spin and orbital Yu-Shiba-Rusinov states and delocalization control of magnetic order in parallel double quantum dot Josephson junctions

Construction of hydrophilic and hydrophobic hybrid interface to achieve controlled zinc deposition for aqueous Zn-ion batteries

Aqueous Zn-ion batteries show superior development prospects and competitiveness with high theoretical capacity, abundant reserves and low potential. However, the inhomogeneous electrodeposition of zinc anodes and zinc dendrite growth highly limit their commercialization. To address these issues, multifunctional hybridized interfaces consisting of layered double hydroxide (LDH) and graphene quantum dots (GQDs) are constructed on the surface of zinc metal anodes. LDH with hydrophobicity will effectively shield the corrosion of aqueous electrolyte on the zinc anode, and simultaneously serve as a mechanical skeleton for zinc deposition. The hydrophilic GQDs will control the coordination environment of solvated Zn2+, reduce the reactivity of water and promote the uniform deposition of zinc ions along the (002) crystal surface. The interfacially modified Zn anode achieves more than 1000 h and 1200 h of stable cycling at 1 and 2 mA cm−2, respectively. Moreover, the assembled Zn@LDH@GQDs//NH4V4O10 full cells achieve 2000 stable cycles at a high current density of 10 A g−1. The present work reveals the intrinsic mechanism of inhibiting zinc dendrites by the protective layer of multifunctional hybrid materials, which provides an important idea for stabilizing zinc anodes.

Universality and the thermoelectric transport properties of a double quantum dot system: Seeking for conditions that improve the thermoelectric efficiency

Employing universal relations for the Onsager coefficients in the linear regime at the symmetric point of the single impurity Anderson model, we calculate the conditions under which the quantum scattering phase shift should satisfy to produce the asymptotic Carnot’s limit for the thermoelectric efficiency. We show that a single quantum dot connected by metallic leads at the Kondo regime cannot achieve the best thermoelectric efficiency. We study a serial double quantum dot system, with the quantum dots immersed in ballistic conduction channels. Each QD exhibits a strong but finite local electronic correlation UU. We show that maintaining one dot in the electron-hole symmetric point and allowing charge fluctuations in the other QD makes it possible to drive the system to the maximum thermoelectric efficiency. We also show that the bound states in the continuum (BICs) and quasi-BICs associated with the quantum scattering interference process drive the DQD to the maximum thermoelectric efficiency. We identify two types of quasi-BICs that occur at low and high temperatures: The first is associated with single Fano resonances, and the last is with several Fano processes. We also discussed possible temperature values and conditions that could be linked with the experimental realization of the results.

Quantum phase transitions and cat states in cavity-coupled quantum dots

We study double quantum dots coupled to a quasistatic cavity mode with high mode-volume compression allowing for strong light-matter coupling. Besides the cavity-mediated interaction, electrons in different double quantum dots interact with each other via dipole-dipole (Coulomb) interaction. There is a first-order cavity-induced ferroelectric quantum phase transition when the attractive dipolar interaction is smaller than the critical value defined by the energy splitting in DQDs and a smooth transition, otherwise. We show that, in the smooth transition region, both the ground and the first excited states of an array of double quantum dots are cat states. Such states are actively discussed as high-fidelity qubits for quantum computing, and thus our proposal provides a platform for semiconductor implementation of such qubits. We also calculate gauge-invariant observables such as the net dipole moment, the optical conductivity, and the absorption spectrum beyond the semiclassical approximation. The results are robust against cavity losses and variations of system parameters. Published by the American Physical Society 2024

Coupling and characterization of a Si/SiGe triple quantum dot array with a microwave resonator

Abstract Scaling up spin qubits in silicon-based quantum dots is one of the pivotal challenges in achieving large-scale semiconductor quantum computation. To satisfy the connectivity requirements and reduce the lithographic complexity, utilizing the qubit array structure and the circuit quantum electrodynamics (cQED) architecture together is expected to be a feasible scaling scheme. A triple-quantum dot (TQD) coupled with a superconducting resonator is regarded as a basic cell to demonstrate this extension scheme. In this article, we investigate a system consisting of a silicon TQD and a high-impedance TiN coplanar waveguide (CPW) resonator. The TQD can couple to the resonator via the right double-quantum dot (RDQD), which reaches the strong coupling regime with a charge–photon coupling strength of g 0/(2π) = 175 MHz. Moreover, we illustrate the high tunability of the TQD through the characterization of stability diagrams, quadruple points (QPs), and the quantum cellular automata (QCA) process. Our results contribute to fostering the exploration of silicon-based qubit integration.