Triple Quantum Dot Research Articles

Compressed Optimization of Device Architectures for Semiconductor Quantum Devices

Recent advances in nanotechnology have enabled researchers to manipulate small collections of quantum mechanical objects with unprecedented accuracy. In semiconductor quantum dot qubits, this manipulation requires controlling the dot orbital energies, tunnel couplings, and the electron occupations. These properties all depend on the voltages placed on the metallic electrodes that define the device, whose positions are fixed once the device is fabricated. While there has been much success with small numbers of dots, as the number of dots grows, it will be increasingly useful to control these systems with as few electrode voltage changes as possible. Here, we introduce a protocol, which we call the Compressed Optimization of Device Architectures (CODA), in order to both efficiently identify sparse sets of voltage changes that control quantum systems, and to introduce a metric which can be used to compare device designs. As an example of the former, we apply this method to simulated devices with up to 100 quantum dots and show that CODA automatically tunes devices more efficiently than other common nonlinear optimizers. To demonstrate the latter, we determine the optimal lateral scale for a triple quantum dot, yielding a simulated device that can be tuned with small voltage changes on a limited number of electrodes.

Physically defined triple quantum dot systems in silicon on insulator

We report characterizations of two types of Si triple quantum dot (TQD) devices with charge sensors, with the aim of integrating spin qubits. The QDs of a single TQD device are connected in line to adjacent QD(s), while all QDs are tunnel-coupled to each other in the other device to form a triangle. Both TQD systems are physically defined on silicon-on-insulator substrates using electron beam lithography and dry etching. From electron transport measurements of each type of TQD system at 4.2 K, we demonstrate the formation of tunnel-coupled TQD systems and the tunability of their electric potentials.

Long-range overlapping of Kondo clouds in open triple quantum dots

We study the phenomena of overlapping of Kondo clouds in an open triple quantum dots (OTQDs) system by using the dissipaton equation of motion (DEOM) theory. Motivated by the long-rang interaction of the TQDs system demonstrated in Cheng et al (2017 Phys. Rev. B 95 155417), we present a comprehensive picture of the long-range overlapping behavior of Kondo clouds via investigation of the spectral functions, spin–spin correlation, dot occupancies and susceptibility. For the configuration , a conduction electron peak occurs in the spectral function of intermediate QD in the Kondo regime. This peak results from the overlapping of the two Kondo clouds forming from between the two peripheral QDs and leads, enhances with decreasing temperature and increasing dot-lead coupling. Both the spin–spin correlations between the two adjacent QDs and the two peripheral QDs own negative values. It also confirms the physical picture of the overlapping between left and right Kondo clouds via the intermediate QD. To understand the physical insight, we examine also the electron occupacies and the spectral functions, with their dependence on the temperature and dot-lead coupling. In addition, a distinct nonmonotonic behavior of the susceptibility associated with the Kondo clouds is characterized.

Fast optical control of a coded qubit in a triple quantum dot

In this work, we study strategies for the optical control, within the dipole approximation, of a qubit encoded in the total intrinsic angular momentum states of three electrons confined in a triple quantum dot. The system is modeled using effective confining potentials, and its electronic structure is calculated using the configuration interaction method. Optimal control theory (OCT) was applied to design low-fluence time-dependent electric fields controlling the qubit in times shorter than a nanosecond. The resulting pulses produce transitions between the qubit states for experimentally available field amplitudes with high fidelity. Moreover, studying the dominant frequencies present in the OCT pulses, we show that the switching between the qubit states can also be achieved with simpler combinations of sinusoidal driving fields that produce transitions to intermediate states. The limitations of an extended Hubbard description for the model and its dynamic behavior are also discussed.

Maximal thermal entanglement using three-spin interactions

Three-spin interactions in three-qubit systems at thermal equilibrium can be used for simple and efficient creation of maximally entangled states. We do not require set of gates to achieve this goal; rather, maximal thermal entanglement naturally arises by appropriately tuning the interactions present in the system. Within the broad range of parameter regimes found, we identify the ones accessible in triple quantum dot and triangular optical lattice, thus opening a way toward simple implementation of maximally entangled states with different types of three-spin interactions. Our results suggest tight connection between the presence of W type of entanglement and magnetization, enabling experimental detection of the W state.

Temperature-Dependent Photoluminescence in CdTe/ZnTe Triple Quantum Dots

We studied the optical properties of CdTe/ZnTe quantum dots (QDs) in single, double, and triple QD structures by measuring the temperature-dependent photoluminescence (PL). On going from the single to the triple QD structures, the PL peak position of the QDs was blue shifted. This blue-shift was attributed to compressive strain imparted by the separation layers and to material intermixing. Temperature-dependent PL measurements of the triple QD structures showed that, as the temperature increased, the PL peak intensity for the 4.5-monolayer (ML) QDs decreased less than it did for the 3.0- and the 3.5-ML QDs. The activation energy of the 4.5-ML QDs in the triple QD structures was larger than that of the corresponding 4.5-ML QDs in the single QD structures. The reason for this is that, as the temperature increases, some carriers disappear due to non-radiative recombination, but many carriers in the 3.0- and the 3.5-ML QDs move to the 4.5-ML QDs in the triple QD structures, and those carriers then undergo radiative recombination.

Leakage and dephasing in Si28 -based exchange-only spin qubits

Exchange-only spin qubits hosted in $^{28}$Si-based triple quantum dots do not suffer from decoherence caused by randomly fluctuating nuclear-spin ensembles and can be relatively robust against electrical noise when operated at a sweet spot. Remaining sources of decoherence are qubit relaxation, leakage out of the qubit subspace, and dephasing due to residual effects of charge noise, the latter two of which are the focus of this work. We investigate spin-orbit-mediated leakage rates to the three-spin ground state accompanied by virtual (i) tunneling, (ii) orbital excitation, and (iii) valley excitation of an electron. We find different power-law dependencies on the applied magnetic field $B$ for the three mechanisms as well as for the two leakage rates, ranging from $\propto B^5$ to $\propto B^{11}$, and identify the sweet spot as a point of minimal leakage. We also revisit the role of electrical noise at the sweet spot, and show that it causes a decay of coherent qubit oscillations that follows a power law $\propto 1/t$ (as opposed to the more common exponential decay) and introduces a $\pi/2$ phase shift.

Spin–orbit hybrid entangled channel for spin state quantum teleportation using genetic algorithms

We present a physical model of spin quantum teleportation protocol (QTP) in a triple quantum dot array using a genetic algorithm approach. The information to teleport is spin-coded in one electron confined in a single quantum dot (SQD). The remaining double quantum dot (DQD) system has just an electron with spin that includes spin–orbit interaction. Charge and spin of the electron get hybridized with the site occupancy having two intrinsic quantum degrees of freedom. The DQD is prepared in a hybrid spin–orbit entangled (HES) Bell-like state with tunneling and site energies as time-dependent control parameters that are optimized by means of genetic algorithms (GAs). The hybrid entangled resources that we obtained allow spin-charge quantum state teleportation with a fidelity of 0.9972 and are used as a resource channel to establish the QT protocol. The spin state of the electron in the SQD interacts with the DQD spin–orbit entangled channel via a modulated exchange interaction to emulate Alice’s joint measurement required for QT with GA parameter control. A charge detection measurement in one of the DQD systems is sufficient to have the spin state teleported up to a unitary transformation. A specific joint measurement and unitary transformation were selected to test the protocol, and we obtain fidelity of 0.99 for the QTP. The quantum circuit models for both the spin–orbit entangled state and the teleportation are determined from the analysis of the stages of the controlled quantum dynamics obtained from the GA approach.

Spin Seebeck Effect in a Multiple Quantum Dot Molecule with Spin-Dependent Interdot Coupling

We study the spin Seebeck effect in a circularly connected triple quantum dot (TQD) structure taking the spin-dependent interdot coupling and magnetic flux into consideration. Particular attention is paid on the generation and manipulation of the 100% spin-polarized and pure spin thermopowers, which denote the arisen spin voltage in response of an infinitely small temperature gradient applied across the system. This can be realized by adjusting the peaks’ positions in the spin-up and spin-down thermopowers with the help of the spin polarization of the interdot couplings. At low temperature, a large value of pure spin thermopower is obtained even under very weak spin polarization of the interdot coupling. Strong spin polarization of it is favorable for 100% spin-polarized thermopower whose magnitude can reach as large as that of the charge one. We also find that a sign change of the considered two quantities can be realized by adjusting the magnetic flux penetrating through the TQDs. The present results could be useful in designing high-efficiency pure spin energy conversion and spin filter devices.

The Zeno effect and relaxation rates in a triple quantum dot system

We study the quantum Zeno effect (QZE) and relaxation rates in a three quantum dot system with a mesoscopic detector near one of the three dots. The evolution of three dot states is analyzed under different conditions. For small energy differences, we find that quantum anti-Zeno effect (QAZE) occurs because measurements cannot localize the electron in the initial dot state at arbitrary voltage or temperature, but accelerate quantum transition of the electron. For large energy gaps, dot states remain the initial values, namely, Zeno effect occurs. With increasing voltage or temperature, the relaxation rates which are related to quantum transition between eigenstates increase. Furthermore, it is demonstrated that they are not absolutely dependent on the eigen energy and the difference of eigen energy. The voltage and temperature play a similar role on the relaxation rates, but a different role on occupation probabilities. In addition, it is proved that the voltage induces relaxation at zero temperature. Moreover, we demonstrate that the change rates of occupation probabilities under eigenstate and dot state are related to the energy differences. In both dot-state and eigenstate representations, the first derivatives of occupation probabilities versus voltage change obviously when the voltage is matched with the difference of eigen energy (We use the unit system of e = kB = ℏ = 1), but the first derivatives of occupation probabilities versus temperature change obviously when the temperature is matched with the difference of dot-state energy. Especially, for large energy gaps, the first derivatives of occupation probabilities versus voltage change rapidly when the voltage is matched with the difference of dot-state energy. The temperatures, at which the first derivatives of occupation probabilities versus temperature change rapidly, are independent of the differences of both dot-state and eigen energy.

Tunable self-focusing and self-defocusing effects in a triple quantum dot via the tunnel-enhanced cross-Kerr nonlinearity.

Kerr-nonlinearity induced self-focusing or self-defocusing effect provides the opportunity for exploring fundamental phenomena related to the light-matter interactions. Here we show that the linear and nonlinear dispersion responses are significantly sensitive to both the detunings and the tunneling strengths of the indirect-excitonic (IX) states in an asymmetric triple quantum dot system. In particular, the nonlinear dispersion properties are dominated by the tunnel-enhanced cross-Kerr nonlinearity from one of the IX states. Meanwhile, by varying the detunings of other IX states, we reveal that the tunnel-enhanced cross-Kerr nonlinearity gives rise to the realization of the self-focusing and self-defocusing effects. Moreover, by taking into account the effect of the longitudinal-acoustic-phonon induced dephasing of the IX states, it is possible to modulate the height and position of the peak of the self-focusing or self-defocusing effect. Our results may have potential applications in nonlinear-optics and quantum-optics devices based on the tunnel-enhanced nonlinearities in this solid-state system.

Fast creation and transfer of coherence in triple quantum dots by using shortcuts to adiabaticity.

Motivated by the progress on shortcuts to adiabaticity, we propose three schemes for speeding up (fractional) stimulated Raman adiabatic passage, and achieving rapid and non-adiabatic creation and transfer of maximal coherence in a triple-quantum-dot system. These different but relevant protocols, designed from counter-diabatic driving, dress-state method, and resonant technique, require their own pumping fields, applied gate voltages and varying tunneling couplings between two spatially separated dots. Such fast and reliable shortcuts not only allow for feasibly experimental realization in solid-state architectures but also may have potential applications in quantum information processing and quantum control.

Spin-Polarized Transport and Spin Seebeck Effect in Triple Quantum Dots with Spin-Dependent Interdot Couplings

We study the spin-dependent electronic and thermoelectric transport through a structure composed of triple quantum dots (TQDs) coupled to two metallic leads in the presence of spin-dependent interdot couplings, which is reliable by applying a static magnetic field on the tunnel junctions between different dots. When the TQDs are serially connected, a 100 % spin-polarized conductance and thermopower emerge even for very small spin-polarization of the interdot coupling as the dots are weakly coupled to each other. Whereas if the TQDs are connected in a ring shape, the Fano antiresonance will result in sharp peaks in the conductance and thermopower. In the presence of spin-dependent interdot couplings, the peaks of the spin-up and spin-down thermopowers will shift to opposite directions in the dot level regime, resulting large either 100 % spin-polarized or pure spin thermopowers. The latter generally arises at low temperatures and is robust against the level detuning, the dot-lead coupling, and the system equilibrium temperature.

Phonon-induced decoherence of a charge quadrupole qubit

Many quantum dot qubits operate in regimes where the energy splittings between qubit states are large and phonons can be the dominant source of decoherence. The recently proposed charge quadrupole qubit, based on one electron in a triple quantum dot, employs a highly symmetric charge distribution to suppress the influence of long-wavelength charge noise. To study the effects of phonons on the charge quadrupole qubit, we consider Larmor and Ramsey pulse sequences to identify favorable operating parameters. We show that phonon-induced decoherence increases with the qubit frequency, in contrast to the effects of charge noise. We also show that there is an optimum value of the tunnel coupling of the qubit at which the decohering effects of phonons and charge noise are small enough to be consistent with single qubit gate fidelities > 99.99%.

Tunable charge qubit based on barrier-controlled triple quantum dots

We present a theoretical proposal of a tunable charge qubit, hosted in triple quantum dots. The manipulation is solely performed by changing the heights of the two potential barriers between the three dots, while the energy of all three dots are fixed. We have found that when the relative height of the two barriers are changed, the direction of the axis of rotation in performing single-qubit gates can be varied. On the other hand, the corresponding rotation speed can be tuned by raising or lowering the two barriers at the same time. Our proposal therefore allows for tunability of both the rotation axis and rotating speed for a charge qubit via all-electrical control, which may facilitate realization of quantum algorithms in these devices.

Enhancement of charge and spin Seebeck effect in triple quantum dots coupling to ferromagnetic and superconducting electrodes

We theoretically study the thermoelectric transport properties through a triple quantum dots (QDs) device with the central QD coupled to a ferromagnetic lead, a superconducting one, and two side QDs with spin-dependent interdot tunneling coupling. The thermoelectric coefficients are calculated in the linear response regime by means of nonequilibrium Green's function method. The thermopower is determined by the single-electron tunneling processes at the edge of superconducting gap. Near the outside of the gap edge the thermopower is enhanced while thermal conductance is suppressed, as a result, the charge figure of merit can be greatly improved as the gap appropriately increases. In the same way, charge figure of merit also can be greatly improved near the outside of the gap edge by adjusting interdot tunneling coupling and asymmetry coupling of the side QDs to central QD. Moreover, the appropriate increase of the interdot tunneling splitting and spin polarization of ferromagnetic lead not only can improve charge thermopower and charge figure of merit, but also can enhance spin thermopower and spin figure of merit. Especially, the interdot tunneling splitting scheme provides a method of controlling charge (spin) figure merit by external magnetic field.

Photon-Assisted Electronic Transport Through an Asymmetrically Coupled Triple-Quantum Dot Interferometer

We present theoretically the quantum electronic transport through an interferometer asymmetrically coupled with triple quantum dots. Using the Keldysh non-equilibrium Green’s function method, the photon-assisted transport properties through the asymmetric quantum system are numerically analyzed. The sidebands of the photon-assisted tunneling process appear when driven by the time-modulated field. The average current spectra are simulated as a function of quantum dot energy to understand the roles of side-coupling strength and time-modulated field in sideband effect and electron tunneling. This is helpful in future design of the basic structures required for quantum computation applications.

Bound states in the continuum poisoned by Majorana fermions

In this work, we study the bound states in the continuum (BICs) in a system formed by a triple quantum dot array embedded between two one-dimensional topological superconductors (TSCs), both hosting Majorana bound states (MBSs) at their ends. The results show the formation of BICs with topological characteristics due to the presence of MBSs. This is a consequence of the interplay between the BIC arising from quantum dots states by means of energy level symmetry breaking through gate voltages, and MBSs leaked into the quantum dots. The BIC is not observed when both TSCs are in long wire limit, i.e. for vanishing inter MBSs coupling, while it projects into the electronic transmission whenever the inter MBSs couplings are away from zero, regardless if they have different strength and/or the phase difference between both TSCs. We study the behavior of BICs poisoned by MBSs as a function of the parameters that are controlling the system. We believe our findings could be useful to implement a protection tool for BICs using MBSs based on tunable gate voltages.

Single, double, and triple quantum dots in Ge/Si nanowires

We report highly tunable control of holes in Ge/Si core/shell nanowires. We demonstrate the ability to create single quantum dots of various sizes, with low hole occupation numbers and clearly observable excited states. For the smallest dot size, we observe indications of single-hole occupation. Moreover, we create double and triple tunnel-coupled quantum dot arrays. In the double quantum dot configuration, we observe Pauli spin blockade. These results open the way to perform hole spin qubit experiments in these devices.

Thermal rectification with interacting electronic channels: Exploiting degeneracy, quantum superpositions, and interference

This work explores different mechanisms that induce thermal rectification in the nanoscale. The presence of interacting energy channels combined with simple asymmetries is sufficient for promoting the desired behavior. We use simple quantum dot configurations, identifying the basic properties that enhance rectification for each case: the size of a quantum dot state space (which suggests the use of scaled up systems with many interacting channels), tunneling asymmetries due to coherent tunneling in a double quantum dot, or quantum interference in a triangular triple quantum dot. An efficient and tunable thermal diode is proposed using a channel capacitively coupled to a mesoscopic switch.