Double Quantum Dot Research Articles

Majorana Qubits and Non-Abelian Physics in Quantum Dot–Based Minimal Kitaev Chains

The possibility of engineering artificial Kitaev chains in arrays of quantum dots coupled via narrow superconducting regions has emerged as an attractive way to overcome the disorder issues that complicate the realization and detection of topological superconducting phases in other platforms. Although a true topological phase would require long chains, a two-site chain realized in a double quantum dot can already be tuned to points in parameter space where it hosts zero-energy states that seem identical to the Majorana bound states that characterize a topological phase. These states have been named “poor man’s Majorana bound states” (PMMs) because they lack formal topological protection. In this work, we propose a pathway for next-generation experiments on PMMs. The pathway starts with experiments to characterize a single pair of PMMs by measuring the Majorana quality and then moves on to initialization and readout of the parity of a PMM pair, which allows the measurement of quasiparticle poisoning times. The next step is to couple two PMM systems to form a qubit. We discuss measurements of the coherence time of such a qubit, as well as a test of Majorana fusion rules in the same setup. Finally, we propose and analyze three different types of braidinglike experiments that require more complex device geometries. Our conclusions are supported by calculations based on a realistic model with interacting and spinful quantum dots, as well as by simpler models to gain physical insight. Our calculations show that it is indeed possible to demonstrate non-Abelian physics in minimal two-site Kitaev chains despite the lack of a true topological phase. However, our findings also reveal that doing so requires some extra care, appropriately modified protocols, and awareness of the details of this particular platform. Published by the American Physical Society 2024

Strong coupling between a microwave photon and a singlet-triplet qubit

Combining superconducting resonators and quantum dots has triggered tremendous progress in quantum information, however, attempts at coupling a resonator to even charge parity spin qubits have resulted only in weak spin-photon coupling. Here, we integrate a zincblende InAs nanowire double quantum dot with strong spin-orbit interaction in a magnetic-field resilient, high-quality resonator. The quantum confinement in the nanowire is achieved using deterministically grown wurtzite tunnel barriers. Our experiments on even charge parity states and at large magnetic fields, allow us to identify the relevant spin states and to measure the spin decoherence rates and spin-photon coupling strengths. We find an anti-crossing between the resonator mode in the single photon limit and a singlet-triplet qubit with a spin-photon coupling strength of g/2π = 139 ± 4 MHz. This coherent coupling exceeds the resonator decay rate κ/2π = 19.8 ± 0.2 MHz and the qubit dephasing rate γ/2π = 116 ± 7 MHz, putting our system in the strong coupling regime.

Double quantum dot spin resonances with a time-dependent spin-orbit coupling

Two-spin resonances in double quantum dots with a constant spin-orbit coupling (SOC) can be induced by an ac field which periodically modulates the energy detuning. But a large ac field amplitude may unintentionally trigger an ac modulation in the SOC, which in turn can alter the frequency of the resonances and the form of the current peaks. Here, we demonstrate that depending on the applied magnetic field and the double quantum dot parameters a weak time dependence in the SOC can have a completely different effect on the resonant peaks; in particular, it can enhance the peaks, but it can also completely suppress them.

Minimal Kitaev-transmon qubit based on double quantum dots

Minimal Kitaev-transmon qubit based on double quantum dots

Electron confinement study in a double quantum dot by means of Shannon entropy information

In this work, we use the Shannon informational entropies to study an electron confined in a double quantum dot; we mean the entropy in the space of positions, Sr, in the space of momentum, Sp, and the total entropy, St=Sr+Sp. We obtain Sr, Sp and St as a function of the parameters A2 and k which rules the height and the width, respectively, of the internal barrier of the confinement potential. We conjecture that the entropy Sr maps the degeneracy of states when we vary A2 and also is an indicator of the level of decoupling/coupling of the double quantum dot. We study the quantities Sr and Sp as measures of delocalization/localization of the probability distribution. Furthermore, we analyze the behaviors of the quantities Sp and St as a function of A2 and k. Finally, we carried out an energy analysis and, when possible, compared our results with work published in the literature.

Minimal quantum dot based Kitaev chain with only local superconducting proximity effect

The possibility to engineer a Kitaev chain in quantum dots coupled via superconductors has recently emerged as a promising path toward topological superconductivity and possibly non-Abelian physics. Here we show that it is possible to avoid some of the main experimental hurdles on this path by using only local proximity effect on each quantum dot in a geometry that resembles a two-dot version of the proposal in Fulga []. There is no need for narrow superconducting couplers, additional Andreev bound states, or spatially varying magnetic fields; it suffices with spin-orbit interaction and a constant magnetic field in combination with control of the superconducting phase to tune the relative strengths of elastic cotunneling and an effective crossed-Andreev-reflection-like process generated by higher-order tunneling. We use a realistic spinful, interacting model and show that high-quality Majorana bound states can be generated already in a double quantum dot. Published by the American Physical Society 2024

Non-standard quantum algebras and finite dimensional -symmetric systems

In this work, -symmetric Hamiltonians defined on quantum algebras are presented. We study the spectrum of a family of non-Hermitian Hamiltonians written in terms of the generators of the non-standard Hopf algebra deformation of . By making use of a particular boson representation of the generators of , both the co-product and the commutation relations of the quantum algebra are shown to be invariant under the -transformation. In terms of these operators, we construct several finite dimensional -symmetry Hamiltonians, whose spectrum is analytically obtained for any arbitrary dimension. In particular, we show the appearance of Exceptional Points in the space of model parameters and we discuss the behaviour of the spectrum both in the exact -symmetry and the broken -symmetry dynamical phases. As an application, we show that this non-standard quantum algebra can be used to define an effective model Hamiltonian describing accurately the experimental spectra of three-electron hybrid qubits based on asymmetric double quantum dots. Remarkably enough, in this effective model, the deformation parameter z has to be identified with the detuning parameter of the system.

Cu+-doped lead-free double perovskite quantum dots for enhancing the photovoltaic performance of carbon-based Cs2AgInCl6 perovskite solar cells.

Lead-free double perovskite solar cells (PSCs) have received widespread attention because of their non-toxic nature and three-dimensional structure. However, their photovoltaic efficiency is limited by their large bandgap, including indirect or direct forbidden. Herein, Cu+ ions are incorporated into Cs2AgInCl6 double perovskite quantum dots, following which the bandgap is effectively decreased from 3.6 to 2.9 eV. Meanwhile, a facile method of drop-coating is employed to fabricate Cs2AgInCl6 films and carbon electrodes. A carbon electrode derived from a by-product of the cane sugar industry (molasses) is used to replace the expensive hole-transport materials and metal electrodes. A 0.5% Cu+-doped Cs2AgInCl6, device fabricated using carbon-based PSCs with a stacked-architecture achieves a power conversion efficiency of 1.77%, which is 2.9 times higher than that of the original device, and displays a better stability compared with that of the control one. This study provides guidance for preparing PSCs using a low-cost, facile strategy.

Novel NiCoP nanoarray modified by NiFeCo layered double hydroxide quantum dots as highly efficient bifunctional electrocatalysts in anion exchange membrane water electrolysis

The prepared LDHQDs/NiCoP NA demonstrates outstanding overall water splitting performance. The coupling effect between the NiCoP NA and LDHQDs facilitates efficient electron transport and modulates the adsorption and desorption of intermediates.

Donor Impurity Related Linear and Nonlinear Optical Properties of Symmetric and Asymmetric Double Triangular Quantum Dots

Donor Impurity Related Linear and Nonlinear Optical Properties of Symmetric and Asymmetric Double Triangular Quantum Dots

Decoupling of Majorana bound states in T-shaped double-quantum-dot structure with ferromagnetic leads

The significant potential applications of Majorana bound state (MBS) in topological quantum computing manifest the importance and necessity of relevant in-depth research. To understand the physical properties of MBS, the most practical approach is to integrate it to a mesoscopic circuit and then investigate its quantum transport behaviors. In this work, we investigate the transport properties in the systems with MBS, and provide theoretical support for its further understanding and detection, by utilizing the nonequilibrium Green’s function method and scattering matrix theory. Specifically, we investigate theoretically the transport properties in a T-shaped double-quantum dot structure, by considering MBS to be coupled to the dot in the main channel, which shows that in the linear transmission region, when the level of side-coupled dot is tuned to the Fermi energy level, the contribution of MBS to the conductance is eliminated under weak and strong Coulomb interaction. The side-coupled dot is far away from the Fermi energy level, leading to different results. When Majorana zero mode is added, the linear conductance is independent of the level of the side-coupled quantum dot, and the conductance plateau appears. However, with coupling between the MBSs, the linear conductance is the same as that without coupling between the MBSs. The decoupling phenomenon of the MBS remains strong. Therefore, the signature of the MBS can be eliminated by adjusting the level of the side-coupled quantum dot or the inter-MBS coupling. When ferromagnetic leads are introduced, the appearance or disappearance of the conductance plateau is clearly dependent on the difference between the magnetic field direction and the lead polarization direction in the system, whereas the decoupling behavior of the MBS is still existent. This work contributes to further explaining the decoupling phenomenon of MBSs in a T-shaped double-quantum-dot system, and presents a theoretical approach to more in-depth understanding and detection of the MBS.

Fano and Dicke Effects in Parallel-Coupled Quantum Dots Embedded Between Two Leads

In this article, electronic transport through a double quantum dot attached in parallel to a metal lead and with the presence of a magnetic flux is studied with the help of the Green-function formalism and the equation of motion approach. A detail formula is obtained for the density of states and the conductance. The Fano and Dicke effects in the conductance spectra of the double quantum dots system DQD is found and controlled by tuning the coupling (Γ) of the two dots to right (left) Leads and by changing the external magnetic flux.

In English

In mini-review, deals with the theory of exciton quasimolecules in a nanosystem consisting of double quantum dots of germanium synthesized in a silicon matrix. An exciton quasimolecule was formed as a result of the interaction of two spatially indirect excitons. It is shown that, depending on the distance D between the surfaces of the quantum dots, spatially indirect excitons and of exciton quasimolecules was formedin the nanosystem.The binding energy of the singlet ground state of the exciton quasimolecule has been gigantic exceeding the binding energy of the biexciton in a silicon single crystal by almost two orders of magnitude. The emergence of a band of localized electron states in the band gap of the silicon matrix was found. This band of localized electron states appeared as a result of the splitting of electron levels in the chain of germanium quantum dots. The nature of formation in the Ge/Si heterostructures was analyzed depending on the distance D between the surfaces of QDs SIEs and of exciton quasimolecules.It was shown that the binding energy of the ground singlet state of an exciton quasimolecule was gigantic, exceeding the binding energy of a biexciton in a silicon single crystal by almost two orders of magnitude.The possibility of using quasimolecules of excitons to create elements of silicon infrared nanooptoelectronics, including new infrared sensors, was established. The emergence of a band of localized electron states in the band gap of the silicon matrix was found.In this case, the band of localized electron states appeared as a result of the splitting of electron levels in the chain of germanium QDs.It was shown that the movement of an electron along the zone of localized electron states in the linear chain of germanium QDs caused an increase in photoconductivity.The effect of increasing photoconductivity can make a significant contribution in the process of converting the energy of the optical range in photosynthesizing nanosystems.

Tunneling induced transparency and slow light in an asymmetric double quantum dot molecule—Metal nanoparticle hybrid

We investigate the optical properties appearing in a nanostructure that is composed of an asymmetric double semiconductor quantum dot (SQD) molecule and a metal nanoparticle (MNP). The profile of the total linear absorption spectrum is proportional to the SQD contribution, while the MNP contribution is important. The profile of the doublet of resonances detected on the total linear absorption spectrum creates a transparency window. The doublet is asymmetric for small SQD-MNP distances and has a narrow peak and a wide peak. The width of the transparency window is increased, either with the enhancement of the rate at which the electron tunneling effect takes place within the double SQD molecule or with the decrease of the distance that separates the SQD molecule from the center of the MNP. The steep slope detected on the linear dispersion spectrum for frequencies laying within the transparency window owes its presence to the tunneling induced transparency and leads to slow light production. The corresponding value of the slow down factor is maximized for low values of the electron tunneling rate as well as for low center-to-center distances between the components of the hybrid nanostructure.

Capacitive crosstalk in gate-based dispersive sensing of spin qubits

In gate-based dispersive sensing, the response of a resonator attached to a quantum dot gate is detected by a reflected radio frequency signal. This enables fast readout of spin qubits and tune up of arrays of quantum dots but comes at the expense of increased susceptibility to crosstalk, as the resonator can amplify spurious signals and induce fluctuations in the quantum dot potential. We attach tank circuits with superconducting NbN inductors and internal quality factors Qi>1000 to the interdot barrier gate of silicon double quantum dot devices. Measuring the interdot transition in transport, we quantify radio frequency crosstalk that results in a ring-up of the resonator when neighboring plunger gates are driven with frequency components matching the resonator frequency. This effect complicates qubit operation and scales with the loaded quality factor of the resonator, the mutual capacitance between device gate electrodes, and with the inverse of the parasitic capacitance to ground. Setting qubit frequencies below the resonator frequency is expected to substantially suppress this type of crosstalk.

High Brightness Electroluminescence of Non‐Carrier‐Injection QLEDs with Precise Layer Processing by Spontaneous Spreading Method

AbstractThe luminescent properties and mechanisms of non‐carrier injection (NCI) mode quantum dot light‐emitting diodes (QLEDs) are explored in this work. The intermediate insulator electric layer, Poly(vinylidene fluoride‐trifluoroethylene‐chlorofluoroethylene) (P(VDF‐TrFE‐CFE)), effectively blocks carrier injection from the electrodes. Carriers for radiative recombination in the quantum dot (QD) layer are generated by the corresponding carrier generation complex layer under an AC electric field. In this investigation, the emission layer (EML), comprising distinct layers of Cd‐based quantum dots, is precisely regulated using the spontaneous spreading (SS) method. The work reveals that the thickness of the QDs in NCI‐QLEDs significantly influences the device's luminescent performance. In NCI‐QLEDs with a double QD layer as the EML, the device exhibits a maximum brightness of 1003.6 cd m⁻2 and a start‐up voltage of 7 root mean square voltage (VRMS). This brightness level represents the highest reported for vertical emission NCI‐QLEDs. All devices exhibit a broad range of driven voltages. Interestingly, luminescence is detected only during a half‐cycle of the driven signal, as indicated by transient time‐resolved spectrum test results. A system is established to analyze the luminescence mechanism comprehensively. Finally, a proposed carrier compounding mechanism sheds light on the behavior of NCI‐QLED devices.

Josephson Junction π-0 Transition Induced by Orbital Hybridization in a Double Quantum Dot.

In this Letter, we manipulate the phase shift of a Josephson junction using a parallel double quantum dot (QD). By employing a superconducting quantum interference device, we determine how orbital hybridization and detuning affect the current-phase relation in the Coulomb blockade regime. For weak hybridization between the QDs, we find π junction characteristics if at least one QD has an unpaired electron. Notably the critical current is higher when both QDs have an odd electron occupation. By increasing the inter-QD hybridization the critical current is reduced, until eventually a π-0 transition occurs. A similar transition appears when detuning the QD levels at finite hybridization. Based on a zero-bandwidth model, we argue that both cases of phase-shift transitions can be understood considering an increased weight of states with a double occupancy in the ground state and with the Cooper pair transport dominated by local Andreev reflection.

Single-Electron Occupation in Quantum Dot Arrays at Selectable Plunger Gate Voltage.

The small footprint of semiconductor qubits is favorable for scalable quantum computing. However, their size also makes them sensitive to their local environment and variations in the gate structure. Currently, each device requires tailored gate voltages to confine a single charge per quantum dot, clearly challenging scalability. Here, we tune these gate voltages and equalize them solely through the temporary application of stress voltages. In a double quantum dot, we reach a stable (1,1) charge state at identical and predetermined plunger gate voltage and for various interdot couplings. Applying our findings, we tune a 2 × 2 quadruple quantum dot such that the (1,1,1,1) charge state is reached when all plunger gates are set to 1 V. The ability to define required gate voltages may relax requirements on control electronics and operations for spin qubit devices, providing means to advance quantum hardware.

Classification and magic magnetic field directions for spin-orbit-coupled double quantum dots

Classification and magic magnetic field directions for spin-orbit-coupled double quantum dots

Simultaneous detection of AFB1 and aflD gene by "Y" shaped aptamer fluorescent biosensor based on double quantum dots.

The developed method for simultaneous detection of aflatoxin B1 (AFB1) and aflD genes can effectively monitor from the source and reduce the safety problems and economic losses caused by the production of aflatoxin, which can be of great significance for food safety regulations. In this paper, we constructed a sensitive and convenient fluorescent biosensor to detect AFB1 and aflD genes simultaneously based on fluorescence resonance energy transfer (FRET) between quantum dots (QDs) and a black hole quenching agent. A stable "Y" shaped aptasensor was employed as the detection platform and a double quantum dot labeled DNA fragment was utilized to be the sensing element in this work. When the targets of AFB1 and aflD genes were presented in the solution, the aptamer in the "Y" shaped probe is specifically recognized by the target. At this time, both Si-carbon quantum dots (Si-CDs) and CdTe QDs are far away from the BHQ1 and BHQ3 to recover the fluorescence. The linear range of the prepared fluorescence simultaneous detection method was as wide as 0.5-500ng·mL-1 with detection lines of 0.64ng·mL-1 for AFB1 and 0.5-500nM with detection lines of 0.75nM for aflD genes (3σ/k). This fabricated fluorescent biosensor was further validated in real rice flour and corn flour samples, which also achieved good results. The recoveries were calculated by comparing the known and found amounts of AFB1 which ranged from 88.4 to approximately 115.32% in the rice flour samples and 90.7 ~ 102.58% in the corn flour samples. The recoveries of aflD genes ranged from 84.32 to approximately 109.3% in the rice flour samples and 89.48 ~ 100.99% in the corn flour samples. Therefore, the proposed biosensor can significantly improve food safety and quality control through a simple, fast, and sensitive agricultural product monitoring and detection system.