Interdot Tunnel Coupling Research Articles

Magnetic transport measurements of spin–orbit and hyperfine interactions in a Ge hut wire double quantum dot

Ge quantum dots are emerging as an attractive platform to realize high-fidelity qubits because of their strong spin–orbit interaction (SOI) and weak hyperfine interaction (HFI). Here, we measure and analyze the transport spectra on a highly tunable double quantum dot (DQD) defined in Ge hut wires with an external out-of-plane magnetic field. At a high magnetic field, the SOI induced spin-flip tunneling lifts the Pauli spin blockade and maps out the energy spectrum of the DQD. At a low magnetic field, the HFI mixes the (1, 1) states and leads to an enhanced leakage current, which is affected by the inter-dot tunnel coupling.

Kondo induced π -phase shift of microwave photons in a circuit quantum electrodynamics architecture

Mesoscopic systems constitute appealing platforms to study many-body physics with light and matter degrees of freedom. The Kondo effect refers to the screening of a spin-1/2 impurity by a cloud of conduction electrons, then forming a many-body Fermi liquid ground state. The Kondo resonance produces a phase shift in the transmitted electronic wave packet which depends on the symmetry and nature of the many-body ground state. Theoretical calculations suggest that the Kondo resonance can interact with the irradiation photon field and should give rise to a {\pi}-phase shift of the photon signal in the case where the ground state is a Fermi liquid. This {\pi}-phase shift of microwave photon is driven from the Korringa-Shiba relation of quantum impurity Fermi-liquid ground states. We report the first observation of such a {\pi}-phase shift in a graphene double quantum dot within a circuit quantum electrodynamics architecture where the microwave photons couple to the pseudo-spin or charge degrees of freedom. The observed Kondo temperature TK ~ 550 mK is in agreement with DC conductance measurements. All our results support the formation of a Kondo resonance located above the Fermi level of the electronic reservoirs and the occurrence of an SU(4) Fermi-liquid ground state. We finally study how the Kondo-photon interactions can be tuned by inter-dot electron tunnel coupling strengths. Our experimental achievements may contribute to a better understanding of many-body physics in hybrid circuit systems, and open up new applications in atomic thin materials from the light-matter interaction.

Tunable interdot coupling in few-electron bilayer graphene double quantum dots

We present a highly controllable double quantum dot device based on bilayer graphene. Using a device architecture of interdigitated gate fingers, we can control the interdot tunnel coupling between 1 and 4 GHz and the mutual capacitive coupling between 0.2 and 0.6 meV, independent of the charge occupation of the quantum dots. The charging energy and, hence, the dot size remain nearly unchanged. The tuning range of the tunnel coupling covers the operating regime of typical silicon and GaAs spin qubit devices.

Microwave spectroscopy of spin–orbit coupled states: Energy detuning vs interdot coupling modulation

We study the AC field induced current peaks of a spin-blockaded double quantum dot with spin–orbit interaction. The AC field modulates either the interdot tunnel coupling or the energy detuning, and we choose the AC field frequency range to induce two singlet–triplet transitions giving rise to two current peaks. We show that for a large detuning, the two current peaks can be significantly stronger when the AC field modulates the tunnel coupling, thus making the detection of the spin–orbit gap more efficient. We also demonstrate the importance of the time dependence of the spin–orbit interaction.

Nonequilibirum noise spectrum and Coulomb blockade assisted Rabi interference in a double-dot Aharonov-Bohm interferometer

We investigate the charge-states coherence underlying the nonequilibirum transport through a spinless double-dot Aharonov-Bohm (AB) interferometer. Both the current noise spectrum and real-time dynamics are evaluated with the well-established dissipaton-equation-of-motion method. The resulted spectrums show the characteristic peaks and dips, arising from coherent Rabi oscillation dynamics, with the environment-assisted indirect inter-dot tunnel coupling mechanism. The observed spectroscopic features are in a quantitative agreement to the real-time dynamics of the reduced density matrix off-diagonal element between two charge states. As the aforementioned mechanism, these characteristics of coherence are very sensitive to the AB phase. While this is generally true for cross-correlation spectrum, the total circuit noise spectrum that is experimentally more accessible shows remarkably rich interplay between various mechanisms. The most important finding of this work is the existence of Coulomb-blockade-assisted Rabi interference, with very distinct signatures arising from the interplay between the AB interferometer and the interdot Coulomb interaction induced Fano resonance.

Efficient Orthogonal Control of Tunnel Couplings in a Quantum Dot Array

Electrostatically-defined semiconductor quantum dot arrays offer a promising platform for quantum computation and quantum simulation. However, crosstalk of gate voltages to dot potentials and inter-dot tunnel couplings complicates the tuning of the device parameters. To date, crosstalk to the dot potentials is routinely and efficiently compensated using so-called virtual gates, which are specific linear combinations of physical gate voltages. However, due to exponential dependence of tunnel couplings on gate voltages, crosstalk to the tunnel barriers is currently compensated through a slow iterative process. In this work, we show that the crosstalk on tunnel barriers can be efficiently characterized and compensated for, using the fact that the same exponential dependence applies to all gates. We demonstrate efficient calibration of crosstalk in a quadruple quantum dot array and define a set of virtual barrier gates, with which we show orthogonal control of all inter-dot tunnel couplings. Our method marks a key step forward in the scalability of the tuning process of large-scale quantum dot arrays.

Selective tuning of spin-orbital Kondo contributions in parallel-coupled quantum dots

We use co-tunneling spectroscopy to investigate spin-, orbital-, and spin-orbital Kondo transport in a strongly confined system of InAs double quantum dots (QDs) parallel-coupled to source and drain. In the one-electron transport regime, the higher symmetry spin-orbital Kondo effect manifests at orbital degeneracy and no external magnetic field. We then proceed to show that the individual Kondo contributions can be isolated and studied separately; either by orbital detuning in the case of spin-Kondo transport, or by spin splitting in the case of orbital Kondo transport. By varying the inter-dot tunnel coupling, we show that lifting of the spin degeneracy is key to confirming the presence of an orbital degeneracy, and to detecting a small orbital hybridization gap. Finally, in the two-electron regime, we show that the presence of a spin-triplet ground state results in spin-Kondo transport at zero magnetic field.

Single-Electron Double Quantum Dots in Bilayer Graphene.

We present transport measurements through an electrostatically defined bilayer graphene double quantum dot in the single-electron regime. With the help of a back gate, two split gates, and two finger gates, we are able to control the number of charge carriers on two gate-defined quantum dots independently between zero and five. The high tunability of the device meets requirements to make such a device a suitable building block for spin-qubits. In the single-electron regime, we determine interdot tunnel rates on the order of 2 GHz. Both, the interdot tunnel coupling as well as the capacitive interdot coupling increase with dot occupation, leading to the transition to a single quantum dot. Finite bias magneto-spectroscopy measurements allow to resolve the excited-state spectra of the first electrons in the double quantum dot and are in agreement with spin and valley conserving interdot tunneling processes.

Adiabatic electron charge transfer between two quantum dots in presence of 1/f noise

Controlled adiabatic transfer of a single electron through a chain of quantum dots has been recently achieved in GaAs and Si/SiGe based quantum dots, opening prospects for turning stationary spin qubits into mobile ones, and solving in this way the problem of long-distance communication between quantum registers in a scalable quantum computing architecture based on quantum dots. We consider theoretically the process of such an electron transfer between two tunnel-coupled quantum dots, focusing on control by slowly varying the detuning of energy levels in the dots. We take into account the fluctuations in detuning caused by $1/f$-type noise that is ubiquitous in semiconductor nanostructures, and analyze their influence on probability of successful transfer of an electron in a spin eigenstate. With numerical and analytical calculations we show that probability of electron not being transferred due to $1/f^\beta$ noise in detuning is $\propto \sigma^2 t^{\beta-1}/v$, where $\sigma$ characterizes the noise amplitude, $t$ is the interdot tunnel coupling, and $v$ is the detuning sweep rate. Interestingly, this means that the noise-induced errors in charge transfer are independent of $t$ for $1/f$ noise. For realistic parameters taken from experiments on silicon-based quantum dots, we obtain the minimal probability of charge transfer failure between a pair of dots is limited by $1/f$ noise in detuning to be the on order of $0.01$. This means that in order to reliably transfer charges across many quantum dots, charge noise in the devices should be further suppressed, or tunnel couplings should be increased, in order to allow for faster transfer (and less exposure to noise), while not triggering the deterministic Landau-Zener excitation.

Controlling collision properties of solitons in five-level M-type triple quantum dot electromagnetically induced transparency medium by inter-dot tunneling coupling

<sec>Experimentally, the triple-quantum-dots system can be produced on a GaAs <inline-formula><tex-math id="M7">\begin{document}$ \left[ {001} \right]$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M7.png"/></alternatives></inline-formula> substrate by molecular beam epitaxy or in-situ atomic layer precise etching, thus enabling a triangle triple quantum dot (QD) aligned along the <inline-formula><tex-math id="M8">\begin{document}$ \left[ {1\bar 10} \right]$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M8.png"/></alternatives></inline-formula> direction. According to this, we first propose a five-level M-type triple QD electromagnetically induced transparency (EIT) model which consists of a triple QD molecule interacting with a weakly linearly polarized probe field with two orthogonal polarization components under the action of a magnetic field parallel to the light propagation direction. Subsequently, by using the multiple-scale method combined with the Fourier integration method, the propagation characteristics of the optical solitons and the collision characteristics of two solitons in the system are studied. It is shown that the optical solitons can form and propagate stably in this system under the action of quantum inter-dot tunneling coupling whose formation mechanism is different from the soliton-forming mechanism in ultra-cold atomic, single QD, and double QD EIT system. This is because the necessary condition for forming a soliton is to use a strong light beam to modulate a weak light beam, whether it is in an ultra-cold atom system, or a single quantum dot EIT medium or a double quantum dot EIT medium. In a word, the formation of soliton in previous EIT systems needs an additional strong controlling field, while the five-level M-type triple QD EIT system is dependent on the inter-dot tunneling.</sec><sec>Since the solitons can propagate stably, the collision properties of the solitons may be studied in this system. Finally, by applying Fourier integration method, it is found that the collision behaviors of two solitons are determined by their initial phase difference. When their initial phase difference is 0, the collision behavior between the solitons is periodic elastic collision. While their initial phase difference is separately <inline-formula><tex-math id="M9">\begin{document}$ {\rm{\pi }}/4$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M9.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M10">\begin{document}$ \text{π}/2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M10.png"/></alternatives></inline-formula>, and <inline-formula><tex-math id="M11">\begin{document}$ \text{π}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M11.png"/></alternatives></inline-formula>, the collision behaviors exhibit separation phenomenon due to repulsive effect. Interestingly, the collision characteristics of two solitons are controlled by the inter-dot tunneling strength. With the increase of inter-dot tunneling strength, the collision period of two solitons with the initial phase difference of 0 decreases, and the repulsive force of two solitons with the initial phase difference being separately π/4, π/2 and π increases. This provides some theoretical basis for experimentally controlling the soliton dynamical properties in semiconductor quantum dot devices.</sec>

Computer-automated tuning procedures for semiconductor quantum dot arrays

As with any quantum computing platform, semiconductor quantum dot devices require sophisticated hardware and controls for operation. The increasing complexity of quantum dot devices necessitates the advancement of automated control software and image recognition techniques for rapidly evaluating charge stability diagrams. We use an image analysis toolbox developed in Python to automate the calibration of virtual gates, a process that previously involved a large amount of user intervention. Moreover, we show that straightforward feedback protocols can be used to simultaneously tune multiple tunnel couplings in a triple quantum dot in a computer automated fashion. Finally, we adopt the use of a “tunnel coupling lever arm” to model the interdot barrier gate response and discuss how it can be used to more rapidly tune interdot tunnel couplings to the gigahertz values that are compatible with exchange gates.

Site-Selective Quantum Control in an Isotopically Enriched Si28/Si0.7Ge0.3 Quadruple Quantum Dot

Silicon spin qubits are a promising quantum computing platform offering long coherence times, small device sizes, and compatibility with industry-backed device fabrication techniques. In recent years, high fidelity single-qubit and two-qubit operations have been demonstrated in Si. Here, we demonstrate coherent spin control in a quadruple quantum dot fabricated using isotopically enriched 28Si. We tune the ground state charge configuration of the quadruple dot down to the single electron regime and demonstrate tunable interdot tunnel couplings as large as 20 GHz, which enables exchange-based two-qubit gate operations. Site-selective single spin rotations are achieved using electric dipole spin resonance in a magnetic field gradient. We execute a resonant-CNOT gate between two adjacent spins in 270 ns.

High-fidelity and robust two-qubit gates for quantum-dot spin qubits in silicon

A two-qubit controlled-not (cnot) gate, realized by a controlled-phase (C-phase) gate combined with single-qubit gates, has been experimentally implemented recently for quantum-dot spin qubits in isotopically enriched silicon, a promising solid-state system for practical quantum computation. In the experiments, the single-qubit gates have been demonstrated with fault-tolerant control fidelity, but the infidelity of the two-qubit C-phase gate is, primarily due to the electrical noise, still higher than the required error threshold for fault-tolerant quantum computation (FTQC). Here, by taking the realistic system parameters and the experimental constraints on the control pulses into account, we construct experimentally realizable high-fidelity cnot gates robust against electrical noise with the experimentally measured $1/{f}^{1.01}$ noise spectrum and also against the uncertainty in the interdot tunnel coupling amplitude. Our fine-tuned optimal cnot gate has about two orders of magnitude improvement in gate infidelity over the ideal C-phase gate constructed without considering any noise effect. Furthermore, within the same control framework, high-fidelity and robust single-qubit gates can also be constructed, paving the way for large-scale FTQC.

Anti-Resonance in a Laterally Coupled Triple-Quantum-Dot Chain

Employing the nonequilibrium Green’s function technique, electron transport characteristics through a laterally coupled triple-quantum-dot chain are investigated. The conductance versus electron energy is numerically calculated. Two anti-resonance points change into two anti-resonance bands with increasing the size of triple-quantum-dot chain. The widths of two anti-resonance bands are the same and can be regulated via adjusting the inter-dot tunneling coupling strength in the main chain or its attachment. Spin-polarized window emerges due to Zeeman splitting as an external magnetic flux is introduced. Our studies demonstrate that the system can be used as a quantum switch or an efficient spin filtering.

Tuning Methods for Semiconductor Spin Qubits

We present efficient methods to reliably characterize and tune gate-defined semiconductor spin qubits. Our methods are designed to target the tuning procedures of semiconductor double quantum dot in GaAs heterostructures, but can easily be adapted to other quantum-dot-like qubit systems. These tuning procedures include the characterization of the inter-dot tunnel coupling, the tunnel coupling to the surrounding leads and the identification of the various fast initialization points for the operation of the qubit. Since semiconductor-based spin qubits are compatible with standard semiconductor process technology and hence promise good prospects of scalability, the challenge of efficiently tuning the dot's parameters will only grow in the near future, once the multi-qubit stage is reached. With the anticipation of being used as the basis for future automated tuning protocols, all measurements presented here are fast-to-execute and easy-to-analyze characterization methods. They result in quantitative measures of the relevant qubit parameters within a couple of seconds, and require almost no human interference.

Tunneling-induced ultraslow solitons in triangular quantum dot molecules

We theoretically investigate the propagation of a weak probe laser pulse in a triangular quantum dot molecules scheme based on the tunneling induced transparency. We find that the ultraslow optical solitons can be realized due to the destructive quantum interference induced by the interdot tunneling coupling which can be adjusted by the gate voltage appropriately. This work may provide practical applications such as electro-optic modulated devices and other information processes in semiconductor quantum dots structure.

Tuning the Two-Electron Hybridization and Spin States in Parallel-Coupled InAs Quantum Dots.

We study spin transport in the one- and two-electron regimes of parallel-coupled double quantum dots (DQDs). The DQDs are formed in InAs nanowires by a combination of crystal-phase engineering and electrostatic gating, with an interdot tunnel coupling (t) tunable by one order of magnitude. Large single-particle energy separations (up to 10meV) and |g^{*}| factors (∼10) enable detailed studies of the B-field-induced transition from a singlet-to-triplet ground state as a function of t. In particular, we investigate how the magnitude of the spin-orbit-induced singlet-triplet anticrossing depends on t. For cases of strong coupling, we find values of 230 μeV for the anticrossing using excited-state spectroscopy. Experimental results are reproduced by calculations based on rate equations and a DQD model including a single orbital in each dot.

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.

Thermally Driven Out-of-Equilibrium Two-Impurity Kondo System.

The archetypal two-impurity Kondo problem in a serially coupled double quantum dot is investigated in the presence of a thermal bias θ. The slave-boson formulation is employed to obtain the nonlinear thermal and thermoelectrical responses. When the Kondo correlations prevail over the antiferromagnetic coupling J between dot spins, we demonstrate that the setup shows negative differential thermal conductance regions behaving as a thermal diode. In addition, we report a sign reversal of the thermoelectric current I(θ) controlled by t/Γ (t and Γ denote the interdot tunnel and reservoir-dot tunnel couplings, respectively) and θ. All these features are attributed to the fact that at large θ both Q(θ) (heat current) and I(θ) are suppressed regardless of the value of t/Γ because the double dot decouples at high thermal biases. Finally, for a finite J, we investigate how the Kondo-to-antiferromagnetic crossover is altered by θ.

Automated tuning of inter-dot tunnel coupling in double quantum dots

Semiconductor quantum dot arrays defined electrostatically in a 2D electron gas provide a scalable platform for quantum information processing and quantum simulations. For the operation of quantum dot arrays, appropriate voltages need to be applied to the gate electrodes that define the quantum dot potential landscape. Tuning the gate voltages has proven to be a time-consuming task, because of initial electrostatic disorder and capacitive cross-talk effects. Here, we report on the automated tuning of the inter-dot tunnel coupling in gate-defined semiconductor double quantum dots. The automation of the tuning of the inter-dot tunnel coupling is the next step forward in scalable and efficient control of larger quantum dot arrays. This work greatly reduces the effort of tuning semiconductor quantum dots for quantum information processing and quantum simulation.