InAs Nanowire Quantum Dots Research Articles

Characterization of electrostatically defined bottom-heated InAs nanowire quantum dot systems

Conversion of temperature gradients to charge currents in quantum dot systems enables probing various concepts from highly efficient energy harvesting and fundamental thermodynamics to spectroscopic possibilities complementary to conventional bias device characterization. In this work, we present a proof-of-concept study of a device architecture where bottom-gates are capacitively coupled to an InAs nanowire and double function as local joule heaters. The device design combines the ability to heat locally at different locations on the device with the electrostatic definition of various quantum dot and barrier configurations. We demonstrate the versatility of this combined gating- and heating approach by studying, as a function of the heater location and bias, the Seebeck effect across the barrier-free nanowire, fit thermocurrents through quantum dots for thermometry and detect the phonon energy using a serial double quantum dot. The results indicate symmetric heating effects when the device is heated with different gates and we present detection schemes for the electronic and phononic heat transfer contribution across the nanowire. Based on this proof-of-principle work, we propose a variety of future experiments.

Rapid Detection of Coherent Tunneling in an InAs Nanowire Quantum Dot through Dispersive Gate Sensing

Dispersive sensing is a powerful technique that enables scalable and high-fidelity readout of solid-state quantum bits. In particular, gate-based dispersive sensing has been proposed as the readout mechanism for future topological qubits, which can be measured by single electrons tunneling through zero-energy modes. The development of such a readout requires resolving the coherent charge tunneling amplitude from a quantum dot in a Majorana-zero-mode host system faithfully on short time scales. Here, we demonstrate rapid single-shot detection of a coherent single-electron tunneling amplitude between InAs nanowire quantum dots. We have realized a sensitive dispersive detection circuit by connecting a sub-GHz, lumped element microwave resonator to a high-lever arm gate on one of dots. The resulting large dot-resonator coupling leads to an observed dispersive shift that is of the order of the resonator linewidth at charge degeneracy. This shift enables us to differentiate between Coulomb blockade and resonance, corresponding to the scenarios expected for qubit state readout, with a signal to noise ratio exceeding 2 for an integration time of 1 microsecond. Our result paves the way for single shot measurements of fermion parity on microsecond timescales in topological qubits.

Probing electron-phonon interactions in the charge-photon dynamics of cavity-coupled double quantum dots

Electron-phonon coupling is known to play an important role in the charge dynamics of semiconductor quantum dots. Here we explore its role in the combined charge-photon dynamics of cavity-coupled double quantum dots. Previous work on these systems has shown that strong electron-phonon coupling leads to a large contribution to photoemission and gain from phonon-assisted emission and absorption processes. We compare the effects of this phonon sideband in three commonly investigated gate-defined quantum dot material systems: InAs nanowires and GaAs and Si two-dimensional electron gases (2DEGs). We compare our theory with existing experimental data from cavity-coupled InAs nanowire and GaAs 2DEG double quantum dots and find quantitative agreement only when the phonon sideband and photoemission processes during lead tunneling are taken into account. Finally we show that the phonon sideband also leads to a sizable renormalization of the cavity frequency, which allows for direct spectroscopic probes of the electron-phonon coupling in these systems.

Parallel-Coupled Quantum Dots in InAs Nanowires.

We use crystal-phase tuning during epitaxial growth of InAs nanowires to create quantum dots with very strong confinement. A set of gate electrodes are used to reproducibly split the quantum dots into even smaller pairs for which we can control the populations down to the last electron. The double quantum dots, which are parallel-coupled to source and drain, show clear and stable odd-even level pairing due to spin degeneracy and the strong confinement. The combination of hard-wall barriers to source and drain, shallow interdot tunnel barriers, and very high single-particle excitation energies allow an order of magnitude tuning of the strength for the first intramolecular bond. We show examples for nanowires with different facet orientations, and suggest possible mechanisms behind the reproducible double-dot formation.

Magnetoresistance engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates

We present magnetoresistance (MR) experiments on an InAs nanowire quantum dot device with two ferromagnetic sidegates (FSGs) in a split-gate geometry. The wire segment can be electrically tuned to a single dot or to a double dot regime using the FSGs and a backgate. In both regimes we find a strong MR and a sharp MR switching of up to 25\% at the field at which the magnetizations of the FSGs are inverted by the external field. The sign and amplitude of the MR and the MR switching can both be tuned electrically by the FSGs. In a double dot regime close to pinch-off we find {\it two} sharp transitions in the conductance, reminiscent of tunneling MR (TMR) between two ferromagnetic contacts, with one transition near zero and one at the FSG switching fields. These surprisingly rich characteristics we explain in several simple resonant tunneling models. For example, the TMR-like MR can be understood as a stray-field controlled transition between singlet and a triplet double dot states. Such local magnetic fields are the key elements in various proposals to engineer novel states of matter and may be used for testing electron spin-based Bell inequalities.

Single-electron transport in InAs nanowire quantum dots formed by crystal phase engineering

We report electrical characterization of quantum dots formed by introducing pairs of thin wurtzite (WZ) segments in zinc blende (ZB) InAs nanowires. Regular Coulomb oscillations are observed over a wide gate voltage span, indicating that WZ segments create significant barriers for electron transport. We find a direct correlation of transport properties with quantum dot length and corresponding growth time of the enclosed ZB segment. The correlation is made possible by using a method to extract lengths of nanowire crystal phase segments directly from scanning electron microscopy images, and with support from transmission electron microscope images of typical nanowires. From experiments on controlled filling of nearly empty dots with electrons, up to the point where Coulomb oscillations can no longer be resolved, we estimate a lower bound for the ZB-WZ conduction-band offset of 95 meV.

Quantum dots in InAs nanowires induced by surface potential fluctuations

Back-gated InAs nanowire field-effect transistors are studied focusing on the formation of intrinsic quantum dots, i.e. dots not intentionally defined by electrodes. Such dots have been studied before, but the suggested explanations for their origin leave some open questions, which are addressed here. Stability diagrams of samples with different doping levels are recorded at electron temperatures below 200 mK, allowing us to estimate the number and size of the dots as well as the type of connection, i.e. in series or in parallel. We discuss several potential physical origins of the dots and conclude that they are most probably induced by potential fluctuations at the nanowire surface. Additionally, we show that via gate voltage and doping, the samples can be tuned to different regimes of Coulomb blockade.

Suppression of Zeeman Gradients by Nuclear Polarization in Double Quantum Dots

We use electric dipole spin resonance to measure dynamic nuclear polarization in InAs nanowire quantum dots. The resonance shifts in frequency when the system transitions between metastable high and low current states, indicating the presence of nuclear polarization. We propose that the low and the high current states correspond to different total Zeeman energy gradients between the two quantum dots. In the low current state, dynamic nuclear polarization efficiently compensates the Zeeman gradient due to the g-factor mismatch, resulting in a suppressed total Zeeman gradient. We present a theoretical model of electron-nuclear feedback that demonstrates a fixed point in nuclear polarization for nearly equal Zeeman splittings in the two dots and predicts a narrowed hyperfine gradient distribution.

Counting statistics in an InAs nanowire quantum dot with a vertically coupled charge detector

A gate-defined quantum dot (QD) in an InAs nanowire is fabricated on top of a quantum point contact realized in a two-dimensional electron gas. The strong coupling between these two quantum devices is used to perform time-averaged as well as time-resolved charge detection experiments for electron flow through the quantum dot. We demonstrate that the Fano factor describing shot noise or time-correlations in single-electron transport depends in the theoretically expected way on the asymmetry of the tunneling barriers even in a regime where the thermal energy kBT is comparable to the single-particle level spacing in the dot.

Nonequilibrium cotunneling through a three-level quantum dot

We calculate the nonlinear cotunneling conductance through a quantum dot with 3 electrons occupying the three highest lying energy levels. Starting from a 3-orbital Anderson model, we apply a generalized Schrieffer-Wolff transformation to derive an effective Kondo model for the system. Within this model we calculate the nonequilibrium occupation numbers and the corresponding cotunneling current to leading order in the exchange couplings. We identify the inelastic cotunneling thresholds and their splittings with applied magnetic field, and make a qualitative comparison to recent experimental data on carbon nanotube and InAs quantum-wire quantum dots. Further predictions of the model like cascade resonances and a magnetic-field dependence of the orbital level splitting are not yet observed but within reach of recent experimental work on carbon nanotube and InAs nanowire quantum dots.

Giant Fluctuations and Gate Control of the g-Factor in InAs Nanowire Quantum Dots

We study the g-factor of discrete electron states in InAs nanowire based quantum dots. The g values are determined from the magnetic field splitting of the zero bias anomaly due to the spin 1/2 Kondo effect. Unlike to previous studies based on 2DEG quantum dots, the g-factors of neighboring electron states show a surprisingly large fluctuation: g can scatter between 2 and 18. Furthermore electric gate tunability of the g-factor is demonstrated.

Measuring current by counting electrons in a nanowire quantum dot

We measure current by counting single electrons tunneling through an InAs nanowire quantum dot (QD). The charge detector is realized by fabricating a quantum point contact in close vicinity to the nanowire. The results based on electron counting compare well to a direct measurements of the QD current, when taking the finite bandwidth of the detector into account. The ability to detect single electrons also opens up possibilities for manipulating and detecting individual spins in nanowire QDs.

Spin dynamics in InAs nanowire quantum dots coupled to a transmission line

We study theoretically electron spins in nanowire quantum dots placed inside a transmission line resonator. Because of the spin-orbit interaction, the spins couple to the electric component of the resonator electromagnetic field and enable coherent manipulation, storage, and readout of quantum information in an all-electrical fashion. Coupling between distant quantum-dot spins, in one and the same or different nanowires, can be efficiently performed via the resonator mode either in real time or through virtual processes. For the latter case, we derive an effective spin-entangling interaction and suggest means to turn it on and off. We consider both transverse and longitudinal types of nanowire quantum dots and compare their manipulation time scales against the spin relaxation times. For this, we evaluate the rates for spin relaxation induced by the nanowire vibrations (phonons) and show that, as a result of phonon confinement in the nanowire, this rate is a strongly varying function of the spin operation frequency and thus can be drastically reduced compared to lateral quantum dots in GaAs. Our scheme is a step forward to the formation of hybrid structures where qubits of different nature can be integrated in a single device.

Self-Aligned Charge Read-Out for InAs Nanowire Quantum Dots

A highly sensitive charge detector is realized for a quantum dot in an InAs nanowire. We have developed a self-aligned etching process to fabricate in a single step a quantum point contact in a two-dimensional electron gas and a quantum dot in an InAs nanowire. The quantum dot is strongly coupled to the underlying point contact that is used as a charge detector. The addition of one electron to the quantum dot leads to a change of the conductance of the charge detector by typically 20%. The charge sensitivity of the detector is used to measure Coulomb diamonds as well as charging events outside the dot. Charge stability diagrams measured by transport through the quantum dot and charge detection merge perfectly.

Kondo-Enhanced Andreev Tunneling in InAs Nanowire Quantum Dots

We report measurements of the nonlinear conductance of InAs nanowire quantum dots coupled to superconducting leads. We observe a clear alternation between odd and even occupation of the dot, with subgap peaks at |V(sd)| = Delta/e markedly stronger (weaker) than the quasiparticle tunneling peaks at |V(sd)| = 2Delta/e for odd (even) occupation. We attribute the enhanced Delta peak to an interplay between Kondo correlations and Andreev tunneling in dots with an odd number of spins, and we substantiate this interpretation by a poor man's scaling analysis.

Direct Measurement of the Spin-Orbit Interaction in a Two-Electron InAs Nanowire Quantum Dot

We demonstrate control of the electron number down to the last electron in tunable few-electron quantum dots defined in catalytically grown InAs nanowires. Using low temperature transport spectroscopy in the Coulomb blockade regime, we propose a method to directly determine the magnitude of the spin-orbit interaction in a two-electron artificial atom with strong spin-orbit coupling. Because of a large effective g factor |g(*)|=8+/-1, the transition from a singlet S to a triplet T+ ground state with increasing magnetic field is dominated by the Zeeman energy rather than by orbital effects. We find that the spin-orbit coupling mixes the T+ and S states and thus induces an avoided crossing with magnitude Delta(SO)=0.25+/-0.05 meV. This allows us to calculate the spin-orbit length lambda(SO) approximately 127 nm in such systems using a simple model.

Top-gate defined double quantum dots in InAs nanowires

The authors present low temperature transport measurements on double quantum dots in InAs nanowires grown by metal-organic vapor phase epitaxy. Two dots in series are created by lithographically defined top gates with a procedure involving no extra insulating layer. The authors demonstrate the full tunability from strong to weak coupling between the dots. The quantum mechanical nature of the coupling leads to the formation of a molecular state extending over both dots. The excitation spectra of the individual dots are observable by their signatures in the nonlinear transport.

Tunable few-electron quantum dots in InAs nanowires

Quantum dots realized in InAs are versatile systems to study the effect of spin–orbitinteraction on the spin coherence, as well as the possibility to manipulate single spins usingan electric field. We present transport measurements on quantum dots realized in InAsnanowires. Lithographically defined top gates are used to locally deplete the nanowire andto form tunnelling barriers. By using three gates, we can form either single quantum dots,or two quantum dots in series along the nanowire. Measurements of the stability diagramsfor both cases show that this method is suitable for producing high quality quantum dotsin InAs.

Tunable Double Quantum Dots in InAs Nanowires Defined by Local Gate Electrodes

We report on low-temperature transport measurements on single and double quantum dots defined using local gates to electrostatically deplete InAs nanowires grown by chemical beam epitaxy. This technique allows us to define multiple quantum dots along a semiconducting nanowire and tune the coupling between them.