Semiconductor Quantum Devices Research Articles

Codebase release 1.1 for QDsim

We introduce QDsim, a Python package tailored for the rapid generation of charge stability diagrams in large-scale quantum dot devices, extending beyond traditional double or triple dots. QDsim is founded on the constant interaction model from which we rephrase the task of finding the lowest energy charge configuration as a convex optimization problem. Therefore, we can leverage the existing package CVXPY, in combination with an appropriate powerful solver, for the convex optimization which streamlines the creation of stability diagrams and polytopes. Through multiple examples, we demonstrate how QDsim enables the generation of large-scale dataset that can serve a basis for the training of machine-learning models for automated tuning algorithms. While the package currently does not support quantum effects beyond the constant interaction model, QDsim is a tool that directly addresses the critical need for cost-effective and expeditious data acquisition for better tuning algorithms in order to accelerate the development of semiconductor quantum devices.

QDsim: A user-friendly toolbox for simulating large-scale quantum dot devices

We introduce QDsim, a Python package tailored for the rapid generation of charge stability diagrams in large-scale quantum dot devices, extending beyond traditional double or triple dots. QDsim is founded on the constant interaction model from which we rephrase the task of finding the lowest energy charge configuration as a convex optimization problem. Therefore, we can leverage the existing package CVXPY, in combination with an appropriate powerful solver, for the convex optimization which streamlines the creation of stability diagrams and polytopes. Through multiple examples, we demonstrate how QDsim enables the generation of large-scale dataset that can serve a basis for the training of machine-learning models for automated tuning algorithms. While the package currently does not support quantum effects beyond the constant interaction model, QDsim is a tool that directly addresses the critical need for cost-effective and expeditious data acquisition for better tuning algorithms in order to accelerate the development of semiconductor quantum devices.

(Invited) Engineering Sn-Based Group IV Quantum Materials

In recent years, direct band gap GeSn semiconductors have gained major attention for the monolithic integration of infrared emitters and detectors on a Si wafer.[1] A variety of GeSn infrared sensing and imaging technologies are now being developed using industrial-compatible fabrication processes. In addition to the high potential for integration in photonic technologies, GeSn quantum materials have been proposed for the design of novel solid-state qubits architectures.[2-3] Tensile-strained Ge/GeSn quantum wells (QWs) grown on a Si wafer result in a confined 2D hole gas with light-hole (LH) states lying above the heavy-hole (HH) ones. The LH-based nature of the Ge/GeSn QW is in striking difference with canonical Si/SiGe and Ge/SiGe QWs where electrons and HH are confined, respectively.[4] Achieving LH confinement paves the way to engineering ultrafast gate-defined spin qubits and integrate them into a spin-photon interface by leveraging the direct band gap absorption in the Ge/GeSn QW. Scalable all-group IV semiconductor quantum devices will thus be at reach by using the existing large-scale Si manufacturing processing.In this presentation, we will discuss the epitaxial growth of Ge/Ge0.86Sn0.14 QWs using a CVD reactor, reaching sharp interfaces and in-plane biaxial tensile strain up to 1.65 %.[2] Coherent QW epitaxial growth with a Ge layer thickness in the 1.5-12 nm range will be discussed by combining cross-sectional transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurements. Atom probe tomography (APT) measurements will be shown to precisely probe the sharpness of the Ge/GeSn interfaces and to show the absence of impurities in the QW down to the atomic level. Spectroscopic ellipsometry (SE) measurements will be discussed to demonstrate LH confinement in the Ge/GeSn QW. The effect of strain and thickness of the QW on the excitonic optical transitions identified by SE will provide clear signature of LH confinement. The integration of Ge/GeSn QW in practical LH qubits devices will be further motivated through eight-band k·p simulations of the g factor anisotropy, dipole moment, and spin-orbit interaction as a function of the QW parameters and external magnetic fields.[3] Lastly, strategies to engineer both LH and HH spin qubits using Sn-based heterostructures will be discussed to enrich the available architectures for solid-state qubits on Si.[1] O. Moutanabbir, et al. Appl. Phys. Lett. 118, 110502 (2021).[2] S. Assali, et al., Adv. Mater. 34, 2201192 (2022).[3] P. Del Vecchio, O. Moutanabbir, Phys. Rev. B 107, L161406 (2023).[4] G. Scappucci, et al., Nature Reviews Mater. 6, 926-943 (2021).

Break-even point of the phase-flip error correcting code

In this theoretical study, we explore the use of quantum code-based memories to enhance the lifetime of qubits and exceed the break-even point, which is critical for the implementation of fault-tolerant quantum computing. Specifically, we investigate the quantum phase-flip repetition code as a quantum memory and theoretically demonstrate that it can preserve arbitrary quantum information longer than the lifetime of a single idle qubit in a dephasing-time-limited system, e.g. in semiconductor qubits. Our circuit-based analytical calculations show the efficiency of the phase-flip code as a quantum memory in the presence of relaxation, dephasing, and faulty quantum gates. Moreover, we identify the optimal repetition number of quantum error correction cycles required to reach the break-even point by considering the gate error probabilities of current platforms for quantum computing. Our results provide guidelines for developing quantum memories in semiconductor quantum devices.

Probing details of spin-orbit coupling through Pauli spin blockade

Spin-orbit interaction (SOI) plays a fundamental role in many low-dimensional semiconductor and hybrid quantum devices. In the rapidly evolving field of semiconductor spin qubits, SOI is an essential ingredient that can allow for ultrafast qubit control. The exact manifestation of SOI in a given device is, however, often both hard to predict theoretically and probe experimentally. Here, we develop a detailed theoretical connection between the leakage current through a double quantum dot in Pauli spin blockade and the underlying SOI in the system. We present a general analytic expression for the leakage current, which allows to connect experimentally observable features to both the magnitude and orientation of an effective spin-orbit field acting on the moving carriers. Motivated by the large recent interest in hole-based quantum devices, we further zoom in on the case of Pauli blockade of hole spins, assuming a strong transverse confinement potential. In this limit we also find an analytic expression for the current at low external magnetic field, that includes the effect of hyperfine coupling of the hole spins to randomly fluctuating nuclear spin baths. This result can be used to extract information about both hyperfine and spin-orbit coupling parameters for hole spins in devices with a significant fraction of nonzero nuclear spins.

Tuning the Plasma Etching Mode for the Atomic-Scale Smoothing of Single-Crystal Silicon

Atomic-scale smooth surfaces of single-crystal silicon (Si) are indispensable for cutting-edge applications, such as semiconductor chips, quantum devices, and X-ray optics. Here, we vary the CF4/O2 reactant gas ratio to tune the etching mode from isotropic and orientation-selective etching to atom-selective etching in an atmospheric inductively coupled plasma (ICP). At low CF4/O2 ratios, the diffusion of the etching species dominates, resulting in isotropic etching. By contrast, the kinetics of ICP etching becomes dominant upon increasing the CF4/O2 ratio to between 1:1 and 2:1, inducing orientation-selective etching. Notably, CF4/O2 ratios above 2:1 result in atom-selective etching, whereby atoms around rough surface sites can be selectively removed. The atom-selective etching mode was used to achieve an atomically smooth surface with a Sa roughness of 0.14 nm. The results of this study demonstrate that atom-selective etching is an efficient and effective approach for manufacturing Si atomic surfaces.

Quantum-correlated photons generated by nonlocal electron transport

Since the realization of high-quality microwave cavities coupled to quantum dots, one can envisage the possibility to investigate the coherent interaction of light and matter in semiconductor quantum devices. Here we study a parallel double quantum dot device operating as single-electron splitter interferometer, with each dot coupled to a local photon cavity. We explore how quantum correlation and entanglement between the two separated cavities are generated by the coherent transport of a single electron passing simultaneously through the two different dots. We calculate the covariance of the cavity occupations by use of a diagrammatic perturbative expansion based on Keldysh Green's functions to the fourth order in the dot-cavity interaction strength, taking into account vertex diagrams. In this way, we demonstrate the creation of entanglement by showing that the classical Cauchy-Schwarz inequality is violated if the energy levels of the two dots are almost degenerate. For large level detunings or a single dot coupled to two cavities, we show that the inequality is not violated.

Even-denominator fractional quantum Hall state in conventional triple-gated quantum point contact

The even-denominator states have attracted considerable attention owing to their possible applications in future quantum technologies. In this letter, we first report a 3/2 diagonal resistance, indicating the existence of a 3/2 state in a nanometer-sized triple-gated quantum point contact (QPC) fabricated on a high-mobility (not ultra-high-mobility) single-layer two-dimensional (2D) GaAs wafer. The center gate plays a crucial role in realizing the QPC’s 3/2 state. Our observation of the 3/2 state using a conventional QPC device, which is a suitable building block for semiconductor quantum devices, paves a new path for the development of semiconductor-based quantum technologies.

Shadow-wall lithography of ballistic superconductor\u2013semiconductor quantum devices

The realization of hybrid superconductor–semiconductor quantum devices, in particular a topological qubit, calls for advanced techniques to readily and reproducibly engineer induced superconductivity in semiconductor nanowires. Here, we introduce an on-chip fabrication paradigm based on shadow walls that offers substantial advances in device quality and reproducibility. It allows for the implementation of hybrid quantum devices and ultimately topological qubits while eliminating fabrication steps such as lithography and etching. This is critical to preserve the integrity and homogeneity of the fragile hybrid interfaces. The approach simplifies the reproducible fabrication of devices with a hard induced superconducting gap and ballistic normal-/superconductor junctions. Large gate-tunable supercurrents and high-order multiple Andreev reflections manifest the exceptional coherence of the resulting nanowire Josephson junctions. Our approach enables the realization of 3-terminal devices, where zero-bias conductance peaks emerge in a magnetic field concurrently at both boundaries of the one-dimensional hybrids.

Spin Digitizer for High-Fidelity Readout of a Cavity-Coupled Silicon Triple Quantum Dot

An important requirement for spin-based quantum information processing is reliable and fast readout of electron spin states, allowing for feedback and error correction. However, common readout techniques often require additional gate structures hindering device scaling or impose stringent constraints on the tuning configuration of the sensed quantum dots. Here, we operate an in-line charge sensor within a triple quantum dot, where one of the dots is coupled to a microwave cavity and used to readout the charge states of the other two dots. Owing to the proximity of the charge sensor, we observe a near-digital sensor response with a power signal-to-noise ratio >450 at an integration time of $t_{\rm int}$ = 1 $\mu$s. Despite small singlet-triplet splittings $\approx$40 $\mu$eV, we further utilize the sensor to measure the spin relaxation time of a singlet-triplet qubit, achieving an average single-shot spin readout fidelity >99%. Our approach enables high-fidelity spin readout, combining minimal device overhead with flexible qubit operation in semiconductor quantum devices.

Quantum transport and its manipulation in semiconductor quantum devices

Quantum transport and its manipulation in semiconductor quantum devices

Multicomponent Model of Charge Transport in Quantum Semiconductor Devices

A model that allows taking into account the influence of quantum and non-equilibrium effects to the characteristics of semiconductor devices is presented. The model was successfully used for calculation the characteristics of resonant-tun-neling diodes, electronic, thermionic and optoelectronic devices based on nanowires. In a quasi-classical approximation it goes into a drift-diffusion model.

Band splitting induced by momentum-quantization in semiconductor nanostructures: Observation of emission lines in Indium Phosphide (InP) nanotubes

Two spectral emission lines originated from band splitting in Indium Phosphide nanotubes are observed experimentally. The nanotubes are grown by Metal-Organic Chemical Vapour Deposition on patterned template developed by electron beam lithography. The splitting effect arising from spatial confinement-induced momentum quantization on the band structure of nanotubes is investigated by developing a theoretical model. The incorporation of momentum quantization in sp3s*-model with spin-orbit coupling indicates the splitting of Γ-point into two different symmetric points, which emit two spectral lines as confirmed by cathodoluminescence spectra. The observation provides novel insight into the band structure based transport behaviour of semiconductor quantum devices.

Quantum hydrodynamic simulation of hysteresis in the resonant tunneling diode at 300 K

The smooth quantum hydrodynamic model is an extension of the classical hydrodynamic model for semiconductor devices that can handle in a mathematically rigorous way the discontinuities in the potential energy that occur at heterojunction barriers in quantum semiconductor devices. Smooth quantum hydrodynamic model simulations of a GaAs resonant tunneling diode at 300 K with a 5-nm quantum well and 2.5-nm 280 meV double quantum barriers are presented that successfully produce realistic negative differential resistance and hysteresis in the current–voltage curve. This is the first quantum hydrodynamic simulation of hysteresis in the resonant tunneling diode at 300 K, demonstrating that fluid dynamical concepts can model this fundamental quantum mechanical effect even at room temperature. A dispersive quantum term $$\hbar ^2 n u_{xxx}/(8 m)$$ in the energy transport equation, where n is the electron density, u is the electron velocity, and m is the effective electron mass, is essential here in obtaining a realistic hysteresis loop in the current–voltage curve.

Controllable double tunneling induced optical soliton storage in linear triple quantum dot molecules

We propose an asymmetric linear triple quantum dot molecule model for a recent experimental device with a double neighboring dot-dot tunneling coupling, of which the linear and nonlinear dynamical properties are analytically studied by using an amplitude variable approach combined with multi-scale method. It is shown that double transparency windows are formed by the double tunneling coupling. Both dark and bright optical solitons are then obtained, of which both the types and the propagating velocity can be controlled by the two tunneling coupling strengths. Interestingly, the propagating velocity of the solitons can approach to zero at certain tunneling coupling strengths, the motionless optical solitons appearing there. The results indicate potential applications of the semiconductor quantum devices for optical soliton storage.

Machine learning enables completely automatic tuning of a quantum device faster than human experts

Variability is a problem for the scalability of semiconductor quantum devices. The parameter space is large, and the operating range is small. Our statistical tuning algorithm searches for specific electron transport features in gate-defined quantum dot devices with a gate voltage space of up to eight dimensions. Starting from the full range of each gate voltage, our machine learning algorithm can tune each device to optimal performance in a median time of under 70 minutes. This performance surpassed our best human benchmark (although both human and machine performance can be improved). The algorithm is approximately 180 times faster than an automated random search of the parameter space, and is suitable for different material systems and device architectures. Our results yield a quantitative measurement of device variability, from one device to another and after thermal cycling. Our machine learning algorithm can be extended to higher dimensions and other technologies.

Comparison of Strain Effect between Aluminum and Palladium Gated MOS Quantum Dot Systems

As nano-scale metal-oxide-semiconductor devices are cooled to temperatures below 1 K, detrimental effects due to unintentional dots become apparent. The reproducibility of the location of these unintentional dots suggests that there are other mechanisms in play, such as mechanical strains in the semiconductor introduced by metallic gates. Here, we investigate the formation of strain-induced dots on aluminum and palladium gated metal oxide semiconductor (MOS) quantum devices using COMSOL Multiphysics. Simulation results show that the strain effect on the electrochemical potential of the system can be minimized by replacing aluminum with palladium as the gate material and increasing the thickness of the gate oxide.

Improving mobility of silicon metal-oxide–semiconductor devices for quantum dots by high vacuum activation annealing

To improve mobility of fabricated silicon metal-oxide–semiconductor (MOS) quantum devices, forming gas annealing is a common method used to mitigate the effects of disorder at the Si/SiO2 interface. However, the importance of activation annealing is usually ignored. Here, we show that a high vacuum environment for implantation activation is beneficial for improving mobility compared to nitrogen atmosphere. Low-temperature transport measurements of Hall bars show that peak mobility can be improved by a factor of two, reaching using high vacuum annealing during implantation activation. Moreover, the charge stability diagram of a single quantum dot is mapped, with no visible disturbance caused by disorder, suggesting the possibility of fabricating high-quality quantum dots on commercial wafers. Our results may provide valuable insights into device optimization in silicon-based quantum computing.

On-chip mid-infrared and THz frequency combs for spectroscopy

Frequency combs have revolutionized time and frequency metrology and in recent years, new frequency comb lasers that are highly compact or even on-chip have been demonstrated in the mid-infrared and THz regions of the electromagnetic spectrum. The emerging technologies include electrically pumped quantum and interband cascade semiconductor devices, as well as high-quality factor microresonators. In this guest editorial, the authors summarize recent advances in the field, the potential for rapid broadband spectroscopy, as well as the challenges and prospects for use in molecular gas sensing.

Novel Semiconductor Quantum Devices Shaping Our Century [Guest Editorial

The three articles in this special section report on novel and new semiconductor quantum devices. The responsible management of energy resources is an important concern. Effective measures for better management of energy resources are urgently needed as the world’s population increases and prosperity reaches more people. Novel semiconductor quantum devices hold the promise of enabling more-efficient energy conversion, the production of cheap and energy-saving integrated photonics, and reliable global connectivity and communication. Researchers across the world are racing to develop technologies that could potentially be worth billions of dollars and create hundreds of thousands of new jobs at all levels in society. However, even as the world economy grows, our planet is in peril, as the media constantly reminds us. One material class, the IIIAs/ P/N semiconductors, may hold the key to both efficiently managing resources and saving our planet.