Session C29: Semiconductor Qubits I

Live

A periodically driven quantum system with avoided-level crossing experiences both nonadiabatic transitions and wave-function phase changes. These result in coherent interference fringes in the system’s occupation probabilities. For qubits, with repelling energy levels, such interference, named after Landau-Zener-Stückelberg-Majorana, displays arc-shaped resonance lines. In the case of a multilevel system with an avoided-level crossing of the two lower levels, we demonstrate that the shape of the resonances can change from convex arcs to concave heart-shaped and harp-shaped resonance lines. Indeed, the whole energy spectrum determines the shape of such resonance fringes and this also provides insight into the slow-frequency system spectroscopy. As a particular example, we consider this for valley-orbit silicon quantum dots, which are important for the emerging field of valleytronics.

[1] A. I. Ryzhov, S. N. Shevchenko, F. Nori, Phys. Rev. B 98, 195434 (2018).   

Live

In silicon spin qubits, the qubit must be tuned away from the spin-valley hotspot to prevent fast qubit relaxation. We study in detail how the valley splitting depends on the electric and magnetic fields for both ideal and disordered Si/SiGe interfaces. Importantly, our modeling makes it possible to analyze the effect of arbitrary configurations of interface steps. We find, depending on where the interface steps are located, the magnetic field can increase or suppress the valley splitting. Moreover, the valley splitting can scale linearly or, in the presence of interface steps, non-linearly with the electric field [1].

We then extend our analysis by developing a valley-dependent envelope function theory. One crucial set of parameters for understanding the behavior of spin relaxation is the inter-valley and intra-valley dipole matrix elements. Our analysis enables us to calculate the dipole matrix elements as a function of interface roughness, the in-plane orbital splitting, and the electromagnetic field. Interestingly, we find some specific highly disordered interfaces where the valley splitting is suppressed but remains finite whereas the inter-valley dipole matrix elements can vanish. In this case, we find that the spin-valley hotspot also disappears.

[1] arXiv: 2007.00332   

Live

We analyze a new type of discrete time crystalline behavior that is obtained by periodically driving exchange interactions in a Heisenberg spin chain. This behavior is shown to benefit the protection and manipulation of multi-spin quantum states. We use these ideas to further construct a time-crystal-inspired CZ gate between singlet-triplet qubits with high fidelity. Our results indicate that periodically driving exchange couplings can enhance the performance of quantum dot spin systems for quantum information applications.   

Live

The presence of degenerate conduction band valleys and how they are mixed by interfaces play critical roles in determining electron interaction and spectrum in a silicon nanostructure. Here we investigate how the valley phases affect the exchange interaction in a symmetric two-electron silicon double quantum dot. Within the Heitler-London approximation limited to the ground orbital states, exchange splitting is suppressed at a finite value of valley phase difference between the two dots and reaches its minimum value (~0) when the phase difference is π. Such a suppression can be explained using the Hubbard model and the absence of difference in double occupation. The contributions of the higher orbital states thus play a vital role in determining the value of the exchange energy in general.   

Live

Silicon (Si) quantum dot (QD) spin qubits have great potential for application in large-scale quantum circuits as they share many similarities with classical transistors that represent the prototypical example for scalable electronic platforms. However, for QD formation and control, additional gates are required that add to device complexity and thus hinder upscaling. Here, we meet this challenge by demonstrating the scalable integration of a multilayer-gate-stack in Si QD devices using self-alignment, which allows for ultra-small gate lengths and intrinsically perfect layer-to-layer alignment [Geyer et al., arXiv:2007.15400 (2020)]. We study hole transport through a double QD and observe Pauli spin blockade (PSB). Application of a small magnetic field leads to lifting of PSB and reveals the presence of spin-orbit interaction. From the magnitude of a singlet-triplet anticrossing at high magnetic field we estimate a spin-orbit energy of 37µeV, which corresponds to a spin-orbit length of just 48nm. This work paves the way for scalable spin-based quantum circuits with fast, all-electrical qubit control and operation temperatures above 1K.   

Live

The transport of quantum information between different nodes of the device is crucial for a quantum processor. In the context of spin qubits, this can be realized by coherent electron spin shuttling between quantum dots. Here we theoretically study a minimal version of spin shuttling between two quantum dots (QDs) occupied by one electron. We analyze the possibilities and limitations of spin transport during a detuning sweep in a silicon double QD. This research is motivated by recent experimental progress [1,2]. Spin-orbit interaction and an inhomogeneous magnetic field play an important role for spin shuttling and are included in our model. Interactions that couple the position, spin and valley degrees of freedom open avoided crossings in the spectrum allowing for diabatic transitions and interfering paths. The outcomes of single and repeated spin shuttling protocols are explored by means of numerical simulations and an approximate analytic model based on the Landau-Zener model. We find that fast high-fidelity spin-shuttling is feasible for optimal choices of parameters or protected by constructive interference.

[1] T. Fujita et al., npj Quantum Information 3, 22 (2017)
[2] A. R. Mills et al., Nature Communs 10, 1063 (2019)   

Live

The design, fabrication, and characterization of MOS quantum dot devices surface gated with a single Ti/Pd metal layer are presented and discussed. The motivation is to avoid using fragile gate dielectrics that are sometimes seen to break down in multilayer devices and obtain simpler process flows. However, as a tradeoff, we expect reduced electrostatic gate control in single layer design. We observed quantum dots forming in the device channel and gate capacitances consistent with modelling, so that dot positions can be located by triangulation in capacitance modelling. In some cases, non-deliberate dot positions and more dots formed than intended, indicate non-lithographic features in the channel. Nonetheless, the gate capacitance ratios are sufficient to enable charge sensing and persistent feedback mode is used to map charge stability regions over wide gate voltage ranges. Granularity and delamination of the Pd layer in the fine region shown by SEM and AFM, combined with possible Si/SiO₂ interface defects, may account for some of the irregular features in the transport and charge sensing measurements. Overall, these metal single gate layer devices provide ample data for analyzing benefits and costs of this design, and help advancing future implementation of diagnostic qubits.   

Live

All-electrical control of a singlet-triplet qubit in a silicon metal-oxide-semiconductor double quantum dot may be realized through the combination of the exchange interaction and intrinsic spin-orbit effects at a silicon/silicon-dioxide interface. Such a qubit operates independently of ancillary components, such as micromagnets or microwave resonators, that are typically required for qubit control. However, the native control axes of this qubit are non-orthogonal and for quantum computation it is desirable to have orthogonal logic gates. In this work, we demonstrate a set of orthogonal gates that are optimal electrical control solutions incorporating composite exchange and spin-orbit dominated gates. We assess the performance of these composite orthogonal gates using gate set tomography and Clifford randomized benchmarking.   

Live

Silicon qubits have attracted much attention as a building block of quantum computers (QCs) because of their huge potential for mass production. However, at this stage, the number of silicon qubits in operation is limited because an appropriate qubit design methodology is lacking. Therefore, a new simulator for silicon qubit design is strongly required to accelerate the development of QCs.
A precise analysis of the quantum state in the qubit region is a fundamental part of the qubit design. However, an analysis of the semi-classical device region surrounding the qubit is also essential for qubit simulation because it controls the qubit state through the confinement potential and applied field and dynamically couples with the qubit. Consequently, consistent handling of both regions is indispensable for qubit design.
In this presentation, we integrate newly developed methods for qubit simulation into a single device simulation platform and perform self-consistent simulations of qubit characteristics considering a whole device structure. Using this simulator, we achieve simultaneous simulation of quantum transport and qubit operation.   

Live

In this talk I describe two recent advances in the control and measurement of Si/SiGe quantum dot qubits. In the first part of the talk, I describe how relatively small changes in gate voltage can lead to dramatic tunability of 2-electron energy eigenstates, which are important for readout of Loss-Divincenzo, singlet-triplet, and quantum dot hybrid qubits. For the latter qubit, the ability to choose, on demand, singlet-triplet splittings in neighboring quantum dots is particularly useful, because this splitting determines (in the first dot) the qubit zero-to-one energy splitting and (in the second dot) the energy gap for qubit readout. I will show data from the same device demonstrating singlet-triplet splittings varying from below 5 GHz up to 15 GHz. In the low frequency end of this range we demonstrate the key role of electron-electron correlations and Wigner molecule physics. I also present data on the use of three-dimensional integration in dispersive readout measurement schemes for Si/SiGe quantum dot qubits, and I discuss the implications for scaling of qubit numbers.   

Live

Spin qubits in Si/SiGe heterostructures offer a promising platform on which to build a scalable quantum computer. However, fabricating these heterostructures and devices on an all-optical, 300mm process line presents many challenges. One challenge is in creating lateral confinement of the quantum dots. Intel’s SiMOS device architecture is based on fins for lateral confinement, but the formation of fins on Si/SiGe would degrade the strain of the buried silicon quantum well. In this talk, we present a planar device architecture built on a 300mm platform that does not rely on fins for confinement. This planar device is fabricated on a Si/SiGe heterostructure, allowing quantum dots to be formed in a strained silicon quantum well. We will detail progress made on the SiGe virtual substrate and heterostructure, as well as preliminary low temperature measurements.   

On Demand

Intel has established a 300mm process line for the fabrication of spin qubit devices using all optical lithography. The customized QC test-chip provides a diversity of test structures: Transistors, Hall Bars, Quantum Dots, and qubit devices at varying dimensions and 1-D array size. Each wafer has ~80 die with thousands of devices, which allows statistical process control of yield and performance. We have implemented testing at industrial scale, with devices/processes that can be characterized in-house at room temperature and low temperature (1.6K). In this talk, we will provide an overview of integration scheme and yield. We will also discuss key device metrics at both room and low temperatures, in addition to the standard QD/qubit metrics which are normally used. We will present the progress from Transistor to QD devices, and for the first time, we will demonstrate full 300mm spin qubit devices with performance comparable to leading academic devices.