Session W74: Semiconducting Qubits IV

Building small, fast and hot hole spin qubits in Si and Ge

Quantum computers hold the potential to solve tasks exponentially faster than classical computers.

Hole spins in Ge/Si core/shell nanowires offer exceptionally strong yet electrically tunable spin-orbit interaction (SOI), the direct-Rashba SOI. This allows unprecedented control and facilitates scaling. The Rabi frequency can be tuned with gate voltages, going from fast manipulation to idle modes, demonstrating a “spin-orbit switch”. Spin-flip times as short as ~1 ns are obtained, approaching the strong driving regime, due to a SOI length down to a few nm. This qubit can also operate up to temperatures of 2 K. We are also implementing an exchange based CROT gate and dispersive qubit readout approaching the single shot regime.

 

The leading transistor for classical scaling is the silicon fin field-effect transistor (FinFET), integrating billions of FinFETs on a chip. Here, we show that a Si FinFET can host a hole spin qubit and operate above 4 K, potentially allowing in-situ integration of the control electronics. We achieve all-electrical control and 1Q gate fidelities at the fault-tolerance threshold. Further, we demonstrate a CROT gate with spin-orbit induced anisotropic exchange interaction, opening the door to high fidelity and fast 2Q gates, thus taking an important step towards quantum information processing.

    

Modelling of planar germanium hole qubits in electric and magnetic fields

Hole-based spin qubits in strained planar germanium quantum wells have received considerable attention due to their favourable properties and remarkable experimental progress. The sizeable spin-orbit interaction in this structure allows for efficient electric qubit operations while also coupling the qubit to electrical noise.

In this talk we show simulations of a heterostructure hosting these hole spin qubits [1]. We solve the effective mass equations for a realistic heterostructure, provide a set of analytical basis wave functions, and compute the effective g-factor of the heavy-hole ground-state. Our investigations reveal a strong impact of highly excited light hole states located outside the quantum well on the g-factor. We find that sweet spots in out-of-plane magnetic fields are shifted to impractically large electric fields. However, for magnetic fields close to in-plane alignment, sweet spots at low electric fields are recovered. This work will be helpful in understanding and improving coherence of germanium hole spin qubits.

[1] Wang, Scappucci, Veldhorst and Russ, arXiv:2208.04795   

Hole spin qubits in semiconductor quantum dots

Hole spin qubits in semiconductor quantum dots allow ultrafast electrical control, and are considered an intriguing candidate as the building block for a scalable quantum computer. Here we develop a theory with the objective of understanding how an alternating electric field can couple the two heavy hole states in a two-dimensional quantum dot to allow electric dipole

spin resonance. We work within the effective mass approximation and the envelope function approach, and explore influences of system parameters such as the interface electric field and form of confinement potential.   

Towards Si finFET quantum devices with reproducible behavior

Hole spin qubits in Si finFETs show great promise for a quantum computing platform exhibiting fast and all electrical qubit control, even when operated at temperatures above 1K. While charge noise affects all platforms, disorder is one of the main limitations in current Si-MOS qubit technology. 

Here, we present our work towards reducing disorder and trapped charges by investigating material stacks utilizing Hall bar and percolation density measurements. Additionally, fabricating quantum dot devices with ultra-small gate length and pitch will enable a path towards mitigating disorder. We show Si finFET quantum dot devices exhibiting sub 20 nm gate lengths with a small pitch, fabricated with high-resolution e-beam lithography. The reproducibility of quantum dot formation is assessed through transport measurements, RF gate reflectometry, and charge sensing. From these results we draw conclusions on gate stack and fin shape optimization.   

Quantum mechanical modeling of electron spins in realistic gate defined double quantum dots.

Spin qubits in semiconductors are front candidates for quantum computation, given their long coherence times [1]. Benefiting from decades of development in CMOS technologies [2], electrons in gate-defined quantum dots promise a platform for large scale integration of spin qubits. With recent demonstrations of high fidelity two-qubit gates[3,4], comes the need for active control over the exchange interaction that drives them. As such, a more comprehensive understanding of the exchange interaction in realistic devices and its dependence on control and environmental parameters is critical for spin-based quantum computation. The conditions for qubits' operation, on a small or large scale, are dictated by the underlying device architecture, choice of material, externally applied voltages and charge noise environment which has been shown to be a limiting factor for spin qubits. In this work, we propose a modelling framework that bridges a devices physical and operational conditions to the qubit energy space. The model combines electrostatic simulations and full configuration interaction (FCI) methods to estimate exchange interaction. Furthermore, the inclusion of any external potentials allows to study the effect of charge noise. This model opens a window for a deeper insight into spin qubit operation in semiconductors by estimating a critical metric represented in exchange coupling, its dependence on a device’s physical parameters and its sensitivity to the performance-limiting charge noise.

[1] Veldhorst, M. et al. Nat. Nanotechnology (2014)

[2] Li, R et al, IEDM (2020)

[3] Noiri,A. et al, Nature (2022)

[4] Xue, X. et al, Natue (2022)   

Characterization of ultra-low charge noise Silicon MOS quantum dots fabricated in a full 300mm CMOS platform

Silicon spin qubits are among the most promising candidates for large scale quantum computers, due to their excellent coherence and compatibility with CMOS technology for upscaling [1-4]. Advanced industrial CMOS processes are engineered to allow wafer-scale uniformity and high device yield. However, these processes cannot be directly carried over to spin qubits due to different designs and operating conditions.

By leveraging these processes, we have developed an optimized MOS gate stack specifically for spin qubit applications. We have characterized the MOS gate stack with gated hall-bars and used the optimized flow to fabricate quantum dot devices. We report highly uniform quantum dot operation and ultra-low noise at the SiSiO₂ interface at milli-Kelvin temperatures. These results confirm the excellent control over interface quality and set the basis for the large integration of quantum dot devices with low variability across the silicon wafer.

 

[1] X. Xue et al. Nature (2022)

[2] A. Noiri et al. Nature (2022)

[3] A. M. J. Zwerver et al. Nature Electronics (2022)

[4] R. Li et al. IEDM (2020)

    

Author not Attending

Charge noise in quantum dots has been observed to have a 1/f spectrum. We propose a model in which a pair of quantum dots are coupled to a 2D bath of fluctuators that have electric dipole moments and that interact with each other, i.e., with the other fluctuators. We simulate the bath of electric dipole fluctuators with a 2D Ising spin glass in the presence of a ground plane representing metal gates above the oxide layer containing the fluctuators. We calculate the resulting fluctuations in the electric potential at the two quantum dots that lie below the oxide layer. We report on the electric potential noise spectra at the quantum dots and the cross correlation in the noise between the two quantum dots.   

Energy dissipation in macroscopic quantum tunnel junctions

We study energy dissipation, in large quantum tunnel junctions, by applying voltages just below barrier height up to break down voltages. Moreover, by lowering the temperature and adjusting the applied voltage to the junction, the effect on dissipation caused by the variation in barrier height relative to the energy of the incident particle is empirically examined. Here, we propose a model that considers the energy dissipated during quantum tunneling processes in solid state junctions.

The results indicate two effects. Firstly, as the barrier height decreases with increasing temperature the fraction of energy dissipated also decreases. Furthermore, it is found that for lower barrier heights junctions, a drop in the junction resistance around breakdown voltage correlates with large tunneling currents across the junction. This can be explained as lower barrier heigh junctions dissipate less energy and thus, able to sustain larger currents before reaching break down voltages.

    

Effects of leakage on the realization of a discrete time crystal in a chain of singlet-triplet qubits

We consider the effects of leakage on the ability to realize a discrete time crystal (DTC) in a semiconductor quantum dot linear array being operated as a chain of singlet-triplet (ST) qubits. This system realizes an Ising model with an effective applied magnetic field, plus additional terms that can cause leakage out of the computational subspace. We demonstrate that, in the absence of these leakage terms, this model theoretically realizes a DTC phase over a broad parameter regime for six and eight qubits, with a broader parameter range for the eight-qubit case. We then reintroduce the leakage terms and find that the DTC phase disappears entirely over the same parameter range if the system is only subject to a uniform magnetic field, which does not suppress leakage. However, we find that the DTC phase can be restored if the system is instead subject to a magnetic field that alternates from qubit to qubit, which suppresses leakage. We thus show that leakage is a serious problem for the realization of a DTC phase in a chain of ST qubits, but is by no means insurmountable. Our work suggests that experiments manifesting small-system stable DTC should be feasible with currently existing quantum dot spin qubits.   

A realizable time crystal of four silicon quantum dot qubits

We demonstrate that exciting possible realizations of quantum Floquet matter are within reach for modern silicon spin qubits based in quantum dots, most notably the discrete time crystal (DTC).

This is significant given that spin qubits have fallen behind other qubit architectures in terms of size and control.

However, silicon spin qubits are especially well suited to this task, as the charge noise that usually foils gate operations can now be leveraged as an asset in this time crystal realization.

We illustrate differences between prethermal phenomena and true time-crystalline spatiotemporal order.

We demonstrate that even for a spin chain of four qubits, rich phase structures can be established by observing signatures of the discrete time crystal and the Floquet symmetry-protected topological phase (FSPT), both distinct from the thermal phase.

We also analyze the persistence of these signatures at longer chain lengths, showing that the DTC lifetime grows exponentially with the system length, and that these signatures may even be detectable for chains as small as three qubits.

We also discuss the effects of longer pulse durations and the effectiveness of pulse sequences for converting the exchange interaction to an Ising model.

Our theoretical predictions are well-suited for immediate experimental implementations using currently existing quantum dot spin qubit systems.   

Optimizing charge dynamics in quantum dot systems

The current mission to design, develop and implement solid-state quantum computing processes invokes the need to control the charge dynamics within a device coherently. Devices built on quantum dot technology can be simulated as combinations of single-/many-particle quantum wells that could be implemented as computational qubits. Here we solve, via exact diagonalization, the states of a single-electron quantum dot system, utilizing the OCTOPUS software package. Following, we implement from the same software, quantum optimal control theory (QOCT), to determine an optimal laser field that transitions a single charge from an initial state to the desired target state, (i) minimizing the time of transition and (ii) maximizing the overlap between the obtained final state and chosen target state. Both goals can be met while remaining within the range of experimentally accessible frequencies, aiding in bridging the gap between theory and experiment. Consequently, these results can provide insight into the development, initialization, and control of quantum dot systems for use in the design of solid-state quantum information devices.   

Entanglement dynamics of an Ising spin chain coupled to a central ancilla

Characterizing the spread of entanglement in many-body quantum systems is of both fundamental and applied interest. Coupling a controllable ancilla to a many-body system could allow us to quantify the entanglement dynamics of a system. Two important questions that arise are: 1) what information can we gain about the system by measuring the ancilla? and 2) does the ancilla influence the entanglement dynamics of the system? Szabo and Trivedi have characterized the entanglement dynamics of the transverse field Ising chain with the spins coupled uniformly to a central bosonic ancilla using different entanglement metrics including a novel one called multipartite entanglement loss (MEL) [1]. In this work, we augment this investigation using the eigenstate entanglement spectrum (EES) and the entanglement spectrum statistics (ESS) and compare the insights to those from the MEL. EES characterizes the bipartite entanglement of eigenstates of the many-body Hamiltonian. ESS characterizes the growth of bipartite entanglement during time evolution of an initial product state under the Hamiltonian. We use exact diagonalization to explore the transition of the statistics from a Gaussian Orthogonal Ensemble (GOE) to a Poisson distribution as the transverse field strength and the coupling strength to the ancilla are varied. Finally, we replace the central bosonic ancilla with a spin qubit and explore the consequences of non-uniform spin-ancilla couplings.

 

[1] J. C. Szabo and N. Trivedi, Phys. Rev. A 105, 052431 (2022)