arXiv subjects

Lars R. Schreiber

Publications and source records attributed to Lars R. Schreiber.

35 records · Page 2Linked to original sources

arXiv2024

Noise reduction by bias cooling in gated Si/SixGe1-x quantum dots

Silicon-Germanium heterostructures are a promising quantum circuit platform, but crucial aspects as the long-term charge dynamics and cooldown-to-cooldown variations are still widely unexplored quantitatively. In this letter we present the results of an extensive bias cooling study performed on gated silicon-germanium quantum dots with an Al2O3-dielectric. Over 80 cooldowns were performed in the course of our investigations. The performance of the devices is assessed by low-frequency charge noise measurements in the band of 200 micro Hertz to 10 milli Hertz. We measure the total noise power as a function of the applied voltage during cooldown in four different devices and find a minimum in noise at 0.7V bias cooling voltage for all observed samples. We manage to decrease the total noise power median by a factor of 6 and compute a reduced tunneling current density using Schrödinger-Poisson simulations. Furthermore, we show the variation in noise from the same device in the course of eleven different cooldowns performed under the nominally same conditions.

cond-mat.mes-hall↗

arXiv2023

Mapping of valley-splitting by conveyor-mode spin-coherent electron shuttling

In Si/SiGe heterostructures, the low-lying excited valley state seriously limit operability and scalability of electron spin qubits. For characterizing and understanding the local variations in valley splitting, fast probing methods with high spatial and energy resolution are lacking. Leveraging the spatial control granted by conveyor-mode spin-coherent electron shuttling, we introduce a method for two-dimensional mapping of the local valley splitting by detecting magnetic field dependent anticrossings of ground and excited valley states using entangled electron spin-pairs as a probe. The method has sub-μeV energy accuracy and a nanometer lateral resolution. The histogram of valley splittings spanning a large area of 210 nm by 18 nm matches well with statistics obtained by the established but time-consuming magnetospectroscopy method. For the specific heterostructure, we find a nearly Gaussian distribution of valley splittings and a correlation length similar to the quantum dot size. Our mapping method may become a valuable tool for engineering Si/SiGe heterostructures for scalable quantum computing.

quant-ph↗

arXiv2023

Local laser-induced solid-phase recrystallization of phosphorus-implanted Si/SiGe heterostructures for contacts below 4.2 K

Si/SiGe heterostructures are of high interest for high mobility transistor and qubit applications, specifically for operations below 4.2 K. In order to optimize parameters such as charge mobility, built-in strain, electrostatic disorder, charge noise and valley splitting, these heterostructures require Ge concentration profiles close to mono-layer precision. Ohmic contacts to undoped heterostructures are usually facilitated by a global annealing step activating implanted dopants, but compromising the carefully engineered layer stack due to atom diffusion and strain relaxation in the active device region. We demonstrate a local laser-based annealing process for recrystallization of ion-implanted contacts in SiGe, greatly reducing the thermal load on the active device area. To quickly adapt this process to the constantly evolving heterostructures, we deploy a calibration procedure based exclusively on optical inspection at room-temperature. We measure the electron mobility and contact resistance of laser annealed Hall bars at temperatures below 4.2 K and obtain values similar or superior than that of a globally annealed reference samples. This highlights the usefulness of laser-based annealing to take full advantage of high-performance Si/SiGe heterostructures.

physics.app-ph↗

arXiv2023

Spin-EPR-pair separation by conveyor-mode single electron shuttling in Si/SiGe

Long-ranged coherent qubit coupling is a missing function block for scaling up spin qubit based quantum computing solutions. Spin-coherent conveyor-mode electron-shuttling could enable spin quantum-chips with scalable and sparse qubit-architecture. Its key feature is the operation by only few easily tuneable input terminals and compatibility with industrial gate-fabrication. Single electron shuttling in conveyor-mode in a 420 nm long quantum bus has been demonstrated previously. Here we investigate the spin coherence during conveyor-mode shuttling by separation and rejoining an Einstein-Podolsky-Rosen (EPR) spin-pair. Compared to previous work we boost the shuttle velocity by a factor of 10000. We observe a rising spin-qubit dephasing time with the longer shuttle distances due to motional narrowing and estimate the spin-shuttle infidelity due to dephasing to be 0.7 % for a total shuttle distance of nominal 560 nm. Shuttling several loops up to an accumulated distance of 3.36 $μ$m, spin-entanglement of the EPR pair is still detectable, giving good perspective for our approach of a shuttle-based scalable quantum computing architecture in silicon.

quant-ph↗

arXiv2023

Si/SiGe QuBus for single electron information-processing devices with memory and micron-scale connectivity function

The connectivity within single carrier information-processing devices requires transport and storage of single charge quanta. Our all-electrical Si/SiGe shuttle device, called quantum bus (QuBus), spans a length of 10 $\mathrmμ$m and is operated by only six simply-tunable voltage pulses. It operates in conveyor-mode, i.e. the electron is adiabatically transported while confined to a moving QD. We introduce a characterization method, called shuttle-tomography, to benchmark the potential imperfections and local shuttle-fidelity of the QuBus. The fidelity of the single-electron shuttle across the full device and back (a total distance of 19 $\mathrmμ$m) is $(99.7 \pm 0.3)\,\%$. Using the QuBus, we position and detect up to 34 electrons and initialize a register of 34 quantum dots with arbitrarily chosen patterns of zero and single-electrons. The simple operation signals, compatibility with industry fabrication and low spin-environment-interaction in $^{28}$Si/SiGe, promises spin-conserving transport of spin qubits for quantum connectivity in quantum computing architectures.

cond-mat.mes-hall↗

arXiv2023

The SpinBus Architecture: Scaling Spin Qubits with Electron Shuttling

Quantum processor architectures must enable scaling to large qubit numbers while providing two-dimensional qubit connectivity and exquisite operation fidelities. For microwave-controlled semiconductor spin qubits, dense arrays have made considerable progress, but are still limited in size by wiring fan-out and exhibit significant crosstalk between qubits. To overcome these limitations, we introduce the SpinBus architecture, which uses electron shuttling to connect qubits and features low operating frequencies and enhanced qubit coherence. Device simulations for all relevant operations in the Si/SiGe platform validate the feasibility with established semiconductor patterning technology and operation fidelities exceeding 99.9 %. Control using room temperature instruments can plausibly support at least 144 qubits, but much larger numbers are conceivable with cryogenic control circuits. Building on the theoretical feasibility of high-fidelity spin-coherent electron shuttling as key enabling factor, the SpinBus architecture may be the basis for a spin-based quantum processor that meets the scalability requirements for practical quantum computing.

quant-ph↗

arXiv2023

Sensing dot with high output swing for scalable baseband readout of spin qubits

A crucial requirement for quantum computing, in particular for scalable quantum computing and error correction, is a fast and high-fidelity qubit readout. For semiconductor based qubits, one limiting factor for local low-power signal amplification, is the output swing of the charge sensor. We demonstrate GaAs and Si/SiGe asymmetric sensing dots (ASDs) specifically designed to provide a significantly improved response compared to conventional charge sensing dots. Our ASD design features a strongly decoupled drain reservoir from the sensor dot, which mitigates negative feedback effects found in conventional sensors. This results in a boosted output swing of $3\,\text{mV}$, which exceeds the response in the conventional regime of our device by more than ten times. The enhanced output signal paves the way for employing very low-power readout amplifiers in close proximity to the qubit.

cond-mat.mes-hall↗

arXiv2023

Blueprint of a scalable spin qubit shuttle device for coherent mid-range qubit transfer in disordered Si/SiGe/SiO$_2$

Silicon spin qubits stand out due to their very long coherence times, compatibility with industrial fabrication, and prospect to integrate classical control electronics. To achieve a truly scalable architecture, a coherent mid-range link that moves the electrons between qubit registers has been suggested to solve the signal fan-out problem. Here, we present a blueprint of such a $\approx 10\,μ$m long link, called a spin qubit shuttle, which is based on connecting an array of gates into a small number of sets. To control these sets, only a few voltage control lines are needed and the number of these sets and thus the number of required control signals is independent of the length of this link. We discuss two different operation modes for the spin qubit shuttle: A qubit conveyor, i.e. a potential minimum that smoothly moves laterally, and a bucket brigade, in which the electron is transported through a series of tunnel-coupled quantum dots by adiabatic passage. We find the former approach more promising considering a realistic Si/SiGe device including potential disorder from the charged defects at the Si/SiO$_2$ layer, as well as typical charge noise. Focusing on the qubit transfer fidelity in the conveyor shuttling mode, we discuss in detail motional narrowing, the interplay between orbital and valley excitation and relaxation in presence of $g$-factors that depend on orbital and valley state of the electron, and effects from spin-hotspots. We find that a transfer fidelity of 99.9 \% is feasible in Si/SiGe at a speed of $\sim$10 m/s, if the average valley splitting and its inhomogeneity stay within realistic bounds. Operation at low global magnetic field $\approx 20$ mT and material engineering towards high valley splitting is favourable for reaching high fidelities of transfer.

cond-mat.mes-hall↗

arXiv2023

Tailoring potentials by simulation-aided design of gate layouts for spin qubit applications

Gate-layouts of spin qubit devices are commonly adapted from previous successful devices. As qubit numbers and the device complexity increase, modelling new device layouts and optimizing for yield and performance becomes necessary. Simulation tools from advanced semiconductor industry need to be adapted for smaller structure sizes and electron numbers. Here, we present a general approach for electrostatically modelling new spin qubit device layouts, considering gate voltages, heterostructures, reservoirs and an applied source-drain bias. Exemplified by a specific potential, we study the influence of each parameter. We verify our model by indirectly probing the potential landscape of two design implementations through transport measurements. We use the simulations to identify critical design areas and optimize for robustness with regard to influence and resolution limits of the fabrication process.

cond-mat.mes-hall↗

arXiv2021

Conveyor-mode single-electron shuttling in Si/SiGe for a scalable quantum computing architecture

Small spin-qubit registers defined by single electrons confined in Si/SiGe quantum dots operate successfully and connecting these would permit scalable quantum computation. Shuttling the qubit carrying electrons between registers is a natural choice for high-fidelity coherent links provided the overhead of control signals stays moderate. Our proof-of-principle demonstrates shuttling of a single electron by a propagating wave-potential in an electrostatically defined 420 nm long Si/SiGe quantum-channel. This conveyor-mode shuttling approach requires independent from its length only four sinusoidal control signals. We discuss the tuning of the signal parameters, detect the smoothness of the electron motion enabling the mapping of potential disorder and observe a high single-electron shuttling fidelity of $99.42\pm0.02\,\%$ including a reversal of direction. Our shuttling device can be readily embedded in industrial fabrication of Si/SiGe qubit chips and paves the way to solving the signal-fanout problem for a fully scalable semiconductor quantum-computing architecture.

cond-mat.mes-hall↗

arXiv2021

Robust and fast post-processing of single-shot spin qubit detection events with a neural network

Establishing low-error and fast detection methods for qubit readout is crucial for efficient quantum error correction. Here, we test neural networks to classify a collection of single-shot spin detection events, which are the readout signal of our qubit measurements. This readout signal contains a stochastic peak, for which a Bayesian inference filter including Gaussian noise is theoretically optimal. Hence, we benchmark our neural networks trained by various strategies versus this latter algorithm. Training of the network with 10$^{6}$ experimentally recorded single-shot readout traces does not improve the post-processing performance. A network trained by synthetically generated measurement traces performs similar in terms of the detection error and the post-processing speed compared to the Bayesian inference filter. This neural network turns out to be more robust to fluctuations in the signal offset, length and delay as well as in the signal-to-noise ratio. Notably, we find an increase of 7 % in the visibility of the Rabi-oscillation when we employ a network trained by synthetic readout traces combined with measured signal noise of our setup. Our contribution thus represents an example of the beneficial role which software and hardware implementation of neural networks may play in scalable spin qubit processor architectures.

quant-ph↗

arXiv2021

How to solve problems in micro- and nanofabrication caused by the emission of electrons and charged metal atoms during e-beam evaporation

We discuss how the emission of electrons and ions during electron-beam-induced physical vapor deposition can cause problems in micro- and nanofabrication processes. After giving a short overview of different types of radiation emitted from an electron-beam (e-beam) evaporator and how the amount of radiation depends on different deposition parameters and conditions, we highlight two phenomena in more detail: First, we discuss an unintentional shadow evaporation beneath the undercut of a resist layer caused by the one part of the metal vapor which got ionized by electron-impact ionization. These ions first lead to an unintentional build-up of charges on the sample, which in turn results in an electrostatic deflection of subsequently incoming ionized metal atoms towards the undercut of the resist. Second, we show how low-energy secondary electrons during the metallization process can cause cross-linking, blisters, and bubbles in the respective resist layer used for defining micro- and nanostructures in an e-beam lithography process. After the metal deposition, the cross-linked resist may lead to significant problems in the lift-off process and causes leftover residues on the device. We provide a troubleshooting guide on how to minimize these effects, which e.g. includes the correct alignment of the e-beam, the avoidance of contaminations in the crucible and, most importantly, the installation of deflector electrodes within the evaporation chamber.

cond-mat.mes-hall↗

arXiv2020

Large, tunable valley splitting and single-spin relaxation mechanisms in a Si/Si$_x$Ge$_{1-x}$ quantum dot

Valley splitting is a key figure of silicon-based spin qubits. Quantum dots in Si/SiGe heterostructures reportedly suffer from a relatively low valley splitting, limiting the operation temperature and the scalability of such qubit devices. Here, we demonstrate a robust and large valley splitting exceeding 200 $μ$eV in a gate-defined single quantum dot, hosted in molecular-beam epitaxy-grown $^{28}$Si/SiGe. The valley splitting is monotonically and reproducibly tunable up to 15 % by gate voltages, originating from a 6 nm lateral displacement of the quantum dot. We observe static spin relaxation times $T_1>1$ s at low magnetic fields in our device containing an integrated nanomagnet. At higher magnetic fields, $T_1$ is limited by the valley hotspot and by phonon noise coupling to intrinsic and artificial spin-orbit coupling, including phonon bottlenecking.

cond-mat.mes-hall↗

arXiv2019

Low-frequency spin qubit detuning noise in highly purified $^{28}$Si/SiGe

The manipulation fidelity of a single electron qubit gate-confined in a $^{28}$Si/SiGe quantum dot has recently been drastically improved by nuclear isotope purification. Here, we identify the dominant source for low-frequency qubit detuning noise in a device with an embedded nanomagnet, a remaining $^{29}$Si concentration of only 60$\,$ppm in the strained $^{28}$Si quantum well layer and a spin echo decay time $T_2^{\text{echo}}=128\,μ$s. The power spectral density (PSD) of the charge noise explains both the observed transition of a $1/f^2$- to a $1/f$-dependence of the detuning noise PSD as well as the observation of a decreasing time-ensemble spin dephasing time from $T_2^* \approx 20\,μ$s with increasing measurement time over several hours. Despite their strong hyperfine contact interaction, the few $^{73}$Ge nuclei overlapping with the quantum dot in the barrier do not limit $T_2^*$, as their dynamics is frozen on a few hours measurement scale. We conclude that charge noise and the design of the gradient magnetic field is the key to further improve the qubit fidelity.

quant-ph↗

arXiv2018

Transfer of a quantum state from a photonic qubit to a gate-defined quantum dot

Interconnecting well-functioning, scalable stationary qubits and photonic qubits could substantially advance quantum communication applications and serve to link future quantum processors. Here, we present two protocols for transferring the state of a photonic qubit to a single-spin and to a two-spin qubit hosted in gate-defined quantum dots (GDQD). Both protocols are based on using a localized exciton as intermediary between the photonic and the spin qubit. We use effective Hamiltonian models to describe the hybrid systems formed by the the exciton and the GDQDs and apply simple but realistic noise models to analyze the viability of the proposed protocols. Using realistic parameters, we find that the protocols can be completed with a success probability ranging between 85-97%.

quant-ph↗

arXiv2018

Calculation of tunnel-couplings in open gate-defined disordered quantum dot systems

Quantum computation based on semiconductor electron-spin qubits requires high control of tunnel-couplings, both across quantum dots and between the quantum dot and the reservoir. The tunnel-coupling to the reservoir sets the qubit detection and initialization bandwidth for energy-resolved spin-to-charge conversion and is essential to tune single-electron transistors commonly used as charge detectors. Potential disorder and the increasing complexity of the two-dimensional gate-defined quantum computing devices sets high demands on the gate design and the voltage tuning of the tunnel barriers. We present a Green's formalism approach for the calculation of tunnel-couplings between a quantum dot and a reservoir. Our method takes into account in full detail the two-dimensional electrostatic potential of the quantum dot, the tunnel barrier and reservoir. A Markov approximation is only employed far away from the tunnel barrier region where the density of states is sufficiently large. We calculate the tunnel-coupling including potential disorder effects, which become increasingly important for large-scale silicon-based spin-qubit devices. Studying the tunnel-couplings of a single-electron transistor in Si/SiGe as a showcase, we find that charged defects are the dominant source of disorder leading to variations in the tunnel-coupling of four orders of magnitude.

cond-mat.mes-hall↗

arXiv2018

30 GHz-voltage controlled oscillator operating at 4 K

Solid-state qubit manipulation and read-out fidelities are reaching fault-tolerance, but quantum error correction requires millions of physical qubits and thus a scalable quantum computer architecture. To solve signal-line bandwidth and fan-out problems, microwave sources required for qubit manipulation might be embedded close to the qubit chip, typically operating at temperatures below 4 K. Here, we perform the first low temperature measurements of a 130 nm BiCMOS based SiGe voltage controlled oscillator. The device maintains its functionality from 300 K to 4 K. We determined the dependence of frequency and output power on temperature and magnetic field up to 5 T and measured the temperature influence on noise performance. While the output power tends to increase, the frequency shift is 3 % for temperature and 0.02 % for the field dependence, respectively, both relevant for highly coherent spin qubit applications. We observe no improvement on output noise, but increased output flickering.

quant-ph↗