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

Ran Xue, Max Beer+7 more|arXiv (Cornell University)|Jun 28, 2023

Quantum and electron transport phenomena31 references4 citations

TL;DR

This paper presents a fully electrical Si/SiGe quantum bus (QuBus) that enables high-fidelity, micron-scale single-electron shuttling over 10 µm using only six tunable voltage pulses. The device achieves a record shuttle fidelity of (99.7 ± 0.3)% for a round-trip transport of 19 µm, demonstrating scalable, spin-conserving charge transport for quantum computing with potential for large-scale spin qubit architectures.

ABSTRACT

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.

Motivation & Objective

• To develop a scalable, all-electrical single-electron transport device with memory and micron-scale connectivity for quantum information processing.
• To overcome the limitations of conventional wires in quantum devices, where charge localization is restricted to ~100 nm due to disorder.
• To enable spin-conserving transport of electrons in 28Si/SiGe heterostructures, which are promising for long spin coherence times in quantum computing.
• To demonstrate reliable initialization and detection of up to 34 electrons in a sequence of quantum dots using a minimal set of control signals.
• To introduce a novel characterization method, shuttle tomography, to benchmark local shuttle fidelity and potential imperfections in large-scale quantum devices.

Proposed method

• The QuBus device is fabricated in a Si/SiGe heterostructure with a 10 µm-long, undoped SiGe/Si/SiGe quantum well, forming a one-dimensional electron channel (1DEC) confined by a split-gate.
• Over 100 clavier gates on two metal layers above the 1DEC create movable quantum dots (QDs) via electrostatic gating, enabling conveyor-mode shuttling of single electrons.
• Shuttling is achieved using six control terminals (S1–S4, TLP, TLB1), with voltage pulses applied in sequence to adiabatically transport an electron along the channel.
• A single-electron transistor (SET) at the left end detects the electron’s presence in QD₀ with high spatial and charge resolution.
• Shuttle tomography is introduced as a characterization protocol: by measuring detection fidelity after repeated shuttle cycles, the method isolates loading, detection, and per-step shuttle errors.
• Error analysis uses the relation $ F(n) = (1 - ho_{ ext{LD}}) imes (1 - ho_{ ext{shuttle}})^{2n} $, where $ F(n) $ is the observed fidelity after $ 2n $ shuttle steps, enabling extraction of individual error probabilities.

Experimental results

Research questions

• RQ1 Can high-fidelity, all-electrical single-electron shuttling be achieved over micron-scale distances in Si/SiGe heterostructures?
• RQ2 To what extent does the shuttle fidelity degrade over a 10 µm channel, and can it be maintained at the level required for scalable quantum computing?
• RQ3 How can the local potential disorder and gate imperfections in large-scale quantum devices be probed and quantified using a single-electron probe?
• RQ4 Can a minimal set of six voltage pulses control the initialization, transport, and detection of up to 34 electrons in a sequence of quantum dots?
• RQ5 Does conveyor-mode shuttling in Si/SiGe preserve spin coherence, enabling its use in spin-based quantum computing architectures?

Key findings

• The QuBus achieves a single-electron shuttle fidelity of (99.7 ± 0.3)% for a total transport distance of 19 µm (10 µm forward, 9 µm back), demonstrating high-fidelity long-range charge transport.
• The average error per shuttle step is $ ho_{ ext{shuttle}} = 1 - F_{ ext{step}} = 0.004 ext{%} $, corresponding to a per-step fidelity of $ F_{ ext{step}} = 99.996 ext{%} $ with a 95% confidence interval.
• The loading and detection error $ ho_{ ext{LD}} $ is measured at $ 0.7 ext{%} $, with the detection process being highly reliable, as no false positives were observed in thousands of reference measurements.
• The device supports initialization and detection of up to 34 electrons in a sequence of quantum dots, with arbitrary patterns of single- and zero-electron occupation.
• The use of only six control terminals enables complex multi-electron state preparation and transport, significantly reducing control complexity compared to conventional gate arrays.
• The low spin-environment interaction in 28Si/SiGe, combined with adiabatic transport, suggests that spin qubits can be transported with high fidelity, enabling scalable quantum connectivity.