In an article recently published in Nature Communications, researchers demonstrated coherent shuttling of spin qubits through germanium (Ge) quantum dots.
Study: Coherent Spin Qubit Shuttling in Germanium Quantum Dots. Image Credit: Production Perig/Shutterstock.com
Importance of Coherent Shuttling
The leading approach for realizing fault-tolerant quantum computation using semiconductor spin qubits is based on the quantum network concept, where quantum links interconnect the qubit registers. Specifically, the inclusion of mid-range and short-range quantum links could effectively establish qubit connectivity, addressability, and scalability.
In spin qubit devices, the coherent shuttling of hole or electron spins is a promising concept for integrating such quantum links. Short-range coupling can provide local addressability and flexible qubit routing. It is implemented by shuttling a spin qubit in an array through quantum dots.
By shuttling spins through several quantum dots, mid-range links are implemented. These links allow qubit operations at dedicated locations and entangle distant qubit registers for networked computing. The potential of the shuttling-based quantum buses has stimulated studies on shuttling electron charge and spin.
Networked quantum computers will require qubit shuttling and control integration through quantum dot chains, incorporating quantum dots that possess at least two neighbors. Quantum dots in strained Ge heterostructures can serve as a suitable platform for hole spin qubits.
The platform's high quality has allowed rapid development of singlet-triplet qubits and single-spin qubits. Although the robust spin-orbit interaction enables all-electrical and fast control, the resultant anisotropic g-tensor can challenge a quantum bus's feasibility.
The Study
In this study, researchers aimed to demonstrate that a spin qubit could be shuttled through several quantum dots while preserving its quantum information. They used hole spin qubits in Ge to achieve these results despite the existence of strong spin-orbit interaction.
Additionally, researchers operated in a regime where they could implement single-qubit logic and transfer spin qubits coherently through an intermediate quantum dot. The shuttling experiments were performed in a 2 × 2 Ge quantum dot array using two-hole spin qubits.
The device was fabricated on a strained Ge/silicon (Si)Ge heterostructure grown using chemical vapor deposition and composed of a less than 1-nm thick Si cap, a 55 nm Si₀.₂Ge₀.₈ spacer layer, a strained 16 nm Ge quantum well, a 500 nm relaxed Si₀.₂Ge₀.₈ layer, a 1-μm step graded Si₁₋ₓGeₓ (x =1 to 0.8) layer, and a 1.6-μm thick relaxed Ge layer.
30 nm of aluminum was deposited on the heterostructure after oxidized Si cap etching to fabricate the contacts to the quantum well. A 7 nm aluminum oxide layer was deposited using atomic layer deposition to isolate the contacts from the gate electrodes. The gates were defined by Ti/Pd bilayer deposition.
Waiting times up to 10 ns were included on both sides of every microwave pulse in the shuttling experiments. These waiting times were short compared to the qubit coherence times and microwave pulse times. Moreover, experiments with sub-nanosecond time resolution and precise voltage pulses mitigated the spin-orbit interaction-induced finite qubit rotations.
Significance of this Study
Researchers successfully displayed coherent spin qubit shuttling through Ge quantum dots. Although holes in Ge offered challenges owing to an anisotropic g-tensor, the spin basis states could be shuttled n* = 2230 times, while the coherent states could be shuttled up to n* = 67 times and n* = 350 times while using echo pulses.
The high uniformity and small effective mass of strained Ge allowed a relatively large quantum dot spacing of 140 nm, which leads to effective length scales for coherent shuttling of lcoh = 9 μm and for shuttling basis states of lspin = 312 μm. Moreover, the effective length scale increased to lcoh = 49 μm by including echo pulses. These results favorably compared to effective lengths obtained in Si.
However, extrapolating the shuttling experiment performance over a few sites to predict the practical shuttling links' performance requires caution, as quantum dot chains that allow coupling spin qubits over considerable length scales will put greater demands on uniformity, tuning, and the ability to tune all couplings, increasing the challenges of shuttling optimization.
To summarize, the findings of this study demonstrated the feasibility of the qubit shuttling approach to route qubits within registers and to establish quantum links between registers. The findings also paved the way to conveyor-mode shuttling in Ge, where qubits will be coherently displaced using shared gate electrodes in propagating potential wells. Additionally, integrating quantum links based on spin qubits and shuttling in quantum circuits can advance the development of networked quantum computing.
Journal Reference
Wang, C., et al. (2024). Coherent spin qubit shuttling through germanium quantum dots. Nature Communications. DOI: 10.1038/s41467-024-49358-y, https://www.nature.com/articles/s41467-024-49358-y
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