Interfacing spin qubits in quantum dots and donors - hot, dense and coherent (1612.05936v1)

Abstract: Semiconductor spins are one of the few qubit realizations that remain a serious candidate for the implementation of large-scale quantum circuits. Excellent scalability is often argued for spin qubits defined by lithography and controlled via electrical signals, based on the success of conventional semiconductor integrated circuits. However, the wiring and interconnect requirements for quantum circuits are completely different from those for classical circuits, as individual DC, pulsed and in some cases microwave control signals need to be routed from external sources to every qubit. This is further complicated by the requirement that these spin qubits currently operate at temperatures below 100 mK. Here we review several strategies that are considered to address this crucial challenge in scaling quantum circuits based on electron spin qubits. Key assets of spin qubits include the potential to operate at 1 to 4 K, the high density of quantum dots or donors combined with possibilities to space them apart as needed, the extremely long spin coherence times, and the rich options for integration with classical electronics based on the same technology.

• The paper demonstrates the integration challenges of routing precise control signals to spin qubits while outlining scalable wiring solutions.
• The paper details methodologies using sub-nanosecond pulse dynamics and advanced microwave control to ensure high qubit manipulation and readout fidelity.
• The paper explores strategies for operating spin qubits at elevated temperatures (1–4 K) to enhance quantum-classical interfacing and practical deployment.

Interfacing Spin Qubits in Quantum Dots and Donors: Operational Challenges and Prospective Solutions

The paper by Vandersypen et al. addresses critical challenges and proposes potential solutions for interfacing spin qubits in semiconductor quantum dots and donors. As spin qubits are promising candidates for scalable quantum computation, primarily due to their lithographic definition and electrical signal control analogous to classical semiconductor devices, their integration poses distinct challenges in wiring and interconnects, differing significantly from classical circuits. Importantly, scaling quantum circuits demands operational temperatures under 100 mK, but with the potential to extend to a few Kelvin under certain conditions.

Spin qubits in quantum dots and donors benefit from semiconductor technology's scalability, long coherence times, and integration potential with classical electronics. However, the crux of scalability lies in efficiently routing multiple control signals to each qubit, necessary for precision computation and error correction. As the paper outlines, effective quantum-classical interfacing is essential due to quantum bits' fragility and the necessity of continual error-checking and correction. It projects that scaling quantum processors could require between $10^6$ to $10^8$ qubits, a scale comparable to current transistor technologies, yet manageable within practical input-output limits.

Spin Qubit Architecture

Quantum dot and donor spin qubit systems exhibit robust scalability facilitated by sizeable valley-orbit splitting, which is crucial for managing electron spins at elevated temperatures. The canonical encoding employs electron spins $|#1{\uparrow}$ and$|#1{\downarrow}$, utilizing static magnetic fields for qubit definition with potential alternative encodings in multi-electron spin states. The paper extensively discusses the pulse dynamics, necessitating sub-nanosecond precision and tailored microwave fields for effective qubit manipulation, emphasizing the complex coordination required across qubits.

Interfacing and Readout Technological Considerations

Addressing multi-qubit systems hinges on resolving specific control challenges. Notably, the precision in control signals, initialization, and read-out fidelity is foundational. The dense array poses significant fan-out challenges, which can potentially be mitigated by cross-bar addressing, optimizing charge storage based on concepts borrowed from DRAM technology. This requires capacitors of >160 fF per gate to ensure stability against thermal noise, which can undermine gate voltage accuracy.

Alternatively, sparse arrays utilizing distant qubit coupling through capacitors, resonators, or microwave shuttles can allow broader integration with classical electronics. This configuration permits increased flexibility and potential integration of classical processing closer to the qubit plane, alleviating traditional wiring constraints. However, these solutions necessitate advanced multiplexing and novel approaches to efficiently downscale data communication to manageable rates.

Towards Hot Qubit Operation

A significant advancement discussed is the potential operation of spin qubits at 1-4 K, which could tremendously simplify quantum-classical integration within commercially viable cooling systems. Higher temperature operations would leverage robust energy scales within Si-MOS-based quantum dots, benefiting from enhanced initialization and read-out capabilities inherent in strongly confined electronic states. However, such advancements necessitate further exploration into qubit decoherence dynamics, especially concerning hyperfine interactions and noise-inducing mechanisms at these temperatures.

Implications and Future Outlook

The discussions and analyses presented suggest multiple feasible pathways to scale semiconductor spin qubits to levels necessary for practical quantum computing, particularly benefiting from synchronous advancements in semiconductor technology. Innovations in quantum-classical interfaces, coupled with the operational transition to higher temperatures, underscore the potential to overcome prevailing scalability challenges. Destined for further validation, these approaches may hold the key to realizing large-scale, universally programmable quantum processors driven by integrated spin qubit arrays. As the field progresses, strategic pageantry between electronics miniaturization and quantum coherence maintenance remains a quintessential scientific and engineering pursuit.