Silicon Chip Controls Four Electrons for Quantum Computing Advance

Mathieu Darnas and colleagues at CNRS and CEA-Leti and imec have created a multiplexed cryo-CMOS circuit that reliably controls a silicon double quantum dot at 0.5K. The circuit confirms compatibility between sample-and-hold multiplexing, a technique reducing wiring complexity, and the demanding stability and speed requirements of quantum dot operation. Achieving deterministic control and stable access to multiple charge configurations, alongside the resolution of single-electron tunneling, provides a key step towards scalable cryogenic control architectures for future large-scale spin-qubit processors.

Deterministic control unlocks all charge states in a silicon double quantum dot

Stable access to all five charge configurations within a silicon double quantum dot has been achieved, a strong improvement over prior systems limited to fewer configurations or lacking pulsing capabilities. Double quantum dots are nanoscale structures confining electrons, and their charge state, the number of electrons present, dictates their quantum behaviour and serves as a fundamental unit of quantum information. Previous attempts at controlling these dots often struggled to reliably access and maintain all necessary charge states, hindering complex quantum operations. This new research demonstrates deterministic loading and isolation of four electrons, despite sequentially refreshing voltages, a feat previously unattainable due to the stringent stability and speed demands of quantum dot operation. The ability to precisely control the number of electrons is crucial for defining and manipulating the quantum state of the qubits encoded within the double dot. This is achieved through careful tuning of the voltages applied to the dot’s gates, which modulate the potential landscape and control electron flow. Exploiting an isolated regime, single-electron tunneling events were resolved, validating the compatibility of sample-and-hold multiplexed control with both static biasing and dynamic pulsing of these isolated quantum dots. Single-electron tunneling, the quantum mechanical phenomenon where an electron passes through a barrier even without sufficient classical energy, is a sensitive indicator of the control system’s precision and stability, confirming the ability to resolve individual electron movements.

A cryo-CMOS circuit maintains voltages with a drift of only 5.5 μV/s, potentially enabling bias of over 10 million independent gates from a single input. Conventional cryogenic control systems require a dedicated voltage line for each gate controlling a quantum dot, leading to a significant wiring bottleneck as the number of qubits increases. This new circuit employs sample-and-hold (SH) multiplexing, a technique where a single input line is sequentially connected to multiple gates via switches. The cryo-CMOS circuit, fabricated using complementary metal-oxide-semiconductor technology optimised for cryogenic temperatures, maintains voltages with a drift of only 5.5 μV/s. This exceptional stability is achieved through careful circuit design and optimisation for minimal noise and drift at 0.5K. The low voltage drift, coupled with the multiplexing scheme, theoretically allows biasing of over 10 million independent gates using a single input and a 1MHz clock, while maintaining a maximum voltage error below 100 μV. This represents a substantial reduction in wiring complexity and thermal load, crucial for scaling up quantum processors. The increased parasitic capacitance within the circuit contributes to this five-fold improvement in stability compared to earlier designs. Parasitic capacitance acts as a charge reservoir, smoothing out voltage fluctuations and enhancing stability. Further investigation focused on the circuit’s ability to apply detuning pulses with a response time shorter than 1ms. The impact of multiplexed control on spin coherence or gate fidelity remains a significant hurdle to practical quantum computation. Detuning pulses are used to precisely control the energy levels within the quantum dot, enabling qubit manipulation and gate operations.

Coherence limitations and potential impacts of sequential voltage refreshing

A key question remains regarding the impact of this multiplexing approach on coherence, while elegantly addressing the wiring bottleneck inherent in scaling up quantum dot arrays. Quantum coherence, the ability of a qubit to exist in a superposition of states, is fundamental to quantum computation. Long coherence times, the duration quantum information persists, are vital for complex calculations, and any disruption caused by the process could severely limit the potential of this architecture. The sequential nature of voltage refreshing in the multiplexed system introduces a potential source of decoherence. Each time a gate voltage is switched, it may introduce noise or disturbances that disrupt the delicate quantum state of the qubit. Understanding and mitigating these effects is crucial for realising practical quantum computers. The current demonstration does not yet offer insight into how sequentially refreshing voltages affects the delicate quantum states within the dot. Detailed characterisation of coherence times under multiplexed control is necessary to assess the viability of this architecture for complex quantum algorithms.

Despite acknowledged concerns about potential coherence disruption, this demonstration of stable multiplexed control remains significant. The ability to reliably bias and pulse a double quantum dot, accessing all charge configurations, validates the architecture for scaling up complex quantum processors. This directly addresses the practical challenge of wiring density in cryogenic systems, reducing the number of physical connections to simplify fabrication and lower thermal load, important for building larger, more powerful quantum computers. Reducing the thermal load is particularly important as the heat generated by wiring can significantly degrade the performance of quantum devices. This advance paves the way for more densely packed and scalable quantum circuits. The reduction in wiring complexity also simplifies the fabrication process, lowering manufacturing costs and improving yield.

Reliable control of a silicon double quantum dot at 0.5 Kelvin has been demonstrated using a multiplexed cryo-CMOS circuit, establishing a pathway towards scaling up quantum processors. Sequential voltage refreshing allowed the circuit to access all charge configurations within the dot, essential for manipulating quantum information. This advance overcomes limitations imposed by complex wiring and heat, opening questions regarding the impact of this dynamic control on maintaining long quantum coherence times, a crucial factor for complex calculations. Future research will focus on characterising coherence properties under multiplexed control and optimising the circuit design to minimise noise and maximise qubit fidelity, bringing scalable quantum computing closer to reality. The development of robust error correction techniques will also be essential to mitigate the effects of decoherence and ensure reliable quantum computation.

Researchers successfully demonstrated reliable control of a silicon double quantum dot at 0.5 Kelvin using a multiplexed cryo-CMOS circuit. This is important because it allows access to all charge configurations within the dot while reducing the complexity of wiring needed for cryogenic systems. The findings confirm that this method of dynamic voltage refreshing is compatible with biasing and pulsing quantum dots, addressing a key challenge in building larger quantum processors. Further work will focus on characterising coherence properties to optimise the circuit design and improve qubit fidelity.