Silicon quantum processor unit cell operation above one Kelvin (1902.09126v2)

Abstract: Quantum computers are expected to outperform conventional computers for a range of important problems, from molecular simulation to search algorithms, once they can be scaled up to large numbers of quantum bits (qubits), typically millions. For most solid-state qubit technologies, e.g. those using superconducting circuits or semiconductor spins, scaling poses a significant challenge as every additional qubit increases the heat generated, while the cooling power of dilution refrigerators is severely limited at their operating temperature below 100 mK. Here we demonstrate operation of a scalable silicon quantum processor unit cell, comprising two qubits confined to quantum dots (QDs) at $\sim$1.5 Kelvin. We achieve this by isolating the QDs from the electron reservoir, initialising and reading the qubits solely via tunnelling of electrons between the two QDs. We coherently control the qubits using electrically-driven spin resonance (EDSR) in isotopically enriched silicon $^{28}$Si, attaining single-qubit gate fidelities of 98.6% and coherence time $T_2^*$ = 2$\mu$s during `hot' operation, comparable to those of spin qubits in natural silicon at millikelvin temperatures. Furthermore, we show that the unit cell can be operated at magnetic fields as low as 0.1 T, corresponding to a qubit control frequency of 3.5 GHz, where the qubit energy is well below the thermal energy. The unit cell constitutes the core building block of a full-scale silicon quantum computer, and satisfies layout constraints required by error correction architectures. Our work indicates that a spin-based quantum computer could be operated at elevated temperatures in a simple pumped $^4$He system, offering orders of magnitude higher cooling power than dilution refrigerators, potentially enabling classical control electronics to be integrated with the qubit array.

• The paper demonstrates scalable quantum computing potential by operating silicon qubits at 1.5 K with 98.6% gate fidelity.
• The study employs electrically-driven spin resonance with cobalt micromagnets to control quantum dot electron spins without an external reservoir.
• Robust coherence and low-field operation (0.1 T) underscore the integration of classical electronics with quantum processors at elevated temperatures.

Silicon Quantum Processor Unit Cell Operation Above One Kelvin

This paper investigates a critical challenge in scaling quantum computers: operating quantum processors at elevated temperatures. The authors explore the feasability of operating a silicon quantum processor unit cell at approximately 1.5 Kelvin, a marked increase from the sub-100 millikelvin temperatures typically required for most solid-state qubit technologies. The research indicates that leveraging the increased cooling power available at higher temperatures could mitigate some of the fundamental engineering constraints of scaling quantum computers, such as heat dissipation and the integration of classical control electronics.

Key to this approach is the operation of two qubits confined to quantum dots without a tunnel-coupled reservoir for initialization and readout. The authors utilize a configuration where electron spins in silicon quantum dots serve as qubits, controlled via electrically-driven spin resonance (EDSR) with placed cobalt micromagnets. The reported single-qubit gate fidelities reach 98.6%, with coherence times comparable to those previously observed in natural silicon at much lower temperatures. Furthermore, the device can function at magnetic fields as low as 0.1 Tesla, where qubit energy is well below thermal energy, yet coherence and gate fidelities remain robust.

For accurate control and measurement within the isolated QD system, an elementary unit cell layout interests itself in achieving and maintaining specific charge configurations throughout operations. Uniquely, the measured device maintains electron occupation across initialization, control, and readout phases, bypassing the need for an external electron reservoir. This configuration may potentially simplify error-correction architecture and support the construction of large-scale 2D quantum processor arrays.

Practical implications of this paper suggest that higher operating temperatures, achievable using a simple pumped ⁴He system, provide viable alternatives for quantum data processing implementation at a larger scale. While the research demonstrates qubit operation at elevated temperatures exhibiting comparable performance to operations at lower temperatures, the decline in coherence and relaxation times with rising temperatures necessitates further work on noise reduction for optimized operation.

Future directions may involve extending isotopic enrichment to mitigate nuclear spin-induced magnetic noise and pursuing new readout mechanisms that surpass the limitations of traditional single-electron transistor (SET) charge sensing. The integration of superior dispersive readout, for instance, could enhance signal-to-noise ratios and promote efficient readout architectures conducive to scalability.

In conclusion, this research underscores the potential for scalable quantum computing using silicon-based technology at practically feasible temperatures, aligning quantum computation with classical computing systems and moving towards large-scale, functional quantum processors.