Luca Petit

We investigate hole spin relaxation in the single- and multi-hole regime in a 2x2 germanium quantum dot array. We find spin relaxation times T₁ as high as 32 ms and 1.2 ms for quantum dots with single- and five-hole occupations respectively, setting benchmarks for spin relaxation times for hole quantum dots. Furthermore, we investigate qubit addressability and electric field sensitivity by measuring resonance frequency dependence of each qubit on gate voltages. We can tune the resonance frequency over a large range for both single and multi-hole qubits, while simultaneously finding that the resonance frequencies are only weakly dependent on neighbouring gates. In particular, the five-hole qubit resonance frequency is more than twenty times as sensitive to its corresponding plunger gate. Excellent individual qubit tunability and long spin relaxation times make holes in germanium promising for addressable and high-fidelity spin qubits in dense two-dimensional quantum dot arrays for large-scale quantum information.

Quantum computation requires many qubits that can be coherently controlled and coupled to each other¹. Qubits that are defined using lithographic techniques have been suggested to enable the development of scalable quantum systems because they can be implemented using semiconductor fabrication technology^2,3,4,5. However, leading solid-state approaches function only at temperatures below 100 millikelvin, where cooling power is extremely limited, and this severely affects the prospects of practical quantum computation. Recent studies of electron spins in silicon have made progress towards a platform that can be operated at higher temperatures by demonstrating long spin lifetimes⁶, gate-based spin readout⁷ and coherent single-spin control⁸. However, a high-temperature two-qubit logic gate has not yet been demonstrated. Here we show that silicon quantum dots can have sufficient thermal robustness to enable the execution of a universal gate set at temperatures greater than one kelvin. We obtain single-qubit control via electron spin resonance and readout using Pauli spin blockade. In addition, we show individual coherent control of two qubits and measure single-qubit fidelities of up to 99.3 per cent. We demonstrate the tunability of the exchange interaction between the two spins from 0.5 to 18 megahertz and use it to execute coherent two-qubit controlled rotations. The demonstration of ‘hot’ and universal quantum logic in a semiconductor platform paves the way for quantum integrated circuits that host both the quantum hardware and its control circuitry on the same chip, providing a scalable approach towards practical quantum information processing.

Extremely long coherence times, excellent single-qubit gate fidelities, and two-qubit logic have been demonstrated with silicon metal-oxide-semiconductor spin qubits, making it one of the leading platforms for quantum information processing. Despite this, a long-standing challenge in this system has been the demonstration of tunable tunnel coupling between single electrons. Here we overcome this hurdle with gate-defined quantum dots and show couplings that can be tuned on and off for quantum operations. We use charge sensing to discriminate between the (2,0) and (1,1) charge states of a double quantum dot and show excellent charge sensitivity. We demonstrate tunable coupling up to 13 GHz, obtained by fitting charge polarization lines, and tunable tunnel rates down to <1 Hz, deduced from the random telegraph signal. The demonstration of tunable coupling between single electrons in a silicon metal-oxide-semiconductor device provides significant scope for high-fidelity two-qubit logic toward quantum information processing with standard manufacturing.

The abrupt metal-insulator transition in vanadium dioxide (VO₂) offers novel performance and functionality for beyond CMOS switches, enabling simultaneous high ON current and ultra-steep subthreshold slope with low temperature dependence. We developed a field-enhanced design of 2-terminal VO₂ switches that allows decreasing their actuation voltage without affecting their performance and reliability. Exploiting this design, we characterized VO₂ switches with extremely abrupt transitions (<; 1 mV/dec) until 60°C and a reduction in actuation voltage up to 38.3% with respect to conventional devices.