Fast Gate-Based Readout of Silicon Quantum Dots Using Josephson Parametric Amplification.

Spins in silicon quantum devices are promising candidates for large-scale quantum computing. Gate-based sensing of spin qubits offers a compact and scalable readout with high fidelity, however, further improvements in sensitivity are required to meet the fidelity thresholds and measurement timescales needed for the implementation of fast feedback in error correction protocols. Here, we combine radio-frequency gate-based sensing at 622 MHz with a Josephson parametric amplifier, that operates in the 500-800 MHz band, to reduce the integration time required to read the state of a silicon double quantum dot formed in a nanowire transistor. Based on our achieved signal-to-noise ratio, we estimate that singlet-triplet single-shot readout with an average fidelity of 99.7% could be performed in 1 µs, well below the requirements for fault-tolerant readout and 30 times faster than without the Josephson parametric amplifier. Additionally, the Josephson parametric amplifier allows operation at a lower radio-frequency power while maintaining identical signal-to-noise ratio. We determine a noise temperature of 200 mK with a contribution from the Josephson parametric amplifier (25%), cryogenic amplifier (25%) and the resonator (50%), showing routes to further increase the readout speed.

Dispersively Detected Pauli Spin-Blockade in a Silicon Nanowire Field-Effect Transistor.

We report the dispersive readout of the spin state of a double quantum dot formed at the corner states of a silicon nanowire field-effect transistor. Two face-to-face top-gate electrodes allow us to independently tune the charge occupation of the quantum dot system down to the few-electron limit. We measure the charge stability of the double quantum dot in DC transport as well as dispersively via in situ gate-based radio frequency reflectometry, where one top-gate electrode is connected to a resonator. The latter removes the need for external charge sensors in quantum computing architectures and provides a compact way to readout the dispersive shift caused by changes in the quantum capacitance during inter-dot charge transitions. Here, we observe Pauli spin-blockade in the high-frequency response of the circuit at finite magnetic fields between singlet and triplet states. The blockade is lifted at higher magnetic fields when intra-dot triplet states become the ground state configuration. A line shape analysis of the dispersive phase shift reveals furthermore an intra-dot valley-orbit splitting Δvo of 145 µeV. Our results open up the possibility to operate compact complementary metal-oxide semiconductor (CMOS) technology as a singlet-triplet qubit and make split-gate silicon nanowire architectures an ideal candidate for the study of spin dynamics.

Hybrid optical-electrical detection of donor electron spins with bound excitons in silicon.

Electrical detection of spins is an essential tool for understanding the dynamics of spins, with applications ranging from optoelectronics and spintronics, to quantum information processing. For electron spins bound to donors in silicon, bulk electrically detected magnetic resonance has relied on coupling to spin readout partners such as paramagnetic defects or conduction electrons, which fundamentally limits spin coherence times. Here we demonstrate electrical detection of donor electron spin resonance in an ensemble by transport through a silicon device, using optically driven donor-bound exciton transitions. We measure electron spin Rabi oscillations, and obtain long electron spin coherence times, limited only by the donor concentration. We also experimentally address critical issues such as non-resonant excitation, strain, and electric fields, laying the foundations for realizing a single-spin readout method with relaxed magnetic field and temperature requirements compared with spin-dependent tunnelling, enabling donor-based technologies such as quantum sensing.

Probing the limits of gate-based charge sensing.

Quantum computation requires a qubit-specific measurement capability to readout the final state of individual qubits. Promising solid-state architectures use external readout electrometers but these can be replaced by a more compact readout element, an in situ gate sensor. Gate-sensing couples the qubit to a resonant circuit via a gate and probes the qubit's radiofrequency polarizability. Here we investigate the ultimate performance of such a resonant readout scheme and the noise sources that limit its operation. We find a charge sensitivity of 37 µe Hz(-1/2), the best value reported for this technique, using the example of a gate sensor strongly coupled to a double quantum dot at the corner states of a silicon nanowire transistor. We discuss the experimental factors limiting gate detection and highlight ways to optimize its sensitivity. In total, resonant gate-based readout has advantages over external electrometers both in terms of reduction of circuit elements as well as absolute charge sensitivity.

An exchange-coupled donor molecule in silicon.

We present a combined experimental-theoretical demonstration of the energy spectrum and exchange coupling of an isolated donor pair in a silicon nanotransistor. The molecular hybridization of the atomic orbitals leads to an enhancement of the one- and two-electron binding energies and charging energy with respect to the single donor case, a desirable feature for quantum electronic devices. Our hydrogen molecule-like model based on a multivalley central-cell corrected effective mass theory incorporating a full configuration interaction treatment of the 2-electron spectrum matches the measured data for an arsenic diatomic molecule with interatomic distance R = 2.3 ± 0.5 nm.