arXiv subjects

JJ Nelson

Publications and source records attributed to JJ Nelson.

6 recordsLinked to original sources

arXiv2022

Charge-noise spectroscopy of Si/SiGe quantum dots via dynamically-decoupled exchange oscillations

Electron spins in silicon quantum dots are promising qubits due to their long coherence times, scalable fabrication, and potential for all-electrical control. However, charge noise in the host semiconductor presents a major obstacle to achieving high-fidelity single- and two-qubit gates in these devices. In this work, we measure the charge-noise spectrum of a Si/SiGe singlet-triplet qubit over nearly 12 decades in frequency using a combination of methods, including dynamically-decoupled exchange oscillations with up to 512 π pulses during the qubit evolution. The charge noise is colored across the entire frequency range of our measurements, although the spectral exponent changes with frequency. Moreover, the charge-noise spectrum inferred from conductance measurements of a proximal sensor quantum dot agrees with that inferred from coherent oscillations of the singlet-triplet qubit, suggesting that simple transport measurements can accurately characterize the charge noise over a wide frequency range in Si/SiGe quantum dots.

cond-mat.mes-hall↗

arXiv2020

Low-frequency charge noise in Si/SiGe quantum dots

Electron spins in silicon have long coherence times and are a promising qubit platform. However, electric field noise in semiconductors poses a challenge for most single- and multi-qubit operations in quantum-dot spin qubits. Here, we investigate the dependence of low-frequency charge noise spectra on temperature and aluminum-oxide gate dielectric thickness in Si/SiGe quantum dots with overlapping gates. We find that charge noise increases with aluminum oxide thickness. We also find strong dot-to-dot variations in the temperature dependence of the noise magnitude and spectrum. These findings suggest that each quantum dot experiences noise caused by a distinct ensemble of two-level systems, each of which has a non-uniform distribution of thermal activation energies. Taken together, our results suggest that charge noise in Si/SiGe quantum dots originates at least in part from a non-uniform distribution of two-level systems near the surface of the semiconductor.

cond-mat.mes-hall↗

arXiv2020

Rapid high-fidelity spin state readout in Si/SiGe quantum dots via radio-frequency reflectometry

Silicon spin qubits show great promise as a scalable qubit platform for fault-tolerant quantum computing. However, fast high-fidelity readout of charge and spin states, which is required for quantum error correction, has remained elusive. Radio-frequency reflectometry enables rapid high-fidelity readout of GaAs spin qubits, but the relatively large resistances and capacitances of accumulation-mode Si quantum dot devices have made radio-frequency reflectometry challenging in these platforms. In this work, we implement radio-frequency reflectometry in a Si/SiGe quantum dot device with overlapping gates by making minor device-level changes that eliminate these challenges. We demonstrate charge state readout with a fidelity above 99.9% in an integration time of 300 ns. We measure the singlet and triplet states of a double quantum dot via both conventional Pauli spin blockade and a charge latching mechanism, and we achieve maximum fidelities of 82.9% and 99.0% in 2.08 $μ$s and 1.6 $μ$s integration times, respectively. We also use radio-frequency reflectometry to perform single-shot readout of single-spin states via spin-selective tunneling in microsecond-scale integration times.

cond-mat.mes-hall↗

arXiv2019

Infinite-randomness fixed point of the quantum superconductor-metal transitions in amorphous thin films

The magnetic-field-tuned quantum superconductor-insulator transitions of disordered amorphous indium oxide films are a paradigm in the study of quantum phase transitions, and exhibit power-law scaling behavior. For superconducting indium oxide films with low disorder, such as the ones reported on here, the high-field state appears to be a quantum-corrected metal. Resistance data across the superconductor-metal transition in these films are shown here to obey an activated scaling form appropriate to a quantum phase transition controlled by an infinite randomness fixed point in the universality class of the random transverse-field Ising model. Collapse of the field-dependent resistance vs. temperature data is obtained using an activated scaling form appropriate to this universality class, using values determined through a modified form of power-law scaling analysis. This exotic behavior of films exhibiting a superconductor-metal transition is caused by the dissipative dynamics of superconducting rare regions immersed in a metallic matrix, as predicted by a recent renormalization group theory. The smeared crossing points of isotherms observed are due to corrections to scaling which are expected near an infinite randomness critical point, where the inverse disorder strength acts as an irrelevant scaling variable.

cond-mat.supr-con↗

arXiv2018

Digital coherent control of a superconducting qubit

High-fidelity gate operations are essential to the realization of a fault-tolerant quantum computer. In addition, the physical resources required to implement gates must scale efficiently with system size. A longstanding goal of the superconducting qubit community is the tight integration of a superconducting quantum circuit with a proximal classical cryogenic control system. Here we implement coherent control of a superconducting transmon qubit using a Single Flux Quantum (SFQ) pulse driver cofabricated on the qubit chip. The pulse driver delivers trains of quantized flux pulses to the qubit through a weak capacitive coupling; coherent rotations of the qubit state are realized when the pulse-to-pulse timing is matched to a multiple of the qubit oscillation period. We measure the fidelity of SFQ-based gates to be ~95% using interleaved randomized benchmarking. Gate fidelities are limited by quasiparticle generation in the dissipative SFQ driver. We characterize the dissipative and dispersive contributions of the quasiparticle admittance and discuss mitigation strategies to suppress quasiparticle poisoning. These results open the door to integration of large-scale superconducting qubit arrays with SFQ control elements for low-latency feedback and stabilization.

quant-ph↗

arXiv2015

Metallic State of Low Mobility Silicon at High Carrier density induced by an Ionic Liquid

High mobility and dilute two-dimensional electron systems exhibit metallic behavior down to the lowest experimental temperatures. In studies of ionic liquid gated insulating silicon, we have observed transitions to a metallic state in low mobility samples at much higher areal carrier densities than found for samples of high mobility. We have also observed a mobility peak in metallic samples as the carrier density was increased beyond $10^{13} \text{cm}^{-2}$.

cond-mat.mes-hall↗