Charge noise in silicon quantum dot spin qubits

Short answer: Charge traps near the interface of a silicon quantum dot can dephase spin qubits, because two-qubit gates rely on the exchange interaction, which charge noise modulates. A 2018 npj Quantum Information paper finds that tunneling noise can dominate over detuning noise in a controlled-phase gate, and a 2022 paper finds that non-linear coupling of charge noise to exchange is significant. These are theory studies, compared with experiments by their authors.

How does charge noise affect silicon spin qubits?

Spin qubits in silicon quantum dots rely on the exchange interaction for manipulation, which makes them susceptible to charge noise. Charge traps near the interface of a silicon heterostructure cause fluctuations in the quantum-dot confinement and barrier potentials. These fluctuations lead to gating errors and two-spin dephasing. While bulk silicon offers weak hyperfine coupling, the presence of defects such as Pb centers and the oxide layer are well-known sources of charge noise. Even if charge fluctuations do not significantly affect single spins, coupled spins are adversely affected because the exchange interaction is Coulombic in nature.

What types of noise models are used?

Theoretical models distinguish between random telegraph noise from a few traps and 1/f noise from many traps. Random telegraph noise is relevant for dots grown on submicron wafers, while 1/f noise is appropriate for larger wafers suitable for quantum dot arrays. Researchers quantify these effects using realistic noise models to estimate gate errors for single-spin qubit architectures and dephasing in singlet-triplet qubits. The choice of noise model depends on the scale of the fabrication, with larger arrays typically facing different noise characteristics than isolated small-scale devices.

Why is tunneling noise significant in two-qubit gates?

In double quantum dots used for two-qubit controlled-phase gates, spin decoherence can be dominated by tunneling noise from 1/f charge noise rather than detuning noise. This occurs because the effect of tunneling noise on the spin qubit is first order in the charge admixture, whereas the effect of detuning noise is only second order. This difference in order results in different detuning dependence of the decoherence, which provides a method to identify the noise source. The authors report that decoherence in a recent two-qubit experiment was dominated by this tunneling noise, and say this matters for designing spin-qubit operation.

How does non-linear coupling alter decoherence?

The quality of entangling gates is reduced by noise from nearby defects acting as two-level fluctuators. Previous calculations often assumed a Gaussian distribution of exchange interaction frequencies, equivalent to a linear coupling of charge noise with the exchange interaction. However, the actual coupling can differ significantly from this linear approximation depending on inter-qubit tunnel coupling, detuning, and noise magnitude. Analytical expressions for frequency spectra encompassing both linear and non-linear coupling show that decoherence times and decay profiles vary considerably. The authors say that, compared with recent experiments, non-linear charge-noise coupling is significant and requires consideration to characterize and optimize exchange-based entangling gates.

FAQ

Does charge noise always dominate over hyperfine coupling?

No. The papers do not claim that. The 2009 paper asks whether the good spin coherence of bulk silicon survives near an interface, and says defects and the oxide are known sources of charge noise that affect coupled spins through the Coulombic exchange interaction.

Is linear coupling a valid approximation for all noise levels?

No. The coupling can differ significantly from the linear-coupling approximation depending on inter-qubit tunnel coupling, detuning, and the magnitude of charge noise. Non-linear effects are significant and must be considered to accurately characterize exchange-based entangling gates.

How can researchers identify the source of decoherence?

By observing the detuning dependence of the decoherence. Tunneling noise has a first-order effect on charge admixture, while detuning noise has a second-order effect. This difference in order creates distinct detuning dependencies that allow identification of the noise source.

Are these effects limited to specific wafer sizes?

The 2009 paper links noise types to fabrication scales. Random telegraph noise from few traps is relevant for submicron wafers, while 1/f noise from many traps is appropriate for larger wafers suitable for quantum dot arrays.

What is the primary mechanism of decoherence in exchange gates?

In the models discussed, the main mechanism is the modulation of the exchange interaction by charge noise. Because the exchange interaction is Coulombic in nature, fluctuations in confinement and barrier potentials from charge traps induce dephasing and gating errors in coupled spin systems.