Measuring and correcting nanosecond pulse distortions in quantum-dot spin qubits

TL;DR

The paper develops a cryogenic diagnostic for baseband pulse distortions in semiconductor spin qubits by employing detuning-axis pulsed spectroscopy (DAPS) in a silicon double quantum dot. By extracting the device-level step and impulse responses, the authors design finite-impulse-response pre-distortion filters that compensate distortions down to the nanosecond scale and validate the corrections with measurements of singlet-triplet exchange oscillations. The work also provides a physics-based model linking detuning noise, tunnel coupling, and dephasing to the observed dynamics, and demonstrates mitigation of frequency chirp across multiple qubit operations. The resulting approach is scalable, tuning-efficient, and directly characterizes distortion effects at the device level, enabling higher-fidelity spin-qubit control in large-scale quantum-dot processors.

Abstract

Gate-defined semiconductor quantum dots utilize fast electrical control to manipulate spin and charge states of individual electrons. Electrical pulse distortions can limit control fidelities but are difficult to measure at the device level. Here, we use detuning-axis pulsed spectroscopy to characterize baseband pulse distortions in a silicon double quantum-dot. We extract the gate-voltage impulse response and apply a digital pre-distortion filter to eliminate pulse distortions on timescales longer than 1~ns. With the pre-distortion, we reduce the frequency chirp of coherent exchange oscillations in a singlet-triplet qubit. Our results suggest a scalable and tuning-efficient method for characterizing pulse distortions in quantum-dot spin qubits.

Figures (12)

• Figure 1: (a) Experimental setup. A scanning electron microscope image of a device similar to the one used here. Ohmic contacts are indicated with white boxes. The DQD is confined in dots 0 and 1, with plunger gates P$_0$ and P$_1$, respectively, and B$_1$ is the inter-dot tunneling gate. We use a single-electron transistor (SET) for spin and charge readout. P$_0$, P$_1$, and B$_1$ are connected through individual drive lines to room temperature electronics. Each drive line has its own transfer function $\mathcal{H}$, describing the distortions of that line. (b) The ideal square pulse, programmed inside the AWG. (c) Example distorted square pulse, delivered to the DQD.
• Figure 2: (a) Charge stability diagram. The detuning axis is defined along its unit vector $\hat{\epsilon} = (0.74788, -0.66384)$, where $\vec{\epsilon}_x$ and $\vec{\epsilon}_y$ are the horizontal and vertical components, respectively. We define $\epsilon=0$ at the initialization and measurement point, where the three vectors intersect. (b) Energy level diagram of the DQD along the detuning axis. The dashed line in the middle indicates the charge transition boundary. The DAPS pulse sequence, consisting of initialization, evolution, and measurement segments, is illustrated along the time axis on the right. (c) One-electron DAPS data along the horizontal $\vec{\epsilon}_x$ axis. The extracted peaks of the $|g_-\rangle\leftrightarrow|e_+\rangle$ transitions are highlighted. For each horizontal line, we subtract the value at $\epsilon_x = 0$ mV. (d) Horizontal trace of the DAPS data from (c) along the dashed line. (e) Extracted step response function $s(t)$ and (f) impulse response $h(t)$ of the P$_0$ drive line.
• Figure 3: (a) Square-pulse waveform with and without pre-distortion. (b) One-electron DAPS data along the detuning axis $\vec{\epsilon}$ with pre-distortion applied to both plunger gates. The inset shows the result without pre-distortion. (c) Four-electron DAPS along the detuning axis $\vec{\epsilon}$ with pre-distortion. The insert shows the result without pre-distortion.
• Figure 4: (a) Sequence to measure pulsed exchange oscillations in a singlet-triplet qubit. The adiabatic ramp position $\vec{\epsilon}_\text{ad}$ is along the detuning axis $\vec{\epsilon}$ with its amplitude $\epsilon_\text{ad}$. (b), (e) Pulsed exchange oscillations (b) with, and (e) without pre-distortion. (c), (f) Corresponding one-dimensional time traces at $\epsilon = 2$ and $3$ mV. (d), (g) Oscillation frequencies extracted by fitting the data within sliding windows. The horizontal axes represent the initial timestamp $nt_s$ of the sliding windows with $t_s = 1$ ns and $n=0,1,2,\cdots$. The error bars are the mean standard error of the fitted frequencies.
• Figure S1: Device and fridge wiring.