Single-shot latched readout of a quantum dot qubit using barrier gate pulsing

Abstract

Latching techniques are widely used to enhance readout of qubits. These methods require precise tuning of multiple tunnel rates, which can be challenging to achieve under realistic experimental conditions, such as when a qubit is coupled to a single reservoir. Here, we present a method for single-shot measurement of a quantum dot qubit with a single reservoir using a latched-readout scheme. Our approach involves pulsing a barrier gate to dynamically control qubit-to-reservoir tunnel rates, a method that is readily applicable to the latched readout of various spin-based qubits. We use this method to enable qubit state latching and to reduce the qubit reset time in measurements of coherent Larmor oscillations of a Si/SiGe quantum dot hybrid qubit.

Figures (2)

• Figure 1: Device layout and latched readout mechanism for the quantum dot hybrid qubit (QDHQ). a False-colored SEM micrograph of a device lithographically identical to the one under study. b Energy dispersion diagram for the 5-electron QDHQ. The latched readout mechanism via the (4,2) charge state is represented in the gray region. $\Gamma_L$ and $\Gamma_R$ are the tunnel rates of the P1 and P2 dots, respectively. $E^*_{\text{orb}}$ is the multi-electron orbital splitting of the P1 dot, and $E_{ST}$ is the single-triplet splitting of the P2 dot. $2\Delta_1$, $2\Delta_2$, and $2\Delta_3$ are the splittings at the anti-crossings between the (4,1) and (3,2) charge states. c Latched readout window of the QDHQ on the charge stability diagram, enclosed by dashed lines A and B. Line A extends from the (4,2)-(3,2) transition line, and line B is energetically positioned $E^*_{\text{orb}}$ apart from the polarization line. d Latching signal within the readout window, acquired with time-averaged measurement using a lock-in amplifier while applying a triangular latching pulse (inset) to gate P1.
• Figure 2: A barrier gate pulsing scheme for single-shot latched readout and coherent Larmor oscillations of the QDHQ. a Diagram of the manipulation and readout process of the logical state $\ket{1}$ during the Larmor experiments, with the detunings are exaggerated for visual clarity. b Pulse shapes applied to gates P1 and B1 throughout the experiment stages (i) to (vi), as outlined in a. Note that the Larmor pulse in stages (iii) and (iv) is not to scale for demonstration purposes. c Pulsed-gate crosstalk measurement for gates P1 and B1. The arrow marks the point where the ground state (GS) electron-loading line intersects with the electron-unloading line, indicating that the effect of the P1 pulse on the QDHQ is fully canceled by the B1 pulse. d Example charge sensor time traces. The qubit state is identified as $\ket{1}$ when latching occurs and its signal trace surpasses a threshold. e Probability density plot for qubit latching, measured during qubit readout stage. Two peaks for logical states $\ket{0}$ and $\ket{1}$ are well separated with signal-to-noise ratio (SNR) of 10.2 and charge sensitivity of 3.10$\times 10^{-3} \text{e}/\sqrt{\text{Hz}}$. f Larmor oscillations of the QDHQ. Qubit parameters are extracted from its fast Fourier transform (FFT) displayed at the bottom: $\Delta_1 \approx$ 750 MHz and $E_{\text{ST}} \approx$ 4.0 GHz. The curve shown in the FFT data aligns with the energy splitting of the QDHQ (white dashed line). g Probability density plot for qubit reset, measured 2 ms after qubit readout. The initialization probabilities are 98$\%$ with the reset pulse and 80$\%$ without any reset pulse (1000 iterations each).