Charge Sensing Spin Qubit Readout

Short answer: Reading out an electron spin in a silicon quantum dot usually means converting the spin state into a charge or tunnelling signal that a sensor can detect in a single shot. Published experiments report readout fidelities from 73% for an early gate-based method in 2019 up to an estimated 99.94% for a double-latching scheme in 2024. These are laboratory results on individual devices, and the fidelity figures are the authors' own.

What the readout has to do

A 2023 Physical Review X paper names three key metrics for readout in quantum processors: measurement speed, fidelity and footprint. It says fast, high-fidelity readout enables mid-circuit measurements, which many dynamic algorithms and quantum error correction need, and that a small footprint helps scalable, highly connected architectures. For spins in silicon quantum dots, single-shot readout has typically been done by converting spin to charge and detecting the charge with an on-chip sensor.

Gate-based sensing, 2019

A 2019 Nature Nanotechnology paper starts from a scaling problem. Conventional charge sensors capacitively coupled to the dots become impractical as qubit numbers grow, because of their footprint, complexity and required proximity to the dots. Radio-frequency reflectometry on a gate electrode that defines the dot itself could cut the gate count, but single-shot spin readout had not been achieved with it. The authors detect single-electron tunnelling in a double quantum dot and report single-shot spin readout with an average fidelity of 73%, which they describe as a key step towards reading many spin qubits in parallel with a compact gate design.

A single-electron box sensor, 2023

The 2023 paper presents two demonstrations of single-shot readout of spins in silicon quantum dots using a radio-frequency single-electron box, a compact dispersive charge sensor. The authors say it needs fewer electrodes than conventional detectors and reaches a spin readout fidelity of 99.2% in under 6 microseconds, fitted from a physical model. They report that low-loss high-impedance resonators strongly coupled to the sensing dot, together with Josephson parametric amplification, were instrumental in this. They also quantify the benefit of Pauli spin blockade over spin-dependent tunnelling to a reservoir for the spin-to-charge conversion, and conclude that dispersive charge sensing is at the forefront of readout methods for scalable semiconductor spin processors.

Double latching, 2024

A 2024 npj Quantum Information paper notes that latching, which maps spin states to metastable charge states with longer lifetimes and a stronger charge signal, has given readout fidelity above 99% in silicon and germanium systems, but that errors still arise from charge-state leakage and spin relaxation. The authors propose a double-latching scheme using two consecutive gate-voltage pulses to enhance the charge signal and suppress both. They estimate a fidelity as high as 99.94% from observed spin-dependent tunnel rates, and describe it as the highest reported for spin qubits. That figure is an estimate and the superlative is the authors' own.

Reading the numbers

The papers use different devices and methods, and some fidelities are fitted or estimated rather than measured directly, so they are not a clean ranking. None of them describes a processor in routine use.

FAQ

What is single-shot spin readout?

Determining a spin state in one measurement, without averaging repeated runs.

Why use gate-based sensing?

The 2019 paper says it can cut the gate count and the footprint of conventional charge sensors.

What fidelity has been reported?

73% in 2019 for gate-based sensing, 99.2% in under 6 microseconds in 2023, and an estimated 99.94% in 2024.

What does double latching add?

Two gate-voltage pulses that enhance the charge signal and suppress spin relaxation and charge-state leakage.

Is this used in commercial processors?

These papers report laboratory experiments only and make no such claim.