Dispersive gate sensing for spin qubit readout in three papers

Short answer: Dispersive gate sensing reads out a semiconductor qubit by watching how the device's own gate electrode changes the response of a resonant circuit, so no separate charge sensor is needed. A 2013 paper benchmarked it against a quantum point contact detector, a 2019 paper used it on a silicon spin qubit device, and another 2019 paper reported single-shot spin readout with 73 percent average fidelity. These are laboratory device results.

Why look for a different readout?

The 2019 Nature Nanotechnology abstract explains the problem. Qubit measurements for algorithms and error detection are ideally made in a single shot, which is presently achieved with on-chip charge sensors capacitively coupled to the quantum dots. As the number of qubits grows, the authors say this approach becomes impractical because of the footprint and complexity of the sensors and the required proximity to the dots. An alternative is to detect the complex impedance of spin-dependent tunnelling between dots using radiofrequency reflectometry on a gate electrode, which would reduce the gate count and architectural complexity. The abstract says that until that work it had not been possible to achieve single-shot spin readout this way.

What did the 2013 paper show?

The 2013 Physical Review Letters paper reports dispersive charge-state readout of a double quantum dot in the few-electron regime, using the in situ gate electrodes as sensitive detectors. The authors benchmark the technique against the established quantum point contact charge detector and find comparable performance, with a bandwidth of about 10 MHz and an equivalent charge sensitivity of about 6.3 x 10^-3 electron charges per root hertz. They say it alleviates the burden of separate charge detectors and promises readout of qubits in scaled-up arrays.

What did the 2019 papers add?

The Nature Communications paper reports gate-coupled radiofrequency reflectometry for dispersive readout of a fully functional spin qubit device. The device is a p-type double-gate transistor made with industry-standard silicon technology: the first gate confines a hole quantum dot that encodes the qubit, and the second confines a helper dot for readout. The qubit state is measured through the phase response of a lumped-element resonator to spin-selective interdot tunnelling. The authors say this scheme needs no coupling to a Fermi reservoir, offering a compact and potentially scalable solution whose operation may be extended above 1 K.

The Nature Nanotechnology paper detects single-electron tunnelling in a double quantum dot and shows that gate-based sensing can read out the electron spin state in a single shot, with an average readout fidelity of 73 percent. The authors describe this as a key step towards reading out many spin qubits in parallel with a compact gate design.

How to read this

The three results use different devices, and a 73 percent single-shot fidelity leaves room for improvement, so it is a step and not a finished scheme. The above-1 K statement is a possibility the authors raise. This post is based on abstracts and visible summary text and full texts were not checked. It makes no claim about products.

FAQ

What is dispersive gate sensing?

Reading a quantum dot's state from the phase or impedance response of a resonant circuit connected to a gate electrode, per the three abstracts.

How did it compare to a quantum point contact?

Comparable performance in the 2013 benchmark, with about 10 MHz bandwidth.

How accurate was single-shot spin readout?

An average fidelity of 73 percent in the 2019 Nature Nanotechnology paper.

Does it work above 1 K?

The 2019 Nature Communications authors say operation may be extended above 1 K. They do not report doing so.