Gate Reflectometry Distinguishes Key Processes for Forming Kitaev Chains in Quantum Dots

The pursuit of stable quantum bits, or qubits, relies on innovative materials and readout techniques, and recent work focuses on harnessing exotic properties of superconducting circuits. Yining Zhang, Ivan Kulesh, and Sebastiaan L. D. ten Haaf, alongside colleagues at QuTech and the Kavli Institute of Nanoscience, Delft University of Technology, demonstrate a powerful new method for probing these qubits using a technique called gate reflectometry. The team successfully applies this fast and non-invasive method to a specially designed system of quantum dots coupled to a superconductor, effectively creating an artificial Kitaev chain, a promising platform for building robust qubits. Their measurements not only resolve key characteristics of the system’s electronic behaviour, distinguishing between different quantum processes, but also reveal a signal directly linked to the switching of quantum parity, offering a significant step towards controlling and reading out these potentially groundbreaking quantum devices.

This work utilizes gate reflectometry to probe two quantum dots coupled via a semiconductor-superconductor hybrid segment, demonstrating the ability to resolve charge stability diagrams and clearly distinguish between elastic cotunneling and crossed-Andreev reflection, the two key processes enabling the formation of a Kitaev chain. Importantly, this information remains accessible even when the system is completely decoupled from normal leads, with the observed quantum capacitance signal serving as an indicator of parity.

Flux-Controlled Kitaev Chain Characterization in Quantum Wells

This supplementary material details the experimental setup, data analysis, and supporting evidence for creating and characterizing a flux-controlled two-site Kitaev chain using InAs/GaSb quantum wells. Researchers fabricated a double quantum dot and coupled it to a superconducting circuit to achieve this, meticulously characterizing the system using microwave resonators and sophisticated data analysis techniques. Key components include high-mobility two-dimensional electron gases, the precisely controlled double quantum dot, and a superconducting circuit designed to induce superconductivity, with external magnetic fields used to tune parameters and radio-frequency capacitance-voltage measurements characterizing charge states.

Detailed analysis of scanning electron microscope images reveals the device fabrication process, illustrating the aluminum loop, quantum well structure, and gate electrodes. Measurements of the Josephson junction’s response to radio frequencies provide information about the superconducting properties, while Coulomb diamonds confirm the formation of well-defined quantum dots and allow estimation of gate voltage sensitivity. The experimental setup, including a vector network analyzer, arbitrary waveform generator, and microwave circuit, is clearly illustrated, demonstrating the flow of signals. Data analysis involved fitting the phase response of resonators to extract key parameters, identifying Coulomb blockade and resonant tunneling, and investigating interactions between the quantum dots and Andreev bound states.

The supplementary material provides a comprehensive overview of the experimental setup, data analysis, and supporting evidence for the creation and characterization of a flux-controlled two-site Kitaev chain, representing a promising platform for topological superconductivity and the search for Majorana zero modes.

Kitaev Chains Probed with Radiofrequency Reflectometry

Researchers have demonstrated a highly sensitive method for probing and controlling quantum dots coupled through a superconducting material, paving the way for more robust quantum computing architectures. The team successfully used radio-frequency gate reflectometry to examine the interactions between these quantum dots, revealing crucial information about electron tunneling. This technique allows for detailed examination of the system without direct electrical contact, which can disrupt delicate quantum states, focusing on creating artificial Kitaev chains, a promising platform for building qubits.

By tuning the system’s properties, they could control the dominant electron interaction, offering precise control over the quantum state. Importantly, the team showed that gate reflectometry can detect subtle changes in the quantum state, even when the system is completely isolated, crucial for protecting qubits from environmental noise. Measurements revealed a clear signal corresponding to changes in the parity, confirming accurate monitoring of the quantum state, with the sensitivity of this method suggesting potential for detecting and correcting errors in quantum computations and creating more complex, scalable architectures.

Kitaev Chain Parity Measured via Reflectometry

This work demonstrates the successful application of gate reflectometry to probe a Kitaev chain device formed by coupling two quantum dots via superconducting elements. Researchers distinguished between elastic cotunneling and crossed-Andreev reflection, even when the system was isolated from external electrical leads. Measurements reveal that the observed capacitance signal reflects changes in the quantum state, specifically switching between even and odd ground states, confirming the method’s ability to capture essential features of interdot coupling and parity dynamics. The team also identified quasiparticle poisoning events as a source of rapid switching in ground state parity, originating from the normal reservoirs connected to the quantum dots, and acknowledge that understanding the mechanisms limiting parity lifetime is crucial for future development.