Researchers Achieve Deterministic Parity Control in Andreev Molecules with 3 Regimes

Researchers are tackling a fundamental challenge in building robust quantum computers: controlling and reading the parity of electron states within complex superconducting circuits. Shang Zhu, Xiaozhou Yang, and Mingli Liu, alongside colleagues from the Beijing National Laboratory for Condensed Matter Physics, have now demonstrated deterministic, non-local control of this parity in an ‘Andreev molecule’ , a novel hybrid structure , offering a significant step towards scalable quantum devices. Their work establishes a validated framework for ‘parity engineering’, crucially bypassing the limitations of local control methods and eliminating the need for complex, additional sensors by utilising supercurrent as an intrinsic probe. This breakthrough, detailed through careful experimentation and supported by theoretical modelling, unlocks new possibilities for designing and manipulating multi-quantum dot superconducting architectures, paving the way for more powerful and reliable quantum technologies.

This breakthrough, detailed through careful experimentation and supported by theoretical modelling, unlocks new possibilities for designing and manipulating multi-quantum dot superconducting architectures, paving the way for more powerful and reliable quantum technologies. Researchers are0.1(b).

The device was measured in a dilution refrigerator at a base temperature of ~ 40 mK. The measurement configuration is as follows: the tunneling spectrum of QD1 is acquired while tuning the gate voltage V. Figure 1(d) displays I-V curves for different values of V indicated by the vertical bars in Fig0.1(f), showing only positive-current portions. The corresponding differential resistance (dV/dI) curves (Fig0.1(e)) are then obtained. In a QD-SC system, the competition between superconductivity and Coulomb interactions governs the nature of its ground state (GS) and transport properties.
Superconducting pairing favors even-occupancy, spin-singlet states, whereas Coulomb charging facilitates the stabilization of odd-occupancy states with a spin-doublet GS. The relative strength of the QD, SC coupling (Γ) determines whether the system resides in an even or odd parity sector, as summarized by the phase diagram shown in Fig0.1(c). To experimentally demonstrate how coupling strength governs the GS, they tune the gate voltage V and acquire the tunneling spectrum of QD1, as shown in Fig0.1(f). The evolution of the zero-bias conductance (ZBC) is detailed by a linecut (Fig0.1(g), red line) at the corresponding position indicated in Fig0.1(f).

As V is decreased, the ZBC evolves from a single peak into a split double-peak structure, signaling a transition between two distinct regimes, as shown in Fig0.1(g). In the single-peak regime (cyan region in Fig0.1(g)), the ABSs do not cross zero energy (see Fig0.1(f)), consistent with an EPC hosting a pure even GS. In contrast, the double-peak regime (orange region) exhibits zero-energy crossings of the ABSs (Fig0.1(f)), indicative of an EOPC including an odd-occupancy state. This spectral evolution directly reflects a reduction of the QD-SC coupling strength, which drives the system from the EPC into the EOPC. These experimental regimes are qualitatively consistent with the theoretical phases depicted by the cyan and orange lines in the phase diagram of Fig0.1(c). Moreover, the single peak exhibits higher conductance compared to the double peaks, because enhanced Γ favors Cooper pair tunneling.

Electrostatic Control of Quantum Dot Parity Configurations

Tests prove that this supercurrent-based detection encodes parity information through the global superconducting response of the entire device, unlike subgap conductance measured via normal leads. The device, fabricated using molecular beam epitaxy, consists of two QDs formed in an InAs nanowire proximitized by a 5nm-thick epitaxial aluminum layer, this ensures a pristine superconductor, semiconductor interface. QD2 was defined using three bottom finger gates after selective wet-etching of the aluminum shell, while QD1 remained unetched, allowing for independent electrostatic control over each dot. The team established coherent superconducting coupling between the QDs, essential for realizing non-local parity manipulation, and incorporated a superconducting loop around QD2 to fix the superconducting phase difference. Furthermore, the research establishes a validated physical framework for parity engineering, offering a key building block for scalable, multi-QD superconducting architectures.

Non-local parity control in quantum dot networks offers

This achievement circumvents limitations imposed by restricted local accessibility in scaling superconductor-semiconductor hybrid systems towards artificial Kitaev chains. The authors acknowledge that their current experiments operate in the low-frequency limit, but suggest future work could incorporate greater tunability of the intermediate hybrid segment to realise longer topological Kitaev chains. This research signifies a validated physical framework for parity engineering, offering a crucial building block for scalable, multi-QD superconducting architectures. By providing a powerful, sensor-free strategy for parity management and detection, this method alleviates wiring bottlenecks and naturally suppresses quasiparticle poisoning, issues common with normal-metal probes. Furthermore, the inherent compatibility with circuit-QED architectures suggests straightforward integration with fast readout techniques, positioning Andreev-molecule-based architectures as a promising pathway for developing scalable, parity-controlled superconducting quantum devices.