Signatures of Andreev Blockade in a Double Quantum Dot Coupled to a Superconductor
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-01-25 |
| Journal | Physical Review Letters |
| Authors | Po Zhang, Hao Wu, Jun Chen, Sabbir A Khan, Peter Krogstrup |
| Institutions | Microsoft (Denmark), University of Pittsburgh |
| Citations | 17 |
| Analysis | Full AI Review Included |
Evidence of Andreev Blockade in a Double Quantum Dot Coupled to a Superconductor
Section titled âEvidence of Andreev Blockade in a Double Quantum Dot Coupled to a SuperconductorâExecutive Summary
Section titled âExecutive Summaryâ- Core Achievement: Experimental demonstration of Andreev blockade (AB) in a hybrid superconductor-semiconductor double quantum dot (DQD) system.
- Mechanism: AB is the suppression of Andreev reflection (Cooper pair transfer) when the DQD is in a spin-triplet configuration, preventing the formation of a spin-singlet Cooper pair in the superconducting lead.
- Device Structure: A DQD is electrostatically defined in an InAs nanowire segment, with one dot (QDS) coupled to an epitaxial Al shell (superconductor) and the other (QDN) coupled to a normal gold lead.
- Key Signatures Observed: The experiment confirmed four predicted signatures: current confined to single triangular regions in the stability diagram, an alternating pattern of high/low current (blockade/no-blockade) controlled by the QDN occupation, reversal of this pattern upon source-drain bias flip, and the disappearance of all signatures when superconductivity is suppressed.
- Leakage Current: Current was not completely blocked, indicating partial lifting of the blockade, which was successfully modeled by introducing finite temperature effects (thermally excited quasi-particles).
- Relevance: This phenomenon provides a new tool for studying spin-resolved transport and spin pairing in superconductors, essential for developing spin-dependent elements in Andreev and topological quantum computing architectures.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Base Measurement Temperature | 40 | mK | Dilution refrigerator operating temperature. |
| Bulk Al Critical Temperature (Tc) | 1.2 | K | Temperature above which blockade signatures disappear. |
| Induced Superconducting Gap (Î/e) | 0.2 | mV | Consistent with hard gap in the InAs/Al hybrid. |
| Inter-Dot Charging Energy (ENS) | 10 | ”eV | Used in numerical simulations for weakly coupled DQD regime. |
| Gate Pitch | 60 | nm | Electrostatic gate patterning resolution. |
| Dielectric Layer Thickness (HfO2) | 10 | nm | Grown by Atomic Layer Deposition (ALD). |
| Gate Metal Bilayer | 1.5/6 | nm | Ti/PdAu thickness. |
| Lead Metal Bilayer | 10/130 | nm | Ti/Au thickness. |
| Al Etch Developer | MF CD-26 / DI water (1:20) | Ratio | Selective wet etching solution for Al shell removal. |
| Magnetic Field (Suppression Test) | 0.6 | T | Field applied in the sample plane to suppress superconductivity. |
| QDN Charging Energy (Ec) | 4 | mV | Estimated from Coulomb diamond size (Fig. S4). |
Key Methodologies
Section titled âKey Methodologiesâ- Nanowire Synthesis: InAs nanowires with epitaxial Al shells were grown using Molecular Beam Epitaxy (MBE) via the Vapor-Liquid-Solid mechanism, ensuring high interface quality.
- Gate Definition: Electrostatic gates were patterned using 100 kV E-Beam Lithography (EBL) with 60 nm pitch. A 1.5/6 nm Ti/PdAu bilayer was evaporated for the gates.
- Dielectric Passivation: A 10 nm HfO2 dielectric layer was deposited over the gates using Atomic Layer Deposition (100 cycles).
- Nanowire Integration: Nanowires were transferred onto the gate chip using a micro-manipulator.
- Selective Al Etching: EBL defined windows for Al etching. The Al shell was selectively etched using a 1:20 solution of MF CD-26 developer/DI water for 2 minutes at room temperature to create the normal dot (QDN) region.
- Lead Fabrication: Source and drain leads (10/130 nm Ti/Au) were defined using EBL, followed by Ar cleaning and e-beam evaporation.
- Transport Measurement: Differential conductance (dI/dV) and current (I) measurements were performed in a dilution refrigerator at a base temperature of 40 mK, using gate voltages (VN, VS) to define and tune the DQD states.
Commercial Applications
Section titled âCommercial Applicationsâ- Quantum Computing Hardware:
- Andreev and Topological Qubits: The DQD-S structure is a fundamental building block for Andreev qubits and is relevant for hosting Majorana zero modes (topological qubits).
- Spin-State Readout: Andreev blockade can be utilized as a spin-dependent transport or transition rate element to detect the state of a qubit or quantum emulator.
- Advanced Materials Characterization:
- Superconductor Spin Structure Probes: The blockade mechanism offers a means to study spin-resolved transport at zero magnetic field, allowing detailed investigation of spin pairing (triplet vs. singlet) and spin textures (e.g., LOFF states) in novel superconducting materials.
- Quantum Sensing and Metrology:
- Spin-Sensitive Current Probes: The technology could be adapted to create highly sensitive spin-dependent current probes, potentially extending the precision of quantum current standards by leveraging spin parity effects.
- Hybrid Nanodevice Development:
- Crossed Andreev Reflection: The principles of Andreev blockade are relevant for designing two-arm devices (two parallel DQDs) to serve as spin-sensitive probes for crossed Andreev reflection phenomena.
View Original Abstract
We investigate an electron transport blockade regime in which a spin triplet localized in the path of current is forbidden from entering a spin-singlet superconductor. To stabilize the triplet, a double quantum dot is created electrostatically near a superconducting Al lead in an InAs nanowire. The quantum dot closest to the normal lead exhibits Coulomb diamonds, and the dot closest to the superconducting lead exhibits Andreev bound states and an induced gap. The experimental observations compare favorably to a theoretical model of Andreev blockade, named so because the triplet double dot configuration suppresses Andreev reflections. Observed leakage currents can be accounted for by finite temperature. We observe the predicted quadruple level degeneracy points of high current and a periodic conductance pattern controlled by the occupation of the normal dot. Even-odd transport asymmetry is lifted with increased temperature and magnetic field. This blockade phenomenon can be used to study spin structure of superconductors. It may also find utility in quantum computing devices that use Andreev or Majorana states.