Spin Dynamics of a Solid-State Qubit in Proximity to a Superconductor
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-01-05 |
| Journal | Nano Letters |
| Authors | Richard Monge, Tom Delord, Nicholas V. Proscia, Zav Shotan, Harishankar Jayakumar |
| Institutions | Max Planck Institute for the Physics of Complex Systems, Japan Advanced Institute of Science and Technology |
| Citations | 15 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Core Achievement: Demonstrated a 1.5-fold enhancement of the Nitrogen-Vacancy (NV) center spin coherence lifetime (T2) when the diamond probe is brought into close proximity (~150 nm) to a high-critical-temperature (Tc) superconductor film (TBCCO) at 69 K.
- Mechanism Identified: The T2 enhancement is tentatively attributed to the suppression of electric noise originating from fluctuating charge carriers on the diamond surface, caused by a superconductor-induced redistribution of these carriers.
- Novel Imaging: Built upon this coherence enhancement to demonstrate one-dimensional T2-weighted relaxometry imaging, providing a new contrast mechanism for mapping superconductor boundaries.
- Noise Suppression: Theoretical modeling suggests that the observed effect is dominated by electric noise suppression, ruling out conventional magnetic Meissner shielding as the primary cause for the T2 gain in this configuration.
- Methodology: Utilized an all-diamond scanning probe integrated into a cryogenic confocal/ODMR microscope, employing Hahn-echo (HE) and CPMG protocols with specialized microwave recalibration to ensure constant spin control during scanning.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Superconductor Material | Tl2Ba2CaCu2O8 (TBCCO) | N/A | High-Tc film |
| Critical Temperature (Tc) | ~105 | K | TBCCO film specification |
| Operating Temperature | 69 | K | Experimental temperature (well below Tc) |
| TBCCO Film Thickness | 500 | nm | Film deposited on LAO substrate |
| External Magnetic Field (BM) | ~6 | mT | Uniform field applied during measurement |
| NV-Superconductor Distance (z) | 150 to 350 | nm | Typical working range for coherence measurements |
| Hahn-Echo T2 (Away from SC) | 28.3 ± 3.0 | ”s | Measured on Lanthanum Aluminate (LAO) substrate |
| Hahn-Echo T2 (On SC) | 42.8 ± 2.9 | ”s | Measured on TBCCO film |
| T2 Enhancement Factor | ~1.5 | N/A | Coherence gain due to SC proximity |
| Estimated Surface Resistivity (Away) | ~4 x 1013 | Ω | Diamond surface (oxygen-terminated) |
| Estimated Surface Resistivity (On SC) | ~2 x 1013 | Ω | Halved due to proximity effect |
| NV Center Depth (Simulated) | 5-23 | nm | Based on 6-12 keV implantation energy |
Key Methodologies
Section titled âKey Methodologiesâ- Cryogenic Scanning Probe Setup: An all-diamond scanning probe (hosting shallow NV centers) was integrated into a closed-cycle cryo-workstation (69 K operation) featuring a confocal/ODMR microscope and Atomic Force Microscope (AFM) capabilities for precise tip-sample distance control (150 nm minimum).
- Sample Fabrication: The TBCCO film (500 nm thick, Tc ~105 K) was patterned into square patches on a Lanthanum Aluminate (LAO) substrate using optical lithography and wet etching.
- Vector Magnetometry: Optically-Detected Magnetic Resonance (ODMR) was performed using two differently-oriented NVs to reconstruct the full vector map of the external magnetic field (BM) and confirm Meissner shielding near the TBCCO boundaries.
- Spin Coherence Measurement: Pulsed magnetic resonance protocols, including Hahn-Echo (HE), Ramsey, and Carr-Purcell-Meiboom-Gill (CPMG-n), were used to measure the transverse relaxation time (T2) and dephasing time (T2*) of the NV spin.
- T2-Weighted Imaging Protocol: A specialized protocol was developed for scanning the TBCCO boundary, requiring automated, real-time recalibration of the microwave (mw) power and frequency at every spatial point. This ensured constant spin rotation (Ï-pulse duration) and resonant excitation, eliminating signal distortions caused by local mw field variations.
- Noise Modeling: Theoretical formalism was developed to compare the impact of alternative noise sources (magnetic spin noise vs. electric charge noise). The electric noise model incorporated Ohmic conduction and mirror charges to account for the superconductorâs effect on surface charge fluctuations.
Commercial Applications
Section titled âCommercial Applicationsâ- Hybrid Quantum Systems: Designing and optimizing interfaces between solid-state qubits (like NV centers) and superconducting circuits (flux qubits, resonators) for quantum computing and microwave-to-optical transduction.
- Cryogenic Quantum Sensing: Developing high-sensitivity, nanoscale magnetometers and electrometers capable of operating at cryogenic temperatures to characterize noise sources (1/f noise, vortex creep) in superconducting devices.
- Superconductor Characterization: Non-invasive, high-resolution imaging of high-Tc superconductor films, providing contrast based on coherence time (T2-weighted imaging) rather than just magnetic field magnitude (ODMR), offering complementary dynamic information.
- Diamond Material Science: Utilizing the NV center as a probe to study and engineer the complex charge and spin dynamics at the diamond surface, particularly relevant for optimizing shallow NV sensors used in ambient or cryogenic environments.
- Noise Spectroscopy: Exploiting the NV centerâs sensitivity to both magnetic and electric noise to perform time-resolved noise spectroscopy, allowing for the identification and monitoring of thermally activated processes in complex materials.
View Original Abstract
A broad effort is underway to understand and harness the interaction between superconductors and spin-active color centers with an eye on hybrid quantum devices and novel imaging modalities of superconducting materials. Most work, however, overlooks the interplay between either system and the environment created by the color center host. Here we use a diamond scanning probe to investigate the spin dynamics of a single nitrogen-vacancy (NV) center proximal to a superconducting film. We find that the presence of the superconductor increases the NV spin coherence lifetime, a phenomenon we tentatively rationalize as a change in the electric noise due to a superconductor-induced redistribution of charge carriers near induced redistribution of charge carriers near the NV. We then build on these findings to demonstrate transverse-relaxation-time-weighted imaging of the superconductor film. These results shed light on the dynamics governing the spin coherence of shallow NVs, and promise opportunities for new forms of noise spectroscopy and imaging of superconductors.