Quantum sensing with duplex qubits of silicon vacancy centers in SiC at room temperature
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2025-04-05 |
| Journal | npj Quantum Information |
| Authors | Kosuke Tahara, S. Tamura, Haruko Toyama, Jotaro J. Nakane, Katsuhiro Kutsuki |
| Citations | 5 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled āExecutive SummaryāThis research introduces a novel techniqueāDuplex Qubit Operationāto significantly enhance the performance of Silicon Vacancy (VSi) quantum sensors in Silicon Carbide (SiC) at room temperature.
- Core Problem Addressed: VSi centers, while robust, suffer from low optical readout contrast (a few percent) because non-resonant optical pumping leaves an unaffected background population in the spin quartet (|Ā±1/2> states).
- Innovative Solution: The Duplex Qubit scheme simultaneously operates two distinct spin transitions (the + qubit: {| + 3/2>, | + 1/2>} and the - qubit: {| - 1/2>, | - 3/2>}) using two resonant microwave (MW) frequencies ($f_+$ and $f_-$).
- Contrast Enhancement: This approach effectively utilizes the entire spin quartet, resulting in a near-ideal doubling of the signal contrast in pulse Optically Detected Magnetic Resonance (ODMR) measurements (signal gain of 1.97).
- Sensitivity Improvement: The enhanced contrast directly translates to improved sensitivity in AC magnetometry, achieving a gain of up to 1.99 compared to conventional simplex operation.
- Performance Benchmark: A sensitivity of 1.36 ĀµT/√Hz was demonstrated using a dense VSi cluster (1018 cm-3) with a volume of 0.5 Āµm3.
- General Applicability: The method is applicable to other spin-3/2 (or greater S) color centers exhibiting zero-field splitting, making it a versatile technique for quantum sensing platforms beyond SiC.
Technical Specifications
Section titled āTechnical Specificationsā| Parameter | Value | Unit | Context |
|---|---|---|---|
| Qubit Center | Silicon Vacancy (VSi) | - | Cubic (k) site in 4H-SiC |
| Operating Temperature | Room Temperature | - | - |
| VSi Cluster Volume | 0.5 | Āµm3 | Estimated size of the sensing volume |
| VSi Density (Estimated) | 1 x 1018 | cm-3 | High density ensemble |
| Excitation Wavelength | 785 | nm | Non-resonant laser excitation |
| Fluorescence Wavelength | 900-1000 | nm | Optical readout band |
| Zero-Field Splitting (2D/h) | ~70 | MHz | Magnetic resonance frequency at B=0 |
| Rabi Oscillation Contrast Gain | 1.97 | - | Duplex operation vs. Simplex operation (Ideal gain is 2) |
| Spin Coherence Time (T2) | 2.1 | Āµs | Measured using spin-echo sequence in the dense ensemble |
| AC Magnetometry Sensitivity (Ī·) | 1.36 | ĀµT/√Hz | Best result achieved (with (Ļ/2)+x readout) |
| Sensitivity Gain (AC Mag.) | 1.99 | - | Duplex vs. Simplex (Ā±x readout) |
| Ion Implantation Species | He+ | - | Used for VSi creation |
| Ion Implantation Energy | 0.5 | MeV | Determines stopping range (~1 Āµm depth) |
| Epi-layer Thickness | 6.1 | Āµm | n-type 4H-SiC substrate layer |
Key Methodologies
Section titled āKey Methodologiesā- Sample Preparation: VSi centers were created in a 6.1 Āµm thick n-type 4H-SiC epi-layer (1016 cm-3 N density) by focusing 0.5 MeV He+ ions onto a 1 Āµm spot, resulting in a high estimated VSi density of 1 x 1018 cm-3.
- Confocal Microscopy Setup: All experiments were performed at room temperature using a home-built confocal microscope with a 785 nm excitation laser and a photo-receiver with a narrow bandwidth (1.1 kHz) for fluorescence detection.
- Spin Initialization and Readout: Spin states were initialized into |Ā±1/2> using a non-resonant laser pulse (0.5 Āµs width). Optical readout was performed by recording the average fluorescence intensity during a second laser pulse, where the |Ā±3/2> state exhibits stronger fluorescence.
- Duplex MW Generation: Two independent signal generators (SG1 and SG2) synthesized the two required MW frequencies ($f_+$ and $f_-$) for the + and - qubits. These signals were phase-modulated using IQ functions and combined using a power combiner.
- Rabi Frequency Matching: The output power of SG1 and SG2 was individually calibrated to ensure that the Rabi frequencies ($\omega_1$) for both $f_+$ and $f_-$ transitions matched (target value 10 MHz), enabling simultaneous, coherent control of both qubits.
- AC Magnetometry Sequence: AC magnetic field sensing was demonstrated using a standard spin-echo pulse ODMR sequence (Laser - Ļ/2 - Ļ - Ļ - Ļ - Ļ/2 - Laser). The MW pulse train was synchronized with the AC test signal (769.23 kHz) to measure the accumulated phase.
Commercial Applications
Section titled āCommercial Applicationsā- Quantum Sensing and Metrology: Development of highly sensitive, room-temperature magnetometers for industrial inspection, geological surveys, or medical diagnostics (e.g., magnetoencephalography).
- Integrated Quantum Devices: SiCās maturity in semiconductor fabrication allows for the integration of these enhanced VSi qubits into scalable quantum processors or on-chip sensors.
- High-Power Electronics Monitoring: Utilizing VSiās robustness and SiCās role in power devices for localized, non-invasive thermometry and magnetic field mapping within operating high-voltage components.
- Materials Science Research: High-spatial resolution sensing of magnetic and electric fields to characterize defects, strain, and noise sources in novel semiconductor materials.
- Vector Field Sensing: Exploiting the spin-3/2 nature of VSi to build compact vector magnetometers capable of determining the full 3D orientation and magnitude of magnetic fields.
- Quantum Communication Components: SiC color centers are promising solid-state quantum emitters; improved spin control enhances their utility in quantum networking and memory applications.
View Original Abstract
Abstract The silicon vacancy center in Silicon Carbide (SiC) provides an optically addressable qubit at room temperature in its spin- $$\frac{3}{2}$$ 3 2 electronic state. However, optical spin initialization and readout are less efficient compared to those of spin-1 systems, such as nitrogen-vacancy centers in diamond, under non-resonant optical excitation. Spin-dependent fluorescence exhibits contrast only between $$| m=\pm 3/2\left.\right\rangle$$ ā£ m = Ā± 3 / 2 and $$| m=\pm 1/2\left.\right\rangle$$ ā£ m = Ā± 1 / 2 states, and optical pumping does not create a population difference between $$| +1/2\left.\right\rangle$$ ā£ + 1 / 2 and $$| -1/2\left.\right\rangle$$ ā£ ā 1 / 2 states. Thus, operating one qubit (e.g., $$\left{| +3/2\left.\right\rangle ,| +1/2\left.\right\rangle \right}$$ ā£ + 3 / 2 , ā£ + 1 / 2 states) leaves the population in the remaining state ( $$| -1/2\left.\right\rangle$$ ā£ ā 1 / 2 ) unaffected, contributing to background in optical readout. To mitigate this problem, we propose a sensing scheme based on duplex qubit operation in the quartet, using microwave pulses with two resonant frequencies to simultaneously operate $$\left{| +3/2\left.\right\rangle ,| +1/2\left.\right\rangle \right}$$ ā£ + 3 / 2 , ā£ + 1 / 2 and $$\left{| -1/2\left.\right\rangle ,| -3/2\left.\right\rangle \right}$$ ā£ ā 1 / 2 , ā£ ā 3 / 2 . Experimental results demonstrate that this approach doubles signal contrast in optical readout and improves sensitivity in AC magnetometry compared to simplex operation.