Quantum sensing with duplex qubits of silicon vacancy centers in SiC at room temperature
At a Glance
Section titled âAt a Glanceâ| 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 |
Technical Documentation & Analysis: Quantum Sensing with Duplex Qubits
Section titled âTechnical Documentation & Analysis: Quantum Sensing with Duplex QubitsâReference: Tahara et al., Quantum sensing with duplex qubits of silicon vacancy centers in SiC at room temperature, npj Quantum Information (2025) 11:58.
Executive Summary
Section titled âExecutive Summaryâ- Core Achievement: Demonstration of a duplex qubit operation scheme in Silicon Vacancy (VSi) centers in 4H-SiC, significantly enhancing signal contrast for quantum sensing applications.
- Methodology: Simultaneous operation of two spin transitions ($|+3/2\rangle \leftrightarrow |+1/2\rangle$ and $|-1/2\rangle \leftrightarrow |-3/2\rangle$) using microwave (MW) pulses at two resonant frequencies ($f_{+}$ and $f_{-}$).
- Performance Gain: The duplex operation achieved a signal gain of 1.97, effectively doubling the Optically Detected Magnetic Resonance (ODMR) contrast compared to conventional simplex operation.
- Application: The technique was successfully applied to AC magnetometry using a spin-echo sequence at room temperature.
- Sensitivity Result: A minimum detectable field sensitivity ($\eta$) of $1.36$ ”T/$\sqrt{\text{Hz}}$ was achieved in a dense VSi ensemble ($10^{18}$ cm-3).
- Material Context: The study addresses the inherent drawback of VSi centersâlow ODMR contrast (a few percent)âby introducing a generalizable method applicable to other spin-3/2 color centers, including those in diamond.
- 6CCVD Value Proposition: While SiC was used, 6CCVDâs high-purity Single Crystal Diamond (SCD) offers superior intrinsic contrast (20-30% for NV centers) and longer coherence times, providing a direct path to significantly lower noise floors and enhanced sensitivity ($\eta \propto 1/C$).
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Host Material | 4H-SiC | N/A | n-type epi-layer (6.1 ”m thick) |
| Color Center | VSi (Silicon Vacancy) | N/A | Cubic (k) site, V2 center |
| Operating Temperature | Room | °C | Ambient conditions |
| Excitation Wavelength | 785 | nm | Non-resonant optical pumping |
| Fluorescence Range | 900-1000 | nm | Spin-dependent fluorescence |
| Zero-Field Splitting (2D/h) | $\sim 70$ | MHz | CW-ODMR at zero field |
| Target Rabi Frequency ($\omega_{1}$) | 10 | MHz | Calibrated for simultaneous duplex operation |
| Coherence Time ($T_{2}$) | 2.1 | ”s | Limited by high VSi density ($10^{18}$ cm-3) |
| Duplex Contrast Gain | 1.97 | N/A | Ratio of duplex vs. simplex peak-to-peak amplitude (Ideal gain is 2) |
| AC Magnetometry Sensitivity ($\eta$) | 1.36 | ”T/$\sqrt{\text{Hz}}$ | Best result (Duplex, $\pm x$ readout) |
| AC Test Signal Frequency | 769.23 | kHz | Synchronized with spin-echo pulse sequence |
Key Methodologies
Section titled âKey MethodologiesâThe experiment relied on precise material engineering and synchronized quantum control sequences:
- Substrate Preparation: Used a commercially available 4H-SiC substrate featuring a 6.1 ”m n-type epi-layer with a nitrogen impurity density of $10^{16}$ cm-3.
- Defect Creation: Silicon Vacancy (VSi) centers were generated by focused He+ ion implantation (0.5 MeV acceleration energy) into a $1$ ”m diameter spot, resulting in a high estimated volume density of $1 \times 10^{18}$ cm-3.
- Optical Setup: A home-built confocal microscope was utilized, employing a 785 nm excitation laser and collecting fluorescence (900-1000 nm) through a 20 ”m pinhole.
- MW Pulse Generation: Two independent signal generators (SGs) were used to synthesize the two required resonant frequencies ($f_{+}$ and $f_{-}$) for the duplex transitions. These signals were combined using a power combiner (Mini-Circuits ZN2PD-4R753+).
- Rabi Calibration: MW output power was carefully tuned to ensure the Rabi frequencies ($\omega_{1}$) for both $f_{+}$ and $f_{-}$ transitions were matched at $10$ MHz, enabling simultaneous, synchronized control of the duplex qubits.
- Quantum Sequence: AC magnetometry was performed using a standard spin-echo sequence ($\pi/2 - \tau - \pi - \tau - \pi/2$) synchronized with the external AC magnetic field $B_{z}(t)$.
- Readout Enhancement: The duplex operation doubled the signal contrast by ensuring both the $|+3/2\rangle \leftrightarrow |+1/2\rangle$ and $|-1/2\rangle \leftrightarrow |-3/2\rangle$ transitions contributed equally to the final optical readout signal.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThis research highlights the critical need for high-quality materials and precise engineering to maximize quantum sensor performance. While SiC is a viable host, the low intrinsic contrast of VSi centers (a few percent) compared to NV centers in diamond (20-30%) limits ultimate sensitivity. 6CCVD provides the advanced MPCVD diamond materials and customization services necessary to replicate this work or achieve superior performance in diamond-based quantum sensing.
Applicable Materials for Enhanced Quantum Sensing
Section titled âApplicable Materials for Enhanced Quantum Sensingâ| Research Requirement | 6CCVD Material Solution | Technical Advantage |
|---|---|---|
| High Contrast Qubits | Optical Grade Single Crystal Diamond (SCD) | NV centers in SCD offer inherently higher ODMR contrast (20-30%), leading to a direct, linear improvement in sensitivity ($\eta \propto 1/C$) over VSi in SiC. |
| Long Coherence Time ($T_{2}$) | Isotopically Purified SCD (Low 13C) | Essential for achieving the long $T_{2}$ times (tens to hundreds of ”s) necessary for high-sensitivity magnetometry, overcoming the $T_{2} = 2.1$ ”s limitation observed in the dense SiC ensemble. |
| High-Density Ensembles | Polycrystalline Diamond (PCD) Wafers | For large-area sensing applications requiring high ensemble density, 6CCVD offers PCD plates up to 125mm in diameter, providing scalable platforms for high-volume sensor integration. |
| Electrical Readout/Control | Boron-Doped Diamond (BDD) | For extending research into electrical detection of spin states (as cited in the paper), 6CCVD offers BDD films with controlled conductivity. |
Customization Potential for Advanced Experiments
Section titled âCustomization Potential for Advanced ExperimentsâThe duplex qubit method requires precise MW delivery and optical access, areas where 6CCVDâs customization capabilities are critical:
- Custom Dimensions and Substrates: 6CCVD supplies SCD plates up to 500 ”m thick and PCD wafers up to 125mm in diameter, allowing researchers to select the optimal size and thickness for their specific optical and MW coupling geometries.
- Integrated Metalization: The experiment utilized a thin Cu wire for MW delivery. 6CCVD offers in-house, high-precision metalization services (Au, Pt, Pd, Ti, W, Cu) for depositing custom MW strip lines, coplanar waveguides, or antenna structures directly onto the diamond surface. This integration simplifies the experimental setup and maximizes MW coupling efficiency.
- Surface Quality: We provide ultra-smooth polishing (Ra < 1nm for SCD, < 5nm for inch-size PCD), ensuring minimal scattering losses and optimal coupling efficiency for the confocal microscopy setup used for excitation (785 nm) and fluorescence collection (900-1000 nm).
Engineering Support
Section titled âEngineering Supportâ6CCVDâs in-house PhD team specializes in the material science and quantum physics of diamond color centers. We offer authoritative professional support for projects involving:
- Material selection and defect engineering (e.g., controlled NV or SiV creation via implantation).
- Optimization of surface termination and polishing for enhanced optical readout.
- Design consultation for integrated MW/RF structures for complex pulse ODMR sequences, such as the duplex qubit operation demonstrated here.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
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 $$| m=\pm 3/2\left.\right\rangle$$ ⣠m = ± 3 / 2 and $$| m=\pm 1/2\left.\right\rangle$$ ⣠m = ± 1 / 2 states, and optical pumping does not create a population difference between $$| +1/2\left.\right\rangle$$ ⣠+ 1 / 2 and $$| -1/2\left.\right\rangle$$ ⣠â 1 / 2 states. Thus, operating one qubit (e.g., $$\left{| +3/2\left.\right\rangle ,| +1/2\left.\right\rangle \right}$$ ⣠+ 3 / 2 , ⣠+ 1 / 2 states) leaves the population in the remaining state ( $$| -1/2\left.\right\rangle$$ ⣠â 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 $$\left{| +3/2\left.\right\rangle ,| +1/2\left.\right\rangle \right}$$ ⣠+ 3 / 2 , ⣠+ 1 / 2 and $$\left{| -1/2\left.\right\rangle ,| -3/2\left.\right\rangle \right}$$ ⣠â 1 / 2 , ⣠â 3 / 2 . Experimental results demonstrate that this approach doubles signal contrast in optical readout and improves sensitivity in AC magnetometry compared to simplex operation.