Quasi-continuous cooling of a microwave mode on a benchtop using hyperpolarized NV− diamond
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-12-06 |
| Journal | Applied Physics Letters |
| Authors | Wern Ng, Hao Wu, Mark Oxborrow |
| Institutions | Beijing Institute of Technology, Beijing Academy of Quantum Information Sciences |
| Citations | 20 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Quasi-Continuous Microwave Mode Cooling using NV¯ Diamond
Section titled “Technical Documentation & Analysis: Quasi-Continuous Microwave Mode Cooling using NV¯ Diamond”This document analyzes the research paper “Quasi-Continuous Cooling of a Microwave Mode on a Benchtop using Hyperpolarized NV¯ Diamond” to provide technical specifications and align the material requirements with 6CCVD’s advanced MPCVD diamond capabilities.
Executive Summary
Section titled “Executive Summary”This research demonstrates a significant step toward benchtop quantum instrumentation by achieving sustained cooling of a microwave mode using Nitrogen-Vacancy (NV¯) centers in diamond.
- Core Achievement: Demonstrated quasi-continuous cooling of a 2872 MHz microwave mode from ambient temperature (290 K) down to 188 K using optically hyperpolarized NV¯ diamond.
- Continuous Operation: The cooling effect was sustained for the duration of the optical pump pulse (up to 10 ms demonstrated), confirming the material’s capability for continuous operation, a key advantage over previous pulsed methods (e.g., Pc:PTP).
- Efficiency Advantage: NV¯ diamond requires substantially lower instantaneous optical pump power (2 W CW) compared to organic crystals (Pc:PTP, requiring $\sim$ 5 kW pulsed power) to achieve spin polarization and cooling.
- Simplified Setup: The experiment operates entirely at zero applied DC magnetic field (ZF) on a lab benchtop, eliminating the need for bulky cryogenic systems (dilution refrigerators) or strong external magnets.
- Material Limitation Identified: The current cooling depth is limited by the geometry (brilliant-cut jewel) and the total number of participating spins (NT).
- Path to Improvement: The authors propose increasing the diamond size (e.g., 4 mm rod), optimizing NV concentration, and implementing robust thermal management (heat-sinking) to enhance cooling depth and truly continuous operation.
Technical Specifications
Section titled “Technical Specifications”The following hard data points were extracted from the experimental results and simulation parameters:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Microwave Mode Frequency (fmode) | 2872 | MHz | NV¯ $ |
| Initial Ambient Temperature (T0) | 290 | K | Room temperature operation |
| Cooled Mode Temperature (Tmode) | 188 | K | Achieved under 10 ms continuous pumping |
| Noise Power Reduction ($\Delta$P) | -1.9 | dB | Corresponds to 192 K cooling depth (2 ms pulse) |
| Optical Pump Wavelength ($\lambda_{p}$) | 532 | nm | CW Diode-pumped Nd:YAG laser |
| Optical Pump Power (P(t)) | 2 | W | Instantaneous power used for continuous cooling |
| Diamond NV¯ Concentration | 6 $\times$ 1017 | cm-3 | Estimated concentration in the natural sample |
| Diamond Geometry | Brilliant Cut | - | 2.65 mm diameter, 1.5 mm height |
| STO Resonator Loaded Quality Factor (QL) | 2900 | - | Tuned to 2872 MHz |
| Inhomogeneous Spin-Spin Relaxation Time (T2*) | $\sim$ 3 | µs | Estimated based on substitutional nitrogen defects |
| Thermal Photons (Initial) | 2103 | - | At T0 = 290 K |
| Thermal Photons (Cooled) | 1363 | - | At Tmode = 188 K |
Key Methodologies
Section titled “Key Methodologies”The experiment utilized a combination of specialized diamond material, a custom microwave cavity, and a high-sensitivity heterodyne receiver setup.
- Material Selection: A natural, brilliant-cut diamond jewel was used, which had been “color enhanced” (annealed) to generate a high concentration of NV centers (estimated 6 $\times$ 1017 cm-3).
- Microwave Cavity Design: A cylindrical copper cavity housed a monocrystalline strontium titanate (STO) dielectric ring resonator (7.27 mm OD, 3.5 mm height).
- Mode Tuning: The cavity internal height was adjusted via a tuning screw to set the TE01$\delta$ mode frequency precisely to 2872 MHz, matching the absorptive NV¯ transition peak.
- Optical Pumping: A 2 W continuous-wave 532 nm laser was gated using square pulses (2 ms or 10 ms duration) and focused to a 1.5 mm spot diameter onto the diamond sample.
- Zero-Field Operation: The entire setup was run at ambient temperature (290 K) and zero applied DC magnetic field (ZF), simplifying alignment and instrumentation.
- Measurement System: A high-gain superheterodyne receiver (total gain $\sim$ 100 dB) with a 1.25 MHz bandwidth SAW filter was used to measure the instantaneous reduction in noise power ($\Delta$P) extracted from the microwave mode.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”The research highlights that future performance improvements depend critically on optimizing the diamond material’s geometry, NV concentration, and thermal integration. 6CCVD’s MPCVD capabilities are perfectly suited to meet these advanced engineering requirements.
| Research Requirement / Improvement | 6CCVD Solution & Capability | Technical Advantage |
|---|---|---|
| Material Optimization: Need for high-purity diamond with precisely controlled, high NV concentration (6 $\times$ 1017 cm-3 or higher) to maximize NT. | Optical Grade Single Crystal Diamond (SCD): We offer MPCVD SCD substrates with tailored nitrogen doping during growth, ensuring uniform and controlled NV precursor concentration. | Enables precise replication and optimization of NV density, maximizing the number of participating spins (NT) and cooling depth while maintaining high crystal quality. |
| Geometry & Coupling: Need for larger, rod-shaped samples (e.g., 4 mm diameter) with polished flat end windows to improve magnetic filling factor ($\eta_{fill}$) and optical coupling. | Custom Dimensions & Precision Machining: We supply SCD plates up to 500 µm thick and PCD wafers up to 125 mm. We offer custom laser cutting and grinding services for rod shapes and specific geometries. | Allows researchers to utilize optimal geometries for microwave coupling (Vmode) and optical access, overcoming the limitations of the brilliant-cut jewel used in the study. |
| Surface Quality: Requirement for polished flat end windows to reduce retro-reflection and improve optical efficiency. | Ultra-High Quality Polishing: SCD surfaces are polished to achieve roughness Ra < 1 nm, and inch-size PCD surfaces to Ra < 5 nm. | Minimizes optical losses and scattering, ensuring maximum 532 nm pump power is coupled into the NV centers for efficient spin polarization. |
| Continuous Cooling & Heat Sinking: Requirement for robust thermal anchorage to remove absorbed optical pump power and enable truly continuous operation. | Custom Metalization Services: We provide in-house deposition of high-adhesion, low-resistance metal stacks (Ti/Pt/Au, Cu, W) for thermal and electrical contacts. | Facilitates the integration of the diamond into advanced heat-sinking systems, crucial for maintaining the diamond at a low temperature during high-power CW optical pumping. |
| Engineering Support: Assistance needed for optimizing material parameters (N concentration, T2* lifetime) for specific quantum applications. | In-House PhD Engineering Team: Our experts specialize in correlating CVD growth parameters with specific quantum defect properties (e.g., T2* lifetime and NV charge state stability). | Provides critical consultation to ensure the material specifications meet the demanding requirements of quantum sensing and microwave mode cooling projects. |
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
We demonstrate the cooling of a microwave mode at 2872 MHz through its interaction with optically spin-polarized NV− centers in diamond at zero applied magnetic field, removing thermal photons from the mode. By photo-exciting (pumping) a brilliant-cut red diamond jewel with a continuous-wave 532-nm laser, outputting 2 W, the microwave mode is cooled down to a noise temperature of 188 K. This noise temperature can be preserved continuously for as long as the diamond is optically excited and kept cool. The latter requirement restricted operation out to 10 ms in our preliminary setup. The mode-cooling performance of NV− diamond is directly compared against that of pentacene-doped para-terphenyl, where we find that the former affords the advantages of cooling immediately upon light excitation (whereas pentacene-doped para-terphenyl undesirably mases before it begins cooling) and being able to cool continuously at substantially lower optical pump power.