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 | Imperial College London, Beijing Academy of Quantum Information Sciences |
| Citations | 20 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This analysis outlines the demonstration of quasi-continuous microwave mode cooling using optically hyperpolarized Nitrogen-Vacancy (NV-) centers in diamond, achieving a significant step toward benchtop quantum technology.
- Core Achievement: A microwave mode at 2872 MHz was cooled from ambient temperature (290 K) down to a noise temperature of 188 K, corresponding to a 1.9 dB reduction in noise power.
- Operational Mode: The cooling is quasi-continuous, maintaining the 188 K temperature for the entire duration of the optical pump pulse (demonstrated up to 10 ms), limited only by the experimental setup’s ability to keep the diamond cool.
- Material Advantage (NV- vs. Pc:PTP): NV- diamond offers immediate cooling upon light excitation (no pre-masing required) and operates continuously at substantially lower instantaneous optical pump power (2 W CW).
- Comparison Drawback: While continuous, the cooling depth (188 K) is inferior to the previous pulsed record set by pentacene-doped p-terphenyl (Pc:PTP), which achieved 50 K but only for less than 0.5 ms using 5 kW peak power.
- Setup Simplicity: The experiment operates entirely on a lab benchtop at ambient temperature and requires zero applied DC magnetic field, simplifying the apparatus compared to systems requiring strong magnets or cryogenic environments.
- Future Scaling: Proposed improvements include using larger, rod-shaped diamonds for better magnetic filling factor and actively cooling the diamond to enable truly continuous operation beyond the current 10 ms limit.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Cooled Mode Frequency (fmode) | 2872 | MHz | NV- |
| Initial Mode Temperature (T0) | 290 | K | Ambient |
| Cooled Mode Temperature (Tmode) | 188 | K | Achieved continuously |
| Noise Power Reduction (ΔP) | -1.9 | dB | Corresponds to 188 K |
| NV- Concentration (Estimated) | 6 x 1017 | cm-3 | Diamond sample |
| Optical Pump Wavelength | 532 | nm | Continuous Wave (CW) laser |
| Optical Pump Power (Instantaneous) | 2 | W | NV- experiment |
| Continuous Cooling Duration | > 10 | ms | Limited by thermal management |
| STO Resonator Loaded Q (QL) | 2900 | - | TE01δ mode |
| Pc:PTP Cooled Temperature (Tmode) | 50 ± 8 | K | Previous pulsed record |
| Pc:PTP Cooling Duration | < 0.5 | ms | Previous pulsed record |
| Pc:PTP Optical Pump Power (Peak) | ~5 | kW | Previous pulsed record |
| Spin-Lattice Relaxation Rate (γ02) | 1/0.012 | s-1 | Between |
| Inhomogeneous Relaxation Time (T2*) | ~3 | µs | Estimated for bulk NV- |
| Diamond Path Length (L) | 0.15 | cm | For UV/Vis measurement |
Key Methodologies
Section titled “Key Methodologies”The experiment utilized a targeted mode-cooling approach based on photoexcited spin polarization in diamond, measured via a high-sensitivity heterodyne receiver.
- Sample and Cavity Setup: A brilliant-cut NV- diamond jewel was placed inside a Strontium Titanate (STO) dielectric resonator. The resonator was housed in a copper cavity, and its TE01δ mode was tuned to 2872 MHz, matching the NV- |0> → |2> transition frequency at zero magnetic field.
- Optical Pumping: The diamond was excited from above using a 2 W continuous-wave 532 nm laser, gated into square pulses (up to 10 ms duration) to induce spin polarization.
- Hyperpolarization Mechanism: Laser excitation drives the NV- centers from the 3A2 ground state to the 3E excited state, followed by spin-selective intersystem crossing (ISC) to a metastable singlet state, resulting in a highly polarized spin system (cryogenic spin temperature).
- Microwave Interaction: The cold spin system interacts with the 2872 MHz microwave mode, absorbing thermal photons from the mode and effectively cooling it.
- Receiver Chain: The noise power reduction was measured using a high-gain superheterodyne receiver setup. This chain included an iris for critical coupling, a Low-Noise Amplifier (LNA), a mixer, and a 1.25 MHz bandwidth SAW filter, all operating at ambient temperature.
- Performance Measurement: The instantaneous noise power reduction (ΔP) was measured, and this value was numerically inverted using a detailed noise reduction equation (Eq. 16 in SM) to calculate the resultant microwave mode temperature (Tmode).
- Continuous Operation Test: Long-pulse (10 ms) excitation was used to demonstrate that the mode temperature could be maintained at 188 K for the duration of the pump, confirming the material’s capability for quasi-continuous cooling.
Commercial Applications
Section titled “Commercial Applications”This technology provides a pathway for localized, non-cryogenic cooling of critical microwave components, particularly relevant for quantum and high-sensitivity RF systems.
- Quantum Technology: Enables benchtop operation of quantum sensors and potentially certain quantum computing architectures (e.g., those based on NV centers or hybrid systems) by providing targeted cooling without the need for bulky, power-hungry dilution refrigerators.
- High-Sensitivity RF Receivers: Applicable in the development of ultra-low-noise amplifiers (LNAs) and front-end modules for communications, radar, and radio astronomy, improving signal-to-noise ratios (SNR) in ambient conditions.
- Mobile and Field Sensing: The zero-field operation and potential for miniaturization make this suitable for enhancing the sensitivity of mobile magnetic field sensors or portable spectroscopy equipment.
- EPR/ODMR Spectroscopy: Direct application in enhancing the sensitivity of Electron Paramagnetic Resonance (EPR) and Optically Detected Magnetic Resonance (ODMR) systems by cooling the microwave cavity mode coupled to the sample.
- Miniaturized Cryocoolers: Forms the basis for developing compact, low-power cooling elements that could be integrated into consumer electronics or specialized instrumentation where size and power consumption are critical constraints.
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.