Microwave mode cooling and cavity quantum electrodynamics effects at room temperature with optically cooled nitrogen-vacancy center spins

At a Glance

Metadata	Details
Publication Date	2022-11-02
Journal	npj Quantum Information
Authors	Yuan Zhang, Qilong Wu, Hao Wu, Xun Yang, Shi‐Lei Su
Institutions	Beijing Institute of Technology, Beijing Academy of Quantum Information Sciences
Citations	11
Analysis	Full AI Review Included

Technical Documentation & Analysis: Microwave Mode Cooling and C-QED in NV Diamond

6CCVD Document Reference: NPJQI-2022-8-125 Application Focus: Quantum Information, Solid-State Spin Systems, Cryogen-Free Microwave Cooling

Executive Summary

This research demonstrates the potential for achieving highly efficient microwave mode cooling and robust Cavity Quantum Electrodynamics (C-QED) effects at room temperature using optically cooled Nitrogen-Vacancy (NV) centers in diamond. The findings directly validate the need for high-purity, high-density NV diamond material, a core specialty of 6CCVD.

Core Value Proposition: Theoretical prediction of cryogen-free microwave mode cooling, reducing the effective mode temperature from 293 K (Room T) down to 116 K (261 photons).
Methodology: Utilizes a multi-level Jaynes-Cumming (JC) model to simulate trillions of optically pumped NV centers coupled to a 9.22 GHz dielectric microwave resonator.
Key Achievement: Predicted a five-fold reduction in microwave photon number compared to previous experimental results, achieved by optimizing mode frequency and photon damping rate.
C-QED Demonstration: Calculations predict laser-power controlled C-QED effects (Rabi oscillations and splitting), successfully transitioning the NV ensemble into the strong collective coupling regime.
Material Requirement: Achieving the strong coupling regime requires a significant increase in NV center density (target N > 4 x 10¹⁴), necessitating ultra-high-purity, isotopically controlled Single Crystal Diamond (SCD).
6CCVD Advantage: 6CCVD specializes in providing the necessary Optical Grade SCD substrates with controlled nitrogen incorporation and superior surface quality (Ra < 1 nm) required for high-coherence quantum applications.

Technical Specifications

The following table summarizes the critical hard data points and performance metrics extracted from the analysis, focusing on the predicted optimal performance parameters.

Parameter	Value	Unit	Context
Target Microwave Frequency ($\omega_{m}$)	9.22	GHz	Resonant with NV 0 → +1 spin transition
Initial Ambient Temperature (T)	293	K	Room Temperature
Predicted Minimum Effective Mode Temperature (T_mode)	116	K	Achieved under strong laser pumping
Predicted Minimum Photon Number ($\langle \hat{a}^\dagger \hat{a} \rangle$)	261	Photons	Equivalent to 116 K
Laser Excitation Wavelength	532	nm	Used for optical spin cooling
Resonator Photon Damping Rate ($\kappa$)	1.88	MHz	Used in the simulation setup
Required NV Center Density (N)	4 x 10¹⁴	Centers	Target for strong coupling regime (10x increase over previous work)
Target Collective Coupling Strength ($\sqrt{2J}g$)	2$\pi$ x 1.8	MHz	Required to exceed spin dephasing rate ($\chi_{3}$)
Spin Dephasing Rate ($\chi_{3}$)	2$\pi$ x 0.64	MHz	Rate used in the multi-level JC model

Key Methodologies

The research employed a sophisticated theoretical and numerical approach based on a modified diamond maser setup to model the NV spin ensemble and microwave mode interaction.

System Setup: A diamond sample containing NV centers is excited by a 532 nm laser and coupled to a single-crystal sapphire dielectric ring microwave resonator inside a copper cylindrical cavity.
Spin Transition: A magnetic field Zeeman-splits the NV spin levels, tuning the 0 → +1 spin transition to resonate with the 9.22 GHz microwave mode.
Theoretical Model: A Multi-level Jaynes-Cumming (JC) model was developed to account for all electronic and spin levels of the NV center, providing a more complete treatment than simplified two-level models.
Simulation Technique: The quantum master equation for the coupled system was solved using a mean-field approach (cumulant expansion up to second order) to simulate the collective effects of trillions of NV centers.
Cooling Mechanism: Optical pumping (532 nm laser) polarizes the NV spin ensemble, which then acts as a cold bath to absorb thermal photons from the microwave mode, effectively cooling the resonator.
C-QED Analysis: The laser power was varied to control the collective coupling strength, demonstrating the transition from weak to strong coupling regimes, characterized by Rabi oscillations and Rabi splittings.

6CCVD Solutions & Capabilities

This research highlights the critical dependence of high-performance quantum devices on specialized diamond materials. 6CCVD is uniquely positioned to supply the advanced MPCVD diamond required to replicate and extend these groundbreaking results.

Applicable Materials

To achieve the predicted strong coupling regime ($\sqrt{2J}g \approx 2\pi \times 1.8$ MHz) and the minimum effective temperature (116 K), the research requires diamond with extremely high NV density and low spin dephasing.

Research Requirement	6CCVD Applicable Material	Technical Rationale
High NV Density & Coherence (N > 4 x 10¹⁴)	Optical Grade Single Crystal Diamond (SCD)	Ultra-high purity SCD with controlled nitrogen incorporation during MPCVD growth maximizes NV yield while minimizing strain and parasitic defects.
Low Spin Dephasing (Reducing ¹³C concentration)	Isotopically Pure SCD Substrates	6CCVD offers SCD with controlled isotopic purity, essential for reducing the decoherence caused by the nuclear spin bath, thereby enhancing the spin-resonator coupling ratio.
Potential for Electrical Control (Future gate operations)	Boron-Doped Diamond (BDD)	For future experiments requiring electrical gates or integrated circuits, 6CCVD supplies highly conductive BDD films (PCD or SCD) up to 500 µm thick.

Customization Potential

The integration of diamond into a complex dielectric ring resonator requires precise material engineering, which is a core capability of 6CCVD.

Custom Dimensions: The diamond sample must fit precisely within the 9.22 GHz cavity setup. 6CCVD provides custom-cut SCD plates and wafers up to 125 mm (PCD) and substrates up to 10 mm thick, ensuring compatibility with unique resonator geometries.
Precision Polishing: Achieving optimal coupling and minimizing optical/microwave losses requires exceptional surface quality. 6CCVD guarantees Ra < 1 nm polishing for SCD, which is critical for maintaining high Q-factors in the coupled system.
Custom Metalization: While the paper focuses on optical cooling, future C-QED devices may require integrated microwave transmission lines or electrical contacts. 6CCVD offers in-house deposition of standard quantum metals, including Ti/Pt/Au, Pd, W, and Cu, tailored to specific device layouts.

Engineering Support

The successful replication of this work depends heavily on optimizing the diamond growth recipe to control nitrogen concentration and isotopic purity.

Material Consultation: 6CCVD’s in-house PhD team specializes in the material science of NV centers. We offer expert consultation to assist researchers in selecting the optimal SCD specifications (e.g., nitrogen concentration, post-growth treatment) required to achieve the high NV density and low dephasing rates necessary for room-temperature C-QED effects.
Global Supply Chain: We ensure reliable, global shipping (DDU default, DDP available) of sensitive quantum materials, supporting international research efforts to advance cryogen-free quantum technologies.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41534-022-00642-z