Ultrastrong magnetic light-matter interaction with cavity mode engineering

At a Glance

Metadata	Details
Publication Date	2023-05-18
Journal	Communications Physics
Authors	Hyeongrak Choi, Dirk Englund
Citations	2
Analysis	Full AI Review Included

Technical Analysis: Ultrastrong Magnetic Light-Matter Interaction using MPCVD Diamond

This document analyzes the research on achieving ultrastrong magnetic light-matter interaction through cavity mode engineering, focusing on the critical role of high-purity diamond spin ensembles. It outlines how 6CCVD’s specialized MPCVD diamond materials and fabrication capabilities can support and advance this cutting-edge research in quantum computing and sensing.

Executive Summary

Core Achievement: Demonstrated ultrastrong magnetic light-matter coupling by drastically reducing the magnetic mode volume ($V_B$) and maintaining a high Quality factor ($Q$) in engineered microwave cavities.
Performance Metric: Achieved a theoretical $Q/V_B$ enhancement up to $3 \times 10^{16} \lambda^{-3}$, representing an increase of over $10^{16}$ times compared to free space, essential for high-cooperativity quantum systems.
Engineering Methods: Three primary techniques were employed: Longitudinal Squeezing, Current Engineering (reentrant cavities), and Magnetic Field Expulsion (using thin metallic plates analogous to the Meissner effect).
Material Limitation: The minimum achievable mode volume is ultimately limited only by the material’s electromagnetic penetration depth (e.g., 40 nm for superconducting Niobium).
Experimental Validation: Proof-of-principle experiments utilized an ensemble of Nitrogen-Vacancy (NV) centers embedded in electronic-grade diamond as the magnetic spin qubit.
Application Potential: These methods enable new applications in high-cooperativity microwave-spin coupling, compact Electron Paramagnetic Resonance (EPR) sensors, and fundamental physics searches (e.g., dark matter).

Technical Specifications

The following hard data points were extracted from the analysis and experimental validation:

Parameter	Value	Unit	Context
Maximum Projected $Q/V_B$	$3 \times 10^{16}$	$1/\lambda^{3}$	Combined method (Nb cavity, 1 µm plate thickness)
Experimental Mode Volume ($V_B$)	$1.75 \times 10^{-3}$	$\lambda^{3}$	Doubly reentrant cavity ($h=1.9$ cm)
Experimental Quality Factor ($Q$)	$2,421$	N/A	Measured at $f=2.871$ GHz (Copper cavity)
Experimental Resonance Frequency ($f$)	$2.871$	GHz	Matched to NV center resonance
Diamond NV Density	$300$	ppb	Electronic-grade diamond used for spin ensemble
Diamond Dimensions	$3 \times 3 \times 0.5$	mm$^{3}$	Substrate size for NV ensemble
NV Coherence Time ($T_2$)	Up to $1.5$	ms	Room temperature (Reference 29)
Superconductor Penetration Depth ($\lambda_L$)	$40$	nm	Niobium (Nb) at low temperature
Magnetic Field Enhancement	> $10^{16}$	Times	Compared to free space coupling strength

Key Methodologies

The research focused on manipulating the electromagnetic fields within hollow metallic cavities to maximize the magnetic field strength at the location of the spin ensemble while minimizing ohmic losses.

Cavity Selection: Utilized hollow cylindrical resonators supporting non-Transverse Electromagnetic (non-TEM) modes, specifically focusing on the lowest-order Transverse Magnetic (TM₀₁₀) mode, which allows for longitudinal squeezing.
Longitudinal Squeezing: Reduced the cavity height ($h$) to decrease $V_B$ proportionally, achieving a fourfold reduction in $V_B$ (from $0.140 \lambda^{3}$ to $0.0349 \lambda^{3}$) while maintaining the resonant frequency.
Current Engineering: Employed reentrant and doubly reentrant cavity designs to locally increase current density on the inner surfaces, which strongly enhances the magnetic field ($B$) and reduces $V_B$.
- Inverse-tapering of reentrances further increased current crowding, achieving $V_B$ as low as $5.06 \times 10^{-6} \lambda^{3}$ (doubly reentrant, inverse-tapered).
Magnetic Field Expulsion: Introduced thin metallic plates (thickness $t \approx 10$ µm) into the cavity, exploiting the demagnetization factor (analogous to the Meissner effect) to create magnetic “hot spots” near the plate edges, achieving $V_B \approx 3.18 \times 10^{-5} \lambda^{3}$.
Material Integration: Electronic-grade diamond substrates containing a controlled ensemble of NV centers (300 ppb) were placed at the magnetic antinode for experimental validation via Optically Detected Magnetic Resonance (ODMR).
Loss Mitigation: Theoretical modeling emphasized the necessity of superconducting materials (Niobium, Nb) to achieve ultra-high $Q$ factors ($Q \approx 10^{9}$) by minimizing surface resistance ($R_s$).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-quality diamond materials and precision fabrication services required to replicate and extend this research into functional quantum devices, particularly those relying on high-cooperativity microwave-spin coupling.

Applicable Materials

To replicate the experimental validation and achieve the projected theoretical performance, 6CCVD recommends the following materials:

Nitrogen-Doped Single Crystal Diamond (SCD): Essential for creating the active spin ensemble. We offer precise control over nitrogen concentration during MPCVD growth, enabling the targeted 300 ppb NV density or higher/lower concentrations required for specific quantum applications (e.g., single-spin operation or high-density ensembles).
Optical Grade SCD Substrates: Ideal for use as low-loss dielectric supports in the field-expulsion cavities. SCD is an ultra-low loss material, minimizing parasitic dielectric losses ($Q_{dielectric} \approx 10^{10}$ simulated) that limit overall cavity $Q$.
Polycrystalline Diamond (PCD) Substrates: For larger-scale microwave components or substrates requiring high thermal conductivity and mechanical robustness, 6CCVD can supply PCD wafers up to 125 mm in diameter.

Customization Potential

The success of mode engineering relies on micron-level precision and integration. 6CCVD’s in-house capabilities directly address the critical fabrication needs identified in the paper:

Research Requirement	6CCVD Capability	Benefit to Project
Specific Dimensions	Custom plates/wafers up to 125 mm (PCD) and substrates up to 10 mm thick.	Easily supply the required $3 \times 3 \times 0.5$ mm$^{3}$ NV diamond or custom geometries for reentrant structures.
Thin Film Integration	Internal metalization services: Au, Pt, Pd, Ti, W, Cu.	Apply custom electrodes or thin metallic films directly onto the diamond surface for current engineering or integration with superconducting circuits (e.g., flux qubits).
Ultra-Thin Plates	SCD/PCD thickness control from 0.1 µm to 500 µm.	Supply the ultra-thin (e.g., 1 µm) diamond plates required to achieve the maximum projected $Q/V_B$ enhancement in field-expulsion designs.
Surface Quality	Polishing to Ra < 1 nm (SCD) and Ra < 5 nm (PCD).	Ensures minimal surface scattering and low-loss interfaces, critical for maintaining high $Q$ factors in microwave systems.

Engineering Support

6CCVD’s in-house PhD team specializes in the material science of diamond quantum systems. We offer authoritative professional consultation to assist researchers in selecting the optimal diamond grade, nitrogen doping level, and surface preparation for high-cooperativity microwave-spin coupling and compact EPR sensor projects. We ensure that the material properties (purity, crystal orientation, and NV control) meet the stringent requirements for achieving ultrastrong light-matter interaction.

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/s42005-023-01224-x