Quantum sensing of weak radio-frequency signals by pulsed Mollow absorption spectroscopy

At a Glance

Metadata	Details
Publication Date	2017-10-11
Journal	Nature Communications
Authors	Timo Joas, A. M. Waeber, G. Braunbeck, Friedemann Reinhard
Institutions	Technical University of Munich, Schott (Germany)
Citations	63
Analysis	Full AI Review Included

Technical Documentation & Analysis: Quantum Sensing via Pulsed Mollow Absorption Spectroscopy

Executive Summary

This research demonstrates a significant advancement in solid-state quantum sensing by utilizing pulsed Mollow absorption spectroscopy on Nitrogen-Vacancy (NV) centers in diamond, enabling the detection of weak, high-frequency signals previously inaccessible by standard protocols.

High-Frequency Sensing Breakthrough: The protocol successfully detects GHz microwave fields, overcoming the limitation of conventional dynamical decoupling methods which are typically restricted to signal frequencies below a few MHz.
Enhanced Sensitivity: By harnessing the Mollow triplet splitting effect, the scheme shifts detection sensitivity from the inhomogeneous linewidth limit ($1/T_{2}^{*}$) toward the intrinsic spin coherence limit ($1/T_{2}$).
Robust Pulsed Protocol: The continuous wave (CW) Mollow absorption is converted into a robust pulsed sensing protocol (using sequences like XY8 and CPMG), mitigating decoherence caused by drive field power fluctuations ($\Omega$) by relying on precise pulse timing ($\tau$).
Material Requirement: Optimal performance, achieving $T_{2}$ times up to 10 ms and sensitivities down to 50 pT/√Hz, requires high-purity, isotopically engineered diamond ($^{12}$C < 77 K).
Future Applications: This technique is critical for enabling coherent coupling between solid-state spins and single phonons or microwave photons, and for developing room-temperature MASERs and quantum information systems.
6CCVD Value Proposition: 6CCVD provides the necessary ultra-high purity Single Crystal Diamond (SCD) substrates, including isotopically enriched $^{12}$C material, essential for replicating and extending this $T_{2}$-limited sensing capability.

Technical Specifications

Parameter	Value	Unit	Context
Target Signal Frequency	>100	MHz	Weak RF/Microwave signals
Detected Field Frequency	GHz	-	Microwave fields
Single NV Coherence Time ($T_{2}$)	1	ms	300 K, Natural Abundance
Single NV Coherence Time ($T_{2}$)	10	ms	< 77 K, $^{12}$C Isotopically Pure
Ensemble NV Coherence Time ($T_{2}$)	100	µs	300 K
Single NV Sensitivity ($\eta$)	5	nT/√Hz	300 K, Natural Abundance
Single NV Sensitivity ($\eta$)	50	pT/√Hz	< 77 K, $^{12}$C, Cryogenic
Ensemble NV Sensitivity ($\eta$)	1	pT/√Hz	300 K
Laser Excitation Wavelength	532	nm	Spin initialization/readout
Readout Detection Band	> 650	nm	Fluorescence detection
Diamond Material Used	Electronic Grade IIa	-	Polycrystalline CVD Diamond
Maximum Pulse Count ($N_{max}$)	≈ 1000	-	Limit before pulse errors degrade coherence

Key Methodologies

The experiment relies on precise microwave control and high-quality diamond material to create and probe the Mollow sidebands.

NV Center Preparation: Experiments were performed on single NV centers spontaneously created within a polycrystalline electronic grade IIa diamond substrate.
Quantum Control Generation: Both the strong drive (Mollow dressing field) and the weak signal (probe) were generated using an Arbitrary Waveform Generator (AWG), mixed onto a GHz frequency carrier, and amplified.
Microwave Delivery: The amplified microwave fields were applied to the NV center via a coplanar waveguide (CPW) fabricated on the diamond surface.
Spin Initialization and Readout: The NV spin state was initialized using a 532 nm laser pulse and read out via fluorescence detection (> 650 nm band) using a high-NA confocal microscope.
Pulsed Mollow Absorption Protocol: The strong CW drive was dissected into a train of $\pi$ pulses (dynamical decoupling sequences like XY8, CPMG, or XY4) to create robust dressed states.
Sideband Detection: The weak probe signal was applied, and its detuning ($\Delta$) was translated into periodic inversions of its axis, which are resonantly rectified by the strong drive, allowing detection at the Mollow sideband frequencies ($\omega_{0} \pm \pi/\tau$).
Sensitivity Optimization: Signal decay measurements confirmed that sensitivity is lost if the pulse spacing ($\tau$) exceeds the coherence time ($T_{2}^{*}$), emphasizing the need for short pulse spacing and long intrinsic $T_{2}$.

6CCVD Solutions & Capabilities

This research highlights the critical role of ultra-high purity diamond in achieving state-of-the-art quantum sensing performance. 6CCVD is uniquely positioned to supply the advanced materials and customization required to replicate and extend these results, particularly for scaling up to ensemble sensing or integrating into complex quantum devices.

Applicable Materials

To achieve the maximum coherence time ($T_{2}$ = 10 ms) and highest sensitivity (50 pT/√Hz) demonstrated in the paper, researchers must utilize diamond with minimal magnetic noise and strain.

Research Requirement	6CCVD Material Solution	Technical Specification
Maximized $T_{2}$ (10 ms)	Optical Grade Single Crystal Diamond (SCD)	Ultra-low nitrogen content (< 1 ppb).
Isotopic Purification	Isotopically Engineered SCD ($^{12}$C)	Carbon-12 enrichment > 99.995% for spin bath quenching.
Low Strain/High Quality	Electronic Grade SCD Wafers	Polished surfaces (Ra < 1 nm) suitable for high-resolution microscopy and lithography.
Ensemble Sensing Scale-Up	High-Purity Polycrystalline Diamond (PCD)	Wafers up to 125 mm diameter for large-area sensor arrays.

Customization Potential

The integration of NV centers into practical quantum devices, such as those requiring coupling to coplanar waveguides (CPW) or superconducting circuits, necessitates precise material engineering.

Custom Dimensions and Thickness: 6CCVD provides SCD plates and wafers with custom dimensions and thicknesses ranging from 0.1 µm to 500 µm, ensuring compatibility with specific cryogenic or microwave cavity geometries.
Integrated Metalization Services: The experiment utilized a CPW for microwave delivery. 6CCVD offers in-house metalization capabilities (Au, Pt, Pd, Ti, W, Cu) for direct deposition onto the diamond surface, streamlining the fabrication of integrated microwave circuits and antennas required for high-frequency sensing.
Precision Fabrication: We offer advanced laser cutting and shaping services to produce custom geometries, ensuring precise alignment and integration of the diamond substrate with external microwave components.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD growth parameters and post-processing techniques essential for quantum applications. We offer consultation services to optimize material selection for similar High-Frequency Quantum Sensing projects, including:

NV Density Control: Assistance in selecting the appropriate nitrogen concentration or implantation strategy to balance high sensitivity (ensemble NV) versus long coherence (single NV).
Surface Termination: Guidance on surface preparation to maintain the long $T_{2}$ times required for $T_{2}$-limited sensing protocols.
Cryogenic Compatibility: Material selection and processing optimized for operation at cryogenic temperatures (< 77 K) to achieve peak sensitivity.

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/s41467-017-01158-3