Phase-sensitive quantum spectroscopy with high-frequency resolution

At a Glance

Metadata	Details
Publication Date	2021-08-30
Journal	Physical review. A/Physical review, A
Authors	Nicolas Staudenmaier, Simon Schmitt, Liam P. McGuinness, Fedor Jelezko
Institutions	Australian National University, Center for Integrated Quantum Science and Technology
Citations	20
Analysis	Full AI Review Included

Technical Documentation & Analysis: High Frequency Quantum Spectroscopy

Research Paper Analyzed: Staudenmaier et al., “Phase sensitive quantum spectroscopy with high frequency resolution” (arXiv:2105.08381v2, 2022).

Executive Summary

This research successfully demonstrates a novel quantum spectroscopy protocol, High frequency Qdyne, utilizing a single Nitrogen-Vacancy (NV) center in diamond to achieve full signal reconstruction (phase, amplitude, and frequency) of high-frequency oscillating magnetic fields.

Core Achievement: Construction of an atomic heterodyne detector from a single quantum coherent spin, enabling nanoscale spatial resolution for spectrum analysis up to potentially 100 GHz.
Material Requirement: The protocol relies critically on the use of ultra-high purity, isotopically engineered ¹²C Single Crystal Diamond (SCD) to maximize spin coherence time (T₂ ~50 µs).
Exceptional Precision: Achieved a relative frequency uncertainty of < 10^-12 for a 1.51 GHz signal within 10 seconds of integration time.
High Sensitivity: Demonstrated amplitude sensitivity of 58 nT/√Hz and phase sensitivity of 0.095 rad/√Hz without employing dynamical decoupling.
Methodology: The technique uses a local oscillator (defined by the measurement sequence) to create a beat-note with the signal, allowing FFT analysis for simultaneous phase and amplitude recording.
Applications: Opens pathways for advanced nanoscale characterization in quantum technologies, including electron spin detection, spin wave analysis, and miniaturized circuit diagnostics.

Technical Specifications

The following hard data points were extracted from the research paper characterizing the performance of the High frequency Qdyne protocol:

Parameter	Value	Unit	Context
Amplitude Sensitivity	58	nT/√Hz	Noise floor at T = 1s integration time
Phase Sensitivity	0.095	rad/√Hz	Calculated from full signal reconstruction data
Relative Frequency Uncertainty	< 10^-12	N/A	Achieved for 1.51 GHz signal (10s integration)
Dephasing Time (T₂)	~50	µs	Achieved using 99.999% isotopically purified ¹²C diamond
¹²C Isotopic Purity	99.999	%	Required for ultralong spin coherence
Diamond Overgrowth Layer Thickness	~100	nm	Thin layer of ¹²C SCD containing NV centers
Signal Frequency Measured	1.51	GHz	Close to the NV center resonance frequency
Maximum Dynamic Range (B_max)	0.36	µT	Upper limit for optimal sensing time (τ = T/2 = 50 µs)
DC Magnetic Field Applied	~50	mT	Static field aligned along the NV axis
DC Resonance Shift Applied	-3	MHz	Achieved via ~1 Gauss magnetic field from control stripline

Key Methodologies

The experiment relies on precise material engineering and synchronized microwave control, summarized below:

Diamond Material Fabrication: A hemispherical diamond substrate was overgrown with a thin (~100 nm) layer of 99.999% isotopically purified ¹²C Single Crystal Diamond (SCD) to maximize the NV center spin coherence time (T₂).
Optical Setup: A home-built confocal microscope was used, employing a 532 nm green laser for NV center excitation and an avalanche photodiode (APD) for collecting red-shifted fluorescence (spin-dependent readout).
Magnetic Field Control: A static external magnetic field (~50 mT) was applied along the NV axis to tune the spin resonance frequency. A DC shift (-3 MHz) was applied via constant current through a 20 µm thin copper wire stripline near the NV center.
Microwave Signal Generation: Microwave control pulses and the GHz signal fields were generated using an Arbitrary Waveform Generator (AWG) and a stabilized signal generator, combined and delivered via the copper stripline.
Qdyne Protocol Sequence:
- Initialization: NV center polarized to the |0> state via laser irradiation.
- Preparation: A resonant π/2-pulse (known phase) prepares the NV in a superposition state (| +i>).
- Sensing: The NV center interacts with the near-resonant signal field for a fixed time (τ).
- Readout: Spin population is measured optically, and the resulting fluorescence count trace is recorded by a time-tagged single photon counting card (TTSPC).
Signal Reconstruction: The discrete Fourier transform (FFT) of the sampled population oscillation reveals the beat-note frequency (δ = ν_sig - ν_LO). Lorentzian fitting is used to estimate frequency, amplitude, and phase, allowing full signal reconstruction.

6CCVD Solutions & Capabilities

The success of this high-resolution quantum spectroscopy technique is fundamentally dependent on the quality and customization of the diamond material. 6CCVD is uniquely positioned to supply the necessary high-purity, custom-engineered MPCVD diamond required to replicate and advance this research.

Applicable Materials for Quantum Sensing

To achieve the T₂ coherence times (~50 µs) and spectral resolution demonstrated, the researchers required ultra-high isotopic purity. 6CCVD offers the following materials:

Material Grade	Description	Relevance to Qdyne Protocol
Optical Grade SCD (Isotopically Purified)	Single Crystal Diamond with ultra-low nitrogen concentration and isotopic purity (e.g., < 100 ppb ¹³C).	CRITICAL: Directly enables the long T₂ coherence time necessary for high spectral resolution and sensitivity (T₂ limited bandwidth).
Custom NV Incorporation	Controlled introduction of NV centers (e.g., via delta doping or implantation) at specific depths.	Ensures NV centers are located within the thin, high-purity ¹²C layer and positioned optimally for nanoscale sensing.
High-Purity Substrates	SCD substrates up to 10 mm thick, suitable for high-power optical setups and thermal management.	Provides the robust foundation for the hemispherical or solid immersion lens geometries used to maximize photon collection efficiency.

Customization Potential for Device Integration

The experiment utilized a specific geometry (hemispherical lens) and integrated control lines (copper stripline). 6CCVD provides comprehensive customization services to streamline device fabrication:

Custom Dimensions and Geometry: 6CCVD can supply SCD plates and wafers with custom dimensions and can perform precision laser cutting and shaping required for advanced optical components, such as the hemispherical lenses or solid immersion lenses (SILs) used in the confocal setup.
Precision Thin Film Growth: We offer precise control over SCD layer thickness from 0.1 µm to 500 µm. This capability is essential for replicating the ~100 nm thin, high-purity ¹²C layer used to position NV centers close to the surface for nanoscale spatial resolution.
Integrated Metalization Services: The experiment required a 20 µm copper stripline for delivering MW signals and applying DC shifts. 6CCVD offers in-house metalization capabilities, including deposition of Au, Pt, Pd, Ti, W, and Cu, allowing researchers to receive diamond chips with pre-patterned microwave and DC control lines.
Surface Quality: Our SCD polishing achieves surface roughness Ra < 1 nm, minimizing optical losses and ensuring optimal performance for high-NA confocal microscopy.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD diamond growth and material science for quantum applications. We can assist researchers in material selection and specification for similar NV-based quantum sensing and high-frequency spectroscopy projects, ensuring the optimal balance between isotopic purity, NV density, and surface termination.

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

View Original Abstract

Classical sensors for spectrum analysis are widely used but lack micro- or nanoscale spatial resolution. On the other hand, quantum sensors, capable of working with nanoscale precision, do not provide precise frequency resolution over a wide range of frequencies. Using a single spin in diamond, we present a measurement protocol for quantum probes which enables full signal reconstruction on a nanoscale spatial resolution up to potentially 100 GHz. We achieve 58nT/√Hz amplitude and 0.095rad/√Hz phase sensitivity and a relative frequency uncertainty of 10−12 for a 1.51 GHz signal within 10 s of integration. This technique opens the way to quantum spectrum analysis methods with potential applications in electron spin detection and nanocircuitry in quantum technologies.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physreva.104.l020602