Heterodyne sensing of microwaves with a quantum sensor

At a Glance

Metadata	Details
Publication Date	2021-05-12
Journal	Nature Communications
Authors	Jonas Meinel, Vadim V. Vorobyov, Boris Yavkin, Durga Dasari, Hitoshi Sumiya
Institutions	National Institutes for Quantum and Radiological Science and Technology, Max Planck Institute for Solid State Research
Citations	63
Analysis	Full AI Review Included

Executive Summary

This research introduces a novel heterodyne detection method using Nitrogen-Vacancy (NV) centers in diamond, fundamentally advancing microwave (MW) sensing capabilities for engineering applications requiring ultra-high spectral resolution.

Core Value Proposition: Achieves spectral resolution for MW signals that is independent of the quantum sensor’s intrinsic lifetime (T₁ and T₂), overcoming a major limitation in conventional quantum sensing protocols.
Resolution Breakthrough: Demonstrated a spectral resolution below 1 Hz for a 4 GHz MW signal, achieving a Fourier-limited linewidth of 300 mHz over a 3-second correlation time. This is a 3 orders of magnitude improvement over the sensor’s T₁ lifetime limit (kilohertz range).
Methodology: The protocol mixes the external MW signal with a coherent local oscillator (reference) to measure the relative phase, effectively demodulating the signal into the low-frequency domain.
Interaction Control: The interaction between the MW field and the NV spin is controlled using two dressing techniques:
1. Pulsed Mollow Absorption (Dynamical Decoupling): Leads to improved sensitivity (estimated 203 nT/√Hz).
2. Floquet Dynamics (Strong RF Drive): Provides robust control and creates detection sidebands independent of the system’s resonance frequency.
Projected Performance: Projected sensitivity for optimized MW heterodyne detection is 26 nT/√Hz, comparable to state-of-the-art NV sensing protocols.

Technical Specifications

Parameter	Value	Unit	Context
MW Signal Frequency (Tested)	4139.4	MHz	NV electron spin transition frequency
Spectral Resolution (Achieved)	300	mHz	Fourier-limited linewidth for 3s correlation
Spectral Resolution (Goal)	less than 1	Hz	Demonstrated resolution limit
Projected Sensitivity (Optimized)	26	nT/√Hz	Estimated optimal performance
Measured Sensitivity (Current)	203 ± 15	nT/√Hz	Single NV center, current protocol
Static Magnetic Field (B₀)	250	mT	Applied by superconducting magnet
NV Coherence Time (T₂)	300	µs	Typical lifetime in the diamond slice
NV Relaxation Time (T₁)	50	µs	Typical lifetime in the diamond slice
AWG Sampling Rate	12	GSamples/s	Arbitrary Waveform Generator
Electron Irradiation Energy	2	MeV	Used for creating NV centers
Electron Irradiation Fluence	1.3 x 10¹¹	cm^-2	Total dose for NV creation
Annealing Temperature	1000	°C	Post-irradiation thermal treatment (2 h in vacuum)
Excitation Wavelength	520	nm	Diode laser for initialization and readout

Key Methodologies

The experiment relies on high-quality ¹²C-enriched diamond samples and precise quantum control sequences generated by an Arbitrary Waveform Generator (AWG).

Sample Preparation:
- Used a 2 mm x 2 mm x 80 µm, (111)-oriented diamond crystal enriched with ¹²C (99.995%).
- Grown via the Temperature Gradient Method (TGM) under High-Pressure High-Temperature (HPHT) conditions (5.5 GPa, 1350 °C).
- NV centers were created from intrinsic nitrogen via 2 MeV electron irradiation, followed by annealing at 1000 °C for 2 hours in vacuum.
Confocal Setup and Control:
- A confocal microscope setup was used within the bore of a 250 mT superconducting magnet.
- Optical initialization and readout were performed using a 520 nm laser and an Avalanche Photo-Diode (APD).
- MW and Radio Frequency (RF) fields were generated by a 12 GSamples/s AWG and amplified (MW up to 40 dBm, RF up to 52 dBm).
Heterodyne Sensing Protocol:
- Initialization: NV spin is initialized into the $|0\rangle$ state using a green laser pulse.
- State Preparation: A coherent external MW source applies a π/2 pulse, creating an initial superposition state $|\Psi_{\text{init}}\rangle$ (the reference frame).
- Evolution/Sensing: The spin evolves under the influence of the target MW signal and applied dressing fields (Pulsed Mollow or Floquet RF drive) for time $\tau$.
- Readout: The final state’s z-projection ($\langle S_z \rangle$) is measured via fluorescence photon counting.
Interaction Control Techniques:
- Pulsed Mollow Absorption: Uses Dynamical Decoupling sequences (e.g., CPMG) to control the interaction time and increase the sensor lifetime from T₂ to T_1,p.
- Floquet Dynamics: Applies a strong longitudinal RF drive ($\Omega_{\text{rf}} \gg \omega_{\text{rf}}$) to create new dressed states and detection sidebands ($\omega_s \pm \Delta m \omega_{\text{rf}}$), allowing robust control independent of the NV resonance frequency.
Data Analysis:
- A series of sequential measurements are performed, where the initial phase of the reference or dressing field is systematically varied.
- The autocorrelation $C(n)$ of the measured photon counts is computed.
- The Fast Fourier Transform (FFT) of the autocorrelation yields the demodulated frequency $\delta\omega$, providing the high spectral resolution.

Commercial Applications

The ability to sense weak, high-frequency signals with ultra-high spectral resolution is critical across several advanced technology sectors:

Quantum Sensing and Metrology:
- High-resolution magnetic resonance spectroscopy (NMR/EPR) using nanoscale sensors.
- Sensing weak magnetic fields in complex environments (e.g., biological systems or integrated circuits).
Quantum Computing and Communications:
- Precise control and readout of superconducting qubits (transmons) operating in the MW regime.
- Quantum feedback systems requiring sequential weak measurements of quantum states.
Defense and Aerospace:
- Advanced Quantum Radar and Doppler velocimetry technologies requiring high spectral purity detection.
- Detection of weak, highly coherent signals (e.g., in Masers or specialized communication protocols).
Fundamental Physics:
- Studies of weak cosmic radiation and high-frequency phenomena in cosmology.
- Measuring the quantum behavior of mesoscopic bosonic or fermionic systems at high frequencies.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41467-021-22714-y