Dopant-assisted stabilization of negatively charged single nitrogen-vacancy centers in phosphorus-doped diamond at low temperatures

At a Glance

Metadata	Details
Publication Date	2023-10-27
Journal	npj Quantum Information
Authors	Jianpei Geng, Tetyana Shalomayeva, Mariia Gryzlova, Amlan Mukherjee, S. Santonocito
Institutions	University of Stuttgart, Kyoto University
Citations	13
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a critical advancement in stabilizing the negatively charged nitrogen-vacancy (NV^-) center in diamond, a key requirement for solid-state quantum technologies.

Repump Laser Elimination: Stable NV^- charge state is achieved in phosphorus (P)-doped diamond at 4 K without the need for an external repump laser, overcoming a major bottleneck that causes spectral diffusion and slows down quantum protocols.
Dopant-Assisted Recombination: The stabilization mechanism relies on the photo-ionization of shallow P donors (0.57 eV below the conduction band), which supply free electrons to convert the neutral NV⁰ state back to the desired NV^- state.
Linear Recombination Rate: The recombination rate (NV⁰ → NV^-) exhibits a linear dependence on the resonant laser power, contrasting sharply with the conventional quadratic dependence observed in intrinsic diamond using a repump laser. This linearity confirms the P-assisted mechanism.
High NV^- Population: For growth-created NV centers, a near-unity NV^- population (96%) is maintained under continuous resonant excitation (636 nm).
Cryogenic Operation: The stable photoluminescence-excitation (PLE) spectra and charge dynamics were successfully monitored in real-time at liquid helium temperatures (4 K), enabling applications in low-temperature quantum networks and computation.
Coherence Time Improvement: Growth-created NV centers in the P-doped layer showed a long spin coherence time (T₂ = 1.94 ms), attributed to the suppression of paramagnetic vacancy complexes due to P-induced charging.

Technical Specifications

Parameter	Value	Unit	Context
Operating Temperature	4	K	Liquid helium cryostat
P Donor Concentration	5 x 10¹⁶	atoms cm^-3	Epitaxially grown diamond layer
P Donor Energy Level	0.57	eV	Below conduction band edge
Resonant Excitation Wavelength	636	nm	Used for PLE and charge dynamics
Typical Excitation Power (Growth NV)	148	nW	For time trace measurement
NV^- Population (P_-)	96(2)	%	Growth-created NV at 148 nW
Spin Coherence Time (T₂, Growth NV)	1.94	ms	Hahn Echo measurement
Spin Coherence Time (T₂, Implanted NV)	11.53	µs	Hahn Echo measurement
Ionization Rate Dependence (k_ion)	Quadratic	N/A	Two-photon process (NV^- → NV⁰)
Recombination Rate Dependence (k_rec)	Linear	N/A	P-assisted process (NV⁰ → NV^-)
Ionization Saturation Power (P_s)	4.5	µW	Fit parameter for k_ion
Transverse Strain (Inferred)	~4.5	GHz	From PLE peak analysis
Implantation Energy (¹⁵N⁺)	9.8	keV	For creating additional NV centers
Implantation Dose (¹⁵N⁺)	1.3 x 10¹⁰	atoms cm^-2	For creating additional NV centers
Inferred Electron Density (n_e)	10¹⁰ to 10¹⁴	cm^-3	Estimated from electric field screening

Key Methodologies

The experiments utilized a home-built cryogenic confocal microscope to analyze single NV centers in phosphorus-doped diamond.

Sample Preparation (CVD Growth):
- Phosphorus-doped diamond was epitaxially grown onto Ib-type (111)-oriented diamond substrates using Chemical Vapor Deposition (CVD).
- The target phosphorus concentration was 5 x 10¹⁶ atoms cm^-3.
- NV centers were created both during the growth process and via subsequent ion implantation.
Ion Implantation and Nanostructuring:
- Additional NV centers were created by implanting ¹⁵N⁺ ions at 9.8 keV energy and a dose of 1.3 x 10¹⁰ atoms cm^-2.
- Arrays of nanopillars (400-500 nm apex size) were fabricated to enhance photon collection efficiency for implanted NV centers.
Cryogenic Confocal Microscopy:
- Measurements were performed at 4 K using a Janis ST-500 cold finger Helium flow cryostat.
- Excitation was achieved using a 636 nm resonant laser, focused through a high-NA objective (Nikon LU Plan Fluor 100x, NA0.9).
Photoluminescence-Excitation (PLE) Spectroscopy:
- PLE spectra were recorded by sweeping the resonant laser frequency while monitoring fluorescence (660-800 nm bandpass filter).
- The stability of the NV^- charge state was confirmed by obtaining stable PLE spectra over multiple repetitions without applying a repump laser.
Real-Time Charge Dynamics Monitoring:
- Fluorescence time traces were continuously recorded under resonant excitation (636 nm) to observe blinking events (NV^- ↔ NV⁰ switching).
- Histograms of photon counts were fitted to extract the ionization rate (k_ion) and recombination rate (k_rec) as a function of laser power.
Rate Analysis and Modeling:
- k_ion was fitted using a saturation-modified quadratic function (two-photon process).
- k_rec was fitted using a linear function, confirming the theoretical model where the rate is proportional to the carrier density generated by P photo-ionization.

Commercial Applications

The stabilization of the NV^- charge state at cryogenic temperatures without complex repump schemes is crucial for scaling up diamond-based quantum technologies.

Quantum Computing and Simulation:
- Qubit Initialization: Enables faster, high-fidelity initialization of the NV spin state by removing the time overhead and noise associated with repump lasers.
- Quantum Networks: Simplifies the architecture of remote quantum entanglement protocols (e.g., those requiring high entanglement rates ^18,19) by ensuring the NV center remains in the optically active NV^- state.
Quantum Sensing and Metrology:
- High-Sensitivity Magnetometry: Stable NV^- population, especially for shallow NV centers, improves sensitivity and spatial resolution in nanoscale magnetic sensing and imaging (e.g., NMR, ESR).
- Reduced Spectral Diffusion: Eliminating the repump laser minimizes spurious effects like spectral diffusion, leading to narrower optical linewidths (though P-doping introduces its own broadening), which is essential for high-resolution optical interfaces.
Diamond Material Engineering:
- Optimized Doping Recipes: The developed model provides guidance for optimizing phosphorus concentration to balance charge stability (high P) against electric field noise (low P), leading to better material specifications for quantum applications.
Cryogenic Device Integration:
- Simplified Cryogenic Systems: Reducing the number of required external lasers (repump fields) simplifies the optical setup and integration within complex cryogenic environments.

View Original Abstract

Abstract Charge state instabilities have been a bottleneck for the implementation of solid-state spin systems and pose a major challenge to the development of spin-based quantum technologies. Here we investigate the stabilization of negatively charged nitrogen-vacancy (NV − ) centers in phosphorus-doped diamond at liquid helium temperatures. Photoionization of phosphorous donors in conjunction with charge diffusion at the nanoscale enhances NV 0 to NV − conversion and stabilizes the NV − charge state without the need for an additional repump laser. The phosphorus-assisted stabilization is explored and confirmed both with experiments and our theoretical model. Stable photoluminescence-excitation spectra are obtained for NV − centers created during the growth. The fluorescence is continuously recorded under resonant excitation to real-time monitor the charge state and the ionization and recombination rates are extracted from time traces. We find a linear laser power dependence of the recombination rate as opposed to the conventional quadratic dependence, which is attributed to the photo-ionization of phosphorus atoms.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41534-023-00777-7