Readout and control of an endofullerene electronic spin

At a Glance

Metadata	Details
Publication Date	2020-12-17
Journal	Nature Communications
Authors	Dinesh Pinto, Domenico Paone, Bastian Kern, Tim Dierker, René Wieczorek
Institutions	University of Stuttgart, Osnabrück University
Citations	32
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a critical step toward scalable molecular quantum technologies by achieving single-spin readout and control of an endofullerene molecule (¹⁴N@C₆₀).

Core Achievement: Successful single-spin Electron Paramagnetic Resonance (EPR) readout and control of the electronic spin of ¹⁴N@C₆₀ (Nitrogen encapsulated in a C₆₀ cage).
Sensing Platform: A single, near-surface Nitrogen Vacancy (NV) center in diamond (3-8 nm deep) is used as a noninvasive local magnetic field sensor at 4.7 K.
Interaction Mechanism: The NV center couples to the endofullerene spin via strong magnetic dipolar interaction, measured at a separation of approximately 5.6 nm, corresponding to a coupling strength of 20.29 MHz.
Spin Control Demonstrated: Radio-frequency (RF) pulse sequences successfully drove Rabi oscillations on the endofullerene spin, achieving spin-state switching rates up to 12.47 MHz.
Coherence Measurement: Spin-echo measurements established a lower limit for the endofullerene phase coherence time (T₂) of 1 µs.
Material Effects: Surface adsorption of the ¹⁴N@C₆₀ molecule onto the diamond surface enhanced the isotropic hyperfine constant (A=19 MHz) and induced an axial zero-field splitting (D=1.52 MHz).
Future Scalability: These results enable the integration of robust, individually addressable molecular spins into large-scale quantum architectures using standard positioning or self-assembly methods.

Technical Specifications

Parameter	Value	Unit	Context
Operating Temperature	4.7	K	Cryogenic measurement environment.
Static Magnetic Field (B₀)	9.697	mT	Field used for single-spin EPR spectroscopy.
NV Center Depth	3-8	nm	Near-surface NV centers used as local sensors.
NV-Endofullerene Separation (r)	5.6(1)	nm	Extracted from initial linear dephasing rate.
Dipole-Dipole Coupling Strength (J)	20.29(2)	MHz	NV-¹⁴N@C₆₀ interaction strength.
Maximum Rabi Frequency (V_Rabi)	12.47(1)	MHz	Tunable spin-state switching rate via RF control.
Endofullerene Coherence Time (T₂)	≥ 1	µs	Lower limit measured via spin-echo sequence.
NV Spin-Echo Coherence Time (T_NV)	≈ 2.5	µs	Coherence time of the NV sensor itself.
¹⁴N@C₆₀ Hyperfine Constant (A)	19	MHz	Enhanced value due to surface adsorption (vs. 15.85 MHz bulk).
Axial Zero-Field Splitting (D)	1.52	MHz	Induced by surface adsorption effects.
¹⁴N@C₆₀ Solution Concentration	0.1	µLL^-1	Concentration used for drop-casting.
Probability of Single-Spin Coupling	≈ 4.5	%	Calculated probability within the NV sensing radius (~10 nm).
RF Pulse Duration (Minimum)	1/(π * 12.47) ≈ 25.5	ns	Minimum pulse duration for π-pulse at maximum Rabi frequency.

Key Methodologies

The experiment utilized a home-built low-temperature (4.7 K) and ultrahigh vacuum (10^-10 mbar) setup capable of confocal microscopy and pulsed MW/RF control.

Diamond Substrate Preparation: Used a 30 µm thick electronic grade [100] diamond.
NV Center Creation: Implanted with ¹⁵N at 5 keV energy, followed by annealing at 975 °C for 2 hours.
Optical Enhancement: Tapered nanopillar waveguides (700 nm base, 400 nm tip, 1 µm height) were etched into the diamond to increase optical collection efficiency.
Surface Cleaning and Termination: The diamond surface was cleaned and oxygen-terminated by boiling in a tri-acid mixture (1:1:1 HNO₃:H₂SO₄:HClO₄) at 200 °C for 5 hours.
Endofullerene Deposition: Powder ¹⁴N@C₆₀ (filling factor 10^-4) was dissolved in toluene to a concentration of 0.1 µLL^-1. A 1 µL aliquot was drop-coated onto the cleaned diamond surface under ambient conditions.
NV Initialization and Readout: NV spins were initialized and read out using a 515 nm laser focused via a low-temperature objective (NA = 0.82). Red fluorescence (filtered at 650 nm) was monitored using Hanbury Brown-Twiss (HBT) configuration.
Spin Control: MW and RF pulses were delivered via a 20 µm thick gold wire fabricated across the diamond surface.
Measurement Technique: Pulsed Electron-Electron Double Resonance (PELDOR or DEER) spectroscopy was performed using a double-quantum (DQ) pulse scheme to synchronize the central spin-flip of the NV center with the RF spin-flip pulse on the external ¹⁴N@C₆₀ spin.

Commercial Applications

The ability to individually address and control robust atomic spins packaged within molecular cages (endofullerenes) is crucial for several emerging high-tech sectors.

Quantum Computing and Information Processing (QIP): Endofullerenes act as robust, pre-packaged qubits (electron spin S=3/2, nuclear spin I=1) that can be precisely positioned and integrated into solid-state architectures (like diamond NV centers) for scalable quantum registers.
Solid-State Quantum Memories: The nuclear spin of the encapsulated atom (e.g., ¹⁴N or ³¹P) offers long coherence times, making endofullerenes excellent candidates for quantum memory storage interfaced via the electronic spin “bus.”
Nanoscale Magnetic Sensing and Metrology: The NV center/endofullerene system provides a platform for ultra-sensitive, noninvasive local magnetic field sensing, potentially applicable in materials science or biological imaging.
Molecular Spintronics: Utilizing the intrinsic spin properties of molecular systems for data storage and processing, leveraging the chemical tunability and robust nature of the fullerene cage.
Scalable Quantum Architectures: The demonstrated compatibility with C₆₀ self-assembly techniques and standard positioning methods (STM dragging, CNT packing) provides a pathway for manufacturing large-scale, ordered quantum device arrays.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41467-020-20202-3