The research activities of this team focus on quantum sensors based on spin defects in diamond for optical magnetometry applications.

We develop different techniques based on NV centres in diamond to measure magnetic fields with high sensitivity. For the study of magnetic properties of new 2D materials that requires nanometric resolution, we are building a single spin scanning probe magnetometer and implementing a modulation method combined with quantum protocols to greatly increase the detection sensitivity of DC magnetic fields and reach nT sensitivity. We are also developing magnetic field measurement techniques using ensemble of NV centres when nanometric resolution is not required.

We engineer, fabricate and characterize not only diamond tips with single NV centres for scanning probe magnetometry but also micro-/nano-structures compatible with integrated optics and containing ensembles of NV centres for magnetometry applications in the biological and industrial domains.

Another part of our research consists in investigating the optical and spin properties of NV centres in diamond, as well as their interactions with their environment under a wide range of temperatures (from room to cryogenic temperatures). We are also studying a new promising quantum sensor, the ST1 centre in diamond.

Our expertise ranges from experimental techniques (optical and local probe microscopies, MW and RF technologies, quantum protocols, fabrication technologies (e-beam lithography, ICP-RIE), cryogenic temperature, superconducting vector magnet) to simulation methods and programming (Python, Matlab, Octave programming, 3D-FDTD simulations (MEEP), FEM simulations (Comsol)).

The different research topics are:

The development of new 2D materials requires the study of their magnetic properties with a very high local resolution and sensitivity. This can be achieved by optically detecting the magnetic resonance (ODMR) of the single electronic spin of an NV centre located at the tip of a nanometre-size scanning diamond probe. For this purpose we are building a setup that consists of the combination of an atomic force microscope (AFM) and a confocal microscope. In order to increase the sensitivity of this local magnetometre to the nT scale we are implementing a lock-in detection based on the sample modulation at a given frequency f and a XY8-K decoupling sequence to extract the magnetic field modulated at f and suppress all other field fluctuations.

Keywords: quantum protocols (XY8-K decoupling sequence), python programming, MW and RF technologies, lock-in detection, confocal microscopy, atomic force microscopy, Hanbury Brown and Twiss detection setup.

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AFM/CFM setup with illuminated diamond tip and sample.
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Left: PL emission of a single NV centre located at a the apex of a diamond tip. Right: Measurement of the second order correlation function and the Rabi oscillations of a single NV centre in diamond.

Microwave antennas are necessary to optically detect magnetic resonances (ODMR). A research activity of our group consists in designing MW antennas with high transmission over specific frequency ranges, to fabricate them by optical lithography and to characterise them with spectrum analysis and optical magnetometry.

Keywords: FEM simulations (Comsol), fabrication (photo-/electron-lithography), characterisation (spectral analysis (S measurements) and scanning probe magnetometry with NV centres).

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Left: Ring antenna fabricated by photolithography (Fabrication: Fabian Schöppach). Right: Reflection spectra of the fabricated ring antenna (red curve) and of a Sasaki antenna (blue curve).

Due to the high refractive index of diamond, most of the light emitted by NV centres in bulk samples is radiated inside the substrate. The light collection efficiency is crucial for optical magnetometry devices. To control the photoluminescence emission of NV centres we engineer, fabricate and characterise different kinds of micro-/nano- structures: tips with a single NV centre for scanning probe optical magnetometry and micro-/nano-structures with ensemble of NV centres for magnetometry compatible with integrated optics.

Keywords: Simulations (FDTD, FEM), fabrication (electron beam lithography, ICP-RIE), characterization (SEM, confocal microscopy, AFM, ODMR, Rabi oscillations, spin coherence time measurements).

Diamond tips for scanning optical magnetometry

We use an angle etching technique to fabricate triangular-shaped diamond tips on the surface of ultra pure diamond. This method allows us to create single NV centres in each tip in a targeted manner by nano-implantation.

The tip shapes are firstly engineered to maximize the NV centre photoluminescence radiation towards the objective of our confocal microscope by performing 3D-FDTD simulations (MEEP). Secondly the Faraday cage necessary to fabricate the designed tips is engineered by simulating the plasma chamber with FEM (Comsol). Finally, the tips are fabricated by electron beam lithography, ICP-RIE and ion implantation. This technique of fabrication is rather inexpensive, as the diamond substrate only needs to be polished for reuse.

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Left: 3D-FDTD simulation of the PL emission of a single NV centre located at the apex of a triangular shaped diamond tip. Right: SEM picture of a pattern of tips with different sizes fabricated by e-beam lithography.
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FEM simulations of the plasma chamber containing a Faraday cage. Left: Particle trajectory of the ions. Right: Histograms of the incident angles and kinetic energies of the ions for a specific cage geometry (top with a…

Diamond micro-/nano-structures for integrated optical magnetometry

For magnetic field measurements that do not require high local resolution we manufacture micro-/nano-structures in HPHT diamonds and create ensemble of NV centres by ion implantation and annealing. The structures are designed with 3D-FDTD simulations either to steer the NV centre PL emission towards a free space collection system (lens and photodiode) or to match the PL emission pattern with the mode of an optical fiber. The micro-/nano-structures are finally fabricated by electron beam lithography and ICP-RIE.

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Left: SEM picture of a pattern of diamond pillars with different sizes fabricated by e-beam lithography and ICP-RIE. Right: 3D-FDTD simulation of the PL emission of a single NV centre located inside a diamond pillar.

We are studying the coupling mechanisms of NV centres in diamond. We perform microwave free magnetometry measurements, from liquid He temperature to room temperature, and model the measured photoluminescence emitted by the system with Python simulations.

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Left: MW-free magnetometry performed on NV centre ensemble at cryogenic temperature around the GSLAC. Right: Simulation of the PL emission of NV ensemble around the GSLAC.

We are also investigating ST1 centres in diamond, a new promising quantum sensor. We are studying their spin and optical properties. For example we characterise their hyperfine interactions with 13C atoms of the diamond lattice with CW and pulsed ODMR measurements and retrieve the hyperfine tensors with Python simulations.

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Electron spin resonance frequencies of an ST1 centre as a function of the applied magnetic field amplitude B. Left: For different angles between B and the ST1 axis. Right: With hyperfine coupling with a 13C atom when B…

Keywords: Microwave free magnetometry, superconducting vector magnet, cryogenic temperatures, ODMR (pulsed and CW), confocal microscopy, Python simulations.