How it works

QZabre’s quantum sensor tips and QSM instrument are based on the technique of scanning NV magnetometry:

What is a scanning NV magnetometer?

A scanning NV magnetometer is a next-generation scanning probe microscope that utilizes a so-called nitrogen-vacancy center (NV center) as an atomic size magnetic field sensor. The NV center is a lattice defect in diamond whose magnetic orientation can be measured by optical fluorescence. Because NV centers show quantum behavior even at room temperature, the scanning NV magnetometer can exploit quantum metrology techniques to achieve very high magnetic field sensitivity. Scanning NV magnetometers are therefore expected to enable a breakthrough in high-resolution and passive magnetic analysis of surfaces.

How does the scanning NV magnetometer work?

The scanning NV magnetometer relies on the interaction of the NV center with the magnetic field. As the magnetic field at the location of the NV center increases, the magnetic interaction becomes stronger – more energy is required to flip the magnetic orientation of the NV center. This energy can be probed by the technique of electron paramagnetic resonance (EPR) spectroscopy. For this purpose, the NV center is exposed to microwaves with frequencies of typically 2-4 GHz. If the energy of the microwaves (given by E=hν, where ν is the microwave frequency and h=6.63×10-34 J/Hz the Planck constant) exactly equals the magnetic interaction energy of the NV center, its magnetic orientation is rapidly flipped. Under optical illumination, this reorientation manifests itself by a reduction in fluorescence, which is detected by the optical microscope. Since the dependence of the resonance frequency on the magnetic field is extremely well known, the resonance frequency provides a direct and quantitative measurement of the local magnetic field.

Because NV centers are stable to within a few nanometers from the diamond surface, they can be readily embedded in nanostructures, such as the sharp tips required for scanning probe microscopy. By scanning this tip over a surface, the scanning NV magnetometer can record maps of the magnetic field present over a sample with high sensitivity and spatial resolution.

nsdm_schematic
(Left) Basic principle of the scanning NV magnetometer. A sharp diamond tip containing a single nitrogen-vacancy impurity (NV center) is positioned over the sample under study. The scanning NV magnetometer records the interaction of the NV center with the local magnetic field. Illustration adapted from Ref. 1. (Right) Experimental signal, showing the reduction of the optical fluorescence when microwaves are applied at the spin resonance frequency. The local magnetic field B can be directly deduced from the frequency difference between the two resonance peaks, given by 2γB, where γ = 28 MHz/mT is the transduction factor or gyromagnetic ratio.

What sensitivity and spatial resolution can I expect?

The sensitivity and spatial resolution of the scanning NV magnetometer crucially depend on the diamond probe tip. Since the spatial resolution is roughly determined by the vertical separation between the NV center and the sample surface, the impurity must be located within nanometer from the tip apex, and the tip standoff must be kept as small as possible. The QZabre sensor maximizes the spatial resolution by using shallow ion implantation to form NV centers at ~10 nm from the apex, and by employing a proprietary chip assembly technique that minimizes the tip standoff.

The magnetic sensitivity of the scanning NV magnetometer is determined by a combination of the spin dephasing time T2*, the optical contrast ε and the maximum photon count rate I0. A generic estimate for the sensitivity is given by

Formula B_min

where Bmin is the minimum detectable magnetic field (defined by the field that yields an SNR of unity), γ=28 MHz/mT is the transduction parameter (or gyromagnetic ratio), T is the averaging time, and tacq is the photon integration time (which is typically 300 ns for NV centers). The QZabre sensor implements several advanced features to optimize the sensitivity. High quality single crystal diamond is chosen as the starting material to ensure long intrinsic dephasing and coherence times. A series of nanofabrication and processing steps is used to form diamond tips whose shape is optimized for high photon count rates. Taking conservative numbers for the QZabre sensor (ε=0.2, I0=200 kC/s, T2*=1.5 μs), the minimum detectable field is about 0.5-1 μT for pulsed operation and 5-10 μT for continuous-wave (cw) operation, at an averaging time of one second.

What are the applications of scanning NV magnetometry?

The primary application of the scanning NV magnetometer is the high-resolution, sensitive and passive magnetic analysis of surfaces. Multiple magnetic characteristics can be probed by adjusting sequences of laser and microwave pulses: static stray fields, vector components of the field, magnetic fluctuations, and noise spectra. In principle, even electric fields and temperature changes can be detected. These features lead to an extraordinarily broad range of potential applications:

  • Quantitative analysis of current distributions in semiconductors, graphene devices, photoactive films, integrated circuits.
  • Vector field analysis of magnetic domains, defects, and nanostructures.
  • Spin relaxometry.
  • Nanoscale thermometry.
  • Study of novel materials, including complex oxides, skyrmion systems, and topological insulators.
  • Imaging of biomagnetic structures and nanoparticle markers in living cells under ambient conditions.
  • Label-free, noninvasive mapping of bioelectric signals.

What do I need to operate the QZabre sensor?

The QZabre sensor is designed to fit into existing scanning NV magnetometer instruments, including the QSM. Many researchers also use home-built microscopes or modified commercial instruments, and we are happy to advise users on the choice of instrumentation. Since there is considerable flexibility in configuring the elements of the QZabre sensor, we found that it can be integrated in most systems without the need for major modifications.

In its standard configuration, the QZabre sensor is illuminated from the back side (top side), that is, scanning tip and optical objective are on the same side of the sample surface. This allows for the investigation of opaque samples. An AFM controller that supports either amplitude detection or frequency detection is needed to actuate and readout the tuning fork sensor.

Setup-close
Close-up view of a scanning NV magnetometer with the QZabre sensor in the center, illuminated by green laser light. Microscope objective, microwave antenna and sample holder are also visible.

Literature

  1. R. Schirhagl, K. Chang, M. Loretz, and C. L. Degen, Nitrogen-Vacancy Centers in Diamond: Nanoscale Sensors for Physics and Biology, Annual Reviews in Physical Chemistry 65, 83-105 (2014).
  2. L. Rondin, J. P. Tetienne, T. Hingant, J. F. Roch, P. Maletinsky, and V. Jacques, Magnetometry with nitrogen-vacancy defects in diamond, Reports on Progress in Physics 77, 056503 (2014).
  3. F. Jelezko, and J. Wrachtrup, Single defect centres in diamond: A review, physica status solidi (a) 203, 3207 (2006).