Pushing sensing technologies into previously inaccessible domains is one of the great challenges in physics. Our group’s research is aimed at developing sensors that can image magnetic and electric fields with high sensitivity and nanoscale resolution. This allows us to investigate phenomena occurring at a scale that cannot be interrogated by previously established techniques. In particular, we use the Nitrogen-Vacancy center in diamond as our sensing tool. We consider how these sensors can be improved, leading to work in a wide variety of topics. These include research in quantum optics to improve the photon collection efficiency, and hence accuracy, of the NV spin readout, along with fundamental investigations on how mechanical oscillations can be coupled to the NV center spin, which could lead to higher-precision sensing schemes as well as the generation of complex quantum states. Through the development of these sensing tools we have been able to image superconductors, magnetic materials, and nanoscale devices at a previously unexplored resolution.
The negative charge state of the NV center in diamond is the system we use as our sensor. It is composed of neighbouring carbon atoms within the diamond lattice being replaced by a Nitrogen atom and a nearest-neighbour vacancy. The result is a stable and incredibly long-lived electron spin which can be initialised and measured via illumination with a green laser, making it a very sensitive nanoscale sensor of magnetic fields, electric fields, and temperature. Because of its sensitivity and compatibility with ambient temperature sensing, the NV center has been the focus of a significant research effort in recent years, with a wide range of applications from biological sensing to materials science and nanochemistry.
The magnetometry work of our group involves the use of NV centers in a scanning probe setup. Using an NV center on the end of an atomic force microscope (AFM) tip, we are able to scan the NV center above a sample and measure its magnetic characteristics with nanometer resolution. Along with our fabrication of diamond tips, we have been able to image samples in ways previously not achieved.
Observation of Coherent Oscillations in a Single Electron Spin
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Nanoscale imaging magnetometry with diamond spins under ambient conditions.
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High-sensitivity diamond magnetometer with nanoscale resolution
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A robust scanning diamond sensor for nanoscale imaging with single nitrogen-vacancy centres.
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Nature Nanotechnology 7, 320–324 (2012)
NV color centers in diamond have proven to be excellent magnetic field sensors, with impressive sensitivity and spatial resolution, and capable of robust operation even at room temperature [A,B]. This places them at the forefront of technologies for enabling nanoscale magnetic imaging in ambient conditions. Our work in this area is focused on harnessing the NV center to explore applications of magnetic imaging to novel materials and their fundamental properties. Topics of interest include domain formation in antiferromagnets [C], ferromagnetic resonance , and skyrmions , which are promising systems for next-generation magnetic memories.
[A] P. Appel et al.
Nanoscale microwave imaging with a single electron spin in diamond
New Journal of Physics 17, 112001 (2015)
[B] P. Maletinsky, S. Hong, M. Grinolds, et al.
A robust scanning diamond sensor for nanoscale imaging with single nitrogen-vacancy centres
Nature Nanotechnology 7, 320–324 (2012)
[C] T. Kosub et al.
Purely antiferromagnetic magnetoelectric random access memory
Nature communications 8 (2017)
Cooling the system to low temperatures augments the interaction with the NV center, a step towards all-optical control . But more importantly it allows a whole new variety of samples to be studied. Many physical systems feature exotic quantum phenomena which become apparent only at low enough temperatures.
Two refigerators--a liquid helium bath cryostat (4K) and a closed-cycle dilution refrigerator (<100mK)--are used to investigate strongly correlated electron systems. For example, graphene , oxide interfaces , and coneventional [A] and unconventional superconductors  show magnetism at ultra-low temperatures, which is imaged with high resolution using our NV center scanning probes.
[A] L.Thiel, D. Rohner, et al.
Quantitative nanoscale vortex imaging using a cryogenic quantum magnetometer
Nature Nanotechnology 11, 677-681 (2016)
By controllably coupling mechanical, electromagnetic, and spin properties of a system, a host of new effects can be explored . The variety of electronic and spin transitions available in NV centers in diamond as well as the strong influence of strain on these transitions make them interesting systems for studying the complex interactions between optical, mechanical, and spin properties. Spin-optomechanics in diamond cantilevers shows particular promise in high-precision sensing [A], entanglement generation and spin squeezing , and cooling of the cantilever motion . In our room temperature setup, we examine the ways in which the cantilever motion influences the spin properties of the NV center ground state, particularly the spin coherence time. Similarly, our low temperature system studies how the cantilever motion couples to the optical transitions of the NV center.
[A] J. Teissier, A. Barfuss, P. Appel, E. Neu, P. Maletinsky
Strain Coupling of a Nitrogen-Vacancy Center Spin to a Diamond Mechanical Oscillator
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Hybrid continuous dynamical decoupling: a photon-phonon doubly dressed spin
J. Opt 19, 044003 (2017)
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Strong mechanical driving of a single electron spin
Nature Physics 11, 820-824 (2015)
Quantum applications employing single NV centers are limited by the small generation rate of indistinguishable photons due to their long radiative lifetime, the low fraction of emission of coherent photons, and the low extraction efficiency out of the diamond.
A micro-cavity mitigates these problems by enhancing the light-matter interaction . Radiative recombination of a solid-state emitter can be accelerated and photon extraction efficiency enhanced by exploiting the weak coupling regime of cavity-QED. Solid-state monolithic micro-cavities offer limited tuning of the emitter position and the cavity mode resonance. This lack of tuning represents a problem, particularly in the present development phase where it is important to quantify the effects of the micro-cavity via the detuning dependence.
We are developing a micro-cavity that is fully tunable . It is essentially a highly miniaturized Fabry-Perot cavity: the bottom mirror is a plane mirror; the top mirror is curved to confine the light . The radius of curvature of the top mirror is typically 10 microns, and the distance between the two mirrors is at most a few microns: this results in a nearly diffraction-limited micro-cavity mode. We embed diamond micro-platelets hosting individual NV centers [A]. The interaction of the cavity mode with the NV centers significantly enhances their radiative recombination rate.
[A] D. Riedel, D. Rohner, et al.
Low-Loss Broadband Antenna for Efficient Photon Collection from a Coherent Spin in Diamond
Phys. Rev. Applied 2, 064011 (2014)
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Deterministic enhancement of coherent photon generation from a nitrogen-vacancy center in ultrapure diamond
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Imaging the Spin Texture of a Skyrmion Under Ambient Conditions Using an Atomic-Sized Sensor
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All-optical control of a single electron spin in diamond
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