Dr Marcus Doherty

Doherty, Marcus profile
Position Honorary Professor
Department Physics Education Centre
Quantum Science & Technology
Research group Solid state spectroscopy group
Office phone (02) 612 59276
Email
Office Physics New 1 26
Webpage https://www.researchgate.net/profil...

Mechanical enhancement of optical spin-polarization and readout

The defining feature of the nitrogen-vacancy (NV) centre that underpins its quantum applications is its mechanism for optical spin-polarization and readout. However, until recently this mechanism was not well understood. In collaboration with the Lukin Group at Harvard University, we recently developed and tested a model of the key component of the mechanism: the upper intersystem crossing (ISC). This new insight revealed that it maybe possible to mechanically drive the ISC of a NV centre by embedding it in a mechanical resonator. This mechanical drive has the potential to greatly improve spin polarization fidelity and readout contrast, and thus lead to the universal enhancement of the NV centre's applications. Furthermore, this new concept may be applied to better understand the optical-spin mechanisms of other defects.

The aim of this project is to understand and demonstrate the mechanical enhancement of the optical spin-polarization and readout mechanism of the NV centre.

A spin quantum bus for room-temperature quantum computing in diamond

The nitrogen-vacancy (NV) centre in diamond has been used to realize small-scale quantum computing under ambient conditions. A major barrier in the development of large-scale quantum computing in diamond is the connection of NV spin registers by a quantum bus at room temperature. Given that diamond is expected to be an ideal spin transport material, the coherent transport of spin directly between the spin registers offers a potential solution. Yet, there has been no demonstration of spin transport in diamond due to difficulties in achieving spin injection and detection via conventional methods. In our recent paper, we identified that it may be possible to optically inject and detect spin by combining the electronic properties of dopant centres, such as substitutional nitrogen, with the optical spin-polarization and readout mechanism of the NV centre. Having identified methods for optical spin injection and detection, we proposed scalable architectures for spin quantum buses that connect hybrid defect spin registers.

The aim of this project is to design and demonstrate a spin quantum bus for room-temperature quantum computing in diamond.

Analysis of surface noise sources in quantum microscopy

An outstanding problem in diamond quantum microscopy is that near-surface nitrogen-vacancy (NV) centres have comparatively short spin coherence times, and thus, provide poorer sensitivity. This reduction in spin coherence is hypothesised to be due to the greater proximity of the NV centres to the magnetic and electric noise sources on the surface (such a charge traps and paramagnetic defects) as well as surface modified spin-phonon interactions. However, confirmation of this hypothesis and the systematic study of the different noise sources and how they depend on surface preparation is required. Such an analysis will allow for the possible reduction of the net surface noise and enhanced performance of NV quantum microscopes.

The aim of this project is to analyse the surface noise sources that currently limit quantum microscopy.

Spin-mechanical quantum microscopy

Nitrogen-vacancy (NV) centres in diamond nano-mechanical structures can be used to measure forces at the nanoscale. This is possible because a force applied to such a structure will induce a mechanical stress on embedded NV centres, which perturbs their spin resonances. Owing to this generalisable transduction mechanism,  many force sensing mechanical structures are possible and they may be tailored for the measurement of specific forces. As a consequence, spin-mechanical quantum microscopy may be capable of performing force microscopy in geometries that are inaccessible to conventional techniques and, in doing so, solve long standing problems in biomechanics and the inertial analysis of chemical adsorbates.

The aim of this project is to design and demonstrate techniques of spin-mechanical quantum microscopy.

Design of phononic microstructures for enhanced quantum communications in diamond

The silicon-vacancy (SiV) centre in diamond is an optical defect that holds great promise as a quantum repeater and photon-spin bus in quantum communications and computing, respectively. However, the performance of the centre is currently limited by decoherence induced by electron-phonon interactions with E symmetric phonon modes. There has been recent attempts to eliminate these processes by using nanodiamonds that are too small to support phonons that are resonant with the electronic transitions. However, it was found that any improvement was greatly outweighed by the degradation of the optical properties by surface effects. A new idea is to eliminate these interactions by engineering phononic microstructures (rather than nanostructures) that are sufficiently large to avoid surface effects, but do not support E symmetric phonons that are resonant with the electronic transitions. In this way, symmetry, rather than size, is being used to reduce the electron-phonon processes.

The aim of this project is to design and demonstrate phononic microstructures that enhance the coherence properties of the SiV centre.

Developing a first-principles model of the optical and spin properties of defects in diamond

Despite their impressive applications in quantum technology, the physics of optical defects in diamond is not well understood. In particular, there does not exist a first-principles computational method to model the properties that are critical to quantum applications: fine/ hyperfine interactions and optical spin-polarization and readout mechanisms. The development of such a computational method would provide the necessary understanding to enhance these properties in existing defects, identify desirable properties in new defects and to even engineer defects with bespoke properties. Indeed, there is known to exist hundreds of optical defects in diamond and related materials that are waiting to be studied in depth and their suitability for quantum applications assessed. This task will only be tractable with the aid of a new computational method.

The aim of this project is to develop and demonstrate a first-principles computational method for modelling the optical and spin properties of defects in diamond.

Realising quantum microscopy in extreme conditions

Owing to the properties of the nitrogen-vacancy (NV) centre, it is known that diamond quantum microscopy can be performed in extreme conditions of temperature and pressure. Such conditions of pressure and temperature are routinely studied using diamond anvil cells (DACs) and cryostats, respectively. There is thus a clear opportunity to construct a novel instrument by embedding NV centres directly into a DAC and placing the DAC in a cryostat. Such an instrument would be capable of the nanoscale of imaging magnetic and electric fields as function of temperature and pressure over the approximate ranges of 4-700K and 0.1-60 GPa. This instrument would provide a unique tool to advance the frontiers of material science and image matter phase transitions (eg. superconducting, ferromagnetic, ferroelectric) in currently inaccessbile regions of temperature and pressure.

The aim of this project is to design and demonstrate a new instrument capable of quantum microscopy in extremes of temperature and pressure.

Single molecule electric field and vibrational resonance imaging

Diamond quantum microscopy using the nitrogen-vacancy (NV) centre is the only technique capable of the nanoscale imaging of electric fields with sufficient sensitivity to locate a single electron with nanometre resolution under ambient conditions. We have now developed advanced quantum electrometry techniques that extend this capability to the detection of individual polar chemical bonds via their electric dipole moment. Furthermore, we propose that individual bonds can be imaged with even greater resolution by spectroscopically selecting their individual vibrational resonances. We call this technique vibrational resonance imaging (VRI), since it is analogous to the techniques of single molecule magnetic resonance imaging (MRI). VRI is unique in its combination of the analytical power of vibrational spectroscopy with nanoscale resolution. VRI is also complementary to MRI, such that the combined application of these two techniques will provide complete structural information of a molecule. Combined VRI-MRI thus offers a new tool for decoding the complex molecules at the foundations of life and disease.

The aim of this project is to develop and demonstrate quantum microscopy techniques for single molecule electric field and vibrational resonance imaging.