Quantum-limited sensing

We have projects in two possible areas:

(1) Quantum-scale displacement sensing in quantum cavity optomechanics

The 2017 Nobel prize awarded for the discovery of gravitational waves followed their recent detection, for the first time, by the LIGO experiment.

Although LIGO is very much "Big Science", that work has spawned a generation of table-top experiments which exploit the extraordinary sensitivity offered by optical cavities for detecting quantum-scale displacements. The UCL optomechanics group is attempting to cool levitated nano particles and membranes into quantum regimes for foundational science as well as technological applications such as ultra sensitive accelerometers and force sensors.

In such experiments, the quantum motion of the mechanical device is detected indirectly , via the light emerging from the cavity. The measurement process itself limits sensitivity, because of back-action, imposing the so-called Standard Quantum limit. (SQL) The project investigates means of attaining and even overcoming the SQL, using correlations in the cavity field (Phys Rev Lett, 120, 020503 (2018) in the detection method.

(2) Nanoscale NMR and imaging using colour centres in diamond

The unique properties of colour centres in diamond have the potential to transform and revolutionise NMR and MRI in the future. Within the last few months, nanoscale NMR has been achieved with zepto-liter samples, effectively reducing the number of spins in the detected volume by 12 orders of magnitude relative to conventional NMR .

Notably, the experiments employed an entanglement-based algorithm, storage in long-live quantum memories using nearby nuclear spins which can be augmented by quantum error correction methods. In sum, techniques developed for fields such as quantum information, are gradually being adapted for applications in biophysics and medicine. This has included, for example, dynamical processes such as measurement of currents flowing down neural cells (axons) by a single NV spin sensor which detects the associated nanoTesla magnetic fluctuations within a millisecond .

We are currently investigating and developing the potential of quantum approaches such as Floquet theory to relate the sequences of microwave pulses using in single-spin sensing to the underlying atomic scale structure as an alternative to more usual methods based on signalprocessing approaches. This allows for better understanding of quantum backaction and strong coupling regimes which are less straightforward to analyse in comparison with classical signals or weakly coupled spins.

For further information contact: Prof. T S Monteiro:

TEL.+44 (020)7679 3504 t.monteiro@ucl.ac.uk