Using quantum sensors to enable precision measurements of gravity, inertial forces and magnetic fields
Atomic magnetometry is the monitoring of atomic spin in external magnetic fields, specifically the Larmor atom spin precession. The strength of the magnetic field to be measured is mapped on to the amplitude (radio-frequency magnetometers) or the frequency (dc magnetometers) of the atomic coherence oscillations.
There are three areas of magnetometry research at NPL:
- Research – We are exploring the factors that define fundamental limits to the sensor performance (such as quantum noise and back-action) and the techniques that may reduce their amplitudes (including entanglement and quantum non-demolition measurements).
- Measurement techniques – We are developing the measurement techniques that are used to implement atomic magnetometry technology such as low-field Nuclear Magnetic Resonance and eddy current imaging.
- Instrumentation – We are developing portable radio-frequency atomic magnetometers able to operate in unshielded industrial environments. Portable magnetometers will be suitable for non-destructive imaging of structural defects in steel.
In atom interferometers a matter wave is coherently split into partial waves, which propagate along different paths and are then recombined to produce interference fringes. The spatial position of the fringes depends on relative phase shifts between the partial waves, which are accumulated along the different paths. The phases of the matter waves are very sensitive to inertial and electromagnetic forces, which makes atom interferometers ideal for quantum sensors.
At NPL we are working on inertial sensors based on atom interferometers, which can be used for the measurement of gravity, linear accelerations and rotations.
An NPL gravity gradiometer and absolute gravimeter based on a double rubidium atomic fountain is currently under optimisation. The gradiometer is based on a pair of pulsed atom interferometers, which are produced by two spatially-separated and free-falling clouds of laser-cooled atoms. Use of the same pulsed Raman laser beams for simultaneous coherent momentum splitting and recombining of both atomic clouds makes it possible to reject the common phase noise of the Raman laser beam, including the phase noise caused by the mechanical vibrations of the reference mirror. This is the main advantage of the quantum gravity gradiometer over similar classical devices.
We are also developing a new type of continuous all-optical waveguide atom interferometer based on guiding of ultra-cold atoms along laser beams (waveguides) and their diffraction splitting by optical lattices produced by the optical interference of the crossing waveguides. Such atom interferometers can also be implemented in all-optical atom chips based on guiding of atoms in two-colour evanescent light waves generated near to integrated planar optical waveguides. One of the advantages of such an approach is that the corresponding atomic waveguides can be curved. Therefore, a true Sagnac interferometer for matter waves can be realised, which is very attractive for atom gyroscopes because of its non-sensitivity to linear accelerations. Such interferometers can have sizes of several centimetres, while their sensitivity to inertial forces can be as high as those of much bigger interferometers based on free-falling atoms.