Quantum Sensors

Our sensor development focuses on performing precision inertial sensing, and in particular on the measurement of gravity and gravity gradients, through atom interferometry.

A large portion of our work involves bringing cold atom based gravity sensing to practical use, through improving the readiness, developing the underpinning hardware, and building demonstrators that can operate in application relevant environments – for example, targeting finding underground infrastructure using a cold atom gradiometer. We are the gravity sensing team of the UK National Quantum Technology Hub in Sensors and Metrology, and are leading in transferring knowledge and capability into industry to build up impact across a variety of potential end uses, such as within civil engineering or archaeology. Our work includes a variety of projects, ranging from those performing fundamental research aiming to improve the underpinning techniques to a cross-over between physics and engineering, and those in strong collaboration with industry. 

An atom interferometer operates by dropping or throwing up a cloud of cooled atoms, which act as ideal test masses in freefall. To measure gravity, three precisely timed pulses of light are shone onto the atoms, transferring momentum to the cloud and placing the atoms into a quantum superposition of two momentum states. The first pulse is tailored to give precisely half of the atoms an extra momentum kick. This causes one half to travel more quickly through space, splitting the cloud in two. After a time T has passed, a second pulse is used to invert the momentum difference of the two clouds, causing them to begin to move towards each other once again. Finally, after further time T a third pulse is used to close the interferometer. As shown in the figure, the result is analogous to an optical Mach-Zender interferometer where the roles of atoms and light have been reversed. The first and last pulses act as beam splitters, while the intermediate serves as a mirror. Rather than observing the interference pattern through optical intensity, the population of two atomic states is measured. During the sequence the atoms accumulate a phase difference due to gravity, allowing us to detect local changes in density.

 

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Figure: showing the basic scheme of an atom interferometer. An atom cloud is placed in a superposition of two states through interaction with a laser beam. This gives two separated clouds travelling through space. The two are then recombined to create an interferometer. Measuring the population ratio of two states then provides a sensitive measure of gravity.

Overview of Quantum Technology Hub for Sensors and Metrology research

Professor Kai Bongs introduces the Quantum Technology Hub for Sensors and Metrology in a video produced for the University Annual Meeting in March 2018. Professor Bongs gives an overview of the practical applications of the Hub’s research, and is joined by Dr Nicole Metje, Reader in Infrastructure Monitoring and Professor George Tuckwell, Divisional Director for Geoscience and Engineering at RSK, who specifically talk about the research and development in gravity sensors, which will ultimately help us to see underneath the ground.