Accurate measurements of gravity are needed by geologists and navigators alike. But conventional gravimeters are bulky instruments that take a long time to 'settle' after each measurement. What if you need to track rapid changes in gravity — say, while moving underground or in flight? That calls for a device that is small and responsive.
The authors propose building such a device on a single chip. The core is a superconducting qubit, a quantum 'switch' that is exquisitely sensitive to the magnetic flux through its loop. Next to the qubit, a tiny beam is suspended; under gravity it bends slightly, and this deflection changes the magnetic flux in the loop. The qubit's frequency shifts, and from that shift, you can calculate the gravitational force.
The key trick is how to read out the signal. The qubit quickly loses its quantum 'phase' due to the beam's jitter, but if you measure at moments when the beam returns to its starting position, the noise almost disappears. Such stroboscopic sampling yields sensitivities from hundreds to thousands of nanogals per root hertz, with measurement times under a millisecond.
The device can be tuned electrically and calibrated with a microwave signal — no mechanical standards needed. This paves the way for compact, fast, and accurate chip-scale gravimeters for field work and autonomous navigation.
One nanogal is roughly one billionth of the acceleration due to gravity. A layer of water just three centimeters thick under the instrument creates such a difference.
In the movie 'Interstellar,' the characters use gravitational anomalies for navigation; a chip-scale gravimeter could be the first step toward a pocket device that senses the slightest changes in gravity in real time.