Researchers in France have devised a new technique in quantum sensing that uses trapped ultracold atoms to detect acceleration and rotation. They then combined their quantum sensor with a conventional, classical inertial sensor to create a hybrid system that was used to measure acceleration due to Earth’s gravity and the rotational frequency of the Earth. With further development, the hybrid sensor could be deployed in the field for applications such as inertial navigation and geophysical mapping.
Measuring inertial quantities such as acceleration and rotation is at the heart of inertial navigation systems, which operate without information from satellites or other external sources. This relies on the precise knowledge of the position and orientation of the navigation device. Inertial sensors based on classical physics have been available for some time, but quantum devices are showing great promise. On one hand, classical sensors using quartz in micro-electro-mechanical (MEM) devices have gained widespread use due to their robustness and speed. However, they suffer from drifts – a gradual loss of accuracy over time, due to several factors such as temperature sensitivity and material aging. On the other hand, quantum sensors using ultracold atoms achieve better stability over long operation times. While such sensors are already commercially available, the technology is still being developed to achieve the robustness and speed of classical sensors.
Now, the Cold Atom Group of the French Aerospace Lab (ONERA) has devised a new method in atom interferometry that uses ultracold atoms to measure inertial quantities. By launching the atoms using a magnetic field gradient, the researchers demonstrated stabilities below 1 µm/s2 and 1 µrad/s for acceleration and rotation measurements over 24 hours respectively. This was done by continuously performing a 4 s interferometer sequence on the atoms for around 20 min to extract the inertial quantities. That is equivalent to driving a car for 20 min straight and knowing the acceleration and rotation to the µm/s2 and µrad/s level.
Cold-atom accelerometer–gyroscope
They built their cold-atom accelerometer–gyroscope using rubidium-87 atoms. By holding the atoms in a magneto-optical trap, the researchers cool them down to 2 µK, enabling good control over the atoms for further manipulation. By releasing the atoms from the trap, the atoms freely fall along the gravity direction. This allows the researchers to measure their free falling acceleration using atom interferometry. In their protocol, a series of three light pulses that coherently splits an atomic cloud into two paths, redirects and then recombines it allowing the cloud to interfere with itself. From the phase shift of the interference pattern, the inertial quantities can be deduced.
Measuring their rotation rates however, requires that the atoms have an initial velocity in the horizontal direction. This is done by applying a horizontal magnetic field gradient, which results in a horizontal force on atoms with magnetic moments. The rubidium atoms are prepared in one of the magnetic states known as the Zeeman sublevels. The researchers then use a pair of coils that they called the “launching coils” in the horizontal plane to create the necessary magnetic field gradient to give the atoms a horizontal velocity. The atoms are then transferred back to the ground non-magnetic state using a microwave pulse before performing atom interferometry. This avoids any additional magnetic forces that can affect interferometer’s outcome.
Analysing the launch velocity using laser pulses with tuned frequencies, the researchers are able to discriminate the velocity of the atoms whether it being from the magnetic launching scheme or other effects. The researchers observe two dominant and symmetric peaks associated to the velocity of the atoms due to the magnetic launching. However, they also observe a third smaller peak in between. This peak is due to an unwanted effect from the laser beams that transfers additional velocity to the atoms. Further improvement in the stability of the laser beams’ polarization – the orientation of its oscillating electric field with respect to its propagation axis, as well the current noise in the launching coils will allow for more atoms to be launched.
Using their new launch technique, the researchers operated their cold-atom dual accelerometer–gyroscope for two days straight, averaging down their results to obtain an acceleration measurement of 7×10−7 m/s2 and a rotation rate of 4×10−7 rad/s, limited by residual ground vibration noise.
Best of both worlds
While classical sensors suffer from long term drifts, they operate continuously in comparison to a quantum sensor that requires preparation of the atomic sample and the interferometry process which takes around half a second. For this reason, a classical–quantum hybrid sensor benefits from the long-term stability of the quantum sensor and the fast repetition rate of the classical one. By attaching a commercial classical accelerometer and a commercial classical gyroscope to the atom interferometer, they implemented a feedback loop on the classical sensor’s outputs. The researchers demonstrated a respective 100-fold and three-fold improvement on the acceleration and rotation rates stabilities, respectively, of the classical sensors compared to when they are operated alone.
Operating this hybrid sensor continuously and utilizing their magnetic launch technique, the researchers report a measure of the local acceleration due gravity in their laboratory of 980,881.397 mGal (the milligal is a standard unit of gravimetry). They measured Earth’s rotation rate to be 4.82 × 10−5 rad/s. Cross checking with another atomic gravimeter, they find their acceleration value deviating by 2.3 mGal, which they regard to be due to misalignment of the vertical interferometer beams. Their rotation measurement has a significant error of about 25%, which the team attributes to wave-front distortions for the Raman beams used in their interferometer.
Yannick Bidel, a researcher working on this project, explains how such an inertial quantum sensor has room for improvement. Large-momentum-transfer, a technique to increase the arm separation of the interferometer, is one way to go. He further adds that once they reach bias stabilities of 10−9 to 10−10 rad/s within a compact size atom interferometer, such a sensor could become transportable and ready for in-field measurement campaigns.
The research is described in Science Advances.
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