Figure. Schematic of the magneto-optical trap (MOT). The atoms (green spheres) are captured and cooled using three pairs of opposing laser beams (blue arrows) and a magnetic field generated by coils placed above and below the atoms. Source: Niels Bohr Institute. University of Copenhagen.
by Amara Graps
From Academic work
One of my favorite aspects of the quantum sensors sector is that the measurements provide an immediate relationship to physical constants and fundamentals. Atom interferometry is also multipurpose, as the same fundamental device can be used to perform different sensing functions (time, rotation, magnetic fields, acceleration/gravity) with software directing the laser control pulses.
The atom interferometer’s versatility is evident in Chad Orzel’s recollection, of his post-doc days:
When I was a post-doc, the lab I worked in got funding from the Navy to develop atom-based detectors of rotation and gravity gradients because they might be useful in submarine navigation. A sensitive rotation detector can tell you how you’ve changed the orientation of the submarine, and a gravity sensor could help detect underwater mountains and other obstacles, in both cases without needing to send out sonar pulses that other people could detect.
Professors, consider including lectures on quantum sensors in your upper undergraduate physics courses if you haven’t previously, as you may ‘hook’ a few motivated new researchers. I didn’t learn about quantum sensors in my physics academic coursework, so this statement in GQI’s Quantum Sensing Outlook Report (*) captured my attention.
Academic work has for some time identified the appealing potential of cold atom interferometers to measure gravity/acceleration and gravity gradient.
If you follow the references for this “Academic work”, then you’ll find yourself in the laboratories of Nobel Prize winners. In 1997, the Nobel Prize in Physics went jointly to Steven Chu, Claude Cohen-Tannoudji and William D. Phillips for their developments of methods to cool and trap atoms with laser light. The Nobel Prize committee explained that the three Nobel Laureates set the foundation for advancements: 1) the construction of atomic clocks, 2) the measurement of the acceleration of gravity, and 3) atomic lithography. See also Nature’s: Nobel prizes honor atom-trappers.
To Optical Molasses
Steven Chu’s Stanford students were on the cutting edge, constructing devices demonstrating:
- ‘optical molasses’,
- ‘atomic fountains’,
- ‘optical tweezers’, and
- ‘Sisyphus cooling’,
which points to the second reason I like this field: its delightful, memorable labels.
Unlike cooling, which necessitates a velocity-dependent force, trapping requires the light to impose a position-dependent force on the atoms. The Magneto-Optical Trap (MOT) is the most frequently utilized of the trap types. Three pairs of counter-propagating laser beams and an inhomogeneous magnetic field are used in a MOT to achieve entrapment.
Figure. Optical molasses. Doppler cooling with three pairs of counter-propagating laser beams. In this configuration, an atom, regardless of what direction it is moving, will encounter a friction force. In this way the velocity spread (and the temperature) will be reduced. The action of the laser light on the atoms is like that of a sticky medium, giving rise to the term optical molasses. Source: Wikipedia.
In GQI’s Quantum Sensing Outlook Report (*), the subset of atom interferometers is described in detail amongst the other quantum sensor modalities. The operational principles of each type of Gravimeter, Gradiometer, Accelerometer is explained, compared and summarized.
- Conventional Gravimeters can be broadly classified as relative or absolute gravimeters.
- Relative Gravimeters typically use the displacement of a spring to measure gravity.
- Quantum Absolute Gravimeter: The cloud of neutral atoms is cooled in a MOT. The cloud is in a condition of superposition. The cloud falls freely before being reassembled and measured. The resulting interference fringes enable a sensitive measurement of absolute gravity.
- The Differential Quantum Gravimeter & Gradiometer. A variant is to use two quantum gravimeters close together to measure gravity gradients
See a Summary from the Report in the next Table. The acronyms are ESL = Engineering Severity Level, and SWaP = Size Weight and Power.
(*) GQI’s Quantum Sensing Outlook Report is a 114p analysis, up and down the quantum sensing stack: the physics package, control package, control logic and framework required to access different sensing modalities (time, magnetic field, electric field, gravity & acceleration and others) for advanced applications. If you are interested to learn more, please don’t hesitate to contact info@global-qi.com.
October 1, 2024
