Precision quantum sensing using interferometry of atomic matter waves has been shown for both basic and applied physics applications, from highly accurate measurement of local gravity and acceleration to general relativity testing.
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One of the main ideas of quantum physics is the wave-particle duality of matter. It was first proposed by Louis De Broglie in 1924 and holds that any particle or quantum entity can be classified as either a wave or a particle-based on how its attributes are measured.
Wave-Particle Duality
The fact that quantum matter can exist in multiple locations at once is a natural result of its wave-like nature. The particle's multiple pathways "collapse" into a single traceable journey when experimenters measure the particle's location. When using quantum interferometry, particles are first put in the lowest energy state and then moved through a region of interest.
The phase difference between the two groups is then measured. On Earth, matter-wave interferometers have been shown experimentally using a variety of particles, including atoms, electrons, neutrons, antimatter, and even biomolecules.
A purely quantum state of matter known as a Bose-Einstein condensate (BEC) can be formed by cooling atoms to temperatures below 1 microKelvin, thanks to technological advancements over the past few decades. As a result, ultracold atom interferometers that use this special matter are a top contender for precision inertial force sensing.
Light waves are recombined in optical interferometry after traveling on different routes. Either the light interferes constructively and appears bright, or it interferes destructively and appears dark, depending on the difference in phase of the waves that build along the two routes.
Atom interferometers with BECs, like high-precision optical interferometers - like the Laser Interferometer Gravitational-Wave Observatory [LIGO], which employed highly stable optical waves to detect gravitational waves recently, low-velocity quantum gasses to achieve phase-sensitive, high-precision measurements of fundamental forces, including gravity.
Additionally, they provide hitherto unheard-of rotation sensitivity and acceleration for inertial navigation. Indeed, it is demonstrated that these atom interferometers are sensitive enough to identify changes in a gravitational field, suggesting that they may be employed to map the inner density of planets.
Quantum Gravimeter
Atom interferometry modifies the quantum states of a cloud of cold atoms by applying a series of precisely timed, retro-reflected laser pulses. Because of the accelerations that occur during interferometry along the axis that the lasers probe, the atoms collect a phase. The laser system serves as an optical reference for measuring the atomic quantum phase in precise acceleration measurements.
Acceleration changes caused by local gravity can be detected by a quantum gravimeter. First, a cloud of atoms is captured and laser-cooled to nearly absolute zero in order to make measurements. Following the deactivation of the cooling lasers, the atom cloud drops in space due to gravity, and atom interferometry is conducted by directing three light pulses in the cloud's direction. The cloud is split into two momentum states by the initial pulse of the laser.
To reverse these momentum states, a second pulse is applied after a time, T. After another length T, the clouds reunite, and the last pulse is then applied to cause the atomic cloud to become interfered with. In order to determine the output states of the atoms and, consequently, the local gravity at the device, the accumulated phase is finally measured using a final set of pulses.
Since the vibrations on the retro-reflecting mirror "jumps" the phase of the laser light that the atoms see, the interferometer is extremely sensitive to these vibrations. In order to counteract this, the vibrations are quantified during interferometry using a MEMS accelerometer that is fixed on the mirror. With this knowledge, the data may subsequently be post-corrected to take vibrational phase shifts into account and provide an accurate measurement of gravity.
Quantum Accelerometer
A quantum accelerometer measures accelerations along a horizontal axis using atom interferometry.
During interferometry, the cloud passes across the probing beam as the device measures accelerations perpendicular to gravity. Large beams must thus be employed to guarantee that the cloud receives a constant laser intensity during each of the interferometer's pulses. In order to achieve the laser intensities required to maximize the interferometer's performance, more power is needed.
Atom Interferometry in Space
NASA and the field of fundamental physics had long aimed to accomplish atom interferometry in orbit. In the near future, a new precision regime for inertial force and rotation measurements is anticipated to open up for space-based atom interferometry, which holds the promise of essentially limitless free-fall time and extremely low-temperature gases. This could revolutionize both spacecraft navigation capabilities and current gravity science.
Atom interferometry was first demonstrated on an orbiting platform in May 2020, using an ultracold BEC of rubidium atoms. NASA's Cold Atom Lab (CAL), which has been functioning onboard the International Space Station (ISS) since 2018, carried out this experiment.
Outlook
The successful results obtained from matter-wave interferometry in space portend the widespread application of space-based quantum sensors in the field of cosmic science in the future. Applications of the new technology include gravitational wave and dark energy searches, general relativity testing, spacecraft navigation, and mineral prospecting on planetary bodies.
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References and Further Reading
NASA Science Editorial Team (CAL). (Dec 22, 2020) Quantum Technologies Take Flight
[Online] https://science.nasa.gov/. Available at: https://science.nasa.gov/science-research/science-enabling-technology/technology-highlights/quantum-technologies-take-flight/
Greve, G.P., Luo, C., Wu, B. et al. Entanglement-enhanced matter-wave interferometry in a high-finesse cavity. Nature 610, 472–477 (2022). https://doi.org/10.1038/s41586-022-05197-9
Cronin, A. D., Schmiedmayer, J. & Pritchard, D. E. Optics and interferometry with atoms and molecules. Rev. Mod. Phys. 81, 1051–1129 (2009).
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