Matter has dual nature; everything exists both as particle and wave. At a temperature close to absolute zero, the wave nature of atoms become observable by radiation in visible range. At such ultracold temperatures in nanoKelvin range, the atoms coalesce into a single larger entity and transition to fifth state called Bose Eisenstein Condensate (BEC) which behaves as a wave in a large packet. Like all waves, atoms in this state display the phenomenon of interference and the interference patterns of atom waves can be studied in the laboratories. Atom interferometers deployed in microgravity environment of space act as an extremely precise sensor and provide opportunity to measure most feeble accelerations. The mini fridge sized Cold Atom Laboratory (CAL) orbiting the Earth aboard International Space Station (ISS) is a research facility for the study of ultra-cold quantum gases in the microgravity environment of space. It was upgraded with Atom Interferometer few years ago. As per the report published on 13 August 2024), researchers have successfully conducted pathfinder experiments. They could measure vibrations of the ISS using a three-pulse Mach–Zehnder interferometer on board CAL facility. This was first time a quantum sensor was used in space to detect changes in immediate surrounding. The second experiment involved use of Ramsey shear-wave interferometry to manifest interference patterns in a single run. The patterns were observable for over 150 ms free-expansion time. This was the longest demonstration of the wave nature of atoms in freefall in space. The research team also measured the Bragg laser photon recoil as a demonstration of the first quantum sensor using atom interferometry in space. These developments are significant. As the most precise sensors, the space-based ultracold atom interferometers can measure extremely feeble accelerations hence offer opportunities for researchers to explore the questions (such as dark matter and dark energy, matter-anti-matter asymmetry, unification of gravity with other fields) that General Relativity and the Standard Model of particle physics cannot explain and fill the gap in our understanding of universe.
Waves display the phenomenon of interference, i.e., two or more coherent waves combine to give rise to a resultant wave which may have a higher or lower amplitude depending on the phases of the combining waves. In the case of light, we see resultant waves in the form of dark and light fringes.
Interferometry is a method of measuring characteristics using the phenomenon of interference. It involves splitting the incident wave into two beams which travel different paths then combine to form resultant interference pattern or fringes (in the case of light). The resultant interference pattern is sensitive to changes in the conditions of the travel paths of the beams, for instance, any change in the length of travel path or in any field in relation to wavelength influences interference pattern and can be used for measurements.
de Broglie wave or matter wave
Matter has dual nature; it exists both as particle as well as wave. Every moving particle or object has a wave characteristic given by de Broglie Equation
λ = h/mv = h/p = h/√3mKT
where λ is wavelength, h is Planck’s constant, m is mass, v is velocity of the particle, p is momentum, K is Boltzmann constant, and T is temperature in Kelvin.
The thermal de Broglie wavelength is inversely proportional to the square root of temperature in kelvin meaning λ will be greater at lower temperature.
Study of ultra cold atom waves
For a typical atom, the de Broglie wavelength at the room temperature is in order of angstrom (10−10 m) viz. 0.1 nanometre (1 nm=10−9 m). A radiation of a given wavelength can resolve details in the same size range. Light cannot resolve details smaller than its wavelength hence a typical atom at room temperature cannot be imaged using visible light which has a wavelength in the range of about 400 nm to 700 nm. X-rays can do because of its angstrom range wavelength but its high energy destroys the very atoms that it is supposed to observe. Therefore, the solution lies in the reducing the temperature of the atom (to below 10−6 kelvin) so that the de Broglie wavelengths of the atoms increase and become comparable to the wavelengths of visible light. At ultracold temperatures, the wave nature of the atoms becomes measurable and relevant for interferometry.
As the temperature of atoms is reduced further in nanokelvin range (10−9 kelvin) range to about 400 nK, the atomic bosons transition to the fifth state matter called Bose-Einstein condensate (BCE). At such ultra-low temperatures near absolute zero when the thermal movements of particles become extremely negligible, the atoms coalesce into a single larger entity that behaves as a wave in a large packet. This state of atoms provides opportunity to researchers to study quantum systems on a macroscopic scale. The first atomic BCE was created in 1995 in a gas of rubidium atoms. Since then, this area has seen many improvements in technology. The molecular BEC of NaCs molecules was recently created at an ultracold temperature of 5 nanoKelvin (nK).
Microgravity conditions in space is better for quantum mechanical research
The gravity in the earth-based laboratories requires use of magnetic trap to hold the atoms in place for an effective cooling. Gravity also limits the interaction time with the BECs in the terrestrial laboratories. The formation of BECs in microgravity environment of space-based laboratories overcomes these limitations. Microgravity environment can increase interaction time and reduce disturbances from the applied field, thereby better support quantum mechanical research. BCEs are now routinely formed under microgravity conditions in space.
Cold Atom Laboratory (CAL) at the International Space Station (ISS)
Cold Atom Laboratory (CAL) is a multi-user research facility based at International Space Station (ISS) for the study of ultra-cold quantum gases in the microgravity environment of space. CAL is operated remotely from the operation centre at the Jet Propulsion Laboratory.
At this space-based facility, it is possible to have observation times over 10 seconds and the ultracold temperatures below 100 picoKelvin (1 pK= 10-12 Kelvin) for the study of quantum phenomena.
The Cold Atom Lab was launched on 21 May 2018 and was installed on the ISS in late May 2018. A Bose-Einstein Condensate (BEC) was created in this space-based facility in July 2018. This was the first time; a fifth state of matter was created in Earth orbit. Later, the facility was upgraded following deployment of ultracold atom interferometers.
CAL has achieved many milestones in the recent years. Rubidium Bose–Einstein condensates (BECs) was produced in space in 2020. It was also demonstrated that microgravity environment is advantageous for cold-atom experiment.
Last year, in 2023, researchers produced dual-species BEC formed from 87Rb and 41K and demonstrated simultaneous atom interferometry with two atomic species for the first time in space in Cold Atom Laboratory facility. These achievements were important for quantum tests of universality of free fall (UFF) in space.
Recent advancement in space-based quantum technologies
As per the report published on 13 August 2024), researchers employed 87Rb atoms in the CAL atom interferometer and successfully conducted three pathfinding experiments. They could measure vibrations of the ISS using a three-pulse Mach–Zehnder interferometer on board CAL facility. This was first time a quantum sensor was used in space to detect changes in immediate surrounding. The second experiment involved use of Ramsey shear-wave interferometry to manifest interference patterns in a single run. The patterns were observable for over 150 ms free-expansion time. This was the longest demonstration of the wave nature of atoms in freefall in space. The research team also measured the Bragg laser photon recoil as a demonstration of the first quantum sensor using atom interferometry in space.
Significance of ultracold atom interferometers deployed into space
Atom interferometers harness the quantum nature of atoms and are extremely sensitive to changes in acceleration or fields hence have applications as high precision tools. Earth-based atom interferometers are used to study gravity and in advanced navigation technologies.
Space-based atom interferometers have advantages of persistent microgravity environment which offers free fall conditions with much less influence of fields. It also helps Bose-Einstein condensates (BECs) reach colder temperatures in picoKelvin range and exist for longer duration. The net effect is extended observation time hence better opportunity to study. This endows ultracold atom interferometers deployed into space with high-precision measurement capabilities and make them super-sensors.
Ultracold atom interferometers deployed in space can detect very subtle variations in gravity which is indicative of variation in densities. This can help in study of composition of planetary bodies and any mass changes.
High precision measurement of gravity can also help better understand dark matter and dark energy and in exploration of subtle forces beyond General Relativity and the Standard Model which describe observable universe.
General Relativity and the Standard Model are the two theories that describe observable universe. Standard model of particle physics is basically quantum field theory. It describes only 5 % of the universe, the rest 95% is in dark forms (dark matter and dark energy) that we do not understand. The Standard Model cannot explain dark matter and dark energy. It cannot explain matter-antimatter asymmetry as well. Similarly, gravity could not be unified with the other fields yet. The reality of universe is not fully explained by the current theories and models. Giant accelerators and observatories are unable to shed light on much of these mysteries of nature. As the most precise sensors, the space-based ultracold atom interferometers offer opportunities for researchers to explore these questions to fill the gap in our understanding of universe.
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References:
- Meystre, Pierre 1997. When atoms become waves. Available at https://wp.optics.arizona.edu/pmeystre/wp-content/uploads/sites/34/2016/03/when-atoms.pdf
- NASA. Cold Atom Laboratory – Universe Missions. Available at https://www.jpl.nasa.gov/missions/cold-atom-laboratory-cal & https://coldatomlab.jpl.nasa.gov/
- Aveline, D.C., et al. Observation of Bose–Einstein condensates in an Earth-orbiting research lab. Nature 582, 193–197 (2020). https://doi.org/10.1038/s41586-020-2346-1
- Elliott, E.R., Aveline, D.C., Bigelow, N.P. et al. Quantum gas mixtures and dual-species atom interferometry in space. Nature 623, 502–508 (2023). https://doi.org/10.1038/s41586-023-06645-w
- Williams, J.R., et al 2024. Pathfinder experiments with atom interferometry in the Cold Atom Lab onboard the International Space Station. Nat Commun 15, 6414. Published: 13 August 2024. DOI: https://doi.org/10.1038/s41467-024-50585-6 . Preprint version https://arxiv.org/html/2402.14685v1
- NASA Demonstrates ‘Ultra-Cool’ Quantum Sensor for First Time in Space. Published 13 August 2024.Avaialble at https://www.jpl.nasa.gov/news/nasa-demonstrates-ultra-cool-quantum-sensor-for-first-time-in-space
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