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At any moment, trillions of particles known as neutrinos are flowing through our bodies and everything in our environment, without any observable effect. Smaller than electrons and lighter than photons, these elusive particles are the most plentiful entities with mass in the cosmos.
The precise mass of a neutrino remains largely unknown. These particles are so minuscule and interact so infrequently with matter that measuring them is exceptionally challenging. Researchers strive to achieve this by utilizing nuclear reactors and gigantic particle accelerators to create unstable atoms, which subsequently decay into various byproducts, including neutrinos. Through this method, physicists can generate streams of neutrinos that they can examine for characteristics such as the particle’s mass.
Now, physicists at MIT suggest a far more compact and efficient method to produce neutrinos that could be achieved in a small-scale experiment.
In a publication featured in Physical Review Letters, the physicists unveil the idea of a “neutrino laser” — a surge of neutrinos that could be created by laser-cooling a gas of radioactive atoms to temperatures lower than interstellar space. At such extremely low temperatures, the team anticipates that the atoms will behave as a singular quantum entity and undergo radioactive decay in unison.
The decay of radioactive atoms inherently releases neutrinos, and the physicists assert that in a coherent, quantum state, this decay should intensify, along with the production of neutrinos. This quantum phenomenon should generate an amplified stream of neutrinos, reminiscent of how photons are enhanced to create conventional laser light.
“In our design for a neutrino laser, the neutrinos would be emitted at a significantly faster pace than usual, somewhat akin to how a laser emits photons swiftly,” says study co-author Ben Jones PhD ’15, an associate professor of physics at the University of Texas at Arlington.
For illustration, the team calculated that such a neutrino laser could be realized by confining 1 million atoms of rubidium-83. Usually, these radioactive atoms have a half-life of approximately 82 days, implying that half the atoms decay, emitting an equivalent number of neutrinos every 82 days. The physicists demonstrate that by cooling rubidium-83 to a coherent, quantum state, the atoms should undergo radioactive decay in just minutes.
“This presents a groundbreaking approach to accelerate radioactive decay and neutrino production, which to my knowledge, has never been attempted,” remarks co-author Joseph Formaggio, professor of physics at MIT.
The team aspires to construct a small tabletop demonstration to validate their concept. If successful, they envision that a neutrino laser could serve as a novel mode of communication, allowing particles to be transmitted directly through the Earth to subterranean stations and habitats. The neutrino laser might also provide an effective source of radioisotopes, which, along with neutrinos, are byproducts of radioactive decay. Such radioisotopes could enhance medical imaging and cancer detection.
Coherent condensate
For every atom in the cosmos, there are roughly a billion neutrinos. A significant portion of these invisible particles is believed to have formed in the initial moments after the Big Bang, persisting in what physicists term the “cosmic neutrino background.” Neutrinos are also generated whenever atomic nuclei fuse or split apart, such as during the fusion reactions in the sun’s core and through the normal decay of radioactive substances.
A few years ago, Formaggio and Jones individually contemplated a unique possibility: What if a natural mechanism for neutrino production could be amplified through quantum coherence? Initial explorations revealed fundamental barriers in materializing this idea. Years later, while discussing the properties of ultracold tritium (an unstable hydrogen isotope that undergoes radioactive decay), they pondered: Could enhancing the production of neutrinos occur if radioactive atoms such as tritium were cooled sufficiently to achieve a quantum state referred to as a Bose-Einstein condensate?
A Bose-Einstein condensate, or BEC, is a state of matter that emerges when a gas of certain particles is cooled down to near absolute zero. At this temperature, the particles reach their lowest energy state and cease to move independently. In this extreme cold, the particles begin to “sense” each other’s quantum effects, allowing them to function as a single coherent entity — a distinctive phase that can lead to exotic physics.
BECs have been realized in various atomic species. (One of the initial achievements was with sodium atoms, accomplished by MIT’s Wolfgang Ketterle, who shared the 2001 Nobel Prize in Physics for this result.) However, no one has produced a BEC from radioactive atoms. Achieving this would be particularly challenging, as most radioisotopes possess short half-lives and would completely decay before they could be sufficiently cooled to form a BEC.
Nevertheless, Formaggio speculated that if radioactive atoms could be transformed into a BEC, would this enhance the generation of neutrinos in some manner? Initially working through the quantum mechanical calculations, he found that such an effect was unlikely.
“It turned out to be a red herring — we can’t speed up the radioactive decay process and neutrino generation merely by forming a Bose-Einstein condensate,” Formaggio explains.
In harmony with optics
Years later, Jones revisited the notion, incorporating an additional element: superradiance — a phenomenon of quantum optics that occurs when a group of light-emitting atoms is stimulated to act in unison. In this coherent phase, it’s predicted that the atoms should emit a burst of photons that is “superradiant,” or more intense than when the atoms are typically out of sync.
Jones suggested to Formaggio that perhaps a similar superradiant effect could occur in a radioactive Bose-Einstein condensate, potentially resulting in a comparable burst of neutrinos. The physicists returned to the drawing board to derive the equations of quantum mechanics that govern how light-emitting atoms transition from a coherent starting state to a superradiant state. They employed the same equations to analyze the behavior of radioactive atoms within a coherent BEC state.
“The outcome indicates: You receive significantly more photons at a faster rate, and when you apply the same principles to a system that generates neutrinos, it will result in a much larger quantity of neutrinos produced more rapidly,” Formaggio clarifies. “That’s when the puzzle pieces fell into place, realizing that superradiance in a radioactive condensate could enable this accelerated, laser-like neutrino emission.”
To validate their theoretical concept, the team calculated how neutrinos would be generated from a cloud of 1 million super-cooled rubidium-83 atoms. They discovered that in the coherent BEC state, the atoms underwent radioactive decay at an increasing rate, emanating a laser-like stream of neutrinos within minutes.
Now that the physicists have theorized that a neutrino laser is feasible, they intend to test the concept through a small tabletop setup.
“It should be sufficient to take this radioactive material, vaporize it, trap it using lasers, cool it down, and then convert it into a Bose-Einstein condensate,” Jones remarks. “Then it should begin to exhibit this superradiance spontaneously.”
The duo acknowledges that such an experiment will necessitate numerous precautions and meticulous manipulation.
“If it turns out we can demonstrate this in the laboratory, then people can contemplate: Can we utilize this as a neutrino detector? Or as a revolutionary communication medium?” Formaggio states. “That’s when the real excitement begins.”
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